The Piranshahr pull-apart basin,situated in the northwestern part of the Zagros Main Recent Fault(MRF),is characterized by two right-stepping segments of the dextral MRF.Here,a combination of finite element modeling t...The Piranshahr pull-apart basin,situated in the northwestern part of the Zagros Main Recent Fault(MRF),is characterized by two right-stepping segments of the dextral MRF.Here,a combination of finite element modeling techniques,especially twodimensional numerical modeling using ABAQUS software,along with field-based structural geological methods are used to assess the geometry and evolution of the pull-apart along the releasing stepovers in this strike-slip system.The utilized numerical approach applies two-dimensional(2D)finite-element modeling related to elastic Newtonian rheology to evaluate the distribution of stress and localization of strain within the pull-apart basin.This study provides valuable insights into the factors controlling the shape,as well as exploring the interaction between the pre-existing structures in this right-lateral strike-slip releasing stepover,pull-apart basin development in strike-slip systems,and stress-strain behavior by studying the impact of boundary conditions and fault overlap on the deformation pattern.The models consider three representative geometries of fault segment interactions,including underlapping,neutral,and overlapping stepovers,positioned at angles of 30°,45°,and 60°.The results indicate that increased overlap creates an extensive and elongated deformation pattern,while decrease overlap leads to block rotation and a narrow deformation pattern.In addition,the degree of overlapping between parallel strike-slip faults influences the stress and strain.The mean normal stress within the transtensional basin,located between the fault segments,exhibits an extensional nature,while the region outside the stepover experiences general compressive mean normal stresses.The Piranshahr transtensional pull-apart basin exemplifies the progressive evolution of underlapping stepovers,resulting in displaying an elongated rhomboidal to trapezoidal-shaped geometry over time.展开更多
Natural fractures controlled by faults in ultradeep carbonate strata play substantial roles as both fluid migration channels and storage spaces.However,characterizing the heterogeneous distribution of underground frac...Natural fractures controlled by faults in ultradeep carbonate strata play substantial roles as both fluid migration channels and storage spaces.However,characterizing the heterogeneous distribution of underground fractures within the complex three-dimensional geometry of strike-slip fault zones remains challenging.This study investigates the characteristics of natural fractures controlled by strike-slip faults in the fractured Middle and Lower Ordovician reservoirs of the central and northern Tarim Basin,China.Seismics,cores,and image logs were integrated to quantitatively analyze the intensity and dip angle of natural fractures and findings were verified using published sandbox simulations.The carbonate reservoir contains three main types of natural fractures:tectonic fractures,abnormal high-pressure-related fractures,and stylolites.Strike-slip faults control the distribution and characteristics of tectonic fractures across various scales.Generally,both fracture intensity and porosity exhibit a decreasing trend as the distance from the main fault surface increases.Compared with those in non-stepover zones along a strike-slip fault,natural fractures and faults in stepover zones are more developed along the fault strike,with significantly greater development intensity in central stepover regions than that at its two ends.Furthermore,strike-slip faults influence the dip angles of both natural fractures and secondary faults.The proportion of medium-to-low-dip angle fractures and faults in the stepover zone is greater than that in the non-stepover zone.Additionally,the proportion of medium-to low-dip angle fractures and faults in the middle of the stepover is greater than that at both ends.Therefore,strike-slip fault structures control the dip angle of natural fracture and the heterogeneity of secondary fault and fracture intensity.The linking damage zone in the stepover contains a larger volume of fractured rocks,making it a promising petroleum exploration target.The development of stepovers and the orientation of present-day in-situ stress substantially influence the productivity of fractured reservoirs controlled by strike-slip faults.The analysis in this study reveals that reservoir productivity increases as the angle between the strike-slip fault segment and the maximum horizontal principal stress decreases.This study provides valuable insights for quantitatively evaluating fracture heterogeneity in fractured reservoirs and establishing optimized selection criteria for favorable targets in fault-related fractured reservoirs.展开更多
