Strike-slip Fault Zone Research Articles

Strike-Slip Fault Effects on Diversity of the Ediacaran Mound-Shoal Distribution in the Central Sichuan Intracratonic Basin, China

The largest Precambrian gasfield in China has been found from the Ediacaran (Sinian) carbonates in the central Sichuan Basin. The deep ancient reservoirs were generally attributed to the high-energy mound-shoal body in the carbonate platform. However, there is still little understanding on the distribution of the mound-shoal bodies that hampers further gas exploitation from the deep subsurface. Based on the seismic data, a large strike-slip fault system was identified by new 3D seismic data in the central Sichuan Basin. Further, it was found that the strike-slip fault had some effects on the mound-shoal bodies of the Ediacaran Dengying Formation. First, the platform margin was divided by strike-slip faults into three distinct segments to show two weak-rimmed margins and one interbedded rimmed margin. Second, the platform margin could be offset or migrated with the strike-slip faults. Third, there is varied margin thickness across the strike-slip fault zone. In the inner platform, more carbonate mound-shoal bodies developed behind the weak-rimmed margin that was divided by the strike-slip fault zones. In addition, the mound-shoal bodies may be separated by faulted sag. Further, the mound-shoal bodies may have developed along the faulted higher position in one side of the strike-slip fault zone. These patterns of the mound-shoal bodies suggest that the strike-slip fault had an important role in the sedimentary microfacies’ diversity in the intracratonic carbonates. The low displacement of the strike-slip fault is chiefly responsible for a weaker controlling effect on the microfacies change in the intracratonic basin rather than other tectonic settings.

The Fault Effects on the Oil Migration in the Ultra-Deep Fuman Oilfield of the Tarim Basin, NW China

A giant, ultra-deep (>7000 m) strike-slip fault-related oilfield has been found in the central depression of the Tarim Basin. However, little research has addressed this discovery to understand the fault effects on the oil migration, which constrains the oil exploitation deployment and well optimization. Based on fault modeling and fluid analysis, we present the effects of fault segment on the oil segmentation and migration in the ultra-deep Fuman Oilfield. The results show a distinct fault segmentation and also subsequent variable fault architecture and non-connected fracture network other than a through-going fault zone. There are fault segment-related isolated fractured reservoirs and fluid variation along the fault strike, which indicates that a large oilfield comprises a series of unconnected small oil reservoirs along the fault segments. The fluid segmentation and molecular indicator of dibenzothiophenes reveal that there is a variable petroleum lateral migration along the strike-slip fault segment. The fault segmentation is a major contribution in the varied oil accumulation along the strike-slip fault zone in Fuman Oilfield. This case study suggests that fault segmentation has variable effects on petroleum migration and accumulation along the strike-slip zone.

Investigation on water-drive performance of a fault-karst carbonate reservoir under different well patterns and injection-production modes based on 2D visualized physical models

Recently, a new type of carbonate reservoir, the fault-karst carbonate reservoir has attracted lots of attention, which usually develops large-scale karst caves, vugs, and high-angle fractures along the strike-slip fault zones. Despite containing plenty of hydrocarbon resources, the complex geological structure and the strong heterogeneity make it difficult to develop this kind of reservoir effectively and economically. To perform water-drive experiments and investigate the production performance under different well patterns and injection-production modes, two 2D visualized physical models were designed and fabricated based on the typical geological structures of the fault-karst carbonate reservoir in the vertical and horizontal directions, respectively. According to the experimental results, the average oil recovery can reach 62.96% for the vertical model and 58.18% for the horizontal model. In fact, gravity effect due to density differences can greatly improve the displacement efficiency of oil by water in the vertical direction; hence, the horizontal heterogeneity would have a relatively greater impact on waterflooding development. When water is injected into the reservoir from the middle section, a large amount of un-swept oil will remain at the bottom fractures. Thus, the Bottom Injection & Top Production (BI & TP) mode should be adopted in all well patterns to increase recovery. If non-inverted well patterns are employed, dominant water channeling would form quickly in the horizontal direction, and the swept area will be hugely limited. Therefore, the Center Injection & Edge Production mode (inverted well patterns) is recommended for water-drive applications. It is also found that the water breakthrough time negatively correlates with the effective flooding time for the vertical model and the total oil recovery is more impacted by water breakthrough for both models. Additionally, the oil-water exchange ratio is introduced to evaluate the utilization efficiency of injected water. Calculation results have shown that the BI & TP mode is optimal in terms of either total recovery or oil-water exchange ratio in the vertical direction, but the well pattern layout in the horizontal direction could impact the oil recovery and oil-water exchange ratio, which, in turn, affect the project economics. This study provides novel insights for optimizing well patterns and waterflooding measures in actual fault-karst carbonate reservoirs.

