Formation and evolution of the strike-slip faults in the central Sichuan Basin, SW China

Based on 3D seismic and drilling data, the timing, evolution and genetic mechanism of deep strike-slip faults in the central Sichuan Basin are thoroughly examined by using the U-Pb dating of fault-filled carbonate cement and seismic-geological analysis. The strike-slip fault system was initially formed in the Late Sinian, basically finalized in the Early Cambrian with dextral transtensional structure, was overlaid with at least one stage of transpressional deformation before the Permian, then was reversed into a sinistral weak transtensional structure in the Late Permian. Only a few of these faults were selectively activated in the Indosinian and later periods. The strike-slip fault system was affected by the preexisting structures such as Nanhuanian rifting normal faults and NW-striking deep basement faults. It is an oblique accommodated intracratonic transfer fault system developed from the Late Sinian to Early Cambrian to adjust the uneven extension of the Anyue trough from north to south and matches the Anyue trough in evolution time and intensity. In the later stage, multiple inversion tectonics and selective activation occurred under different tectonic backgrounds.

Control of strike-slip faults on Sinian carbonate reservoirs in Anyue gas field, Sichuan Basin, SW China

The largest Precambrian gas field (Anyue gas field) in China has been discovered in the central Sichuan Basin. However, the deep ancient Ediacaran (Sinian) dolomite presents a substantial challenge due to their tightness and heterogeneity, rather than assumed large-area stratified reservoirs controlled by mound-shoal microfacies. This complicates the characterization of “sweet spot” reservoirs crucial for efficient gas exploitation. By analyzing compiled geological, geophysical and production data, this study investigates the impact of strike-slip fault on the development and distribution of high-quality “sweet spot” (fractured-vuggy) reservoirs in the Ediacaran dolomite of the Anyue gas field. The dolomite matrix reservoir exhibits low porosity (less than 4%) and low permeability (less than 0.5×10^(-3) μm^(2)). Contrarily, fractures and their dissolution processes along strike-slip fault zone significantly enhance matrix permeability by more than one order of magnitude and matrix porosity by more than one time. Widespread “sweet spot” fracture-vuggy reservoirs are found along the strike-slip fault zone, formed at the end of the Ediacaran. These fractured reservoirs are controlled by the coupling mechanisms of sedimentary microfacies, fracturing and karstification. Karstification prevails at the platform margin, while both fracturing and karstification control high-quality reservoirs in the intraplatform, resulting in reservoir diversity in terms of scale, assemblage and type. The architecture of the strike-slip fault zone governed the differential distribution of fracture zones and the fault-controlled “sweet spot” reservoirs, leading to wide fractured-vuggy reservoirs across the strike-slip fault zone. In conclusion, the intracratonic weak strike-slip fault can play a crucial role in improving tight carbonate reservoir, and the strike-slip fault-related “sweet spot” reservoir emerges as a unique and promising target for the efficient development of deep hydrocarbon resources. Tailored development strategies need to be implemented for these reservoirs, considering the diverse and differential impacts exerted by strike-slip faults on the reservoirs.

Analysis of a Pull-Apart Basin and Its Associated Fractures in the Woodford Shale, Central Oklahoma

Pull-apart basins are faulting and folding zones with high intensity of fractures that strongly affect the production in unconventional shale gas. While most observations of pull-apart basins were from surface mapping or laboratory experiments, we investigated a nascent pull-apart basin in the subsurface. We characterized a nascent pull-apart basin along the strike-slip fault within the Woodford Shale by using seismic attributes analyses, including coherence, dip-azimuth, and curvature. The results indicate a 32 km long, N-S striking strike-slip fault that displays a distinct but young pull-apart basin, which is ~1.6 km by 3.2 km in size and is bounded by two quasi-circular faults. The curvature attribute map reveals two quasi-circular folds, which depart from the main strike-slip fault at ~25°, resulting in an elliptical basin. Inside the basin, a series of echelon quasi-circular normal faults step into the bottom of the basin with ~80 m of total subsidence. We propose that the controls of the shape of pull-apart basin are the brittleness of the shale, and we suggest proper seismic attributes as a useful tool for investigating high fracture intensity in the subsurface for hydrofracturing and horizontal drilling within the shale.

