Quantitative multiparameter prediction of fault-related fractures： a case study of the second member of the Funing Formation in the Jinhu Sag, Subei Basin

In this paper, the analysis of faults with different scales and orientations reveals that the distribution of fractures always develops toward a higher degree of similarity with faults, and a method for calculating the multiscale areal fracture density is proposed using fault-fracture self-similarity theory. Based on the fracture parameters observed in cores and thin sections, the initial apertures of multiscale fractures are determined using the constraint method with a skewed distribution. Through calculations and statistical analyses of in situ stresses in combination with physical experiments on rocks, a numerical geomechanical model of the in situ stress field is established. The fracture opening ability under the in situ stress field is subsequently analyzed. Combining the fracture aperture data and areal fracture density at different scales, a calculation model is proposed for the prediction of multiscale and multiperiod fracture parameters, including the fracture porosity, the magnitude and direction of maximum permeability and the flow conductivity. Finally, based on the relationships among fracture aperture,density, and the relative values of fracture porosity and permeability, a fracture development pattern is determined.

Calculation of reverse-fault-related parameters using topographic profiles and fault bedding

Fault-related parameters are critical for studying tectonic evolution, deformation character- istics, active tectonism, and seismic hazards. A new method of calculating reverse-fault- related parameters has been developed, which uses systematic analysis of the geometrical characteristics of normal and reverse scarps of reverse faults together with measurements of topographic profiles and fault bedding. The results show that the most suitable method of calculating fault parameters heavily relies on the specific type of fault scarp. For a reverse scarp, the size of the vertical displacement （VD） of the fault, the vertical separation （VS） of the hanging wall and the footwall, and the fault scarp height （SH）how the relationship VD ≥VS ≥ SH; conversely, for normal scarps, VD ≤ VS ≤ SH. The theoretical equations were used to study fault deformation in the Southwest Tianshan Mountain foreland basin. The results showed that, for every fault, VD ≥ VS ≥SH, which is consistent with our predicted relationship. This finding demonstrates that this method is suitable to explore structural information of reverse faults. In the study area, the vertical displacement is 1.4 times the horizontal displacement, suggesting that fiexural-slip faults may play an important role in transferring local deformation from horizontal shortening to vertical uplift. Therefore, one of the most important steps in correct calculation of reverse-fault-related parameters is selection of the proper equations by identifying the specific type of fault scarp and the corresponding calculation method.

5 Analysis of Fault-Related Folding in South of Birjand

Folds have a significant development in the Cretaceous-Tertiary rock units of the northern part of Bagheran Mountains in southwest Birjand between Lut and Sistan structural zones. The general trends of fold axis and axial surface are E-W and the folds are less exposed by distance from mountain and plain boundary. Geometric and kinematic status investigation of folds (such as f2 fold in the middle part) and faults shows that faulting process has created some of the folds leading to their development. Such structures are described as fault-related folds. Also, analysis of geometry and mechanism of faults indicate that back thrusts have the largest influence on generation and development of folds in this region.

Segment, Linkage, and Extensional Fault-Related Fold in Western Liaodong Bay Subbasin, Northeastern Bohai Sea, China

The western Liaodong （辽东） Bay subbasin displays examples of segment, linkage of extensional fault, and fault-related folds. The Liaoxi （辽西） extensional fault system consists of a series of NNE- and NE-trending segments that were linked through relay ramps. The fault hanging walls are characterized by a series of en echelon synclines with axial traces sub-parallel to the faults. The synclines are doubly plunging located on the hanging wall of normal faults, with the strata dip sub-parallel to the fault. These folds result from along-strike displacement variations of the individual fault segments, as well as from extensional fault-related folding. In the study area, the synclines are separated by transverse intra-basin highs and relay ramps that formed where segment linkage occurred. These hanging wall synclines and their relation to fault displacement variations indicate that they are formed by extensional fault-related fold.

