Articles in press have been peer-reviewed and accepted, which are not yet assigned to volumes /issues, but are citable by Digital Object Identifier (DOI).
Display Method:
Display Method:
2024, 49(7): 2301-2314.
doi: 10.3799/dqkx.2024.070
Abstract:
With the deepening of offshore oil and gas exploration in China, the proportion of tectonic oil and gas reservoirs has decreased significantly, and complex fields such as deep layer, buried hill, and lithology have become new growth poles in the searching of offshore oil and gas. This poses a new challenge to seismic exploration technology. In order to solve the geological problems in these complex fields, seismic acquisition methods such as small bins, high folds, ultra-long offsets, wide/multi azimuth Ocean Bottom Nodes are constantly emerging. The geometry and operation methods for offshore seismic acquisition have become increasingly complex, leading to a substantial increase in the cost of acquisition while enhancing the quality of seismic data under complex offshore geological conditions. It is urgent to practice the concept of value-driven exploration, reduce acquisition cost through technical innovation, and lead the high-quality development of offshore seismic exploration. This paper focuses on introducing several efficient offshore seismic acquisition technologies, including analysis of current status, applications, and prospects of technological development. While improving the quality of offshore seismic data and enhancing exploration efficiency, concerted efforts are made to raise acquisition efficiency and develop an economically and technically integrated offshore seismic exploration technology. This is essential for achieving value-driven exploration and promoting the high-quality development of offshore oil and gas exploration.
With the deepening of offshore oil and gas exploration in China, the proportion of tectonic oil and gas reservoirs has decreased significantly, and complex fields such as deep layer, buried hill, and lithology have become new growth poles in the searching of offshore oil and gas. This poses a new challenge to seismic exploration technology. In order to solve the geological problems in these complex fields, seismic acquisition methods such as small bins, high folds, ultra-long offsets, wide/multi azimuth Ocean Bottom Nodes are constantly emerging. The geometry and operation methods for offshore seismic acquisition have become increasingly complex, leading to a substantial increase in the cost of acquisition while enhancing the quality of seismic data under complex offshore geological conditions. It is urgent to practice the concept of value-driven exploration, reduce acquisition cost through technical innovation, and lead the high-quality development of offshore seismic exploration. This paper focuses on introducing several efficient offshore seismic acquisition technologies, including analysis of current status, applications, and prospects of technological development. While improving the quality of offshore seismic data and enhancing exploration efficiency, concerted efforts are made to raise acquisition efficiency and develop an economically and technically integrated offshore seismic exploration technology. This is essential for achieving value-driven exploration and promoting the high-quality development of offshore oil and gas exploration.
2024, 49(7): 2315-2329.
doi: 10.3799/dqkx.2022.485
Abstract:
In order to discuss the relationship between sedimentary enviroment and organic matter accumulation in black shale of Middle Ordovician Shengping Formation, the Xishuiyuan Section in Wenqiao Town, Quanzhou County, northern Guangxi was chosen. The organic carbon content, kerogen carbon isotope, major and trace elements were analyzed to investigate paleo-redox, paleo-productivity, hydrothermal sedimentation, clastic influx and water limitation of the Middle Ordovician sedimentary environment in the northern Guangxi. The results show that the lower member of Shengping Formation is mainly composed of mud-rich siliceous shale with TOC of 1.45%-3.04%, and the upper member is mainly composed of siliceous shale with TOC of 0.63%-2.69%. The source of organic matter in the lower member of Shengping Formation is mainly type I kerogen, while the source of organic matter in the upper member may be type II kerogen in addition to type I kerogen. The sedimentary period of Shengping Formation in northern Guangxi is a suboxic and anoxic deep-water shelf and basin facies environment. The organic matter accumulation of mud-rich siliceous shale is a dual control pattern of productivity and preservation conditions. The organic matter accumulation of siliceous shale is a preservation condition pattern.
In order to discuss the relationship between sedimentary enviroment and organic matter accumulation in black shale of Middle Ordovician Shengping Formation, the Xishuiyuan Section in Wenqiao Town, Quanzhou County, northern Guangxi was chosen. The organic carbon content, kerogen carbon isotope, major and trace elements were analyzed to investigate paleo-redox, paleo-productivity, hydrothermal sedimentation, clastic influx and water limitation of the Middle Ordovician sedimentary environment in the northern Guangxi. The results show that the lower member of Shengping Formation is mainly composed of mud-rich siliceous shale with TOC of 1.45%-3.04%, and the upper member is mainly composed of siliceous shale with TOC of 0.63%-2.69%. The source of organic matter in the lower member of Shengping Formation is mainly type I kerogen, while the source of organic matter in the upper member may be type II kerogen in addition to type I kerogen. The sedimentary period of Shengping Formation in northern Guangxi is a suboxic and anoxic deep-water shelf and basin facies environment. The organic matter accumulation of mud-rich siliceous shale is a dual control pattern of productivity and preservation conditions. The organic matter accumulation of siliceous shale is a preservation condition pattern.
2024, 49(7): 2330-2345.
doi: 10.3799/dqkx.2023.098
Abstract:
As the main sediments across the Yangtze area during the Ordovician-Silurian (O-S) transition, the Upper Ordovician Wufeng Formation and Lower Longmaxi Formation are widely distributed in the Yangtze Sea, and they are also the main targets for marine shale gas exploration in the South China. However, little attention has been paid to the Wufeng-Longmaxi black rock series in the foreland basin, southeastern Yangtze Block, due to the lack of complete and fresh outcrops caused by intensive erosion and tectonic damage. In this paper, by utilizing the newly obtained drilling cores of the Well TD2 located in the Hunan Province, the sedimentary environment and organic matter accumulation of the Wufeng-Longmaxi black rock series in the foreland basin were investigated based on the systematically petrological and geochemical analysis. The organic-rich shale of the Wufeng-Longmaxi formations is dominated by siliceous shale and carbonaceous shale, in which plenty of terrigenous clast and even thin sandstone can be observed. The black shale with TOC > 2% is about 21 m in thickness. Four sedimentary cycles can be recognized from the O-S black series and they exhibit different lithological and geochemical variation characteristics. The organic shale generally developed in the Late Katian to the Early Rhuddanian, while the black rock series developed during the Late Rhuddanian to Early Aeronian have low TOC content and are discontinuous. Based on the analyses of lithology, mineral composition and geochemistry, tectonism, sea-level and terrigenous supply should be the primary controlling factors for the deposition of the O-S black shale in the foreland basin, Yangtze area. During the Late Katian to Early Rhuddanian, the study area experienced significantly tectonic subsidence and sea-level rise caused by regional tectonic compression and global sea-level rise, then strongly reducing conditions were formed and paleoproductivity was improved in the deep-depression area, which promoted the enrichment of organic matter in black rock series during this stage. Nevertheless, with intensifying tectonic collision between the Yangtze and Cathaysia blocks since Late Rhuddanian, the tectonism in the study area changed from tectonic subsidence to uplift. Persistent tectonic uplift and intensive weathering conditions led to the noticeable increase of terrigenous supply in the foreland depression, as well as gradual sea-level fall, and then, the development of black organic-rich shale in the foreland basin was terminated at the Early Aeronian. It should be noted that during the earliest Aeronian stage, global glacial sea-level rise promotes the development of black shale in the study area again. Finally, a sedimentary model for the O-S black shale in the foreland depression, western Hunan area was proposed. This study is not only beneficial for gaining more geological knowledge about the black shale formation in a marine foreland basin, but also helpful for understanding the sedimentary responses to major geological events occurred in Yangtze area during the O-S transition.
As the main sediments across the Yangtze area during the Ordovician-Silurian (O-S) transition, the Upper Ordovician Wufeng Formation and Lower Longmaxi Formation are widely distributed in the Yangtze Sea, and they are also the main targets for marine shale gas exploration in the South China. However, little attention has been paid to the Wufeng-Longmaxi black rock series in the foreland basin, southeastern Yangtze Block, due to the lack of complete and fresh outcrops caused by intensive erosion and tectonic damage. In this paper, by utilizing the newly obtained drilling cores of the Well TD2 located in the Hunan Province, the sedimentary environment and organic matter accumulation of the Wufeng-Longmaxi black rock series in the foreland basin were investigated based on the systematically petrological and geochemical analysis. The organic-rich shale of the Wufeng-Longmaxi formations is dominated by siliceous shale and carbonaceous shale, in which plenty of terrigenous clast and even thin sandstone can be observed. The black shale with TOC > 2% is about 21 m in thickness. Four sedimentary cycles can be recognized from the O-S black series and they exhibit different lithological and geochemical variation characteristics. The organic shale generally developed in the Late Katian to the Early Rhuddanian, while the black rock series developed during the Late Rhuddanian to Early Aeronian have low TOC content and are discontinuous. Based on the analyses of lithology, mineral composition and geochemistry, tectonism, sea-level and terrigenous supply should be the primary controlling factors for the deposition of the O-S black shale in the foreland basin, Yangtze area. During the Late Katian to Early Rhuddanian, the study area experienced significantly tectonic subsidence and sea-level rise caused by regional tectonic compression and global sea-level rise, then strongly reducing conditions were formed and paleoproductivity was improved in the deep-depression area, which promoted the enrichment of organic matter in black rock series during this stage. Nevertheless, with intensifying tectonic collision between the Yangtze and Cathaysia blocks since Late Rhuddanian, the tectonism in the study area changed from tectonic subsidence to uplift. Persistent tectonic uplift and intensive weathering conditions led to the noticeable increase of terrigenous supply in the foreland depression, as well as gradual sea-level fall, and then, the development of black organic-rich shale in the foreland basin was terminated at the Early Aeronian. It should be noted that during the earliest Aeronian stage, global glacial sea-level rise promotes the development of black shale in the study area again. Finally, a sedimentary model for the O-S black shale in the foreland depression, western Hunan area was proposed. This study is not only beneficial for gaining more geological knowledge about the black shale formation in a marine foreland basin, but also helpful for understanding the sedimentary responses to major geological events occurred in Yangtze area during the O-S transition.