Characteristics of strike-slip inversion structures of the Karatau fault and their petroleum geological significances in the South Turgay Basin,Kazakhstan

The Karatau fault is one of the important strike-slip faults in central Asia,and the South Turgay Basin is located towards its northern end.Detailed seismic interpretation indicated that the strikeslip tectonism of the Karatau fault weakened gradually from west to east in the South Turgay Basin.Typical flower structures developed on the section,and strike-slip faults showed an echelon pattern on planar view.The Karatau strike-slip fault affected the South Turgay Basin in two periods：（1） The South Turgay strike-slip pull-apart rift basin formed as a result of regional extensive stress in the Early-Middle Jurassic,characterized by the juxtaposition of horsts and grabens.The formation of horsts provided favorable reservoir spaces for later hydrocarbon accumulation,and different filling stages of grabens controlled different reservoir-forming factors in grabens.（2） Two stages of tectonic inversion occurred in the Late Jurassic and Late Cretaceous and played a crucial role in the final shape of the structure in the South Turgay Basin.The oil and gas migrated to form reservoirs and mainly concentrated in the horsts,graben slopes and in both sides of the strike-slip fault zone.In the case of the degree of accumulation of petroleum,the factor explaining why horsts are better than grabens is the strike-slip pull-apart of the South Turgay Basin,and the structure inversion of the South Turgay Basin explains why the west graben is better than the east one.Overall,the Karatau strike-slip fault played a very important role in the formation of the South Turgay Basin and its hydrocarbon accumulations.

Characteristics and evolution of strike-slip tectonics of the Liaohe Western Sag,Bohai Bay Basin

Because of its rich oil and gas resources and the special tectonic location of the Liaohe Western Sag （the Tanlu Fault traverses the sag）, Bohai Bay Basin, a detailed study of its strike-slip tectonics is significant in revealing the sag＇s tectonic evolution, its control on hydrocarbon accumulation, and the activity history of the northern section of the Tanlu Fault in the Cenozoic. Through systematic structure analysis of 3D seismic data of the Liaohe Western Sag, combined with balanced section analysis, a variety of structural features in relation to right-lateral strike-slip faults, such as echelon normal faults, ＂comb＂ structure, ＂flower＂ structure,＂interpretable＂ and ＂buried＂ strike-slip faults have been revealed exist in the Liaohe Western Sag. According to the research in this paper, the complex structural phenomena in the Liaohe Western Sag could be reasonably interpreted as right-lateral strike-slip activity and the strike-slip activities of the Liaohe Western Sag began in the early Oligocene. The activity was weak at the beginning （E3s1-2）, then strengthened gradually and reached its strongest level in the late Oligocene （E3d1）. In the Miocene, the strike-slip activity was low and then strengthened significantly once again from the Pliocene to the present. It is speculated that the entire northern section of the Tanlu Fault has had a similar evolution history since the Oligocene.

The formation and evolution of the Qiaojia pull-apart basin,North Xiaojiang Fault Zone,Southwest China

Sedimentary sequences with drastic thickening over short distances have been observed in Qiaojia County,Yunnan Province,Southwest China.These are related to a pull-apart basin controlled by the Xiaojiang strike-slip fault.Our field investigations include determining the surface characteristics of the Qiaojia basin which consists of three terrace sequences and a series of alluvial fans.Several drill holes were used to reveal the internal structure of the basin.The results suggest that the basinal sediments are over 300 m thick.From bottom to top,they can be classified into five different units.We inferred that the units of lacustrine sediments are deposited in a paleolake which was formed by a paleo-landslide.Accelerator mass spectrometry radiocarbon dating(AMS ^(14)C dating) was used to estimate the ages of the terrace and lacustrine sediments.We use the results to infer that the paleo-lake has existed about 15,000 years and that the Qiaojia basin was uplifted at an average rate of 3.3 mm/a.Furthermore,we then model the evolution process of the basin and interpreted 6 phases of development.

