Middle Triassic Leikoupo Formation Research Articles

Utilizing logs to identify complex lithology of tight marl reservoir in the Leikoupo Formation 32 Submember (T2l32) of the Sichuan Basin, China

Recent exploration has revealed that the Middle Triassic Leikoupo Formation 32 Submember (T2l32) in the Sichuan Basin contains unconventional marl reservoirs with significant natural gas potential. Due to limited cores, old wells, and conventional logs, however, the lithological understanding of T2l32 is incomplete and relies solely on inaccurate mud logs. This lack of lithological foundation challenges geology and petroleum research. To identify complex lithology, this paper presents a double-hierarchical workflow to identify seven types of lithology using logs. The first order distinguishes salt, anhydrite, and marl, while the second order further subdivides marl into anhydrite marl, argillaceous limestone, shaly limestone, and limy shale. Different rocks’ logging response characteristics are summarized based on quantity-limited cores and micro-resistivity imaging logs. Lithological identification of 2D and 3D plots is established using sensitive GR, DEN, and RT. Corresponding identification standards are built in two hierarchies. According to these standards, the lithology of T2l32 is identified in a total of 119 wells. Finally, the lithological characteristics of vertical, horizontal, and plane are discussed in the research area. The research results may aid in comprehending the entire lithological characteristics of the complex marl reservoir in T2l32 of the Sichuan Basin. It would help the exploration potential of petroleum systems in turn.

Mechanisms by Which an Evaporated Lagoon Sedimentation System Controls Source–Reservoir Preservation in Lei32 Sub-Member Unconventional Oil and Gas

The muddy limestone in the Lei32 sub-member of the Middle Triassic Leikoupo Formation in the well Chongtan 1 (CT1) of the Sichuan Basin has yielded promising industrial oil and gas production. This discovery has the potential to become a significant strategic reservoir in the future for oil and gas exploration in the Sichuan Basin. However, the understanding of hydrocarbon accumulation in the muddy limestone of the Lei32 sub-member remains insufficient, which poses limitations on further exploration selection and deployment strategies. This study focuses on the analysis of core samples and laboratory data in the wells CT1 and Jianyang 1 (JY1), aiming to investigate the source rock and reservoir characteristics of the muddy limestone in the Lei32 sub-member, as well as the primary controlling factors influencing the development of the source rock and reservoir. The results show that the Lei32 sub-member in the Central Sichuan Basin is an evaporated lagoon deposition; the tight argillaceous limestone and limy mudstone exhibit the characteristic of source–reservoir integration, belonging to a new type of unconventional oil and gas reservoir. The reservoir space of the argillaceous limestone and limy mudstone in the Lei32 sub-member primarily consists of inorganic and organic micro–nanopores and microfractures. The average porosity and permeability are 2.7% and 0.19 mD, indicating a low-porosity and low-permeability unconventional reservoir. The clay minerals and gypsum content are the important factors influencing reservoir porosity, and the fractures are key factors influencing permeability. This study will elucidate the specific features of hydrocarbon accumulation in the muddy limestone reservoirs of the Lei32 sub-member and provide insights into its exploration potential.

The timing of the initial collision between the South and North China blocks constraining from the sediments in the eastern Sichuan Basin

In this study, detrital zircon U–Pb geochronology, trace element and Hf isotopic compositional data from the Early-Middle Triassic clastic rocks in the eastern Sichuan Basin were obtained to distinguish the sediment provenance and constrain the timing of the initial collision between the South China and North China blocks. Detrital zircons from the Early Triassic Feixianguan Formation clastic rocks yield one major age peak at 2476 Ma and three minor age peaks at 1886, 802 and 304 Ma. These detrital zircons may be derived from the South China Block. Detrital zircons from the Early Triassic Jialingjiang Formation clastic rocks yield multiple age peaks at 979, 856, 392 and 269 Ma, indicating a mixed sediment provenance from the South China Block and Qinling Orogenic Belt. This is the first appearance of the detritus with the Qinling Orogenic Belt affinity in the eastern Sichuan Basin. Detrital zircons from the Middle Triassic Leikoupo Formation clastic rocks yield two centralized age peaks at 447 and ca. 245 Ma. These zircons may mainly be derived from the Qinling Orogenic Belt. The results indicate an abrupt change in the detrital zircon U–Pb provenance from the South China Block to the Qinling Orogenic Belt during the Early-Middle Triassic. Integrating the provenance change and other geological characteristics, we suggest that the initial collision in the eastern Qinling Orogenic Belt occurred in the Early Triassic.