Vertical Differential Structural Deformation of the Main Strike‐slip Fault Zones in the Shunbei Area, Central Tarim Basin: Structural Characteristics, Deformation Mechanisms, and Hydrocarbon Accumulation Significance

AbstractVertical differential structural deformation (VDSD), one of the most significant structural characteristics of strike‐slip fault zones (SSFZs) in the Shunbei area, is crucial for understanding deformation in the SSFZ and its hydrocarbon accumulation significance. Based on drilling data and high‐precision 3‐D seismic data, we analyzed the geometric and kinematic characteristics of the SSFZs in the Shunbei area. Coupled with the stratification of the rock mechanism, the structural deformations of these SSFZs in different formations were differentiated and divided into four deformation layers. According to comprehensive structural interpretations and comparisons, three integrated 3‐D structural models could describe the VDSD of these SSFZs. The time‐space coupling of the material basis (rock mechanism stratification), changing dynamic conditions (e.g., changing stress‐strain states), and special deformation mechanism of the en echelon normal fault array uniformly controlled the formation of the VDSD in the SSFZs of the Shunbei area. The VDSD of the SSFZs in this area controlled the entire hydrocarbon accumulation process. Multi‐stage structural superimposing deformation influenced the hydrocarbon migration, accumulation, distribution, preservation, and secondary adjustments.

Cenozoic tectonic evolution of the Bohai Bay Basin: Constraints from strike-slip activities of the Wangjiagang fault zone, NE China

The Wangjiagang (WJG) fault zone is regarded as the northward propagation of the Tan-Lu fault zone (TLFZ) in the Bohai Bay Basin (BBB). We used three-dimensional seismic and borehole data, as well as structure and isopach maps, to describe the structural properties and tectonic development of the WJG fault zone. The properties and thickness of the mechanical layer are the principal factors of differences in the deformation of the strike-slip fault zone. Extensional tectonics and strike-slip faulting occurred in WJG. Not only does strike-slip faulting regulate sand body distribution, but it also plays a critical role in oil migration. WJG provides a record of the tectonic evolution of the BBB and how it responded to plate interactions around the Pacific-Eurasian-Indian plates and/or the strike-slip faulting of the TLFZ. The BBB extended in the early Paleocene as a result of relative relaxation between the Pacific and Eurasian plates and the upwelling of the asthenosphere mantle. Before and after the late Eocene (ca. 42 Ma), the tectonic stress regimes at a nearly N-S-oriented extensional to dextral strike-slip faulting along TLFZ coincided with the strike-slip faulting of the pre-existing fault in WJG. The Pacific Plate's change in direction and rate of subduction, as well as the far-field effect of a hard collision between the Indian and Eurasian plates, caused the TLFZ's two walls to move in opposite directions. The resulting force couple caused a strong simple shear in the TLFZ and adjacent areas, including the WJG fault zone.