The fracture-cave reservoirs controlled by strike-slip faults are the main targets for oil and gas exploration of ultradeep carbonate in the Tarim Basin.It is of great significance to clarify the distribution rules of...The fracture-cave reservoirs controlled by strike-slip faults are the main targets for oil and gas exploration of ultradeep carbonate in the Tarim Basin.It is of great significance to clarify the distribution rules of fractures related to strike-slip faults for guiding the exploration and development of ultra-deep oil and gas.In this study,six groups of strike-slip fault 3D models based on discrete element numerical simulation method have been created to investigate characteristics of fault-related fracture development and distribution law.In addition,we compared the modeling results to the measurement of fractures from the outcrop of a strike-slip fault in the Northern Tarim Basin to verify their validity.The results show that the stress environment is stable in the simple strike-slip section,and fractures intersecting with the strike-slip direction at a small angle are the principal fracture.In the releasing stepover and double-bend sections,the maximum principal stress changes from horizontal to vertical during the formation of pull-apart zones,where the principal fractures intersect the strike-slip direction at a large angle.The maximum principal stress in the restraining stepover and double-bend section remains horizontal,but their strikes change significantly with the increment of fault displacement.Thus,fractures intersecting the strike-slip direction at a small angle will become principal fractures early on,while those parallel to or anti-intersecting the strike-slip direction at a small angle will become principal fractures later.There are obvious differences in the development of fractures in different structural positions of strike-slip faults.Fractures are mainly concentrated in the fault tips,connections,and fault plane,and the magnitude of the fault damage zone is relatively larger in the first two.Compared with fault displacement,the principal damage zone(PDZ)shows stronger control on the distribution and development intensity of fractures.With the increment of fault displacement,the width of the fault damage zone and the fracture density first rapidly increases before the formation of PDZ and then slows down.Moreover,the formation time of PDZ in the restraining double-bend and stepover section is earlier than the simple strike-slip,releasing double-bend,and releasing stepover sections,and absorb more strain before the formation of the principal displacement zone.Thus,the restraining sections have the highest fracture intensity,followed by the pull-apart sections,then the simple strikeslip section.The results play an important role in understanding the development law of fractures related to strike-slip faults in different arrangements and move modes.展开更多
基金the financial support provided by (Shahid Beheshti University)
文摘The Piranshahr pull-apart basin,situated in the northwestern part of the Zagros Main Recent Fault(MRF),is characterized by two right-stepping segments of the dextral MRF.Here,a combination of finite element modeling techniques,especially twodimensional numerical modeling using ABAQUS software,along with field-based structural geological methods are used to assess the geometry and evolution of the pull-apart along the releasing stepovers in this strike-slip system.The utilized numerical approach applies two-dimensional(2D)finite-element modeling related to elastic Newtonian rheology to evaluate the distribution of stress and localization of strain within the pull-apart basin.This study provides valuable insights into the factors controlling the shape,as well as exploring the interaction between the pre-existing structures in this right-lateral strike-slip releasing stepover,pull-apart basin development in strike-slip systems,and stress-strain behavior by studying the impact of boundary conditions and fault overlap on the deformation pattern.The models consider three representative geometries of fault segment interactions,including underlapping,neutral,and overlapping stepovers,positioned at angles of 30°,45°,and 60°.The results indicate that increased overlap creates an extensive and elongated deformation pattern,while decrease overlap leads to block rotation and a narrow deformation pattern.In addition,the degree of overlapping between parallel strike-slip faults influences the stress and strain.The mean normal stress within the transtensional basin,located between the fault segments,exhibits an extensional nature,while the region outside the stepover experiences general compressive mean normal stresses.The Piranshahr transtensional pull-apart basin exemplifies the progressive evolution of underlapping stepovers,resulting in displaying an elongated rhomboidal to trapezoidal-shaped geometry over time.
基金supported by the National Natural Science Foundation of China(No.U21B2062)funding from the Chinese Scholarship Council(CSC)and the American Association of Petroleum Geologists Foundation Grantsin-Aid Program.
