The cumulative expression of multistage deformation is complex multiperiod fractures,which are commonly seen in tectonic zones.The Middle Triassic Leikoupo Formation in the western Sichuan Basin Depression,China,is a ...The cumulative expression of multistage deformation is complex multiperiod fractures,which are commonly seen in tectonic zones.The Middle Triassic Leikoupo Formation in the western Sichuan Basin Depression,China,is a typical marine carbonate reservoir with natural fractures caused mainly by tectonic movements.According to outcrops,drill cores,image logging,and fluid inclusions,the fracture characteristics,types of natural fractures,and interactions of fractures are determined.In total,419 natural fractures in 493.2 m of cores from 7 wells are investigated,which are mainly shear and tensile fractures with a small number of weathering generated fractures.Meanwhile,the results of the stable isotope analysis of δ13C and δ18O,as well as the flow fluid inclusion data,reveal four tectonic periods of fractures with different occurrences.Based on the history of regional tectonic evolution,indicating one period of weathering fractures ascribable to stratal uplift and three periods of structural fractures related to the sequential tectonic movements of the Longmenshan fault belt.By analyzing the interaction relationships of fractures,three types of fracture interaction relationships are observed:cutting,restraining,and overlapping.The four stages fractures are chronologically assigned to(1)the early Indosinian N-S trending compression,(2)the late Indosinian NW-SE compression,(3)the middle Yanshanian NE-SE compression,and(4)the early Himalayan E-W compression.The influence of natural fractures on gas migration and well production in marine carbonates is discussed,and indicates that tectonic fractures could provide seepage channels for gas migration and accumulation from near or distant hydrocarbon source rocks into the Middle Triassic Leikoupo Formation.This study utilizes a pragmatic approach for understanding the fracture genesis mechanism in oil and gas field with multiperiod fractures.展开更多
In 2022,the risk exploration well Chongtan1(CT1)in the Sichuan Basin revealed commercial oil and gas flow during test in a new zone–the marl of the second submember of the third member of Leikoupo Formation(Lei-32)of...In 2022,the risk exploration well Chongtan1(CT1)in the Sichuan Basin revealed commercial oil and gas flow during test in a new zone–the marl of the second submember of the third member of Leikoupo Formation(Lei-32)of Middle Triassic,recording a significant discovery.However,the hydrocarbon accumulation in marl remains unclear,which restricts the selection and deployment of exploration area.Focusing on Well CT1,the hydrocarbon accumulation characteristics of Lei-32 marl are analyzed to clarify the potential zones for exploration.The following findings are obtained.First,according to the geochemical analysis of petroleum and source rocks,oil and gas in the Lei-32 marl of Well CT1 are originated from the same marl.The marl acts as both source rock and reservoir rock.Second,the Lei-32 marl in central Sichuan Basin is of lagoonal facies,with a thickness of 40–130 m,an area of about 40000 km^(2),a hydrocarbon generation intensity of(4–12)×10^(8) m^(3)/km^(2),and an estimated quantity of generated hydrocarbons of 25×10^(12) m^(3).Third,the lagoonal marl reservoirs are widely distributed in central Sichuan Basin.Typically,in Xichong–Yilong,Ziyang–Jianyang and Moxi South,the reservoirs are 20–60 m thick and cover an area of 7500 km^(2).Fourth,hydrocarbons in the lagoonal marl are generated and stored in the Lei-32 marl,which means that marl serves as both source rock and reservoir rock.They represent a new type of unconventional resource,which is worthy of exploring.Fifth,based on the interpretation of 2D and 3D seismic data from central Sichuan Basin,Xichong and Suining are defined as favorable prospects with estimated resources of(2000–3000)×10^(8) m^(3).展开更多
Karst in interbedded carbonates and evaporites has been reported to have important and complex impacts on reservoir. It is significant for exploration and karst geology. Here, we report such a new case from Middle Tri...Karst in interbedded carbonates and evaporites has been reported to have important and complex impacts on reservoir. It is significant for exploration and karst geology. Here, we report such a new case from Middle Triassic Leikoupo Formation of Sichuan Basin, Southwest China. Stratigraphic incompleteness and the occurrence of unconformity provide evidence for the presence of eogenetic karst. Under the impact of this eogenetic karst, residual weathered and solution-collapse breccia, solution pores and silicification and dedolomitization have been observed. Classic stratigraphic zonation of karst is not readily distinguishable, which is ascribed to the stratigraphic collapse of carbonate rocks resulting from the dissolution of evaporites by lateral subsurface fluid flow. In terms of impact on reservoir quality, karst can generally improve the initial physical property of the porous layers in theory. However, subsurface fluid flow dissolved the evarporitic beds and facilitated the collapse of overlying strata. As a consequence, the lateral continuity of the reservoirs would be destroyed, and relatively high-quality reservoirs can only be developed with little collapse of overlying strata, reflecting reservoir heterogeneities. This may be a general feature of reservoir formation under the impact of karst in interbedded carbonates and evaporites.展开更多