The depositional model of the Lower Cretaceous Nantun Formation and its implications for oil accumulation in the northern Wuerxun Sag of Hailar Basin, NE China

The northern Wuerxun Sag is a lacustrine rift basin which experienced multiple faulting and subsidence during the Lower Cretaceous. The tectonic evolution with uplifted regions as the provenances caused the complex sedimentary fill of the Lower Cretaceous Nantun Formation. In this paper, we use well log, core and 3D seismic data to characterise the sedimentary facies and depositional environment of the Nantun Formation, and consider how this affects the distribution of hydrocarbons within the basin. Four depositional systems are recognised in this study: fan deltas located along the steep slopes; braided deltas along the gentle slopes; sublacustrine fans and lacustrine in the centre of the basin. Fan deltas are mainly composed of coarser sediments with lower sediment maturity than braided deltas, and are mainly located in steep slopes with rift border faults. Sublacustrine fans form lenticular depositional bodies in the basin centre, with higher sediment maturity as a result of reworking. Three sediment-transport pathways were recognised which governed the sediment deposition in this basin, which are (1) the steep rift border fault in the northwest, (2) the NE–SW Surennuoer strike-slip fault zone (SSFZ), and (3) the major faults in the south-eastern gentle slope. Most oil is found in sublacustrine fan sandstones which were transported along the SSFZ mainly from shallower fan deltas, with underlying mudstones (SQN1) as effective source rocks and overlying thick and laterally continuous lacustrine mudstones (SQD1) as seal rocks. The good source–reservoir–seal assemblages enable prolific near-source oil distribution in this area, suggesting the sedimentary controls on oil accumulation, which may guide future oil exploration and production activities in this area.

Elastic characteristics of fault damage zones within superdeep carbonates in Tarim Basin, Northwest China

Abstract Superdeep fault-karst carbonate reservoirs discovered over 7 km deep are controlled by strike-slip fault zones and karst collapses in Tarim Basin, Northwest China. The resulting fracture-cave system provides favorable migration channels and reservoir spaces for hydrocarbon, while the characterization of the internal fault structures remains enigmatic. Based on seismic imaging data, we conducted an integrated study on fault damage zones by seismic curvature attributes, velocity anisotropies, and seismic attenuations. The results show that three typical fault-zone patterns can be identified in the study area, including paratactic multiple fault cores, interactive fault cores and one primary-several subsidiary fault cores. These typical patterns can be clearly characterized via curvature attributes. The elastic characteristics of fault damage zones are significantly affected by seismic frequencies, which are manifested from velocity anisotropies and seismic attenuations. The maximum seismic attenuation occurs along with the orientation of fault cores. There is a strong anisotropic characteristic of P-wave phase velocity with incident angle of three fault-zone models. It appears that seismic attributes associated with geological steering are an effective tool for the subsurface characterization of fault damage zones.

Development Characteristics of Silurian Strike-Slip Faults and Fractures and Their Effects on Drilling Leakage in Shunbei Area of Tarim Basin

In recent years, the Ordovician fault-controlled fracture-cavity reservoirs developed in the basement strike-slip fault zone in the Shunbei area of the Tarim Basin has achieved major breakthroughs. However, during the drilling process of the strike-slip fault zone in the Shunbei area, the problem of mud leakage in the frequently interbedded Silurian sandstone and mudstone strata overlying the Ordovician target layer is very significant, and it has seriously affected the normal drilling and wellbore stability. In this study, taking the Silurian of the No. A Strike-slip Fault Zone in the Shunbei area as an example, the development characteristics of strike-slip faults and fractures, the strength of the in-situ stress field, and the influence of these factors on the drilling mud leakage were systematically studied using 3D seismic, logging, drilling, logging, well log, and engineering construction data. The results show that the mud leakage in strata S1t is significantly larger than that in S1k, and the leakage amount in sandy mudstone is the largest; the strong strike-slip extension developed the negative flower-shaped normal faults and the right-order swan-type faults and caused serious stratigraphic fragmentation. The amount of mud leakage increases with the increase of fault distance. Moreover, the closer to the fault, the higher the frequency and amount of mud leakage. When the distance between the wellbore and the fault exceeds 300 m, the possibility of mud leakage decreases significantly. The Silurian S1t is dominated by high-angle and vertical tension-shear fractures with good opening; while the S1k is dominated by low-angle structural and horizontal bedding fractures. The differences in fracture type cause the mud leakage in S1t to be significantly larger than that of S1k. In addition, the fracture development intervals identified by the R/S-FD method are in good agreement with the mud leakage intervals, which further indicates that the degree of fracture development is the key factor leading to the drilling mud leakage. The study also found that the degree of fracture development and the difference in horizontal principal stresses are the dominant factors leading to high Silurian mud leakage.