2024, 49(7): 2346-2358.
doi: 10.3799/dqkx.2023.128
Abstract:
The continental shale in the Permian Fengcheng Formation of the Mahu Sag in the Junggar Basin has a maximum burial depth exceeding 5 000 m, rich in oil and gas resources. The widely developed natural fracture plays a crucial role in the accumulation and exploration of hydrocarbons in these reservoirs. A classification scheme is established for natural fractures in deep continental shale based on their geological genesis and occurrence by observing cores, image logs, thin sections, and SEM samples. Moreover, the development characteristics and effectiveness of different types of natural fractures Were analyzed, and the natural fracture heterogeneity and their contributions to reservoirs are discussed. Accordingly, natural fractures in deep continental shale are divided into tectonic, diagenetic, and abnormal high-pressure related fractures based on their geological genesis. Tectonic fractures can be subdivided into translayer shear, bed-parallel shear, and intralayer open fractures based on their occurrence, while diagenetic fractures are divided into bedding, stylolite, and shrinkage fractures. Tectonic fractures have relatively large scales, obvious groups, and high dipping and nearly vertical angles. Diagenetic fractures mainly develop horizontally, with curved surfaces and easily branched extensions. Intralayer open, bedding, and stylolite fractures are the dominant types of fractures in deep continental shale reservoirs. Natural fractures can be filled in varying degrees by minerals such as calcite and fine-grained mixtures containing organic matter, among which tectonic and bedding fractures are less filled, and stylolites are easier to be filled. Microscopic tectonic fractures have smaller apertures, while diagenetic fractures usually have larger apertures. This study speculates that tectonic fractures mainly provide effective pathways for fluid flow in reservoirs, while bedding fractures are more developed, which are not only seepage channels for reservoir fluid but also an important part of the effective storage space. The research results provide an important reference for improving the classification scheme of natural fractures in deep continental shale and for better understanding of the natural fracture distribution in such reservoirs.
The continental shale in the Permian Fengcheng Formation of the Mahu Sag in the Junggar Basin has a maximum burial depth exceeding 5 000 m, rich in oil and gas resources. The widely developed natural fracture plays a crucial role in the accumulation and exploration of hydrocarbons in these reservoirs. A classification scheme is established for natural fractures in deep continental shale based on their geological genesis and occurrence by observing cores, image logs, thin sections, and SEM samples. Moreover, the development characteristics and effectiveness of different types of natural fractures Were analyzed, and the natural fracture heterogeneity and their contributions to reservoirs are discussed. Accordingly, natural fractures in deep continental shale are divided into tectonic, diagenetic, and abnormal high-pressure related fractures based on their geological genesis. Tectonic fractures can be subdivided into translayer shear, bed-parallel shear, and intralayer open fractures based on their occurrence, while diagenetic fractures are divided into bedding, stylolite, and shrinkage fractures. Tectonic fractures have relatively large scales, obvious groups, and high dipping and nearly vertical angles. Diagenetic fractures mainly develop horizontally, with curved surfaces and easily branched extensions. Intralayer open, bedding, and stylolite fractures are the dominant types of fractures in deep continental shale reservoirs. Natural fractures can be filled in varying degrees by minerals such as calcite and fine-grained mixtures containing organic matter, among which tectonic and bedding fractures are less filled, and stylolites are easier to be filled. Microscopic tectonic fractures have smaller apertures, while diagenetic fractures usually have larger apertures. This study speculates that tectonic fractures mainly provide effective pathways for fluid flow in reservoirs, while bedding fractures are more developed, which are not only seepage channels for reservoir fluid but also an important part of the effective storage space. The research results provide an important reference for improving the classification scheme of natural fractures in deep continental shale and for better understanding of the natural fracture distribution in such reservoirs.
2024, 49(7): 2359-2372.
doi: 10.3799/dqkx.2023.011
Abstract:
In order to fully understand the hydrocarbon generation potential of the Paleocene Yueguifeng, Lingfeng and Mingyuefeng formations in the Lishui Sag, East China Sea Basin, variations in paleoclimate and paleoenvironment and their influence on organic matter enrichment are investigated through analyzing major and trace elements. Results show that the Paleocene sediments are low in terrigenous detrital components and rich in authigenic components, where major and trace elements did not experience diagenetic alteration. Multiple geochemical proxies suggest that paleoclimate during the Yueguifeng, Lingfeng and Mingyuefeng formations depositional periods was humidity, drought and humidity, respectively, while corresponding paleo-salinity was brackish water, saline water and fresh water, and paleowater depth was relatively deep water, relatively shallow water and shallow water, respectively. Water column evolved from reducing condition to oxidizing condition, and then to weak oxidizing-oxidizing condition, respectively. Organic matter productivity and preservation/degradation controlled by co-variations of paleoclimate and paleoenvironment accounted for the difference in organic matter enrichment of the Yueguifeng, Lingfeng and Mingyuefeng formations mudstones. The Yueguifeng Formation mudstone was deposited in semi-deep to deep lake environment with high productivity under warm-humid climate. Considerable planktonic algae were efficiently preserved at a stratified and dysoxic water column, resulting in high organic matter abundance. The Lingfeng Formation mudstone was developed at oxygen-enriched shallow marine environment under arid climate, which was not conducive to the reproduction and preservation of planktonic algae, giving rise to low organic matter abundance. The Mingyuefeng Formation coal-bearing mudstone was deposited in marine-continental transitional environment with low productivity under warm-humid climate, where flat terrain and considerable higher plants contributed to the development of alternated beds of coal and mudstone. The future oil and gas exploration in the Lishui Sag should focus on hydrocarbon generation center of the Yueguifeng Formation with high organic matter abundance and hydrocarbon generation capacity, and select effective structural or lithologic traps.
In order to fully understand the hydrocarbon generation potential of the Paleocene Yueguifeng, Lingfeng and Mingyuefeng formations in the Lishui Sag, East China Sea Basin, variations in paleoclimate and paleoenvironment and their influence on organic matter enrichment are investigated through analyzing major and trace elements. Results show that the Paleocene sediments are low in terrigenous detrital components and rich in authigenic components, where major and trace elements did not experience diagenetic alteration. Multiple geochemical proxies suggest that paleoclimate during the Yueguifeng, Lingfeng and Mingyuefeng formations depositional periods was humidity, drought and humidity, respectively, while corresponding paleo-salinity was brackish water, saline water and fresh water, and paleowater depth was relatively deep water, relatively shallow water and shallow water, respectively. Water column evolved from reducing condition to oxidizing condition, and then to weak oxidizing-oxidizing condition, respectively. Organic matter productivity and preservation/degradation controlled by co-variations of paleoclimate and paleoenvironment accounted for the difference in organic matter enrichment of the Yueguifeng, Lingfeng and Mingyuefeng formations mudstones. The Yueguifeng Formation mudstone was deposited in semi-deep to deep lake environment with high productivity under warm-humid climate. Considerable planktonic algae were efficiently preserved at a stratified and dysoxic water column, resulting in high organic matter abundance. The Lingfeng Formation mudstone was developed at oxygen-enriched shallow marine environment under arid climate, which was not conducive to the reproduction and preservation of planktonic algae, giving rise to low organic matter abundance. The Mingyuefeng Formation coal-bearing mudstone was deposited in marine-continental transitional environment with low productivity under warm-humid climate, where flat terrain and considerable higher plants contributed to the development of alternated beds of coal and mudstone. The future oil and gas exploration in the Lishui Sag should focus on hydrocarbon generation center of the Yueguifeng Formation with high organic matter abundance and hydrocarbon generation capacity, and select effective structural or lithologic traps.
2024, 49(7): 2373-2387.
doi: 10.3799/dqkx.2022.428
Abstract:
The sedimentary filling evolution during the Paleogene rifting process in Zhu Ⅰ depression remains hotly disputed. Tracing source to sink process of the Paleogene was carried out on the sandstones of Wenchang Formation and Enping Formation in Zhu Ⅰ depression, with the method of zircon U-Pb dating. The geochronology, provenance and sedimentary filling evolution in different evolution stages were further explored. The results show that the sags in Zhu Ⅰ depression not only received the intrabasinal provenances from the paleo-uplift area, but also were affected by the terrigenous supply transported from the northern peripheral uplifts in South China Block during rifting stage. The Wenchang Formation sediments were dominated by intrabasinal short-distance provenances, which were derived from paleo-uplifts nearby the sags, including Mesozoic magmatic rocks, Mesozoic sedimentary strata and Cenozoic volcanics. The U-Pb geochronology of all sags was characterized by the dominance of Yanshanian clusters except for multi-peaks in Yanshanian, Indosinian, Caledonian periods in Lufeng Sag. The sediments of the Enping Formation were firstly affected by the materials from the South China Block in the north of Enping Sag and the west of Huizhou Sag. During the deposition period of the upper Enping Formation, the age spectra of Zhu Ⅰ depression sediments show a wide Proterozoic to Mesozoic range with the peak complexity in Yanshanian, Indosinian, Caledonian and Jinningian periods. Thus, the sags were filled by the materials from both the South China Block and peripheral uplifts with the gradual increase of material supply from the South China Block, indicating that the basin provenances had changed from near-source to far-source ones. Meanwhile, the Zhu Ⅰ depression was characterized by large shallow braided deltas that were formed via relatively long-distance transportation.