Dolomitisation under an arid climate at low sea-level: a case study of the Lei 4 Member of the Middle Triassic Leikoupo Formation, Western Sichuan Depression, China

The origin of dolostones from the Lei 4 Member (T2l4) of the Middle Triassic Leikoupo Formation in the Western Sichuan Depression is unclear. The occurrence and genetic evolution of dolostones in T2l4 were analysed by polarised thin-sections and cathodoluminescence, major- and trace-element, scanning electron microscope, and carbon, oxygen and strontium isotope analyses. The study results are summarised as follows. (1) The dolostones were mainly precipitated in three stages of <40 °C, 40–60 °C and 60–80 °C, corresponding with three paleodepth ranges of 167–300 m, 433–1000 m and 1067–1433 m. The micritic dolostones and the fabric-retentive dolostones (algal dolostone, granular dolostone) mainly formed in a near-surface–shallow-burial environment, and the crystalline dolostones were mainly formed under intermediate burial conditions. (2) 87Sr/86Sr ratios are equivalent to that of the Middle Triassic seawater, Sr/Ba and V/Ni ratios >1, Na notably higher than that in coexisting limestones of T2l4, and the dolostones always coexist with evaporative minerals such as gypsum, indicating that dolomitisation fluids mainly originated from evaporative concentrated seawater. (3) The dolostones mainly inherit materials from precursor limestones based on trace-element distribution patterns, and carbon and oxygen isotope values that are consistent with coexisting limestones of T2l4. (4) Mediated by micro-organisms during the syngenetic period, micritic dolostones and some algal dolostones were formed by replacing aragonites and calcites. During shallow burial, concentrated seawater rich in Mg2+ from the supratidal–intertidal zone flowed downward owing to gravity along the platform and replaced the underlying carbonate rocks, promoting continuous growth of the early dolomites. In the intermediate burial period, the Mg2+-depleted dolomitisation fluid caused the early micritic and silt-crystalline dolostones to recrystallise into silt- or fine-crystalline dolostones with larger crystals and altered the fabric-retentive dolostones into crystalline dolostones. (5) The evaporative dolostones deposited in the near-surface environment are characterised by maximal enrichment of Fe, Sr and Na, the highest δ18O values, the lowest order degree and the highest Ca/Mg ratios. The reflux dolostones formed in a shallow-burial environment characterised by the lowest Fe, medium δ18O values and the lowest order degree. The burial dolostones that developed in the intermediate burial environment are characterised by relative enrichment of Fe and Mn, minimal Na, the lowest δ18O values, the highest order degree and medium Ca/Mg ratios. KEY POINTS The fabric-retentive dolostones mainly formed in a near-surface–shallow-burial environment, and the crystalline dolostones mainly formed under intermediate burial conditions. Dolomitisation fluids mainly originated from the evaporative concentrated seawater, and the dolostones inherit materials from the precursor limestones. Micritic dolostones were formed by evaporative dolomitisation; algal dolostone and granular dolostone were formed by evaporative dolomitisation and reflux dolomitisation; and silt-crystalline dolostone were formed by reflux dolomitisation and burial dolomitisation.

Sedimentary structures of microbial carbonates in the fourth member of the Middle Triassic Leikoupo Formation, Western Sichuan Basin, China

In recent years, abundant natural gas has been found in microbial carbonates in the fourth member of the Leikoupo Formation in Western Sichuan Basin. In this study, from the observation of 626 microbial thin sections, four types of microbial carbonates are classified based on the differences of mesostructures. Among them, thrombolites and stromatolites are subdivided into eight types based on the differences of microstructures. Six types of microbial microstructure association (MSA) are identified, and are mainly developed in microbial mounds. The energy of sedimentary environment and hydrodynamic conditions of them from low to high is MSA-5, MSA-1, MSA-6, MSA-3, MSA-4 and MSA-2. Because of the arid climate in the Annie Period, a restricted platform are developed in the upper sub-member of the Leikoupo Formation in Western Sichuan Basin, and the sedimentary facies are lagoon (gypsiferous lagoon or salt lake in evaporate conditions), microbial mounds, shoals, inner platform shoals and open sea from the east to the west. Microbial microstructures not only affect the pore evolution of microbial carbonate reservoirs, but also affect the diagenesis of microbial carbonate reservoirs of the fourth member of the Leikoupo Formation.