Geometry and Kinematics Characteristics of Strike-Slip Fault Zone in Complex Structure Area: A Case Study From the South No. 15 Strike-Slip Fault Zone in the Eastern Sichuan Basin, China

The eastern Sichuan Basin has undergone multiple stages of tectonic evolution and shows complex structural characteristics. It is found that there are a series of NW-SE base-trending strike-slip faults in this area besides NE-SW strike-blocking structures. There are almost no previous studies on strike-slip faults in the eastern Sichuan Basin. This paper fills in the blank of geometry and kinematics research on the strike-slip fault zone in this area. Based on highly precise 3D seismic section, coherent attribute slice and time slice, the geometric and kinematic characteristics of the fault are analyzed. The dynamic characteristics are analysed based on the structural style of the superimposed development model and the nature and activity intensity of strike-slip faults in different periods. and it was found that No. 15 strike-slip fault was a large basement strike-slip fault. The research results show that the profile of the No. 15 strike-slip fault mainly shows a subvertical fracture, positive flower structure and negative flower structure and a composite flower structure with positive and negative flower structures superimposed on each other in the upper and lower planar sections with linearly outspread and zonal distribution characteristics; spatially, there is a “ribbon” effect and a “dolphin” effect. the left strike-slip distance of the TS interface is 0.462 km, and the right strike-slip distance of the TP2 interface is 0.782 km. The strike-slip fault experienced at least three active stages of superimposed deformation during its formation and reconstruction, among which episode II in the middle Caledonian movement was the main active stage of the No. 15 strike-slip fault zone. This study of the structural style and genetic mechanism of a strike-slip fault zone is of guiding significance to the exploration and development of strike-slip fault-controlled reservoirs in the study area.

The thermal history in sedimentary basins: A case study of the central Tarim Basin, Western China

• Heat flow of the Shunbei area decreased since the Ordovician with a Permian peak. • The middle-lower Ordovician reservoirs experienced three stages of heating process. • Hydrocarbon charging periods of No.1, 5, and 7 strike-slip fault zones are different. The reconstruction of thermal history is essential for the superdeep (>6,000 m) hydrocarbon exploration and development in the Central Tarim Basin, Western China. This study first recovered the burial histories of five typical wells in the Shunbei area (the Central Tarim Basin) based on the drilling data and calculated the erosional thicknesses using the well-tie profiles. The thermal histories and hydrocarbon charging periods of the Shunbei area were studied with the measured equivalent vitrinite reflectance ( R equ ) and fluid inclusion assemblage homogenization temperature data, respectively. Except for some abnormal values, most R equ data with 0.45%–1.48% gradually increase with depth. The thermal modeling revealed that the heat flow in the Shunbei area gradually decreased since the Early Ordovician, with an abrupt peak of ∼45–47 mW/m 2 during the Early Permian. The temperature histories of the Ordovician Yijianfang and Yingshan Formation reservoirs in the Shunbei area could be divided into three stages of rapid heating (1.27–1.36 °C/Ma), slow heating (0.07–0.09 °C/Ma) and rapid heating (1.20–1.28 °C/Ma). We recognized three hydrocarbon charging periods in the No. 1 strike-slip fault zone, which occurred at ∼435–420 Ma, ∼263–220 Ma, and 18–3 Ma, respectively, while the No. 5 and 7 strike-slip fault zones only experienced the first two periods. This study provides new insights into the thermal history and hydrocarbon accumulation in the Shunbei area, which would be very useful for further superdeep oil and gas exploration and resource evaluation in the Central Tarim Basin.