文摘Natural fractures controlled by faults in ultradeep carbonate strata play substantial roles as both fluid migration channels and storage spaces.However,characterizing the heterogeneous distribution of underground fractures within the complex three-dimensional geometry of strike-slip fault zones remains challenging.This study investigates the characteristics of natural fractures controlled by strike-slip faults in the fractured Middle and Lower Ordovician reservoirs of the central and northern Tarim Basin,China.Seismics,cores,and image logs were integrated to quantitatively analyze the intensity and dip angle of natural fractures and findings were verified using published sandbox simulations.The carbonate reservoir contains three main types of natural fractures:tectonic fractures,abnormal high-pressure-related fractures,and stylolites.Strike-slip faults control the distribution and characteristics of tectonic fractures across various scales.Generally,both fracture intensity and porosity exhibit a decreasing trend as the distance from the main fault surface increases.Compared with those in non-stepover zones along a strike-slip fault,natural fractures and faults in stepover zones are more developed along the fault strike,with significantly greater development intensity in central stepover regions than that at its two ends.Furthermore,strike-slip faults influence the dip angles of both natural fractures and secondary faults.The proportion of medium-to-low-dip angle fractures and faults in the stepover zone is greater than that in the non-stepover zone.Additionally,the proportion of medium-to low-dip angle fractures and faults in the middle of the stepover is greater than that at both ends.Therefore,strike-slip fault structures control the dip angle of natural fracture and the heterogeneity of secondary fault and fracture intensity.The linking damage zone in the stepover contains a larger volume of fractured rocks,making it a promising petroleum exploration target.The development of stepovers and the orientation of present-day in-situ stress substantially influence the productivity of fractured reservoirs controlled by strike-slip faults.The analysis in this study reveals that reservoir productivity increases as the angle between the strike-slip fault segment and the maximum horizontal principal stress decreases.This study provides valuable insights for quantitatively evaluating fracture heterogeneity in fractured reservoirs and establishing optimized selection criteria for favorable targets in fault-related fractured reservoirs.
基金This work was financially supported by the National Natural Science Foundation of China(Grant No.U21B2062)the 2021 American Association of Petroleum Geologists Foundation Grants-in-Aid Program.
文摘The fracture-cave reservoirs controlled by strike-slip faults are the main targets for oil and gas exploration of ultradeep carbonate in the Tarim Basin.It is of great significance to clarify the distribution rules of fractures related to strike-slip faults for guiding the exploration and development of ultra-deep oil and gas.In this study,six groups of strike-slip fault 3D models based on discrete element numerical simulation method have been created to investigate characteristics of fault-related fracture development and distribution law.In addition,we compared the modeling results to the measurement of fractures from the outcrop of a strike-slip fault in the Northern Tarim Basin to verify their validity.The results show that the stress environment is stable in the simple strike-slip section,and fractures intersecting with the strike-slip direction at a small angle are the principal fracture.In the releasing stepover and double-bend sections,the maximum principal stress changes from horizontal to vertical during the formation of pull-apart zones,where the principal fractures intersect the strike-slip direction at a large angle.The maximum principal stress in the restraining stepover and double-bend section remains horizontal,but their strikes change significantly with the increment of fault displacement.Thus,fractures intersecting the strike-slip direction at a small angle will become principal fractures early on,while those parallel to or anti-intersecting the strike-slip direction at a small angle will become principal fractures later.There are obvious differences in the development of fractures in different structural positions of strike-slip faults.Fractures are mainly concentrated in the fault tips,connections,and fault plane,and the magnitude of the fault damage zone is relatively larger in the first two.Compared with fault displacement,the principal damage zone(PDZ)shows stronger control on the distribution and development intensity of fractures.With the increment of fault displacement,the width of the fault damage zone and the fracture density first rapidly increases before the formation of PDZ and then slows down.Moreover,the formation time of PDZ in the restraining double-bend and stepover section is earlier than the simple strike-slip,releasing double-bend,and releasing stepover sections,and absorb more strain before the formation of the principal displacement zone.Thus,the restraining sections have the highest fracture intensity,followed by the pull-apart sections,then the simple strikeslip section.The results play an important role in understanding the development law of fractures related to strike-slip faults in different arrangements and move modes.