Complex genesis of multiperiod fractures in the Middle Triassic Leikoupo Formation in the Pengzhou gas field, western Sichuan Basin, China

The cumulative expression of multistage deformation is complex multiperiod fractures, which are commonly seen in tectonic zones. The Middle Triassic Leikoupo Formation in the western Sichuan Basin Depression, China, is a typical marine carbonate reservoir with natural fractures caused mainly by tectonic movements. According to outcrops, drill cores, image logging, and fluid inclusions, the fracture characteristics, types of natural fractures, and interactions of fractures are determined. In total, 419 natural fractures in 493.2 m of cores from 7 wells are investigated, which are mainly shear and tensile fractures with a small number of weathering generated fractures. Meanwhile, the results of the stable isotope analysis of δ13C and δ18O, as well as the flow fluid inclusion data, reveal four tectonic periods of fractures with different occurrences. Based on the history of regional tectonic evolution, indicating one period of weathering fractures ascribable to stratal uplift and three periods of structural fractures related to the sequential tectonic movements of the Longmenshan fault belt. By analyzing the interaction relationships of fractures, three types of fracture interaction relationships are observed: cutting, restraining, and overlapping. The four stages fractures are chronologically assigned to (1) the early Indosinian N–S trending compression, (2) the late Indosinian NW-SE compression, (3) the middle Yanshanian NE-SE compression, and (4) the early Himalayan E-W compression. The influence of natural fractures on gas migration and well production in marine carbonates is discussed, and indicates that tectonic fractures could provide seepage channels for gas migration and accumulation from near or distant hydrocarbon source rocks into the Middle Triassic Leikoupo Formation. This study utilizes a pragmatic approach for understanding the fracture genesis mechanism in oil and gas field with multiperiod fractures.

Depositional framework and reservoir characteristics of microbial carbonates in the fourth member of the middle Triassic Leikoupo Formation, western Sichuan Basin, South China

Microbial carbonate reservoirs are the main petroleum exploration targets in the upper submember of the fourth member of the Middle Triassic Leikoupo Formation in the western Sichuan Basin. To improve understanding of the distribution and reservoir characteristics of microbial carbonates, detailed facies descriptions and reservoir characterizations were performed under the framework of cyclostratigraphy. Ten facies can be identified and grouped into four facies associations in two transgressive-regressive cycles, within an epeiric marine carbonate platform in the western Sichuan Basin. In Cycle 1, Emei taphrogeny caused the formation of the Longmenshan subaqueous paleo-uplift, which formed a relatively restricted tidal flat environment and developed large-scale microbialites on its eastern side. In the central area, wavy to domal stromatolitic dolowackestone (F3) and thrombolitic dolomudstone (F4) are developed in the intertidal and subtidal of the central area with steep substrate gradients and restricted seawater circulation. Laminated microbial dolowackestone (F2) is mainly developed in the intertidal of the southern area with low substrate gradients and restricted seawater circulation. Intraclast dolograinstone (F6) are widely developed in the subtidal shoals of the northern area, which is frequently disturbed by tides and waves with better seawater circulation. In Cycle 2, increasing relative sea level weakened the barrier of Longmenshan subaqueous uplifts to waves, the restricted subtidal environment developed the intraclast dolograinstone (F6) and intraclast packstone-grainstone (F7), occupying the platform with better seawater circulation and higher energy. The framework or inter-clot pores of F2, F3 and F4, interparticle pores of F6, and vugs resulting from syn-depositional dissolution are the dominant pore types. High-quality reservoirs are mainly distributed in the regressive microbialites (F2–F4) of Cycle 1 in the central area and intraclast dolograinstone (F6) of Cycle 1 and Cycle 2 in the northern area. The porosity of microbialites and intraclast dolograinstone reservoirs mainly ranges from 2.26% to 10.00%, with a mean value of 4.96%. The permeability mainly ranges from 0.01mD to 13.98mD, with a mean value of 4.63mD. This study proposed a depositional framework of microbialites and evaluated its control on reservoir quality, which is of great significance for the further study of microbial carbonate reservoirs.