Origin and growth mechanisms of strike-slip faults in the central Tarim cratonic basin, NW China

Through fault structure analysis and chronology study, we discuss the origin and growth mechanisms of strike-slip faults in the Tarim Basin.(1) Multiple stages strike-slip faults with inherited growth were developed in the central Tarim cratonic basin. The faults initiation time is constrained at the end of Middle Ordovician of about 460 Ma according to U-Pb dating of the fault cements and seismic interpretation.(2) The formation of the strike-slip faults was controlled by the near N-S direction stress field caused by far-field compression of the closing of the Proto-Tethys Ocean.(3) The faults localization and characteristics were influenced by the pre-existing structures of the NE trending weakening zones in the basement and lithofacies change from south to north.(4) Following the fault initiation under the Andersonian mechanism, the strike-slip fault growth was dominantly fault linkage, associated with fault tip propagation and interaction of non-Andersonian mechanisms.(5) Sequential slip accommodated deformation in the conjugate strike-slip fault interaction zones, strong localization of the main displacement and deformation occurred in the overlap zones in the northern Tarim, while the fault tips, particularly of narrow-deep grabens, and strike-slip segments in thrust zones accumulated more deformation and strain in the Central uplift. In conclusion, non-Andersonian mechanisms, dominantly fault linkage and interaction, resulted in the small displacement but long intraplate strike-slip fault development in the central Tarim Basin. The regional and localized field stress, and pre-existing structures and lithofacies difference had strong impacts on the diversity of the strike-slip faults in the Tarim cratonic basin.

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

Vertical 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.

Strike-Slip Faults and Their Control on Differential Hydrocarbon Enrichment in Carbonate Karst Reservoirs: A Case Study of Yingshan Formation on Northern Slope of Tazhong Uplift, Tarim Basin

Objective Oil and gas are abundant in the Ordovician Yingshan Formation carbonate karst reservoirs on the northern slope of Tazhong uplift in the Tarim Basin, and have extremely complicated oil-gas-water distribution, however. The difference in burial depth of the reservoirs between east and west sides is up to 1000 m. Water-bearing formations exist between oil- and gas-bearing formations vertically and water-producing wells are drilled between oil- and gas-producing wells. Macroscopically, oil and gas occur at low positions, while water occurs at high positiona on the northern slope of Tazhong uplift. The mechanism of differential hydrocarbon enrichment in heterogeneous reservoirs is by far not clarified, which has affected the efficient exploration and development of oil and gas fields in this area.

Tectonic features,genetic mechanisms and basin evolution of the eastern Doseo Basin,Chad

The features of the unconformity,fault and tectonic inversion in the eastern Doseo Basin,Chad,were analyzed,and the genetic mechanisms and basin evolution were discussed using seismic and drilling data.The following results are obtained.First,four stratigraphic unconformities,i.e.basement(Tg),Mangara Group(T10),lower Upper Cretaceous(T5)and Cretaceous(T4),four faulting stages,i.e.Barremian extensional faults,Aptian–Coniacian strike-slip faults,Campanian strike-slip faults,and Eocene strike-slip faults,and two tectonic inversions,i.e.Santonian and end of Cretaceous,were developed in the Doseo Basin.Second,the Doseo Basin was an early failed intracontinental passive rift basin transformed by the strike-slip movement and tectonic inversion.The initial rifting between the African and South American plates induced the nearly N-S stretching of the Doseo Basin,giving rise to the formation of the embryonic Doseo rift basin.The nearly E-W strike-slip movement of Borogop(F1)in the western section of the Central African Shear Zone resulted in the gradual cease of the near north-south rifting and long-term strike-slip transformation,forming a dextral transtension fault system with inherited activity but gradually weakened in intensity(interrupted by two tectonic inversions).This fault system was composed of the main shear(F1),R-type shear(F2-F3)and P-type shear(F4-F5)faults,with the strike-slip associated faults as branches.The strike-slip movements of F1 in Cretaceous and Eocene were controlled by the dextral shear opening of the equatorial south Atlantic and rapid expanding of the Indian Ocean,respectively.The combined function of the strike-slip movement of F1 and the convergence between Africa and Eurasia made the Doseo Basin underwent the Santonian dextral transpressional inversion characterized by intensive folding deformation leading to the echelon NE-SW and NNE-SSW nose-shaped uplifts and unconformity(T5)on high parts of the uplifts.The convergence between Africa and Eurasia caused the intensive tectonic inversion of Doseo Basin at the end of Cretaceous manifesting as intensive uplift,denudation and folding deformation,forming the regional unconformity(T4)and superposing a nearly E-W structural configuration on the Santonian structures.Third,the Doseo Basin experienced four evolutional stages with the features of short rifting and long depression,i.e.Barremian rifting,Aptian rifting–depression transition,Albian–Late Cretaceous depression,and Cenozoic extinction,under the control of the tectonic movements between Africa and its peripheral plates.