Shoaling is a common type of sedimentation in the evolution of carbonate platform,and commonly has poor continuity.This paper presents a newly discovered and rare type of shoaling,i.e.,mega-shoaling in nearly basin sc...Shoaling is a common type of sedimentation in the evolution of carbonate platform,and commonly has poor continuity.This paper presents a newly discovered and rare type of shoaling,i.e.,mega-shoaling in nearly basin scale,which is developed in the Middle Triassic Leikoupo Formation of the Sichuan Basin,southwest China.During the Leikoupo time,the studied Sichuan Basin experienced hot and dry climate conditions and developed a carbonate platform within a restricted epicontinental sea.In B sub-layer of the Lei-1-1 sub-member of the Leikoupo Formation a series of grainstones of shoal facies accumulated throughout almost the entire basin,thereby generating features associated with basin-scale mega-shoaling.By detailed core examination and microscopic observation of thin sections,it is shown that the lithology of this set of grainstones is dominated by doloarenite(calcarenite)followed by oolitic dolomite(limestone).In addition,it contains three types of sedimentary sequences characterized by upward-coarsening and upward-shallowing as the followings:restricted lagoon to platform interior beach;restricted lagoon to platform interior beach and to platform flat;and tidal flat to peritidal beach.Subsequently,a multicyclic stratigraphic division and correlation revealed that this set of grainstones can be well traced and compared horizontally,and is generally isochronous.In addition,a template for logging facies,established based on core calibrations and logging data,was employed to analyze the 235 wells in the basin.The results demonstrate the shoal grainstones to be 10–40 m thick with a15×104km2continuous distribution area.These findings indicate that the carbonate platform developed mega-shoals within a short period of time.The genesis of such a mega-shoaling was investigated by focusing on various shoaling conditions,such as paleo-tectonics,paleo-geomorphology,paleo-climate,sea-level changes,and palaeo-hydrodynamics.A specific combination of independent geological factors creates beneficial geomorphologic conditions for the mega-shoaling including a quiescent paleo-tectonic environment,relatively flat paleo-geomorphology and evaporites filling up and leveling off.In addition,a stably settling carbonate platform underwent sea-level fluctuations through swift transgressions and protracted regressions,which is not only conducive to continuous,multicyclic and superimposed vertical development of grain beaches but also beneficial for the horizontal migration,coalescence and superimposition of individual grain beaches.As a consequence,large-scale and continuously-distributed grain beach sedimentation emerges and mega-shoals develop.展开更多
基金The first,second,and fifth authors acknowledge the financial support provided by the National Natural Science Foundation of China(41672133)The second author also acknowledges the National Natural Science Foundation of China(4207021285).
文摘The cumulative expression of multistage deformation is complex multiperiod fractures,which are commonly seen in tectonic zones.The Middle Triassic Leikoupo Formation in the western Sichuan Basin Depression,China,is a typical marine carbonate reservoir with natural fractures caused mainly by tectonic movements.According to outcrops,drill cores,image logging,and fluid inclusions,the fracture characteristics,types of natural fractures,and interactions of fractures are determined.In total,419 natural fractures in 493.2 m of cores from 7 wells are investigated,which are mainly shear and tensile fractures with a small number of weathering generated fractures.Meanwhile,the results of the stable isotope analysis of δ13C and δ18O,as well as the flow fluid inclusion data,reveal four tectonic periods of fractures with different occurrences.Based on the history of regional tectonic evolution,indicating one period of weathering fractures ascribable to stratal uplift and three periods of structural fractures related to the sequential tectonic movements of the Longmenshan fault belt.By analyzing the interaction relationships of fractures,three types of fracture interaction relationships are observed:cutting,restraining,and overlapping.The four stages fractures are chronologically assigned to(1)the early Indosinian N-S trending compression,(2)the late Indosinian NW-SE compression,(3)the middle Yanshanian NE-SE compression,and(4)the early Himalayan E-W compression.The influence of natural fractures on gas migration and well production in marine carbonates is discussed,and indicates that tectonic fractures could provide seepage channels for gas migration and accumulation from near or distant hydrocarbon source rocks into the Middle Triassic Leikoupo Formation.This study utilizes a pragmatic approach for understanding the fracture genesis mechanism in oil and gas field with multiperiod fractures.
基金Supported by the PetroChina Science and Technology Project(2021DJ0501,2018A-0105).