Major and trace element hydrogeochemistry of fractured igneous aquifers located in an active strike-slip fault zone: example of the Gezin district (Elazığ, Turkey) in the Eastern Anatolian Fault Zone

Major and trace element hydrogeochemistry of fractured igneous aquifers located in an active strike-slip fault zone: example of the Gezin district (Elazığ, Turkey) in the Eastern Anatolian Fault Zone

Geodynamic Characteristics in the Southwest Margin of South China Sea

The strike-slip fault system in the southwestern margin of South China Sea (SCS) lies on the transition zone between the continental shelf and slope of SCS, which is an important ocean–continent boundary. By using submarine heat flow data, a seismic shear wave tomography model, and gravity potential field data, this paper investigates the distribution of submarine heat flow in the southwestern margin of SCS, the thermal–rheological structure of the crust and mantle, the temperature–viscosity characteristics of the upper mantle Vs low-velocity layer, the tangential stress field of the rheological boundary layer at the lithosphere base, and the convective velocity structure of the mantle asthenosphere. Our new results show that the deep geothermal activity in the southwestern margin of SCS is intense, and the high heat flow area of the mantle with Qm/Qs >70% is distributed along an NNE-trending strip. Moreover, both the east and west sides of the strike–slip fault zone correspond to two low-value areas with a viscosity coefficient of 1021–1022 Pa⋅s at Moho depth, and beneath the Nansha Block are strong and cold blocks with a viscosity coefficient of 1024–1025 Pa⋅s. The northward and eastward shear stress components τN and τE of the rheological boundary layer at the base of the lithosphere mantle decrease with depth. At 65-km depth, both τN and τE are greater than 5.5 × 108 N/m2. At 100-km depth, both τN and τE are less than 1 × 108 N/m2. The calculation results based on the seismic shear wave model of the upper mantle and the gravimetric geoid model indicate that the depth of 120–250 km is the low-velocity layer, and the average temperature of the mantle at 180-km depth can be up to 1,300°C. Moreover, the average effective viscosity coefficient is close to 1018 Pa⋅s, which satisfies the temperature and viscosity conditions for partial melting or convective migration of mantle material. The mantle convection calculation results show that the average flow rate is 8.5 cm/a at 200-km depth and 2.2 cm/a at 400-km depth.

Folding style as indicator for strike-slip fault zone deformation in Gebel Sufr El Dara area, Eastern Desert, Egypt.

Folding style as indicator for strike-slip fault zone deformation in Gebel Sufr El Dara area, Eastern Desert, Egypt.

Key oil accumulation periods of ultra-deep fault-controlled oil reservoir in northern Tarim Basin, NW China