Exploration discovery and hydrocarbon accumulation characteristics of the Doseo strike-slip and inverted basin, Chad

Several international oil companies had conducted petroleum exploration, but failed to make any commercially viable discoveries in the Doseo Basin for over 30 years. In this article, an integrated analysis, based on the latest seismic and drilling data combined with exploration practice and tectonic, sedimentary as well as petroleum-geological characteristics of the basin, has been conducted with the aim to disclose the key factors of hydrocarbon accumulation and enrichment and then to find the potential petroleum plays. The Doseo Basin in Chad is a Meso-Cenozoic lacustrine rift basin developed on the Precambrian crystalline basement in the Central African Shear Zone. It is a half graben rift controlled by the strike-slip fault at the northern boundary, and can be divided into two sub-basins, an uplift and a slope. The basin experienced two rifting periods in the Cretaceous and was strongly inverted with the erosion thickness of 800–1000 m during the Eocene, and then entered the depression and extinction period. Structurally, a large number of normal faults and strike-slip faults are identified in the basin, and the boundary faults are inverted faults with normal at first. The main structural styles include inverted anticlines, fault noses, complex fault-blocks and flower structures. The Lower Cretaceous is the main sedimentary strata, which are divided into the Mangara Group, Kedeni, Doba and Koumra Formations from bottom to up. Two transgressive-regressive cycles developed in the Lower Cretaceous indicates with mainly lacustrine, fluvial, delta, braided-delta, fan-delta sandstone and mudstone. The effective source rock in the basin is the deep-lacustrine mudstone of the Lower Cretaceous containing the type Ⅰ and type Ⅱ;organic matters. Furthermore, Inverted anticlines and fault-complicated blocks comprise the main trap types and the Kedeni Uplift is the most favorable play, followed by the Northern Steep Slope and Southern Gentle Slope. Lateral sealing capacity of faults controls the hydrocarbon abundance.

Tectonic deformation features and petroleum geological significance in Yinggehai large strike-slip basin, South China Sea

The subtle strike-slip tectonic deformation and its relationship to deposition, overpressure and hydrocarbon migration were studied on the basis of systematic sorting of tectonic data.(1) The local T(tension) fractures derived from sinistral strike-slip process were formed before 10.5 Ma, large in number in the nose structure of the eastern slope, and reactivated episodically under the effect of fluid overpressure in the late stage, they served as dominant vertical hydrocarbon migration paths in the slope area of basin.(2) The dextral strike-slip extension was conducive to the increase of depositional rate and formation of regional under-compacted seal, and induced generation of local T fractures which triggered the development of diapirs; in turn, the development of diapirs made T fractures grow in size further.(3) The sinistral strike-slip process weakened after 10.5 Ma, causing tectonic movement characterized by compression in the north and rotational extension in the south, and the uplift and erosion of strata in Hanoi sag and a surge in clastics supply for south Yinggehai sag. Finally, migrating slope channelized submarine fans and superimposed basin floor fans were developed respectively on the asymmetrical east and west slopes of the Yinggehai sag.

Pull-apart Basins and the Total Lateral Displacement Along the Haiyuan Fault Zone in Cenozoic

Pull-apart basins of three scales were found along the Haiyuan fault zone. The largest one is more than 50km long, named Laolongwan basin developed in Miocene. A model was built to calculate the amount of pull-apart of an extensional basin. Parameters used in calculation include thickness and length of deposition and depth of detachment. The results of calculation show that the amount of pull-apart of the Laolongwan Basin is about 30 km. Based on previous studies and calculating by using the average slip rate method, amount of pull-apart of the other two smaller basins are 22 km and 8 km, respectively. Thus, the total displacement of strike-slip along the Haiyuan fault zone is about 60 km, which is close to the offset of the Yellow River from Jingtai to Jingyuan.