文摘In 2022,the risk exploration well Chongtan1(CT1)in the Sichuan Basin revealed commercial oil and gas flow during test in a new zone–the marl of the second submember of the third member of Leikoupo Formation(Lei-32)of Middle Triassic,recording a significant discovery.However,the hydrocarbon accumulation in marl remains unclear,which restricts the selection and deployment of exploration area.Focusing on Well CT1,the hydrocarbon accumulation characteristics of Lei-32 marl are analyzed to clarify the potential zones for exploration.The following findings are obtained.First,according to the geochemical analysis of petroleum and source rocks,oil and gas in the Lei-32 marl of Well CT1 are originated from the same marl.The marl acts as both source rock and reservoir rock.Second,the Lei-32 marl in central Sichuan Basin is of lagoonal facies,with a thickness of 40–130 m,an area of about 40000 km^(2),a hydrocarbon generation intensity of(4–12)×10^(8) m^(3)/km^(2),and an estimated quantity of generated hydrocarbons of 25×10^(12) m^(3).Third,the lagoonal marl reservoirs are widely distributed in central Sichuan Basin.Typically,in Xichong–Yilong,Ziyang–Jianyang and Moxi South,the reservoirs are 20–60 m thick and cover an area of 7500 km^(2).Fourth,hydrocarbons in the lagoonal marl are generated and stored in the Lei-32 marl,which means that marl serves as both source rock and reservoir rock.They represent a new type of unconventional resource,which is worthy of exploring.Fifth,based on the interpretation of 2D and 3D seismic data from central Sichuan Basin,Xichong and Suining are defined as favorable prospects with estimated resources of(2000–3000)×10^(8) m^(3).
基金funded by the “13th Five-Year Plan” National Science and Technology Major Project of China(No.2016ZX05004002)
文摘Karst in interbedded carbonates and evaporites has been reported to have important and complex impacts on reservoir. It is significant for exploration and karst geology. Here, we report such a new case from Middle Triassic Leikoupo Formation of Sichuan Basin, Southwest China. Stratigraphic incompleteness and the occurrence of unconformity provide evidence for the presence of eogenetic karst. Under the impact of this eogenetic karst, residual weathered and solution-collapse breccia, solution pores and silicification and dedolomitization have been observed. Classic stratigraphic zonation of karst is not readily distinguishable, which is ascribed to the stratigraphic collapse of carbonate rocks resulting from the dissolution of evaporites by lateral subsurface fluid flow. In terms of impact on reservoir quality, karst can generally improve the initial physical property of the porous layers in theory. However, subsurface fluid flow dissolved the evarporitic beds and facilitated the collapse of overlying strata. As a consequence, the lateral continuity of the reservoirs would be destroyed, and relatively high-quality reservoirs can only be developed with little collapse of overlying strata, reflecting reservoir heterogeneities. This may be a general feature of reservoir formation under the impact of karst in interbedded carbonates and evaporites.
基金supported by National Basic Research Program of China(Grant No.2012CB214803)PetroChina Scientific Innovative Foundation(Grant No.2011D-5006-0105)Key Subject Construction Project of Sichuan Province,China(Grant No.SZD0414)
文摘Shoaling is a common type of sedimentation in the evolution of carbonate platform,and commonly has poor continuity.This paper presents a newly discovered and rare type of shoaling,i.e.,mega-shoaling in nearly basin scale,which is developed in the Middle Triassic Leikoupo Formation of the Sichuan Basin,southwest China.During the Leikoupo time,the studied Sichuan Basin experienced hot and dry climate conditions and developed a carbonate platform within a restricted epicontinental sea.In B sub-layer of the Lei-1-1 sub-member of the Leikoupo Formation a series of grainstones of shoal facies accumulated throughout almost the entire basin,thereby generating features associated with basin-scale mega-shoaling.By detailed core examination and microscopic observation of thin sections,it is shown that the lithology of this set of grainstones is dominated by doloarenite(calcarenite)followed by oolitic dolomite(limestone).In addition,it contains three types of sedimentary sequences characterized by upward-coarsening and upward-shallowing as the followings:restricted lagoon to platform interior beach;restricted lagoon to platform interior beach and to platform flat;and tidal flat to peritidal beach.Subsequently,a multicyclic stratigraphic division and correlation revealed that this set of grainstones can be well traced and compared horizontally,and is generally isochronous.In addition,a template for logging facies,established based on core calibrations and logging data,was employed to analyze the 235 wells in the basin.The results demonstrate the shoal grainstones to be 10–40 m thick with a15×104km2continuous distribution area.These findings indicate that the carbonate platform developed mega-shoals within a short period of time.The genesis of such a mega-shoaling was investigated by focusing on various shoaling conditions,such as paleo-tectonics,paleo-geomorphology,paleo-climate,sea-level changes,and palaeo-hydrodynamics.A specific combination of independent geological factors creates beneficial geomorphologic conditions for the mega-shoaling including a quiescent paleo-tectonic environment,relatively flat paleo-geomorphology and evaporites filling up and leveling off.In addition,a stably settling carbonate platform underwent sea-level fluctuations through swift transgressions and protracted regressions,which is not only conducive to continuous,multicyclic and superimposed vertical development of grain beaches but also beneficial for the horizontal migration,coalescence and superimposition of individual grain beaches.As a consequence,large-scale and continuously-distributed grain beach sedimentation emerges and mega-shoals develop.