Structural characteristics and implications on oil/gas accumulation in north segment of the Longmenshan piedmont,northwestern Sichuan Basin,SW China

By integrating surface geology,seismic data,resistivity sections,and drilling data,the structural deformation characteristics of the frontier fault of thrust nappes were delineated in detail.The frontier fault of thrust nappes in northwest Scihuan Basin is a buried thrust fault with partial exposure in the Xiangshuichang-Jiangyou area,forming fault propagation folds in the hanging-wall and without presenting large-scale basin-ward displacement along the gypsum-salt layer of the Triassic Jialingjiang Formation to the Triassic Leikoupo Formation.The southwestern portion of the frontier fault of thrust nappes(southwest of Houba)forms fault bend folds with multiple ramps and flats,giving rise to the Zhongba anticline due to hanging-wall slip along the upper flat of the Jialingjiang Formation.In contrast,the northeastern portion of the frontier fault of thrust nappes(northeast of Houba)presents upward steepening geometry,leading to surface exposure of Cambrian in its hanging-wall.With the frontier fault of thrust nappes as the boundary between the Longmenshan Mountain and the Sichuan Basin,the imbricated structural belt in the hanging-wall thrusted strongly in the Indosinian orogeny and was reactivated in the Himalayan orogeny,while the piedmont buried structural belt in the footwall was formed in the Himalayan orogeny.In the footwall of the frontier fault of thrust nappes,the piedmont buried structural belt has good configuration of source rocks,reservoir rocks and cap rocks,presenting good potential to form large gas reservoirs.In comparison,the hanging-wall of the frontier fault of thrust nappes north of Chonghua has poor condition of oil/gas preservation due to the surface exposure of Triassic and deeper strata,while the fault blocks in the hanging-wall from Chonghua to Wudu,with Jurassic cover and thicker gypsum-salt layer of the Jialingjiang formation,has relative better oil/gas preservation conditions and thus potential of oil/gas accumulation.The frontier fault of thrust nappes is not only the boundary between the Longmenshan Mountain and the Sichuan Basin,but also the boundary of the oil/gas accumulation system in northwestern Sichuan Basin.

Controls of strike-slip fault on fractures:Insight from 3D discrete element simulation

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.

Geological constraints of giant and medium-sized gas fields in Kuqa Depression

There is a gas-rich and well-charged petroleumsystem in the Kuqa Depression where Triassic and Jurassicsource rocks play important roles. Distributed in an area ofmore than 10000 km and with a thickness of up to 1000 m,they are composed of dark mudstones, carbonaceous mud-stones and coal seams containing 6%, 40% and 90% of TOC,respectively, and are mainly the humic organic matter. Ashigh-quality regional cap rocks, the Neogene and Eogenegypsum rocks and gypseous mudstones matched well withthe underlying Neogene and Cretaceous-Eogene sandstones.They have formed the most favorable reservoir-seal assem-blages in the Kuqa Depression. Also the Jurassic sandstonesand mudstones formed another favorable reservoir-seal as-semblage. The traps are shaped late in the fold-thrust belt,mainly fixed in the Tertiary-Quaternary, where ten structurestyles have been distinguished. These traps spread as a zonein N-S, are scattered like a segmental line in W-E and showtier-styled vertically. The best traps are gypsum-salt coveredfault-bend anticlines related to the passive roof duplex. Thispetroleum system is characterized by late accumulation. Inthe early Himalayan Movement, mainly gas condensate andoil accumulated and were distributed in the outer circularregion of the kitchen; whereas in the middle and late Hima-layan the gas accumulations mainly formed and were dis-tributed in the inner circular region near the kitchen. Theoverpressure of gas pools is common and is formed by sealcapacity of thick gypsum layers, extensive tectonic compres-sion and large uplift. The well-preserved anticline traps un-derlying the high-quality regional cap rocks of the Tertiarygypsum rocks and gypseous mudstones are the main targetsfor the discovery of giant and medium-sized gas fields. Aboveconclusions are important for the petroleum geology theoryand the exploration of the fold-thrust belt in foreland basinsin central and western China.

Quaternary folding in the south piedmont of central segment of Tianshan Mountains

The Tianshan Mountains are an important active structural belt in the interior of Eurasia. By integrated methods of surface geology survey and interpretation of seismic profiles, we distinguish fold scarps located at the south limb of the Kuqatawu anticline and the north limb of the Dongqiulitag anticline in the Kuqa rejuvenation foreland thrust belt, south piedmont of central segment of the Tianshan Mountains. Fold scarp is a newly found structural phenomenon. Because of the bend of thrust plane and the movement of hanging wall above the thrust plane, the original horizontal deposits of hanging wall and their surface become a monocline structure, resulting from the separating and migration of the active and fixed axial surfaces. Measuring the geometry of fold scarp and using the data of age of the deformed deposits, the crustal shortening rate resulting from the deeply seated subsurface thrust is calculated. The crustal shortening rate reflected by the fold scarp located at the north limb of the