Genetic Type and Source Analysis of Natural Gas in the Leikoupo Formation of the Sichuan Basin in China

The Middle Triassic Leikoupo Formation is largely extended through the Sichuan Basin, SW China. In this formation, several commercially exploited gas reservoirs have been discovered in western and central parts of the basin. Due to the complicated geochemical signatures of the natural gases in these reservoirs, there are contrasted interpretations about their sources, which hamper the evaluation and exploration for new gas resources in the area. To obtain complete understanding of the natural gas sources, the Leikoupo Formation gas reservoirs discovered so far in the Zhongba, Yuanba, Longgang, and Moxi gas fields were selected as the research object of this study. The genetic types and sources of the natural gases in the Leikoupo Formation are discussed based on gas geochemistry combined with their geological background. The natural gas in the top members of the Leikoupo Formation (T2l4 or T2l3) is partially originated from a humic kerogen contained in the source rocks from the overlying Upper Triassic Xujiahe Formation and from sapropel kerogen from the source rocks of the Leikoupo Formation itself. The natural gas of T2l1 member in the lower part of the Leikoupo Formation is mainly sapropel-type probably from the source rock of the Permian Wujiaping Formation, where the Permian Longtan Formation undergoes a phase change into the Wujiaping Formation The reversed δ13C1 and δ13C2 trend in the Leikoupo Formation of the Yuanba gas field is due to the more sapropelic source rocks and higher degree of maturity.

Geochemical Characteristics and Origin of Tight Gas in Member Two of the Upper Triassic Xujiahe Formation in Western Sichuan Depression, Sichuan Basin, SW China

Natural gas in the tight sandstone reservoirs in Member two of the Upper Triassic Xujiahe Formation in the Western Sichuan Depression of the Sichuan Basin has complex geochemical characteristics, and the origin and source of the tight gas are revealed based on the geochemical analysis and comparison in this study. The tight gas has the dryness coefficient of 0.950–0.994, which is positively correlated with the CH4 content. The gaseous alkanes display positive carbon isotopic series, with the δ13C1 and δ13C2 values ranging from −35.6‰ to −30.3‰ and from −29.1‰ to −21.0‰, respectively, and the δD1 values range from −176‰ to −155‰. Genetic identification based on the carbon and hydrogen isotopic compositions indicates that a small amount of tight gas is typically coal-derived gas, whereas most of the tight-gas samples have experienced mixing by oil-associated gas. Geochemical comparisons suggest that the tight gas displays distinct differences with the typical oil-associated gas in Member four of the Middle Triassic Leikoupo Formation in the Chuanxi gas field and the typical coal-derived gas in Member five of Xujiahe Formation in the Xinchang gas field. It is also apparently different from the typical coal-derived gas in Member two of Xujiahe Formation in both Zhongba and Qiongxi gas fields. Among the tight gas in Member two of the Xujiahe Formation from the Western Sichuan Depression, the coal-derived gas is generated mainly by the humic mudstone in the Upper Triassic Ma’antang and Xiaotangzi formations, with assistance of the humic mudstone in Member two of the Xujiahe Formation, whereas the oil-associated gas is derived from the sapropelic limestone in the Ma’antang Formation.

Triassic in the eastern Sichuan basin ray slope and badong group strata division and correlation

Triassic strata are widely developed in the eastern part of Sichuan Basin. The Mingyuexia anticline and Huangnitang anticline located in the eastern Sichuan structural belt develop Triassic Jialingjiang Formation to Xujiahe Formation. The middle Triassic Leikoupo Formation and Badong Formation on both sides of the anticline are synchronous and heterogeneous strata. They are distinguished regionally according to their different sedimentary environments and lithologic characteristics, and the phase transition boundary between Leikoupo Formation and Badong Formation is determined.