A giant fault-controlled oilfield has been found in the ultra-deep (greater than 6000 m) Ordovician carbonate strata in the northern Tarim Basin. It is of great significance for hydrocarbon accumulation study and oil exploitation to determine the key oil accumulation periods. Based on detailed petrographic analysis, fluid inclusion association (FIA) in calcite samples filling in fractures from 12 wells were analyzed, and key accumulation periods of the strike-slip fault-controlled oilfield was studied by combining oil generation periods of the source rocks, formation periods of the fault and traps, and the fluid inclusion data. (1) There are multiple types of FIA, among them, two types of oil inclusions, the type with yellow fluorescence from the depression area and the type with yellow-green fluorescence from the uplift area with different maturities indicate two oil charging stages. (2) The homogenization temperature of the brine inclusions in FIA is mostly affected by temperature rises, and the minimum temperature of brine inclusions symbiotic with oil inclusions is closer to the reservoir temperature during its forming period. (3) FIA with yellow fluorescence all have homogenization temperatures below 50 °C, while the FIA with yellow-green fluorescence have homogenization temperatures of 70–90 °C tested, suggesting two oil accumulation stages in Middle–Late Caledonian and Late Hercynian. (4) The Middle–Late Ordovician is the key formation period of the strike-slip fault, fracture-cave reservoir and trap there. (5) The oil generation peak of the main source rock of the Lower Cambrian is in the Late Ordovician, and the oil accumulation stage is mainly the Late Ordovician in the depression area, but is mainly the Early Permian in the uplift area. The key oil accumulation period of the strike-slip fault-controlled reservoirs is the Late Caledonian, the depression area has preserved the primary oil reservoirs formed in the Caledonian, while the uplift area has secondary oil reservoirs adjusted from the depression area during the Late Hercynian. Oil reservoir preservation conditions are the key factor for oil enrichment in the strike-slip fault zone of northern Tarim, and the Aman transition zone in the depression is richer in oil and gas and has greater potential for exploration and development.

Architecture of strike-slip fault zones in the central Tarim Basin and implications for their control on petroleum systems

The strike-slip fault zones (SSFZs) of the central Tarim Basin have great potential for hydrocarbon exploration and development. They can both serve as hydrocarbon migration paths and host hydrocarbons. However, the complexity of SSFZ varies significantly along the fault strike, and strike-slip fault-controlled deeply buried carbonate reservoirs are often extremely heterogeneous, increasing exploration risks. Here, we reconstructed the architecture and setting of the SSFZ of the central Tarim Basin and analyzed the complex and heterogeneous hydrocarbon system that characterizes this exploration area.The integrated seismic and well datasets show that the development of SSFZs has led to the fracturing of tight carbonates and enhancements in the karstification process, secondary porosity and permeability due to the presence of open fractures. Based on the vertical height of the fault in the section and continuity, the SSFZs in the central Tarim Basin have been classified as 1st-order or 2nd-order faults. The 1st-order faults are found in the repeatedly reactivated vertical SSFZ, which is composed of sub-vertical strike-slip faults in the Paleozoic strata and en-echelon normal faults in the shallow layer. The 2nd-order faults develop only in Paleozoic strata without reactivated features. Continuous through-going, hard-linked 1st-order faults are a more valuable target for future exploration than soft-linked 2nd-order faults because 1st-order faults control larger-scale fracture-cavity reservoirs, and their fault activity periods correspond to hydrocarbon accumulation periods. We classified damage zones as wall-, and linking-damage zones, based on their location around the fault. There are two types of linking-damage zones, extensional steps and contractional steps, which develop in the extensional and contractional quadrants of the fault segment, respectively. Linking-damage zones are associated with more structural complexity than narrow and straight wall damage zones. Fractures in the contractional steps are generally closed and have low conductance, which is not suitable for fracture-cavity trap formation and hydrocarbon migration. Because opening-mode fracture development and permeability enhancement are greatest at the extensional steps, these locations are more likely to be important foci for strike-slip fault-controlled carbonate reservoirs. Our findings, in combination with previous research, indicate that fault zones in the central Tarim Basin, act as combined conduit-barrier systems, with damage zones typically featuring highly conductive fracture networks and fault cores acting as seals, resulting in heterogeneous reservoir distributions.