The sedimentary filling evolution during the Paleogene rifting process in Zhu Ⅰ depression remains hotly disputed. Tracing source to sink process of the Paleogene was carried out on the sandstones of Wenchang Formation and Enping Formation in Zhu Ⅰ depression, with the method of zircon U-Pb dating. The geochronology, provenance and sedimentary filling evolution in different evolution stages were further explored. The results show that the sags in Zhu Ⅰ depression not only received the intrabasinal provenances from the paleo-uplift area, but also were affected by the terrigenous supply transported from the northern peripheral uplifts in South China Block during rifting stage. The Wenchang Formation sediments were dominated by intrabasinal short-distance provenances, which were derived from paleo-uplifts nearby the sags, including Mesozoic magmatic rocks, Mesozoic sedimentary strata and Cenozoic volcanics. The U-Pb geochronology of all sags was characterized by the dominance of Yanshanian clusters except for multi-peaks in Yanshanian, Indosinian, Caledonian periods in Lufeng Sag. The sediments of the Enping Formation were firstly affected by the materials from the South China Block in the north of Enping Sag and the west of Huizhou Sag. During the deposition period of the upper Enping Formation, the age spectra of Zhu Ⅰ depression sediments show a wide Proterozoic to Mesozoic range with the peak complexity in Yanshanian, Indosinian, Caledonian and Jinningian periods. Thus, the sags were filled by the materials from both the South China Block and peripheral uplifts with the gradual increase of material supply from the South China Block, indicating that the basin provenances had changed from near-source to far-source ones. Meanwhile, the Zhu Ⅰ depression was characterized by large shallow braided deltas that were formed via relatively long-distance transportation.
2024, 49(7): 2388-2406.
doi: 10.3799/dqkx.2022.438
Abstract:
The strong pressure dissolution resistance of dolostone is the key to the preservation of deep-ultra-deep carbonate reservoirs in the Longwangmiao Formation. However, it is found that there are great differences in the spatial and temporal distribution of dolostone in the Longwangmiao Formation. In order to improve the distribution prediction of dolostone reservoir in the Longwangmiao Formation, the type and distribution of dolostone, dolomitization mechanism and model have become the key issues to be solved. Based on petrology, sedimentology, sequence stratigraphy and geochemistry, this study focuses on the development and distribution of dolostone in the Longwangmiao Formation, Sichuan Basin. As a result, the following understandings are obtained. Five rock types related to dolostone have been identified in the Longwangmiao Formation, Sichuan Basin. The spatial and temporal distribution of dolostone is controlled by the paleogeomorphology and the sea level fluctuation. The dolostone is mainly distributed in the central Sichuan uplift and the central part of northern Sichuan Basin. The vertical distribution of dolostone is controlled by the coupling of the 3rd order and 4th order sea level change. The 4th order sequence, located in the middle and upper parts of the 3rd order sequence, has high dolomitization degree. The dolomitization fluid in the Longwangmiao Formation is seawater or seawater with a certain degree of evaporation, and three penecontemporaneous dolomitization models are established in the Longwangmiao Formation.
The strong pressure dissolution resistance of dolostone is the key to the preservation of deep-ultra-deep carbonate reservoirs in the Longwangmiao Formation. However, it is found that there are great differences in the spatial and temporal distribution of dolostone in the Longwangmiao Formation. In order to improve the distribution prediction of dolostone reservoir in the Longwangmiao Formation, the type and distribution of dolostone, dolomitization mechanism and model have become the key issues to be solved. Based on petrology, sedimentology, sequence stratigraphy and geochemistry, this study focuses on the development and distribution of dolostone in the Longwangmiao Formation, Sichuan Basin. As a result, the following understandings are obtained. Five rock types related to dolostone have been identified in the Longwangmiao Formation, Sichuan Basin. The spatial and temporal distribution of dolostone is controlled by the paleogeomorphology and the sea level fluctuation. The dolostone is mainly distributed in the central Sichuan uplift and the central part of northern Sichuan Basin. The vertical distribution of dolostone is controlled by the coupling of the 3rd order and 4th order sea level change. The 4th order sequence, located in the middle and upper parts of the 3rd order sequence, has high dolomitization degree. The dolomitization fluid in the Longwangmiao Formation is seawater or seawater with a certain degree of evaporation, and three penecontemporaneous dolomitization models are established in the Longwangmiao Formation.
2024, 49(7): 2407-2419.
doi: 10.3799/dqkx.2022.494
Abstract:
The fluid inclusion characteristics and hydrocarbon accumulation process in Lungu area were clarified. Based on the petrological observation of core samples in Lungu area, the analysis and testing of fluid inclusion system and the fluorescence measurement of crude oil, combined with the homogeneous temperature and burial history projection method, the hydrocarbon generation history of source rocks was simulated to restore the hydrocarbon accumulation process. The results show that there are three stages of oil inclusions in the Ordovician in Lunku area, which are yellow brown, yellow green and blue fluorescence, respectively. The homogenization temperature of the associated saline inclusions ranges from 70 ℃ to 100 ℃, 80 ℃ to 110 ℃, and 100 ℃ to 120 ℃, respectively. The three stages of oil inclusions in the whole area correspond to three charging periods, which are Late Caledonian - Early Hercynian, Late Hercynian and Xishan. The western region of Lungu is dominated by Late Caledonian and Early Hercynian oil, the central region of Lungu is dominated by Late Hercynian oil, and the eastern region of Lungu is dominated by Himalayan oil.
The fluid inclusion characteristics and hydrocarbon accumulation process in Lungu area were clarified. Based on the petrological observation of core samples in Lungu area, the analysis and testing of fluid inclusion system and the fluorescence measurement of crude oil, combined with the homogeneous temperature and burial history projection method, the hydrocarbon generation history of source rocks was simulated to restore the hydrocarbon accumulation process. The results show that there are three stages of oil inclusions in the Ordovician in Lunku area, which are yellow brown, yellow green and blue fluorescence, respectively. The homogenization temperature of the associated saline inclusions ranges from 70 ℃ to 100 ℃, 80 ℃ to 110 ℃, and 100 ℃ to 120 ℃, respectively. The three stages of oil inclusions in the whole area correspond to three charging periods, which are Late Caledonian - Early Hercynian, Late Hercynian and Xishan. The western region of Lungu is dominated by Late Caledonian and Early Hercynian oil, the central region of Lungu is dominated by Late Hercynian oil, and the eastern region of Lungu is dominated by Himalayan oil.
2024, 49(7): 2420-2433.
doi: 10.3799/dqkx.2022.411
Abstract:
Formation pressure recovery is of great significance for the analysis of hydrocarbon accumulation process. This study restores the pressure of Jurassic Sangonghe Formation during hydrocarbon accumulation period in Fukang Sag with inclusion observation technology, the inclusion salinity homogenization temperature method and PVTx simulation method. The results show that the overpressure of Jurassic begins from 4 500 m, and the overpressure amplitude increases with the increase in buried depth, in Fukang Sag. At the same depth, the overpressure of the main reservoir of Sangonghe Formation is obvious. There are two stages of hydrocarbon inclusions captured in the Sangonghe Formation. The fluorescence colors of hydrocarbon inclusions captured in the first stage which occur inside the quartz particles are mainly yellow and yellowish green, and the main temperature range of associated aqueous inclusions is 85-95 ℃, corresponding to the hydrocarbon accumulation in the middle of Early Cretaceous. Since Neogene, the fluorescence color of hydrocarbon inclusions captured in the second stage, which occur along the healing seams that cut through secondary enlargement of quartz or the whole the after the secondary, is mainly bluish white, and the number of the gas-liquid two-phase hydrocarbon inclusions significantly increases. The homogenization temperature of associated aqueous inclusions is 105-115 ℃, corresponding to hydrocarbon accumulation since the Neogene. During hydrocarbon migration and accumulation, overpressure developed in the Sangonghe Formation. The pressure coefficient in the first stage is 1.39-1.44 and that in the second stage is as high as 2.11. The pressure presents the evolution mode of "pressurization-decompression-strong pressurization". Strong overpressure represents strong hydrocarbon transportation and movement force. It is the key factor for hydrocarbon accumulation in Jurassic tight reservoirs.
Formation pressure recovery is of great significance for the analysis of hydrocarbon accumulation process. This study restores the pressure of Jurassic Sangonghe Formation during hydrocarbon accumulation period in Fukang Sag with inclusion observation technology, the inclusion salinity homogenization temperature method and PVTx simulation method. The results show that the overpressure of Jurassic begins from 4 500 m, and the overpressure amplitude increases with the increase in buried depth, in Fukang Sag. At the same depth, the overpressure of the main reservoir of Sangonghe Formation is obvious. There are two stages of hydrocarbon inclusions captured in the Sangonghe Formation. The fluorescence colors of hydrocarbon inclusions captured in the first stage which occur inside the quartz particles are mainly yellow and yellowish green, and the main temperature range of associated aqueous inclusions is 85-95 ℃, corresponding to the hydrocarbon accumulation in the middle of Early Cretaceous. Since Neogene, the fluorescence color of hydrocarbon inclusions captured in the second stage, which occur along the healing seams that cut through secondary enlargement of quartz or the whole the after the secondary, is mainly bluish white, and the number of the gas-liquid two-phase hydrocarbon inclusions significantly increases. The homogenization temperature of associated aqueous inclusions is 105-115 ℃, corresponding to hydrocarbon accumulation since the Neogene. During hydrocarbon migration and accumulation, overpressure developed in the Sangonghe Formation. The pressure coefficient in the first stage is 1.39-1.44 and that in the second stage is as high as 2.11. The pressure presents the evolution mode of "pressurization-decompression-strong pressurization". Strong overpressure represents strong hydrocarbon transportation and movement force. It is the key factor for hydrocarbon accumulation in Jurassic tight reservoirs.