Restoration of karst paleogeomorphology and its significance in petroleum geology—Using the top of the Middle Triassic Leikoupo Formation in the northwestern Sichuan Basin as an example

The influence of karstification on gas reservoir formation is an area of interest in petroleum geology, and karst paleogeomorphology restoration is a topic of reservoir research. However, most previous studies of karst paleogeomorphology have focused on karst reservoirs, ignoring the influence of the karst paleogeomorphology on the overlying strata and gas accumulations. Therefore, the value of the petroleum geology information associated with karst paleogeomorphology is not maximized. The carbonate strata of the Leikoupo Formation were uplifted and exposed by the Indosinian movement, leading to the development of a weathered karst stratum (Lei 4 3 sub-member). Many commercial gas wells have been developed in the Lei 4 3 sub-member. Based on the latest drilling, logging, and two- and three-dimensional seismic data for the northwestern Sichuan Basin, we selected the residual thickness and the impression methods to restore the karst paleogeomorphology of the Lei 4 3 sub-member. The karst paleogeomorphology was generally high in the southeast and low in the northwest. The area can be successively divided into three secondary karst geomorphological units, namely, karst highlands, karst slopes, and karst depressions, and several tertiary geomorphological units. The karst geomorphology had the function of controlling reservoir, hydrocarbon source, and gas accumulation distribution. The karst processes were different in the various geomorphological units, further controlling the distribution and development of the karst reservoirs. The inherited geomorphology also controlled the sedimentation of the clastic rocks in the overlying formation. Due to the paleogeomorphological setting of higher in the east and lower in the west, the hydrocarbon generation intensity of the early Xujiahe Formation increases from the karst highland to the karst depression. Due to the cutting and lateral sealing of the formation by faults and karst valleys, there are no unified gas-water boundaries in the Lei 4 3 sub-member. As a result of the overall tectonic setting of high in the east and low in the west, the natural gas is enriched in the karst monadnocks. • Karst palaeogeomorphology at the top of the Leikoupo Formation was restored. • The karst geomorphology controlled the reservoir, source rocks and gas accumulation. • The gas accumulations have characteristics of each karst monadnock is a gas accumulation.

Middle Triassic sedimentary evolution in the Upper Yangtze region with implications for the collision between the South and North China Blocks

Middle Triassic marine carbonate successions deposited on the northwestern Yangtze passive continental margin preserve information about the initial collision between the South China Block (SCB) and the North China Block (NCB). We extracted data from 51 boreholes and 23 outcrops of the Middle Triassic Leikoupo Formation to interpret the spatio-temporal stratigraphic framework, depositional transition, and contractional deformation in the Upper Yangtze region. The sedimentological analysis indicates that the Leikoupo Formation may be divided into nine successions, which were overall deposited in restricted-evaporative marine platform and lagoon environments during the Anisian. From the thickness distributions of each sequence, the Upper Yangtze region is shown to have experienced northwest-southeast horizontal compression and vertical uplift, which drove northwestward migration of the coeval depocenters and marine regression. We propose that the marine deposits of the Leikoupo Formation and the contemporary paleo-uplifts can be considered products of the forebulge and backbulge depozones of a peripheral foreland basin. The Early Triassic passive continental margin in the Upper Yangtze region had thus transformed into a peripheral foreland basin in response to the collision between the SCB and NCB by the Anisian. The northwestward retreat of seawater with synchronous growth of the forebulge further supports the oblique suturing between the two rotating blocks. In response to continuous basin-ward thrusting of the Micangshan-Daba Shan and Longmen Shan belts, new transpressional flexural subsidence developed, leading to the formation of terrestrial deposits in the Late Triassic basin.

Lithofacies paleogeography and characteristics of large-scale reservoirs of the Middle Triassic Leikoupo Formation in Sichuan Basin, China

Material exploration discoveries have been achieved in the Middle Triassic Leikoupo Formation in the Sichuan Basin in recent years, and a large-scale reservoir is one of the key factors controlling reservoir formation. In this paper, the lithofacies paleogeographic map of the Leikoupo Formation is prepared based on field outcrops, exploratory wells, well logs, and seismic data. The characteristics of large-scale reservoirs have been analyzed, which resulted in three new summarized understandings: (1) At the Leikoupo stage, the Sichuan Basin was relatively closed in the south, east and north, and the seawater entered through the west side, so it was a relatively closed carbonate platform; during the sedimentary stage of Lei I and Lei II members, it was a weakly fringed carbonate platform. In the sedimentary period of Lei III (T2l3) and Lei IV (T2l4) members, it was a rimmed carbonate platform. (2) The sedimentary facies including the platform margin, open platform, restricted platform, and evaporative platform were developed in the Leikoupo Formation; these were mainly the restricted platform facies covering the subfacies of the lagoon, intra-platform shoal, and tidal flat. Two types of hydrocarbon reservoirs, namely the platform-margin shoal and intra-platform shoal reservoirs, were developed in this period. (3) Two sets of large-scale reef beach facies reservoirs were developed in the Leikoupo Formation, one is the platform margin reef beach facies of Lei III (T2l3) and Lei IV (T2l4) members in northwestern Sichuan with a large thickness (generally larger than 50 m); the other one is the intra-platform shoal facies in the lower part of Lei I Member (T2l1) in Central Sichuan with a small thickness, stable distribution, and a large area (larger than 20,000 km2) forming large-scale reservoirs. The two types of large-scale reservoirs experienced multi-stage karstification transformation to become reservoirs with a good quality, laying the material basis for large-scale oil & gas reservoir formation. The research results have important theoretical and practical significance to improve the understanding on the sedimentation and reservoirs of the Leikoupo Formation in the Sichuan Basin, and to provide guidance for the oil and gas exploration in this region.