Large Active Faults and the Wharton Basin Intraplate Earthquakes in the Eastern Indian Ocean

In recent years,great earthquakes occurred within the Wharton Basin in the eastern Indian Ocean,and they have been associa-ted with active faulting on the ancient oceanic crust.Large seismogenic faults were thought to be the fault reactivation on the ancient oceanic crust,but these phenomena are still unclear and require examination.This study used high-quality multibeam bathymetry and multichannel seismic data collected over the northern Ninetyeast Ridge to investigate detailed fault geometry,structure,and activity.We recognized 12 large linear active faults by integrating bathymetry maps and multichannel seismic reflection profiles.Our results showed that these faults have high angles,and they all displaced the basement and propagated to the seafloor with distinct fault scarps.They trended NWW-SEE with a spacing of 10–40km and were parallel to each other and the nearby subfault of the 2012 great intraplate earthquake,suggesting similar stress fields.These faults are also in agreement with the orientations of magnetic isochrons,implying their formation by seafloor spreading.Furthermore,regarding the strike-slip focal mechanism of 2012 earthquakes,we proposed that these faults were created early by a normal spreading process and then evolved into a strike-slip pattern since the ancient oceanic crust ap-proached the subduction zones.

Factors controlling the development of carbonate reservoirs of Ordovician Yingshan formation in the Gucheng area, Tarim Basin

Deep carbonate rocks are important targets for oil and gas exploration in China. In recent years, significant oil/gas discoveries have been made in the deep carbonate sequences in the Tarim, Sichuan, Ordos, and Bohai Bay basins. Despite significant oil/gas discoveries, large-scale exploration has not been conducted in the Gucheng area in the Tarim Basin. To break the bottleneck restricting the petroleum exploration in the Gucheng area, this study analyzed the factors controlling the formation of carbonate reservoirs of the Yingshan Formation in the Gucheng area in detail based on the basic geological conditions of the study area and the data from cores, thin sections, well logging, testing, and 3-D seismic survey. The inner shallow-ramp in the Gucheng area acts as the main sedimentary facies zone for the development of high-quality reservoirs. The grainstones formed in the high-energy environment of the inner shallow-ramp laid the foundation for subsequent reservoir development in the study area. The dolomitized shoal grainstones in the inner shallow-ramp have well-developed intercrystalline pores and intercrystalline dissolved pores due to later dolomitization, thus serving as high-quality reservoirs. Strike-slip faults are crucial to reservoir reformation and determine whether high production can be achieved in oil and gas exploitation in the study area. Moreover, later reformation by hydrothermal solutions also plays a constructive role in reservoir formation.

走滑拉分盆地中断裂发育特征的二维离散元数值模拟

走滑断裂带的发育演化过程与拉分盆地的形成演化有着密切的关系,主断层不同叠置范围下所发育的次级断裂及拉分盆地具不同的规模及形态。前人多利用物理模拟对走滑拉分盆地演化过程及断裂发育特征进行研究,缺乏对盆地演化过程中断裂变形特征分析及主断层不同叠置程度下断裂演变规律的探讨。本文基于离散元计算软件PFC 2D,模拟了拉分盆地演化过程中走滑主断层在未叠置(underlapping)、侧接(neutral)、叠置(overlapping)三种情况下断裂的几何形态变化特征及其平面演变规律。结果表明拉分盆地断裂发育经历了从走滑主断层端点处向释压弯曲内部发育、再向外部扩展的过程,且无论走滑主断层叠置程度如何,该规律均有较好体现。此外,断裂发育位置与拉伸区域叠合度高,表明断裂的发育规律对拉分盆地的沉降中心迁移或具有重要指示意义。

Sedimentary and tectonic evolution of the Banquan pull-apart basin and implications for late Cenozoic dextral strike-slip movement of the Tanlu Fault Zone