2024, 49(7): 2434-2447.
doi: 10.3799/dqkx.2022.436
Abstract:
The crude oil in the reservoirs in the Tahe area is mainly derived from the Cambrian Yuertusi source rocks. The discovered oil reservoirs are the result of mixing of crude oils of different stages and different maturities in the reservoirs. Based on the analysis of reservoir oil maturity and fluorescence spectra, this paper uses fluorescence spectrum parameters to establish a quantitative evaluation template for crude oil maturity, and assesses the maturity of crude oil at different episodes in the Tahe area by obtaining oil inclusion fluorescence spectrum parameters. The analysis results of the maturity parameters of crude oil show that the methylphenanthrene parameters and dibenzothiophene parameters in aromatic hydrocarbons are effective indicators for quantitatively evaluating the maturity of crude oil in Tahe area. Using aromatic hydrocarbon maturity parameters to calculate, the crude oil maturity (Ro) in Tahe area ranges from 0.6% to 1.4%, which has a good correlation with the crude oil fluorescence spectrum parameters, and a quantitative evaluation template for crude oil maturity is established based on the relationship between the two. The fluid inclusion analysis results show that there are four periods of petroleum charging in the Ordovician carbonate reservoirs in the Tahe area, corresponding to oil inclusions with orange, bright yellow, light yellow, and blue fluorescence. The fluorescence spectrum parameters of oil inclusions in different periods are also different. Based on the fluorescence spectrum parameters of oil inclusions in different periods and the established fluorescence spectrum parameters-maturity evaluation template of crude oil, it is determined that the maturity (Ro) of oil in orange fluorescent oil inclusions is less than 0.6%, that of oil in bright yellow fluorescent oil inclusions is between 0.6% and 0.8%, that of oil in light yellow fluorescent oil inclusions is between 0.8% and 1.0%, and that of oil in blue fluorescent oil inclusions is between 1.0% and 1.2%.The determined maturity of petroleum in different periods has an important reference for the study of petroleum accumulation process in Tahe area.
The crude oil in the reservoirs in the Tahe area is mainly derived from the Cambrian Yuertusi source rocks. The discovered oil reservoirs are the result of mixing of crude oils of different stages and different maturities in the reservoirs. Based on the analysis of reservoir oil maturity and fluorescence spectra, this paper uses fluorescence spectrum parameters to establish a quantitative evaluation template for crude oil maturity, and assesses the maturity of crude oil at different episodes in the Tahe area by obtaining oil inclusion fluorescence spectrum parameters. The analysis results of the maturity parameters of crude oil show that the methylphenanthrene parameters and dibenzothiophene parameters in aromatic hydrocarbons are effective indicators for quantitatively evaluating the maturity of crude oil in Tahe area. Using aromatic hydrocarbon maturity parameters to calculate, the crude oil maturity (Ro) in Tahe area ranges from 0.6% to 1.4%, which has a good correlation with the crude oil fluorescence spectrum parameters, and a quantitative evaluation template for crude oil maturity is established based on the relationship between the two. The fluid inclusion analysis results show that there are four periods of petroleum charging in the Ordovician carbonate reservoirs in the Tahe area, corresponding to oil inclusions with orange, bright yellow, light yellow, and blue fluorescence. The fluorescence spectrum parameters of oil inclusions in different periods are also different. Based on the fluorescence spectrum parameters of oil inclusions in different periods and the established fluorescence spectrum parameters-maturity evaluation template of crude oil, it is determined that the maturity (Ro) of oil in orange fluorescent oil inclusions is less than 0.6%, that of oil in bright yellow fluorescent oil inclusions is between 0.6% and 0.8%, that of oil in light yellow fluorescent oil inclusions is between 0.8% and 1.0%, and that of oil in blue fluorescent oil inclusions is between 1.0% and 1.2%.The determined maturity of petroleum in different periods has an important reference for the study of petroleum accumulation process in Tahe area.
2024, 49(7): 2448-2474.
doi: 10.3799/dqkx.2022.490
Abstract:
The Tarim Large Igneous Province is characterized by long duration of magmatism, complex rock types and wide distribution of iron-rich alkaline basalts, although the source and petrogenesis of magmatism associated with it remain disputed. Here we chose the Wajilitag alkaline lamprophyres in the northwestern margin of the Tarim Large Igneous Province to precisely determine their emplacement ages, constrain its petrogenesis and discuss the deep dynamic processes involved in the formation of the Tarim Large Igneous Province using LA-ICP-MS zircon U-Pb dating, mineral and whole-rock geochemistry. Zircon U-Pb geochronology indicates that the Wajilitag alkaline lamprophyres formed at 279±1 Ma, belonging to the second phase of Tarim Large Igneous Province magmatism. The lamprophyre is characterized by a panidiomorphic-porphyritic texture imparted by olivine, clinopyroxene, amphibole and biotite set in a groundmass of plagioclase, clinopyroxene, amphibole, biotite and titanomagnetite microcrysts. These rocks have low SiO2 (43.5%-49.4%), high Fe2O3t (9.32%-15.50%), high TiO2 (2.28%-4.58%), and Mg# numbers of 43.6-52.9, and are enriched in mantle compatible elements (Ni, Cr). Meanwhile, they also show high Na2O (2.58%-5.50%) and low K2O/Na2O ratios (0.31-0.78), highlighting their sodic characters. Fractionated chondrite normalized REE patterns indicates involvement of an enriched mantle source from within the garnet stability field whereas slightly negative K, Sr, Ti and Zr-Hf anomalies displayed on the primitive mantle normalized multi-element spidergram highlight involvement of a subducted component in the mantle source. Bulk-rock (87Sr/86Sr)i (0.704 36-0.705 34), εNd(t)(-1.88-+1.10) and (206Pb/204Pb)i (17.19-17.89) of the Wajilitag alkaline lamprophyres indicate derivation from a slightly depleted mantle source similar to that of asthenospheric magmas such as OIB. Notably, the δ26Mg values of the Wajilitag alkaline lamprophyres are typically lighter than those of the normal mantle source. The primitive magmas of the Wajilitag alkaline lamprophyres are likely derived from low degrees melting of carbonated eclogite bearing mantle plume, and possibly affected by the plume-lithosphere interaction. Ultimately, our study suggests that plume-subducted altered oceanic slab interaction could make important contributions to the genesis of the Tarim Large Igneous Province.
The Tarim Large Igneous Province is characterized by long duration of magmatism, complex rock types and wide distribution of iron-rich alkaline basalts, although the source and petrogenesis of magmatism associated with it remain disputed. Here we chose the Wajilitag alkaline lamprophyres in the northwestern margin of the Tarim Large Igneous Province to precisely determine their emplacement ages, constrain its petrogenesis and discuss the deep dynamic processes involved in the formation of the Tarim Large Igneous Province using LA-ICP-MS zircon U-Pb dating, mineral and whole-rock geochemistry. Zircon U-Pb geochronology indicates that the Wajilitag alkaline lamprophyres formed at 279±1 Ma, belonging to the second phase of Tarim Large Igneous Province magmatism. The lamprophyre is characterized by a panidiomorphic-porphyritic texture imparted by olivine, clinopyroxene, amphibole and biotite set in a groundmass of plagioclase, clinopyroxene, amphibole, biotite and titanomagnetite microcrysts. These rocks have low SiO2 (43.5%-49.4%), high Fe2O3t (9.32%-15.50%), high TiO2 (2.28%-4.58%), and Mg# numbers of 43.6-52.9, and are enriched in mantle compatible elements (Ni, Cr). Meanwhile, they also show high Na2O (2.58%-5.50%) and low K2O/Na2O ratios (0.31-0.78), highlighting their sodic characters. Fractionated chondrite normalized REE patterns indicates involvement of an enriched mantle source from within the garnet stability field whereas slightly negative K, Sr, Ti and Zr-Hf anomalies displayed on the primitive mantle normalized multi-element spidergram highlight involvement of a subducted component in the mantle source. Bulk-rock (87Sr/86Sr)i (0.704 36-0.705 34), εNd(t)(-1.88-+1.10) and (206Pb/204Pb)i (17.19-17.89) of the Wajilitag alkaline lamprophyres indicate derivation from a slightly depleted mantle source similar to that of asthenospheric magmas such as OIB. Notably, the δ26Mg values of the Wajilitag alkaline lamprophyres are typically lighter than those of the normal mantle source. The primitive magmas of the Wajilitag alkaline lamprophyres are likely derived from low degrees melting of carbonated eclogite bearing mantle plume, and possibly affected by the plume-lithosphere interaction. Ultimately, our study suggests that plume-subducted altered oceanic slab interaction could make important contributions to the genesis of the Tarim Large Igneous Province.
2024, 49(7): 2475-2489.
doi: 10.3799/dqkx.2023.008
Abstract:
To provide important insights into the petrogenesis of the Early Cretaceous intermediate-acidic magmatic rocks in middle-lower Yangtze River region, it carried out a detailed investigation of Li-Sr-Nd isotopes and geochemistry for the Wushan granodiorite porphyry, quartz diorite porphyry and the basic dyke. The Wushan granodiorite porphyry samples are characterized by the high concentrations of SiO2, MgO, and K2O and low concentration of Na2O, respectively, enrichment in LREEs and LILEs, and relative depletion in Y and Yb. They exhibit isotopic values of (87Sr/86Sr)i=0.703 0-0.707 4, εNd(t)=-5.1--4.6, and δ7Li=+0.9‰-+3.2‰, respectively, which are between the isotopic values of crustal and mantle end-members, revealing a crust-mantle mixing process between felsic melts from a juvenile lower crust and mafic melts from an enriched mantle source. They were subsequently subjected to weakly fractional crystallization of pyroxene and hornblende and minor crustal contamination during the ascent and emplacement. The Early Cretaceous intermediate-acidic magmatic rocks and related deposits in middle-lower Yangtze River region could be related to upwelling of asthenospheric mantle caused by the subduction of the Paleo-Pacific plate and its induced reactivation of sub-continental lithosphere.
To provide important insights into the petrogenesis of the Early Cretaceous intermediate-acidic magmatic rocks in middle-lower Yangtze River region, it carried out a detailed investigation of Li-Sr-Nd isotopes and geochemistry for the Wushan granodiorite porphyry, quartz diorite porphyry and the basic dyke. The Wushan granodiorite porphyry samples are characterized by the high concentrations of SiO2, MgO, and K2O and low concentration of Na2O, respectively, enrichment in LREEs and LILEs, and relative depletion in Y and Yb. They exhibit isotopic values of (87Sr/86Sr)i=0.703 0-0.707 4, εNd(t)=-5.1--4.6, and δ7Li=+0.9‰-+3.2‰, respectively, which are between the isotopic values of crustal and mantle end-members, revealing a crust-mantle mixing process between felsic melts from a juvenile lower crust and mafic melts from an enriched mantle source. They were subsequently subjected to weakly fractional crystallization of pyroxene and hornblende and minor crustal contamination during the ascent and emplacement. The Early Cretaceous intermediate-acidic magmatic rocks and related deposits in middle-lower Yangtze River region could be related to upwelling of asthenospheric mantle caused by the subduction of the Paleo-Pacific plate and its induced reactivation of sub-continental lithosphere.