Gas source and exploration direction of the Middle Triassic Leikoupo Formation in the Sichuan Basin, China

Natural gas exploration in the Middle Triassic Leikoupo Formation (T2l) in the Sichuan Basin of China has achieved a remarkable breakthrough in recent years and thus attracts wide attention. However, there is no consensus on the gas source. The genetic identification and gas–source correlation based on the geochemical characteristics of natural gas indicate that the natural gas in the Leikoupo Formation is mainly from the underlying Upper Permian Longtan/Wujiaping Formation (P3l/P3w) source rocks (Moxi). Nevertheless, it has assistance (Zhongba) or hydrocarbon supply condition (Chuanxi) brought by the source rocks in the 1st Member of the Upper Triassic Xujiahe Formation (T3x), or mainly from the T3x source rocks with a specific contribution by the T2l source rocks (Yuanba and Longgang). The T2l gas pools display three types of source-reservoir assemblages, and they suggest different exploration directions. The P3l source rocks serve as main source rocks that can constitute the lower-generation and upper-accumulation pattern. The positive structures around the deep faults connecting the P3l and T2l are the most favorable areas to explore large-scale T2l gas pools. The source rocks at the bottom of T3x and the reservoirs at the top of the T2l can constitute the side-generation and side-accumulation pattern, which is favorably developed in the residual karst hills or the local tectonic high position of the karst slope. The T2l can constitute self-generation and self-accumulation pattern, which assists in the formation of gas pools.

Constraints of molecular and stable isotopic compositions on the origin of natural gas from Middle Triassic reservoirs in the Chuanxi large gas field, Sichuan Basin, SW China

Abstract The Middle Triassic Leikoupo Formation in the Sichuan Basin is a gypsum-bearing stratum. The Chuanxi (CX) large gas field has been recently discovered in Middle Triassic reservoirs, with a controversial gas source. The gas geochemistry is analyzed in this study to investigate the gas origin and source. The H2S-bearing dry gas in the CX gas field has an H2S content ranging from 1.07% to 4.09%, with a gas souring index (GSI) of 0.0120–0.0438. The δ13C1, δ13C2, and δ2HCH4 values range from –31.8 to –30.4‰, from –32.9 to –29.6‰, and from –153 to –136‰, respectively. Genetic identification based on the geochemical characteristics suggests that the gas is an oil-type and derives from the secondary cracking of oil. According to the GSI and alkane carbon isotopes, the gas experienced weak TSR alteration without ethane. Potential high-quality source rocks include the Upper Permian mudstones and marlstones, as well as the Lower Cambrian mudstones, with a total organic carbon content generally higher than 2.0%. A comparison of the carbon isotopic compositions between ethane in the gas and kerogen and the hydrogen isotopic compositions of methane implies that natural gas has an affinity with the Upper Permian rather than the Lower Cambrian source rocks. The extremely low development of solid bitumen in the reservoirs suggests the inexistence of large-scale paleo-oil pools in the Leikoupo Formation. The Middle Triassic natural gas in the CX gas field originates from the Upper Permian Longtan Formation, where the gas pools are dominated by direct filling of secondary cracking gas.