Structural styles and linkage evolution in the middle segment of a strike-slip fault: A case from the Tarim Basin, NW China

The structural geometric and kinematic analysis of the Shunbei No. 5 fault zone (fault SB5) in the Tarim Basin was conducted based on the fault interpretation on the three-dimensional (3D) seismic sections and coherence slices of several seismic reflecting surfaces in the study area. The fault SB5 evolution model was recovered using the fault pattern analyses and palaeostress reconstruction, which required factors like the width of the damage zone, maximum throw of the main fault, vertical separation, and the shear crack angle along the fault zone. The results show that: (1) Four tectonic layers in the vertical direction are identified according to the difference of structure style, divided by the top and bottom of the Cambrian salt rocks, and the top surface of the Ordovician carbonate. (2) Strike-slip faults in the study area are the middle segment of the large strike-slip fault zone composed of major faults and overlap zones; 11 stepovers in the study area can be classified into three types according to their geometric characteristics. Strain concentration appears on the overlap zone under the continuous shear stress field and shows high activity. Fault SB5 has undergone three major evolutionary stages: the transpressional stage in the Middle Ordovician, the dextral shearing stage in the Early Silurian, and the final evolutionary sinistral process in the Late Devonian.

The tectonic transition from rifting to strike-slip in the Liaodong Bay Depression, offshore China

The Liaodong Bay Depression was shaped by the Bohai Bay rift basin (BBRB) and the Tan-Lu strike-slip fault system (TSFS) in the Cenozoic and has undergone a tectonic transition from rifting to strike-slip. Therefore, it is an ideal region to study the transition from rifting to strike-slip. Based on new 3D serial seismic data interpretation, we investigated the types and formation times of the main faults in the Liaodong Bay Depression and restored them within the palaeogeography of each evolution stage. Our results show that the Liaodong Bay Depression underwent a tectonic transition from rifting to strike-slip: (1) The mantle material upsurge caused by the Izanagi plate trapped in the mantle transition zone led to rifting of the Liaodong Bay Depression from the Palaeocene to the Eocene. (2) In the middle Eocene, the change in the subduction direction of the Pacific plate led to dextral strike-slip movement along the Tan-Lu fault zone. (3) The rifting and strike-slip in the Liaodong Bay depression are related, and the transition from rifting to strike-slip occurred from the middle Eocene to the early Oligocene. The strike-slip faults developed when rifting was weak in the Oligocene. The transition from rifting to strike-slip faulting formed oil-gas accumulation zones along the strike-slip fault zone and thus resulted in the good oil and gas exploration prospects of this area.

Distribution and gas exploration of the strike–slip faults in the central Sichuan Basin

The Sichuan Basin has abundant oil and gas resources and it has been generally acknowledged as a super petroliferous basin. In recent years, immense exploration and development potential of deep natural gas emerges in the Sichuan Basin. The identification of strike–slip faults and their reservoir controlling effects are of great significance to develop a new exploration domain of fault-controlled gas reservoirs. In order to clarify the distribution of strike–slip faults in the Sichuan Basin and their relationships with natural gas exploration, in view that the strike–slip faults in this basin are small and complex with poor seismic data and influenced more by the fold deformation of Triassic gypsum salt beds, this paper identifies strike–slip faults and analyzes their characteristics, distribution and genesis based on the interpretation of continuous 3D seismic data of the central Sichuan Basin. Then, the fault-controlled gas reservoir model of strike-slip fault is established by reviewing old wells and evaluating geological conditions of hydrocarbon accumulation. Finally, the exploration potential and exploration direction of fault-controlled gas reservoirs are discussed. And the following research results are obtained. First, intraplate weak strike–slip faults can be identified by combining the seismic profile marker of strike–slip fault with the plane marker. Second, eight large strike–slip fault zones with a total length of 1280 km are identified and confirmed in the central Sichuan Basin. Third, strike–slip faults are mainly distributed in the Sinian–Permian System, mainly composed of small-displacement oblique faults and echelon faults, and different in stratification, grading, classification and segmentation. Fourth, under the control of the oblique subduction of the Proto-Tethys Ocean and the different structures of the basement, the strike–slip faults in the central Sichuan Basin were formed in the Sinian and there are multiple stages of inherited Caledonian–Hercynian activities. Fifth, the strike–slip faults in the central Sichuan Basin play a role in source–reservoir connection, reservoir stimulation, and gas accumulation and high yield, and constitute the fault-controlled natural gas accumulation system of “three-element controlling reservoir and composite hydrocarbon accumulation” based on source rock, fault and reservoir. In conclusion, a large intraplate strike–slip fault system is developed in the Sichuan Basin, which has the geological conditions of hydrocarbon accumulation to form strike–slip fault-controlled large gas field, and the large source connecting strike-slip fault zones are the main breakthrough directions of natural gas exploration in fault-controlled carbonate rocks.