2024, 49(7): 2490-2507.
doi: 10.3799/dqkx.2022.484
Abstract:
The Baiyin Group, which is the ore-hosted strata in Baiyin ore field, is located at the southeast of North Qilian. The research of isotope chronology, petrology and whole-rock geochemistry was carried out on the bimodal volcanic rocks to reveal their ages, petrogenesis and forming environment. LA-ICP-MS zircon U-Pb dating results show that the group age of the rhyolites are 473.0±1.7 Ma and 473.9±2.1 Ma (206Pb/238U ages), and it can be confirmed that the rhyolite suite formed during the Early Ordovician. The basalt samples, which are regarded as the low-K sub-alkaline tholeiitic series, have geochemical characteristics including high Al2O3, low K2O, TiO2 and P2O5. The rhyolitic rocks (SiO2 > 70%), which belong to cold-wet-oxidized rhyolite, exhibit low FeOT/MgO values (2.44-2.80) and total REE content. With enrichment in incompatible element (Ba, Th, U), obvious negative anomalies of Nb, Ta, Ti and no obvious Eu anomalies (δEu=0.76-0.92), the bimodal volcanic rocks show the geochemical features of island arc magma. The rhyolite zircon εHf(t) values range from -4.14 to 14.78 with Hf Model ages (tDM2) varying from 1 707 Ma to 505 Ma. These features can be concluded that the basalt derived from partial melting of depleted mantle under the effect of subduction fluids and crustal contamination. Meanwhile, the rhyolite may be derived from partial melting of crustal material with a small amount of mantle-derived magma, therefore they are not comagma. The petrology and geochemical information of bimodal volcanic rocks from Baiyin Group indicate they formed in transitional environment from island arc to back-arc basin when northward subduction happened in Early Ordovician.
The Baiyin Group, which is the ore-hosted strata in Baiyin ore field, is located at the southeast of North Qilian. The research of isotope chronology, petrology and whole-rock geochemistry was carried out on the bimodal volcanic rocks to reveal their ages, petrogenesis and forming environment. LA-ICP-MS zircon U-Pb dating results show that the group age of the rhyolites are 473.0±1.7 Ma and 473.9±2.1 Ma (206Pb/238U ages), and it can be confirmed that the rhyolite suite formed during the Early Ordovician. The basalt samples, which are regarded as the low-K sub-alkaline tholeiitic series, have geochemical characteristics including high Al2O3, low K2O, TiO2 and P2O5. The rhyolitic rocks (SiO2 > 70%), which belong to cold-wet-oxidized rhyolite, exhibit low FeOT/MgO values (2.44-2.80) and total REE content. With enrichment in incompatible element (Ba, Th, U), obvious negative anomalies of Nb, Ta, Ti and no obvious Eu anomalies (δEu=0.76-0.92), the bimodal volcanic rocks show the geochemical features of island arc magma. The rhyolite zircon εHf(t) values range from -4.14 to 14.78 with Hf Model ages (tDM2) varying from 1 707 Ma to 505 Ma. These features can be concluded that the basalt derived from partial melting of depleted mantle under the effect of subduction fluids and crustal contamination. Meanwhile, the rhyolite may be derived from partial melting of crustal material with a small amount of mantle-derived magma, therefore they are not comagma. The petrology and geochemical information of bimodal volcanic rocks from Baiyin Group indicate they formed in transitional environment from island arc to back-arc basin when northward subduction happened in Early Ordovician.
2024, 49(7): 2508-2525.
doi: 10.3799/dqkx.2023.010
Abstract:
Pingtian pluton is located in the central part of South China, providing an ideal window for understanding the Triassic tectonic-magmatic activities and the geodynamic setting in South China. Systematic whole-rock geochemistry, zircon U-Pb dating and zircon in situ Hf isotope studies were carried out, taking the Pingtian granite group as the object of study. The results show that the Pingtian pluton consists of coarse-grained porphyritic biotite granite, coarse-grained porphyritic potassium feldspar granite and medium-grained monzonite, with diagenetic ages ranging from 238 to 239 Ma, and it was formed in the Middle Triassic. Geochemical characteristics show that the rocks are enriched in light rare earth elements with obvious europium negative anomalies (δEu average=0.42). It is enriched in Zr, Hf, Y and Ce, and significantly depleted in Sr, P and Ti. It belongs to the metaluminous to weakly peraluminous alkaline granite type, and is A-type granite. The zircon εHf(t) values range from -37.7 to -5.0, and the tDM2 two-stage model ages range from 1 578 to 3 597 Ma. Combined with the whole-rock geochemical characteristics, it reveals that the magmas were derived mainly from the partial melting of felsic material in the crust under low-temperature and high-pressure environment, probably mixed with old crustal material, and experienced crystalline differentiation, and was formed in the post-collisional extensional background. Integrating the geochemical characteristics and spatial distribution of A-type granites and alkaline syenite in South China, we propose that the tectonic evolution of the Triassic was mainly controlled by the collisional interaction of the South China block with the Indochina block and the North China block, and that the tectonic setting changed at around 238 Ma, from a collisional extrusion environment in the Early Triassic to a transition to a post-collisional extensional environment in the Middle to Late Triassic.
Pingtian pluton is located in the central part of South China, providing an ideal window for understanding the Triassic tectonic-magmatic activities and the geodynamic setting in South China. Systematic whole-rock geochemistry, zircon U-Pb dating and zircon in situ Hf isotope studies were carried out, taking the Pingtian granite group as the object of study. The results show that the Pingtian pluton consists of coarse-grained porphyritic biotite granite, coarse-grained porphyritic potassium feldspar granite and medium-grained monzonite, with diagenetic ages ranging from 238 to 239 Ma, and it was formed in the Middle Triassic. Geochemical characteristics show that the rocks are enriched in light rare earth elements with obvious europium negative anomalies (δEu average=0.42). It is enriched in Zr, Hf, Y and Ce, and significantly depleted in Sr, P and Ti. It belongs to the metaluminous to weakly peraluminous alkaline granite type, and is A-type granite. The zircon εHf(t) values range from -37.7 to -5.0, and the tDM2 two-stage model ages range from 1 578 to 3 597 Ma. Combined with the whole-rock geochemical characteristics, it reveals that the magmas were derived mainly from the partial melting of felsic material in the crust under low-temperature and high-pressure environment, probably mixed with old crustal material, and experienced crystalline differentiation, and was formed in the post-collisional extensional background. Integrating the geochemical characteristics and spatial distribution of A-type granites and alkaline syenite in South China, we propose that the tectonic evolution of the Triassic was mainly controlled by the collisional interaction of the South China block with the Indochina block and the North China block, and that the tectonic setting changed at around 238 Ma, from a collisional extrusion environment in the Early Triassic to a transition to a post-collisional extensional environment in the Middle to Late Triassic.
2024, 49(7): 2526-2538.
doi: 10.3799/dqkx.2024.027
Abstract:
The surface of Mars has been sculpted by a diversity of long-lasting aqueous systems and likely had a more habitable environment in the past. During recent decades, the ancient habitable environments and evolutionary history of Mars have been interesting topics of planetary science research and deep space exploration. The Qaidam Basin, located on the northeastern Tibetan Plateau, has been considered to be an ideal analog to ancient Mars due to its limited aqueous activity, coldness, aridity, and high UV radiation. This study combined remote sensing image analysis, short-wave infrared spectroscopy, laser-induced breakdown spectroscopy, X-ray diffraction, and X-ray fluorescence to analyze the spectral characteristics and mineral and elemental compositions of surface soils sampled from representative Mars-like landforms (including alluvial fan, dune, debris flow, gully, yardang, playa, and polygon) across the Qaidam Basin. Results of this study reveal that the mineral composition of Mars-like Qaidam landforms is primarily composed of quartz, albite, gypsum, and calcite, as well as illite, chlorite, microcline, and halite. Samples from the yardang and alluvial fan demonstrated their high preservation capacities of carbonates, clay minerals, and organic matter. These findings offer valuable insights for the interpretation of in-situ spectroscopic data and environmental chemical analyses of paleohydrologic settings on Mars from the perspective of comparative planetology.
The surface of Mars has been sculpted by a diversity of long-lasting aqueous systems and likely had a more habitable environment in the past. During recent decades, the ancient habitable environments and evolutionary history of Mars have been interesting topics of planetary science research and deep space exploration. The Qaidam Basin, located on the northeastern Tibetan Plateau, has been considered to be an ideal analog to ancient Mars due to its limited aqueous activity, coldness, aridity, and high UV radiation. This study combined remote sensing image analysis, short-wave infrared spectroscopy, laser-induced breakdown spectroscopy, X-ray diffraction, and X-ray fluorescence to analyze the spectral characteristics and mineral and elemental compositions of surface soils sampled from representative Mars-like landforms (including alluvial fan, dune, debris flow, gully, yardang, playa, and polygon) across the Qaidam Basin. Results of this study reveal that the mineral composition of Mars-like Qaidam landforms is primarily composed of quartz, albite, gypsum, and calcite, as well as illite, chlorite, microcline, and halite. Samples from the yardang and alluvial fan demonstrated their high preservation capacities of carbonates, clay minerals, and organic matter. These findings offer valuable insights for the interpretation of in-situ spectroscopic data and environmental chemical analyses of paleohydrologic settings on Mars from the perspective of comparative planetology.