Geological structures and potential petroleum exploration areas in the southwestern Sichuan fold-thrust belt, SW China

Based on the latest geological, seismic, drilling and outcrop data, we studied the geological structure, tectonic evolution history and deformation process of the southwestern Sichuan fold-thrust belt to find out the potential hydrocarbon exploration areas in deep layers. During key tectonic periods, the southwestern Sichuan fold-thrust belt developed some characteristic strata and structural deformation features, including the Pre-Sinian multi-row N-S strike rifts, step-shaped platform-margin structures of Sinian Dengying Formation, the western paleo-uplift in the early stage of Late Paleozoic, the Late Paleozoic–Middle Triassic carbonate platform, foreland slope and forebulge during Late Triassic to Cretaceous, and Cenozoic multi-strike rejuvenated fold-thrusting structures. The fold-thrust belt vertically shows a double-layer structural deformation controlled by the salt layer in the Middle Triassic Leikoupo Formation and the base detachment layer at present. The upper deformation layer develops the NE-SW strike thrusts propagating toward basin in long distance, while the deeper deformation layer had near north-south strike basement-involved folds, which deformed the detachment and thrusting structures formed earlier in the upper layer, with the deformation strength high in south part and weak in north part. The southern part of the fold-thrust belt is characterized by basement-involved fold-thrusts formed late, while the central-northern part is dominated by thin-skin thrusts in the shallow layer. The Wuzhongshan anticlinal belt near piedmont is characterized by over-thrust structure above the salt detachment, where the upper over-thrusting nappe consists of a complicated fold core and front limb of a fault-bend fold, while the deep layer has stable subtle in-situ structures. Favorable exploration strata and areas have been identified both in the upper and deeper deformation layers separated by regional salt detachment, wherein multiple anticlinal structures are targets for exploration. Other potential exploration strata and areas in southwestern Sichuan fold-thrust belt include the deep Sinian and Permian in the Wuzhongshan structure, pre-Sinian rifting sequences and related structures, platform-margin belt of Sinian Dengying Formation, and Indosinian paleo-uplift in the east of the Longquanshan structure.

Sedimentary and Diagenetic Features and Their Impacts on Microbial Carbonate Reservoirs in the Fourth Member of the Middle Triassic Leikoupo Formation, Western Sichuan Basin, China

In recent years, the discovery of two gas fields in the fourth member of the Leikoupo Formation in the Western Sichuan Basin of SW China confirmed the exploration potential of microbial carbonates. The aim of the present study is to clarify the formation mechanism of the microbial reservoirs in the Leikoupo Formation. For this purpose, lithofacies, depositional environments, and diagenesis analyses were performed in samples collected from cores of 12 wells. The climate of study area was arid during Anisian time, and the water body was restricted. In such a climate, an evaporitic environment was developed, where ten types of lithofacies, dominated by microbial carbonates and gypsum rocks, were recognized. Thrombolites and stromatolites are the main high-quality reservoirs rock types in the fourth member of the Leikoupo Formation in the Western Sichuan Basin of SW China, which developed as microbial mounds, with reservoir space of microbial inter-clot pores, intra-clot pores, fenestral pores, inter-crystalline pores, and cracks. The microbial inter-clot pores are the main reservoir space, formed by trapping and binding of marls by benthic microbial communities. These pores were partially filled with evaporites because of the arid climate, which were subsequently dissolved (mainly gypsum) in the syn-depositional period, thus greatly improving the quality of reservoirs. Although some pores were occluded by multi-stage cements during the burial stage, major pores were well preserved own to the early dolomitization, rapid burial of the Leikoupo Formation, and early charging of hydrocarbon. The early dolomitization enhanced the anti-compaction ability of microbial carbonates during the burial stage. Rapid burial of the Leikoupo succession slowed down early cementation, and it also accelerated the maturation and expulsion process source rock to promote early charging of hydrocarbon in pores, which created a closed system, inhibiting strong burial cementation.