The Banquan Basin is a pull-apart basin with the largest scale and the most prominent structure due to dextral slip of the Tanlu Fault Zone(TLFZ) in late Cenozoic. The depositional history of the basin records the start time and evolution of the right-lateral strike-slip movement of the TLFZ. This paper studies the sedimentary and tectonic evolution of the Banquan Basin by seismic reflection exploration, borehole detection and cosmogenic nuclide chronology. We analyze the coupling relationship between the pull-apart basin and the strike-slip fault and discuss the start time and tectonic significance of the right-lateral strikeslip of the TLFZ. Our study indicates that the Banquan Basin has undergone three evolutionary stages: weak rifting during the pre-pull-apart period, strong extension during the syn-pull-apart period and subsidence during the post-pull-apart period. This implies that the TLFZ, which controls the evolution of the basin, experienced an evolutionary process of weak activity,intensified activity and migration of activity toward the central basin. The sedimentary filling of the basin has strong response to the episodic pull-apart and extension of the basin. Lying upon the basement of the basin, a thin layer of Miocene mudstone slowly accumulated due to local rifting before the strong pull-apart event. Along with the dextral slipping and pull-apart process, the basin was filled with alluvial fan facies, fluvial facies and floodplain facies strata from bottom to top. The latest tectonic movement of the TLFZ in the North China Block in late Cenozoic was dominated by episodic dextral strike-slip motion, and this deformation pattern started at 4.01±1.27 Ma. The latest tectonic deformation in North China since late Cenozoic was governed by eastward extrusion and tectonic orogenesis of the eastern margin of the Qinghai-Xizang Plateau since late Miocene. The eastward thrusting of the Liupanshan fault zone and sinistral shearing of the Qinling fault zone led to the anticlockwise rotation and pushing of secondary blocks in North China, resulting in a planar bookshelf faulting and rotation pattern. This unique deformation pattern transferred eastwards to the North China Plain at ~4.01 Ma and the process continues to the present time.This planar bookshelf rotation, accompanied with regional sinistral strike-slip movement of the ~EW-trending boundary fault zones to the north and south of the North China Block and dextral strike-slip motion of the NNE-trending boundary faults between secondary blocks, is likely to be the long-range effect of the strong extrusion of the eastern margin of the QinghaiXizang Plateau.

The Cretaceous Songliao Basin:Volcanogenic Succession,Sedimentary Sequence and Tectonic Evolution,NE China

The Songliao basin （SB） is a superposed basin with two different kinds of basin fills. The lower one is characterized by a fault-bounded volcanogenic succession comprising of intercalated volcanic, pyrodastic and epiclastic rocks. The volcanic rocks, dating from 110 Ma to 130 Ma, are of geochemically active continental margin type. Fast northward migration of the SB block occurred during the major episodes of the volcanism inferred from their paleomagnetic information. The upper one of the basin fill is dominated by non-marine sag-style sedimentary sequence of silicidastics and minor carbonates. The basin center shifted westwards from the early to late Cretaceous revealed by the GGT seismic velocity structure suggesting dynamic change in the basin evolution. Thus, a superposed basin model is proposed. Evolution of the SB involves three periods including （1） Alptian and pre- Aptian： a retroarc basin and range system of Andes type related to Mongolia-Okhotsk collisional belt （MOCB）; （2） Albian to Companian： a sag-like strike-slip basin under transtension related to oblique subduction of the Pacific plate along the eastern margin of the Eurasian plate; （3） since Maastrichtian： a tectonic inverse basin under compression related to normal subduction of the Pacific plate under the Eurasian plate, characterized by overthrust, westward migration of the depocenter and eastward uplifting of the basin margin.

Practice and theoretical and technical progress in exploration and development of Shunbei ultra-deep carbonate oil and gas field,Tarim Basin,NW China

In this review on the exploration and development process of the Shunbei ultra-deep carbonate oil and gas field in the Tarim Basin, the progress of exploration and development technologies during the National 13th Five-Year Plan of China has been summarized systematically, giving important guidance for the exploration and development of ultra-deep marine carbonate reservoirs in China and abroad. Through analyzing the primary geological factors of “hydrocarbon generation-reservoir formation-hydrocarbon accumulation” of ancient and superposed basin comprehensively and dynamically, we point out that because the Lower Cambrian Yuertusi Formation high-quality source rocks have been located in a low-temperature environment for a long time, they were capable of generating hydrocarbon continuously in late stage, providing ideal geological conditions for massive liquid hydrocarbon accumulation in ultra-deep layers. In addition, strike-slip faults developed in tectonically stable areas have strong control on reservoir formation and hydrocarbon accumulation in this region. With these understandings, the exploration focus shifted from the two paleo-uplifts located in the north and the south to the Shuntuoguole lower uplift located in between and achieved major hydrocarbon discoveries. Through continuing improvement of seismic exploration technologies for ultra-deep carbonates in desert, integrated technologies including seismic acquisition in ultra-deep carbonates,seismic imaging of strike-slip faults and the associated cavity-fracture systems, detailed structural interpretation of strike-slip faults, characterization and quantitative description of fault-controlled cavities and fractures, description of fault-controlled traps and target optimization have been established. Geology-engineering integration including well trajectory optimization,high efficiency drilling, completion and reservoir reformation technologies has provided important support for exploration and development of the Shunbei oil and gas field.

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 ℃, while the FIA with yellow-green fluorescence have homogenization temperatures of 70–90 ℃ 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.