2024, 49(7): 2539-2551.
doi: 10.3799/dqkx.2024.057
Abstract:
Sedimentary basins are prominent in convergent margin studies. Located near the volcanic arc in the forearc region, the forearc basin is relatively well-preserved after intense orogeny, with records of intact information about orogenic processes. This paper illustrates the tectonic setting, formation mechanism, provenance, and sedimentary-structural characteristics of the forearc basins exemplified by well-studied Cenozoic examples. Forearc basins are located between the volcanic arc and the trench-slope break. It can be formed in an extensional environment where normal fault forms a half-graben basin, or in a compressional environment where thrusts of the accretionary wedge serve as a dam to pond sediments. All kinds of sedimentary facies can be developed in the forearc basin located in a continental arc, while hemipelagic-pelagic facies are dominant in that located in an intra-oceanic arc. Terrigenous debris is mainly derived from the proximal volcanic arc and accretionary wedge and enters the basin through rivers, submarine canyons, and collapse. Sediments in the forearc basin usually grow thicker from the basin margin to the center and from the bottom to the top. Subduction of mid-ocean ridge, seamount, oceanic plateau, and fracture zone could affect the structure of subduction zones to various degrees, leading to inversion, exhumation, sedimentary hiatus, sedimentary provenance changing, and depositional environment changing of the forearc basin. The development of the forearc basin is difficult to be summarized by a single model. Thus, when forearc basin is applied to orogeny analysis, multi-disciplinary data should be considered in a comprehensive analysis of basin geological records.
Sedimentary basins are prominent in convergent margin studies. Located near the volcanic arc in the forearc region, the forearc basin is relatively well-preserved after intense orogeny, with records of intact information about orogenic processes. This paper illustrates the tectonic setting, formation mechanism, provenance, and sedimentary-structural characteristics of the forearc basins exemplified by well-studied Cenozoic examples. Forearc basins are located between the volcanic arc and the trench-slope break. It can be formed in an extensional environment where normal fault forms a half-graben basin, or in a compressional environment where thrusts of the accretionary wedge serve as a dam to pond sediments. All kinds of sedimentary facies can be developed in the forearc basin located in a continental arc, while hemipelagic-pelagic facies are dominant in that located in an intra-oceanic arc. Terrigenous debris is mainly derived from the proximal volcanic arc and accretionary wedge and enters the basin through rivers, submarine canyons, and collapse. Sediments in the forearc basin usually grow thicker from the basin margin to the center and from the bottom to the top. Subduction of mid-ocean ridge, seamount, oceanic plateau, and fracture zone could affect the structure of subduction zones to various degrees, leading to inversion, exhumation, sedimentary hiatus, sedimentary provenance changing, and depositional environment changing of the forearc basin. The development of the forearc basin is difficult to be summarized by a single model. Thus, when forearc basin is applied to orogeny analysis, multi-disciplinary data should be considered in a comprehensive analysis of basin geological records.
2024, 49(7): 2552-2569.
doi: 10.3799/dqkx.2023.009
Abstract:
A comprehensive understanding of the spatial and temporal differences in the activity of north-south trending normal faults is important to understand the formation of the southern Tibet Rifts. Here, we use the interpretation of high-resolution remote sensing images, structural geomorphology, ground survey, Unmanned Aerial Vehicles (UVA) systems for photogrammetric surveys, and published chronological data to investigate the Quaternary activities and spatial and temporal differences of the main north-south graben and its boundary normal faults at the southern end of the Nyalam-Coqen rift. It is found that the Quaternary activity of normal faults at the southern end of the Nyalam-Coqen rift show obvious spatial and temporal differences. In time, activity of normal faults at the southern end of the Nyalam-Coqen rift presents three periods of pre-Quaternary, Early and Middle Pleistocene, and Late Quaternary. In space, the rifting range of the near north-south grabens has shrunk significantly since the Late Miocene, and its rifting range gradually has shrunk from Nyalam area to Peiku Co and Mulin Co, and Drolung. In addition, the spatial and temporal differences and characteristics of the north-south normal faulting in each rift should be considered to analyze the formation mechanism of the southern Tibet rifts.
A comprehensive understanding of the spatial and temporal differences in the activity of north-south trending normal faults is important to understand the formation of the southern Tibet Rifts. Here, we use the interpretation of high-resolution remote sensing images, structural geomorphology, ground survey, Unmanned Aerial Vehicles (UVA) systems for photogrammetric surveys, and published chronological data to investigate the Quaternary activities and spatial and temporal differences of the main north-south graben and its boundary normal faults at the southern end of the Nyalam-Coqen rift. It is found that the Quaternary activity of normal faults at the southern end of the Nyalam-Coqen rift show obvious spatial and temporal differences. In time, activity of normal faults at the southern end of the Nyalam-Coqen rift presents three periods of pre-Quaternary, Early and Middle Pleistocene, and Late Quaternary. In space, the rifting range of the near north-south grabens has shrunk significantly since the Late Miocene, and its rifting range gradually has shrunk from Nyalam area to Peiku Co and Mulin Co, and Drolung. In addition, the spatial and temporal differences and characteristics of the north-south normal faulting in each rift should be considered to analyze the formation mechanism of the southern Tibet rifts.
2024, 49(7): 2570-2588.
doi: 10.3799/dqkx.2024.005
Abstract:
Characteristics of fracture distribution are vital for research of granitic reservoirs. Here, unmanned aerial vehicle (UAV)-based photogrammetric data were used to identify and quantitatively analyze the structural fractures developed in the outcrops of Cretaceous granites from coastal region of South China. Fracture density and connectivity were described in detail to gain a better insight into the nature of fracture distribution developed in multi-phase tectonic events. The results show that the study areas predominantly exhibit two sets of fractures of NE20°‒40° and SE100°‒140°. The NE-trending fractures developed earlier than SE-trending fractures. Multiphase tectonic deformation significantly enhances the connectivity and density of fracture network. Areas of elevated density are concentrated within the extensive intrusion zone of diabase dikes and the interior of fault zones. The linear density, surface density, and connectivity of the structural fracture network are positively correlated with the size of the fault zone. This study allows us to build a structural model to elucidate the development of multi-phase structural fractures in coastal region of South China. It underscores that fracture intersections such as fault zones and intrusions by diabase dikes are areas of high fault network connectivity and density, and thus optimal reservoir conditions. This research provides a guideline for understanding fracture network distribution in granitic reservoirs and for reservoir evaluation in crystalline basement during petroleum exploration.
Characteristics of fracture distribution are vital for research of granitic reservoirs. Here, unmanned aerial vehicle (UAV)-based photogrammetric data were used to identify and quantitatively analyze the structural fractures developed in the outcrops of Cretaceous granites from coastal region of South China. Fracture density and connectivity were described in detail to gain a better insight into the nature of fracture distribution developed in multi-phase tectonic events. The results show that the study areas predominantly exhibit two sets of fractures of NE20°‒40° and SE100°‒140°. The NE-trending fractures developed earlier than SE-trending fractures. Multiphase tectonic deformation significantly enhances the connectivity and density of fracture network. Areas of elevated density are concentrated within the extensive intrusion zone of diabase dikes and the interior of fault zones. The linear density, surface density, and connectivity of the structural fracture network are positively correlated with the size of the fault zone. This study allows us to build a structural model to elucidate the development of multi-phase structural fractures in coastal region of South China. It underscores that fracture intersections such as fault zones and intrusions by diabase dikes are areas of high fault network connectivity and density, and thus optimal reservoir conditions. This research provides a guideline for understanding fracture network distribution in granitic reservoirs and for reservoir evaluation in crystalline basement during petroleum exploration.
2024, 49(7): 2589-2599.
doi: 10.3799/dqkx.2023.107
Abstract:
The study of groundwater pore-scale flow field characteristics is of great significance for the in-depth understanding of groundwater seepage and solute transport. However, the effect of the microstructure of porous media with different particle size characteristics on the pore-scale flow field is yet unclear. This paper constructs porous media with different particle size distribution characteristics and swelling degrees of solid particles using an iterative rearrangement algorithm. On this basis, the effects of the average particle size, particle size variance and solid particle swelling degrees on the characteristics of groundwater flow field are investigated. The results show that under the condition of the same porosity of the porous media, the mean particle size and particle size variance of solid grains have less influence on the flow field characteristics such as the heterogeneity and probability density distribution of flow velocity in porous media. However, in the same medium, when the expansion of solid particles causes a reduction in porosity, a smaller change in average particle size can have a significant effect on the above flow field characteristics. For example, when the expansion degree of solid particles increases, a slight increase in particle size will lead to a large rise in the proportion of the dominant flow area and the stagnant region at the same time, a significant enhancement in the heterogeneity of flow field, and a significant growth in the divergence degree of the flow velocity probability density distribution.
The study of groundwater pore-scale flow field characteristics is of great significance for the in-depth understanding of groundwater seepage and solute transport. However, the effect of the microstructure of porous media with different particle size characteristics on the pore-scale flow field is yet unclear. This paper constructs porous media with different particle size distribution characteristics and swelling degrees of solid particles using an iterative rearrangement algorithm. On this basis, the effects of the average particle size, particle size variance and solid particle swelling degrees on the characteristics of groundwater flow field are investigated. The results show that under the condition of the same porosity of the porous media, the mean particle size and particle size variance of solid grains have less influence on the flow field characteristics such as the heterogeneity and probability density distribution of flow velocity in porous media. However, in the same medium, when the expansion of solid particles causes a reduction in porosity, a smaller change in average particle size can have a significant effect on the above flow field characteristics. For example, when the expansion degree of solid particles increases, a slight increase in particle size will lead to a large rise in the proportion of the dominant flow area and the stagnant region at the same time, a significant enhancement in the heterogeneity of flow field, and a significant growth in the divergence degree of the flow velocity probability density distribution.