Multiphase dolomitization of a microbialite-dominated gas reservoir, the middle Triassic Leikoupo Formation, Sichuan Basin, China

Microbialites have great economic potentials in hosting hydrocarbons throughout the geological record. Diagenesis, particularly dolomitization, has been proven to be crucial in microbialite reservoir development, however, the impacts of dolomitization on microbialite reservoir quality were not yet clearly understood. In this study, core samples covering the Middle Triassic Leikoupo formation were collected, and combination of petrology, fluid-inclusion microthermometry, and stable and radiogenic isotopes analyses was conducted, aiming at unraveling the origin of the dolomites and their reservoir characteristics in the microbialites from the Sichuan Basin, China. Two types of microbial dolostone reservoir were determined and compared in terms of their dolomitization mechanisms and reservoir characteristics. Organogenic dolomitization mediated by microbial activities probably resulted in the initial biotic dolomicirte (D1) precipitation in sediment pile as a consequence of bacterial sulfate reduction (BSR), characterized by significant δ13C depletion compare to the middle Triassic seawater value. An early stage dolomitization, occurred in fluids dominated by seawater was responsible for the precipitation of very fine crystalline dolomite (D2). Burial dolomitization, precipitated medium-to coarsely crystalline dolomite cements (D3) from burial brine in temperatures between 120 °C and 160 °C and characterized by elevated 87Sr/86Sr values and slightly low δ13C values. Last hydrothermal dolomitization event, represented by saddle dolomite cement (D4), has probably occurred and is associated with deeply-seated hydrothermal fluids at temperatures >200 °C. Abiotic microbialite dolomudstone reservoir quality (D2) (porosity = 7.4%; permeability = 24.9 mD) superior to the biotic microbial-mediated dolomudstone reservoir (D1) (porosity = 1.4%; permeability = 0.6 mD). Microbialite dolomudstone reservoirs (D2) are microporous, with the presence of both primary porosity, e.g., framework and interparticle, and secondary porosity, e.g., solution-enlarged and micropores. The wide occurrence of intragranular and micro-fracture, together with the proximity of larger macropores, may increase connectivity and allow for a combined micropore to macropore transport process for natural gases. Micropores in these dolostones were interpreted to be formed by mole to mole replacement of calcite by dolomite. Hence, this study emphasizes the important role of dolomitization in the formation of ancient microporous microbialites reservoirs characterized by low porosity and low permeability.

Reservoir characteristics and forming conditions for the Middle Triassic Leikoupo Formation in the western Sichuan Basin, China

In the past few years, petroleum exploration in the Leikoupo Formation in the western Sichuan Basin of China has obtained fruitful achievements, but composite research on the carbonate rock reservoir and its control factors are lagging behind, which constrains further exploration in the Leikoupo Formation based on field outcrop, core and logging data, while the laboratory analysis, reservoir characteristics and forming conditions of the Leikoupo Formation in the western Sichuan Basin are systematically summarized. The results show: (1) the Leikoupo Formation reservoir mainly involves algae dolomite, grain dolomite and fine powder crystal dolomite with intergranular, intercrystalline and biotrellis pores, and the reservoir generally has low porosity and low permeability; (2) both sedimentary facies and karstification control the development of the Leikoupo Formation reservoir, in which the sedimentary facies is essential to the reservoir, and penecontemporaneous dolomitization and karstification act as a controlling factor to maintain the pore size and secondary pore development, while supergene karstification at the end of the Middle Triassic improved the pores, and the dissolution of organic acid in the burial stage preserved the pores.

Eogenetic Karst in Interbedded Carbonates and Evaporites and Its Impact on Hydrocarbon Reservoir: A New Case from Middle Triassic Leikoupo Formation in Sichuan Basin, Southwest China

Karst in interbedded carbonates and evaporites has been reported to have important and complex impacts on reservoir. It is significant for exploration and karst geology. Here, we report such a new case from Middle Triassic Leikoupo Formation of Sichuan Basin, Southwest China. Stratigraphic incompleteness and the occurrence of unconformity provide evidence for the presence of eogenetic karst. Under the impact of this eogenetic karst, residual weathered and solution-collapse breccia, solution pores and silicification and dedolomitization have been observed. Classic stratigraphic zonation of karst is not readily distinguishable, which is ascribed to the stratigraphic collapse of carbonate rocks resulting from the dissolution of evaporites by lateral subsurface fluid flow. In terms of impact on reservoir quality, karst can generally improve the initial physical property of the porous layers in theory. However, subsurface fluid flow dissolved the evarporitic beds and facilitated the collapse of overlying strata. As a consequence, the lateral continuity of the reservoirs would be destroyed, and relatively high-quality reservoirs can only be developed with little collapse of overlying strata, reflecting reservoir heterogeneities. This may be a general feature of reservoir formation under the impact of karst in interbedded carbonates and evaporites.