Characteristics and main control factors of Ordovician shoal dolomite gas reservoir in Gucheng area, Tarim Basin, NW China

Based on seismic, drilling data and experimental analysis, the characteristics and main controlling factors of shoal dolomite gas reservoir in the third member of Ordovician Yingshan Formation of Gucheng area, Tarim basin were examined. The study shows that the dolomite gas reservoir in Gucheng area is lithologic gas reservoir controlled by shoal and fault jointly, and its formation is mainly attributed to the following factors: (1) The continuously developing paleotectonic structure has been in the direction of gas migration and accumulation; (2) The large area of medium—high energy grain bank deposited in gentle slope environment is the material basis for the formation of dolomite reservoir; (3) Atmospheric water leaching and dolomitization and fluid dissolution in fault zone are the key factors for the formation of high-quality dolomite reservoir; (4) The natural gas comes from cracking of the ancient oil reservoir and hydrocarbon generation of dispersed organic matter in source rocks, and the NNE-trending strike-slip fault is the dominant channel for vertical migration of natural gas; (5) Limestone cap rocks in the first and second members of Yingshan Formation provide direct sealing for the formation of gas reservoir there. On the basis of comprehensive analysis, it is pointed out that the Gucheng area has three grain shoal zones in the third member of Yingshan Formation in nearly S-N direction, which together with seven strike-slip fault zones in NNE direction control the development of shoal dolomite gas reservoir.

Slip rate deficit partitioned by fault-fold system on the active Haiyuan fault zone, Northeastern Tibetan Plateau

This study focuses on the complexity of fault-fold system developed in the Hasishan section of the Haiyuan fault zone in the northeastern margin of the Tibetan Plateau, along which a 237-km-long coseismic surface rupture zone formed during the 1920 M 8.5 Haiyuan earthquake. Interpretations of satellite imageries combined with field investigations and structural analyses of fault geometry and kinematics reveal that the fault-fold system developed in the Hasishan section are characterized by four main elements (i) the left-lateral strike-slip Haiyuan primary fault F1; (ii) a discrete branch fault F2 with an identical sense of movement; (iii) a tension structure, characterized by a NE-trending normal fault F3 developed in piedmont terrace deposits; and (iv) a compression structure P, characterized by a NW-trending en echelon folds within the Cretaceous–Cenozoic sedimentary sequences including the most recent late Pleistocene–Holocene unconsolidated and/or weakly-consolidated deposits. Previous geologic and geophysical observations show that the slip rate along the whole Haiyuan fault zone is non-uniform, indicating an eastward decrease with a gradient of ∼2.65 mm/100 km. In contrast, the slip rate for the primary fault F1 in the Hasishan section was reported to be 3.2 ± 0.2 mm/yr, ∼3 mm/yr less than the result of 6.2 mm/yr inferred from the slip rate gradient. The finding in this study demonstrates that the fault-fold system consisting of faults F2, F3, and fold P has partitioned ∼3 mm/yr slip within the Hasishan section of the Haiyuan fault zone. This study documents that the slip rates at different sections along the primary strike-slip faults are partially dependent on local geometric structure and fault complexity, and the recognition and analyses of related fault-fold system within large strike-slip fault zone are important for better characterizing the total slip rate budget across primary strike-slip faults, which will contribute to improve seismic hazard assessments.