2024, 49(7): 2600-2613.
doi: 10.3799/dqkx.2023.024
Abstract:
In order to quantitatively assess the amount of organic carbon (OC) released by clayey aquitard to adjacent aquifers during compaction, the background values of OC in borehole sediments were used as the constraint condition in the study area of the Chen Lake Wetland. The physical simulation experiments of natural sedimentation and artificial compaction were carried out by collecting surficial undisturbed silt, and a mathematical model representing the difference of release amount of OC at different depths was established. Under natural deposition conditions, sediment OC was released with pore water through mineralization and reduction dissolution of associated minerals; the concentration contribution of OC to the underlying aquifer (about 50-80 m) from clayey aquitard (about 20 m) is 6.99-11.19 mg/L under compaction, which is about 3.9 times of OC release amount under advection and diffusion. Under artificial compaction condition represented by land subsidence, the concentration contribution of organic carbon is 0.19-2.02 mg/L under compaction, which is higher than that of advection and diffusion in the same period. Compaction release of clayey aquitard pore water is an important source of OC in groundwater, which should be paid more attention in the study of natural inferior groundwater.
In order to quantitatively assess the amount of organic carbon (OC) released by clayey aquitard to adjacent aquifers during compaction, the background values of OC in borehole sediments were used as the constraint condition in the study area of the Chen Lake Wetland. The physical simulation experiments of natural sedimentation and artificial compaction were carried out by collecting surficial undisturbed silt, and a mathematical model representing the difference of release amount of OC at different depths was established. Under natural deposition conditions, sediment OC was released with pore water through mineralization and reduction dissolution of associated minerals; the concentration contribution of OC to the underlying aquifer (about 50-80 m) from clayey aquitard (about 20 m) is 6.99-11.19 mg/L under compaction, which is about 3.9 times of OC release amount under advection and diffusion. Under artificial compaction condition represented by land subsidence, the concentration contribution of organic carbon is 0.19-2.02 mg/L under compaction, which is higher than that of advection and diffusion in the same period. Compaction release of clayey aquitard pore water is an important source of OC in groundwater, which should be paid more attention in the study of natural inferior groundwater.
2024, 49(7): 2614-2626.
doi: 10.3799/dqkx.2023.005
Abstract:
To investigate the rock weathering processes in subtropical basin in east China, we analyzed major ion compositions of rivers and precipitation samples in the Qingyi River Basin in the lower reach of the Yangtze River. In this study, the characteristics of weathering processes in the Qingyi River Basin were identified, and the rock weathering rates and consumption rates of atmospheric CO2 were estimated based on water chemistry and the forward model. The results show that the anthropogenic influences on rock weathering were not significant, which means the rock weathering in the study area was mainly induced by carbonic acid while the influence of sulfuric acid and nitric acid could be neglected. The cations of rivers were mainly contributed by weathering of carbonates (59.2%), followed by weathering of silicates (17.9%). Atmospheric precipitation and evaporites contributed 9.6% and 5.6%, respectively. Spatially, the carbonate weathering rates and silicate weathering rates decreased in the order of tributary Huishui River in the upstream mountainous areas (32.04 t·km‒2·a‒1 and 20.97 t·km‒2·a‒1) > main stream of Qingyi River (24.12 t·km‒2·a‒1 and 8.91 t·km‒2·a‒1) > tributary Zhanghe River in the downstream areas (13.68 t·km‒2·a‒1 and 2.85 t·km‒2·a‒1). Similarly, the CO2 consumption rates from carbonates weathering and silicate weathering were in the order of tributary Huishui River (5.86×105 mol·km‒2·a‒1 and 3.29×105 mol·km‒2·a‒1) > main stream of Qingyi River (2.45×105 mol·km‒2·a‒1 and 2.43×105 mol·km‒2·a‒1) > tributary Zhanghe River (0.77×105 mol·km‒2·a‒1 and 1.39×105 mol·km‒2·a‒1). In conclusion, carbonate weathering induced by carbonic acid was dominant in the Qingyi River Basin, with chemical weathering rates slightly lower than those of similar silicate-dominated subtropical areas in east China. The rock weathering rates in the study area differed spatially. In particular, silicate weathering in upstream mountainous areas accounted for more carbon sink of the whole Qingyi River Basin, which is of great importance for the regional carbon cycle.
To investigate the rock weathering processes in subtropical basin in east China, we analyzed major ion compositions of rivers and precipitation samples in the Qingyi River Basin in the lower reach of the Yangtze River. In this study, the characteristics of weathering processes in the Qingyi River Basin were identified, and the rock weathering rates and consumption rates of atmospheric CO2 were estimated based on water chemistry and the forward model. The results show that the anthropogenic influences on rock weathering were not significant, which means the rock weathering in the study area was mainly induced by carbonic acid while the influence of sulfuric acid and nitric acid could be neglected. The cations of rivers were mainly contributed by weathering of carbonates (59.2%), followed by weathering of silicates (17.9%). Atmospheric precipitation and evaporites contributed 9.6% and 5.6%, respectively. Spatially, the carbonate weathering rates and silicate weathering rates decreased in the order of tributary Huishui River in the upstream mountainous areas (32.04 t·km‒2·a‒1 and 20.97 t·km‒2·a‒1) > main stream of Qingyi River (24.12 t·km‒2·a‒1 and 8.91 t·km‒2·a‒1) > tributary Zhanghe River in the downstream areas (13.68 t·km‒2·a‒1 and 2.85 t·km‒2·a‒1). Similarly, the CO2 consumption rates from carbonates weathering and silicate weathering were in the order of tributary Huishui River (5.86×105 mol·km‒2·a‒1 and 3.29×105 mol·km‒2·a‒1) > main stream of Qingyi River (2.45×105 mol·km‒2·a‒1 and 2.43×105 mol·km‒2·a‒1) > tributary Zhanghe River (0.77×105 mol·km‒2·a‒1 and 1.39×105 mol·km‒2·a‒1). In conclusion, carbonate weathering induced by carbonic acid was dominant in the Qingyi River Basin, with chemical weathering rates slightly lower than those of similar silicate-dominated subtropical areas in east China. The rock weathering rates in the study area differed spatially. In particular, silicate weathering in upstream mountainous areas accounted for more carbon sink of the whole Qingyi River Basin, which is of great importance for the regional carbon cycle.
2024, 49(7): 2627-2636.
doi: 10.3799/dqkx.2023.022
Abstract:
During groundwater discharge into the river, Fe2+ and As3+ in anoxic groundwater are brought into the oxygen-containing interaction zone. This study explores the co-migration and transformation mechanism of Fe2+ and As3+with groundwater in the natural sediment medium.The migration and transformation rules of Fe2+ and As3+during groundwater discharge into river water and the transformation rate of Fe2+ and As3+ to solid phase (solidification) in different regions of the interaction zone are studied by laboratory column experiment and batch experiment.The results show that the sediment strongly adsorbs Fe2+ and As3+, and As3+ accelerates the migration of Fe2+. The chemical oxidation and precipitation of Fe2+ occur, and Fe-As bounding minerals are formed when it flows through the interaction zone.The solidification rate of Fe2+ and As3+ is accelerated from the area far from the river to that near the river. In the whole interaction zone, Fe2+ significantly promotes the solidification of As3+, while As3+ slightly inhibits the solidification of Fe2+ near the river bank.In short, during groundwater discharge into river water, the synergistic chemical oxidation and adsorption fixation of Fe2+ and As3+in groundwater occur at different rates in different regions of the interaction zone, which seriously hinders their migration to river water.
During groundwater discharge into the river, Fe2+ and As3+ in anoxic groundwater are brought into the oxygen-containing interaction zone. This study explores the co-migration and transformation mechanism of Fe2+ and As3+with groundwater in the natural sediment medium.The migration and transformation rules of Fe2+ and As3+during groundwater discharge into river water and the transformation rate of Fe2+ and As3+ to solid phase (solidification) in different regions of the interaction zone are studied by laboratory column experiment and batch experiment.The results show that the sediment strongly adsorbs Fe2+ and As3+, and As3+ accelerates the migration of Fe2+. The chemical oxidation and precipitation of Fe2+ occur, and Fe-As bounding minerals are formed when it flows through the interaction zone.The solidification rate of Fe2+ and As3+ is accelerated from the area far from the river to that near the river. In the whole interaction zone, Fe2+ significantly promotes the solidification of As3+, while As3+ slightly inhibits the solidification of Fe2+ near the river bank.In short, during groundwater discharge into river water, the synergistic chemical oxidation and adsorption fixation of Fe2+ and As3+in groundwater occur at different rates in different regions of the interaction zone, which seriously hinders their migration to river water.
2024, 49(7): 2637-2649.
doi: 10.3799/dqkx.2023.130
Abstract:
In order to investigate the reactive-transport patterns of nitrate in hyporheic zone during the hyporheic exchange process under a dynamic water level condition, a vertical two-dimensional numerical model of riverbed dune including (river) water fluctuations and sinuous river bed dune was constructed. By considering three types of river level fluctuations scenarios, river bed slope, aerobic respiration, nitrification and denitrification processes in our model, the effects of bed slope and water level fluctuatio scenarios on spatiotemporal evolution of solute distribution and nitrate conversion efficiency of hyporheic zone were systematically discussed. The results show that larger river bed slope condition can increase the solute exchange flux between surface water and the groundwater flow, and reduce the variation degree of solute concentration, which will consequently decrease the conversion efficiency of NO3‒ in hyporheic zones. Larger subsequent peak level of water fluctuations can prolong hyporheic flow path and increase the variation degree of solute concentration, whereas it can reduce the conversion efficiency of NO3‒ in hyporheic zones. The duration of subsequent water level fluctuations will affect the time response of solute concentration, but will not affect the conversion efficiency of NO3‒. Different delay times of subsequent water level fluctuations will affect the humber of NO3‒ concentration peaks. Furthermore, longer delay time can result in multiple peaks of NO3- concentration.
In order to investigate the reactive-transport patterns of nitrate in hyporheic zone during the hyporheic exchange process under a dynamic water level condition, a vertical two-dimensional numerical model of riverbed dune including (river) water fluctuations and sinuous river bed dune was constructed. By considering three types of river level fluctuations scenarios, river bed slope, aerobic respiration, nitrification and denitrification processes in our model, the effects of bed slope and water level fluctuatio scenarios on spatiotemporal evolution of solute distribution and nitrate conversion efficiency of hyporheic zone were systematically discussed. The results show that larger river bed slope condition can increase the solute exchange flux between surface water and the groundwater flow, and reduce the variation degree of solute concentration, which will consequently decrease the conversion efficiency of NO3‒ in hyporheic zones. Larger subsequent peak level of water fluctuations can prolong hyporheic flow path and increase the variation degree of solute concentration, whereas it can reduce the conversion efficiency of NO3‒ in hyporheic zones. The duration of subsequent water level fluctuations will affect the time response of solute concentration, but will not affect the conversion efficiency of NO3‒. Different delay times of subsequent water level fluctuations will affect the humber of NO3‒ concentration peaks. Furthermore, longer delay time can result in multiple peaks of NO3- concentration.
2024, 49(7): 2650-2661.
doi: 10.3799/dqkx.2023.021
Abstract:
Disintegration and fragmentation effect generally exists in the process of high position rock landslide movement, which can change the material state and motion state of landslide, thus affecting the energy distribution and dynamic transmission characteristics of landslide. By large scale physical model test, this paper deeply studies the slip source block body strength, volume, thickness and joint development degree and slope on the influence of rock disintegration and fragmentation, discusses the disintegration and fragmentation characteristics and the law of detrital energy dissipation of high position rock landslide debris flow, and reveals its momentum transfer mechanism. In the process of dynamic transmission of landslide debris flow, the velocity loss in the front is obviously less than that in the rear, the leading edge has obvious "secondary acceleration", and a large number of fine particles accumulate at the far end. The rear and forward parts of the landslide have obvious velocity and power transfer effect, and the higher the degree of fragmentation, the more significant the dynamic transfer effect. The process of disintegration and fragmentation is accompanied by the transformation, transfer and loss of energy. Under the control of the degree of breakage, the energy dissipation accounts for 3.32%-21.03% of the total potential energy.
Disintegration and fragmentation effect generally exists in the process of high position rock landslide movement, which can change the material state and motion state of landslide, thus affecting the energy distribution and dynamic transmission characteristics of landslide. By large scale physical model test, this paper deeply studies the slip source block body strength, volume, thickness and joint development degree and slope on the influence of rock disintegration and fragmentation, discusses the disintegration and fragmentation characteristics and the law of detrital energy dissipation of high position rock landslide debris flow, and reveals its momentum transfer mechanism. In the process of dynamic transmission of landslide debris flow, the velocity loss in the front is obviously less than that in the rear, the leading edge has obvious "secondary acceleration", and a large number of fine particles accumulate at the far end. The rear and forward parts of the landslide have obvious velocity and power transfer effect, and the higher the degree of fragmentation, the more significant the dynamic transfer effect. The process of disintegration and fragmentation is accompanied by the transformation, transfer and loss of energy. Under the control of the degree of breakage, the energy dissipation accounts for 3.32%-21.03% of the total potential energy.
2024, 49(7): 2662-2674.
doi: 10.3799/dqkx.2022.483
Abstract:
In recent years, small and moderate earthquakes have occurred frequently around Gaoling county, making it a high-incidence area in the Weihe Basin. In order to understand the seismogenic structure and seismogenic background of the seismic area, this paper uses the VELEST and typoDD programs to relocate 235 earthquakes of recent years in the area. By re-picking the seismic phases, GPAT method is applied to calculate 26 focal mechanism solutions of the earthquakes with ML≥2.0. The precision positioning results of small earthquakes indicate that the earthquakes in this area are basically distributed above 15 km, and the vertical profile shows a northward shovel-like fault. Based on the distribution of earthquakes, the scale and activity of faults, and the location of the extreme earthquake zone, this paper considers the seismogenic fault of the Gaoling earthquake group to be the Jingyang-Weinan fault. The calculation results of the focal mechanism of small earthquakes show that the area is dominated by horizontal extension of NNW and tendency of compression in NW direction. The dominant direction of the comprehensive slip angle indicates that the seismogenic fault is dominated by normal fault activity and left-lateral strike-slip. GPS data show that the stress in the study area has a complex four-quadrant distribution, and the Gaoling seismic swarm is in the center of the four quadrants. The article speculates that it is this complex stress background that led to the activity of the Gaoling seismic swarm.
In recent years, small and moderate earthquakes have occurred frequently around Gaoling county, making it a high-incidence area in the Weihe Basin. In order to understand the seismogenic structure and seismogenic background of the seismic area, this paper uses the VELEST and typoDD programs to relocate 235 earthquakes of recent years in the area. By re-picking the seismic phases, GPAT method is applied to calculate 26 focal mechanism solutions of the earthquakes with ML≥2.0. The precision positioning results of small earthquakes indicate that the earthquakes in this area are basically distributed above 15 km, and the vertical profile shows a northward shovel-like fault. Based on the distribution of earthquakes, the scale and activity of faults, and the location of the extreme earthquake zone, this paper considers the seismogenic fault of the Gaoling earthquake group to be the Jingyang-Weinan fault. The calculation results of the focal mechanism of small earthquakes show that the area is dominated by horizontal extension of NNW and tendency of compression in NW direction. The dominant direction of the comprehensive slip angle indicates that the seismogenic fault is dominated by normal fault activity and left-lateral strike-slip. GPS data show that the stress in the study area has a complex four-quadrant distribution, and the Gaoling seismic swarm is in the center of the four quadrants. The article speculates that it is this complex stress background that led to the activity of the Gaoling seismic swarm.
2024, 49(7): 2675-2684.
doi: 10.3799/dqkx.2022.245
Abstract:
Classification of seismic focal mechanism plays an important role in earthquake dynamic analysis. However, focal mechanism classification is quite arbitrary at present, and the existence of the undefined type increases the difficulty of further analysis. To solve this problem, the authors introduce the areal strain (As) of seismic focal mechanism as the standard for the division of focal mechanism types: normal fault type with -1≤As < -0.7, normal strike-slip type with -0.7≤As < -0.3, strike-slip type with -0.3≤As≤0.3, reverse strike-slip type with 0.3 < As≤0.7, and reverse fault type with 0.7 < As≤1. Because the areal strain of horizontal strain rosette divides the focal mechanism according to the proportion of vertical and horizontal deformation caused by the source, the trouble of division by multiple combination of three axial plunge angles is avoided, and the problem of undefined type of focal mechanism is avoided. By presenting this partition strategy in spherical triangle diagram to show the focal mechanism classification, the type classification boundaries of focal mechanism are found to be symmetric. In addition, the overall sliding behavior of earthquake sequence on an active fault or seismic belt is quite important for geodynamic studies. Supposing that the earthquakes occur on a fault belt ruptured as the same focal mechanism, i.e. the released moment tensor dominated by the earthquakes with large moment tensor, and the other focal mechanisms are caused by observational errors, or secondary/minor fault rupture, then the overall focal mechanism can reflect the rupture property of the fault belt. This study proposes averaging the seismic moment tensor weighted by scalar seismic moment of the aftershocks to obtain the overall focal mechanism of earthquakes on an active fault or seismic belt, thus the difference of sliding behavior of the main shock and aftershocks can be analyzed. By adopting the above method in 2021 Qinghai Madoi earthquake sequence and 2022 Qinghai Menyuan earthquake sequence, the difference of slip properties between the main shock and other earthquakes is obtained. It is found that the overall focal mechanism of the 2022 Menyuan aftershocks is almost the same with that of the main shock, while the overall focal mechanism of the aftershocks in 2021 Madoi earthquake sequence has certain differences with that of the main shock. The method provides a tool for fault slip characteristics and geodynamic analysis in an active fault or seismic region.
Classification of seismic focal mechanism plays an important role in earthquake dynamic analysis. However, focal mechanism classification is quite arbitrary at present, and the existence of the undefined type increases the difficulty of further analysis. To solve this problem, the authors introduce the areal strain (As) of seismic focal mechanism as the standard for the division of focal mechanism types: normal fault type with -1≤As < -0.7, normal strike-slip type with -0.7≤As < -0.3, strike-slip type with -0.3≤As≤0.3, reverse strike-slip type with 0.3 < As≤0.7, and reverse fault type with 0.7 < As≤1. Because the areal strain of horizontal strain rosette divides the focal mechanism according to the proportion of vertical and horizontal deformation caused by the source, the trouble of division by multiple combination of three axial plunge angles is avoided, and the problem of undefined type of focal mechanism is avoided. By presenting this partition strategy in spherical triangle diagram to show the focal mechanism classification, the type classification boundaries of focal mechanism are found to be symmetric. In addition, the overall sliding behavior of earthquake sequence on an active fault or seismic belt is quite important for geodynamic studies. Supposing that the earthquakes occur on a fault belt ruptured as the same focal mechanism, i.e. the released moment tensor dominated by the earthquakes with large moment tensor, and the other focal mechanisms are caused by observational errors, or secondary/minor fault rupture, then the overall focal mechanism can reflect the rupture property of the fault belt. This study proposes averaging the seismic moment tensor weighted by scalar seismic moment of the aftershocks to obtain the overall focal mechanism of earthquakes on an active fault or seismic belt, thus the difference of sliding behavior of the main shock and aftershocks can be analyzed. By adopting the above method in 2021 Qinghai Madoi earthquake sequence and 2022 Qinghai Menyuan earthquake sequence, the difference of slip properties between the main shock and other earthquakes is obtained. It is found that the overall focal mechanism of the 2022 Menyuan aftershocks is almost the same with that of the main shock, while the overall focal mechanism of the aftershocks in 2021 Madoi earthquake sequence has certain differences with that of the main shock. The method provides a tool for fault slip characteristics and geodynamic analysis in an active fault or seismic region.
2013, 38(3): 454-460.
doi: 10.3799/dqkx.2013.045
2014, 39(3): 261-270.
doi: 10.3799/dqkx.2014.025
2014, 39(3): 283-292.
doi: 10.3799/dqkx.2014.027
2013, 38(6): 1327-1330.
doi: 10.3799/dqkx.2013.000
2013, 38(5): 963-974.
doi: 10.3799/dqkx.2013.094
