The extensional-convergent tectonic cycle is commonly developed in the carbonate strata of the middle-lower assemblages within small cratons in central and western China. It not only governs the sedimentary differentiation and filling processes but also provides the core driving mechanism for the development and spatio-temporal arrangement of fundamental petroleum geological elements in deep-ultra-deep domains.Taking the Neoproterozoic-Ordovician in the Tarim Basin as an example,by leveraging 42 newly acquired and spliced seismic lines, a 3D data volume spanning 56 000 km² in the Tazhong-Tabei area, more than 70 wells, C-O isotope data (sampled at 5-10 m intervals), and over ten thousand cuttings (core) thin sections, we reconstruct the tectonic-paleogeographic background during key tectonic transformation (sub) stages. Subsequently, lithofacies paleogeographic maps are meticulously compiled in sequence units to clarify the platform type conversions and sedimentary differentiation characteristics within the extensional-convergent cycle and to determine the reservoir-forming assemblages.The results indicate that: (1) The differential sedimentary filling of two types of Nanhua Period paleo-rifts, combined with the inherited paleo-uplifts and the (syndepositional) paleo-uplifts formed by convergent compression in the Middle-Late Ordovician, jointly constitute the foundation for tectonic-paleogeogeographic differentiation. (2) The Sinian-Ordovician can be divided into one first-order sequence and four second-order tectonic sequences, corresponding respectively to four evolutionary stages: the rift-depression transition stage, the cratonic extensional construction stage, the extensional-convergent transition stage, and the convergent strong differentiation-drowned platform stage. During this period, the tectonic-paleogeographic background and paleo-sea level fluctuations controlled the orderly succession and development of muddy slopes, carbonate slopes, rimmed platforms, and drowned platforms. (3) Six sets of large-scale source rocks were developed within the extension-convergence cycle. These source rocks, together with the following five assemblages, constitute five types of source-reservoir-cap assemblages: mudstone of the Cambrian Yuertusi Formation-dolomite of the Upper Sinian, gypsum-salt rock of the Cambrian Miaolingian Series-dolomite of the Cambrian Series 2, tight carbonate rocks and mudstone of the Lower Ordovician-weathered crust of dolomite of the Cambrian Furongian Series, Sangtamu mudstone of the Upper Ordovician-fault-controlled karst weathered crust of the Middle-Upper Ordovician and slope-facies mudstone-collapse body of the Cambrian Furongian Series.
The Permian Taiyuan Formation tight limestone in the Ordos Basin exhibits significant exploration potential. Multiple high-risk exploration wells, such as YT1H well and ZT1H well, have yielded high industrial gas flows, making this formation a key target for natural gas exploration. However, strong reservoir heterogeneity and poorly understood microscopic pore structure characteristics and controlling factors have hindered further gas exploration planning in this area. Based on observations of core observations of, cast thin sections, and field emission scanning electron microscopy (FE-SEM), combined with low-temperature nitrogen adsorption experiments, and high-pressure mercury intrusion porosimetry (MIP), this study systematically investigates the microscopic pore structure characteristics and their controlling factors in the tight limestone reservoirs of the Taiyuan Formation. The influence of differential dissolution during the early diagenetic stage on pore structure is revealed. The results indicate: (1) The Taiyuan Formation develops reservoir rocks including bound bioclastic limestone, micritic bioclastic limestone, and bioclastic micritic limestone. Main types of reservoir space include moldic pores, intraskeletal pores, framework pores, intercrystalline pores, and dissolved micro-fractures. Porosity ranges from 0.38% to 9.34%, and permeability ranges from 0.002×10-3 μm2 to 0.776×10-3 μm2, characterizing a low-porosity, low-permeability tight reservoir. (2) The pore structure exhibits strong heterogeneity, with multimodal pore size distribution showing peaks in ranges such as 3-4 nm, 50-100 nm, and 700 nm-2 μm. The reservoir contains coexisting micro-scale and nano-scale pore systems, where storage capacity is mainly contributed by macropores and micropores, while adsorption capacity is dominated by micropores and mesopores. (3) Reservoir properties are controlled by heterogeneous pore structure. Porosity and permeability show positive correlations with average and median pore-throat radii, but a negative correlation with pore-throat sorting coefficient. Compared to porosity, pore-throat size and uniformity exert a greater influence on permeability. (4) The heterogeneity of the microscopic pore structure is closely related to differential dissolution during early diagenesis. Moderate karstification improves pore structure, whereas excessive dissolution leads to deterioration. In the slight dissolution stage (selective dissolution), pores are mainly micropores and mesopores, with fine throat types. Under moderate dissolution (mottled dissolution), enhanced karstification transforms micropores and mesopores into macropores and micrometer-scale pores, resulting in medium pore-throat systems. In the over-dissolution stage (karst brecciation), the karst system disrupts the bedrock, leading to degraded pore structure characterized by medium pores and fine throats.
The dolomites of the Middle Permian Maokou Formation in the central Sichuan Basin are important targets for deep oil and gas exploration in the Sichuan Basin. To gain an in-depth understanding of their genesis and reservoir formation mechanisms, this study, based on systematic core and thinsection observations combined with geochemical analysis such as rare earth elements, carbon and oxygen isotopes, and U-Pb dating, has yielded the following insights: (1) The dolomites of the Maokou Formation were primarily formed through three stages of dolomitization: residual bioclastic grain dolomite is the product of penecontemporaneous sea water dolomitization; very fine-crystalline and fine-crystalline dolomite formed in a shallow burial environment, controlled by dolomitization of marine-sourced pore water; while medium- to coarse-crystalline dolomites were related to the Emeishan large igneous province event and formed by the replacement of pre-existing dolomite or limestone through shallow burial structural-hydrothermal dolomititation. (2) The reservoir spaces of the Maokou Formation dolomites underwent a complex diagenetic evolution process, with the diagenetic sequence being: cementation→meteoric freshwater dissolution→penecontemporaneous dolomitization→shallow burial dolomitization→mechanical compaction→tectonic fracturing→hydrothermal alteration→pressure dissolution→hydrocarbon charging. (3) The development of the Maokou Formation dolomite reservoirs is controlled by a "sedimentation-diagenesis-tectonics" triple control mechanism: high-energy platform-margin shoals laid the foundation for primary porosity; penecontemporaneous dissolution and dolomitization were key to the development of large-scale porosity; and tectonic-hydrothermal activities played an important adjusting role in the reservoirs through the dynamic balance between dissolution-enhanced porosity and cementation-reduced porosity. Based on the above understanding, it is inferred that large-scale dolomite reservoirs of the Maokou Formation in the central Sichuan Basin are mainly distributed in high-energy platform-margin shoals or areas adjacent to faults. The results of this study enhance the understanding of the formation process of complex dolomite reservoirs and provide clear directions and solid theoretical support for the next phase of deep oil and gas exploration in this area.
The controlling mechanisms of physical properties and distribution laws of deep water and low-permeability reservoirs have become key scientific issues urgently to be solved in China offshore oil and gas exploration and development. Taking the 3rd member of Lingshui Formation reservoir in YL10 structure on the southern slope of Baodao Sag, Qiongdongnan Basin as the research object, this paper systematically studies the petrological characteristics, pore-throat structure and physical property distribution laws of the reservoir by comprehensively using experiments such as cast thin sections, scanning electron microscopy (SEM), high-pressure mercury intrusion and fluid inclusions. The results show that: (1) The 3rd member of Lingshui Formation in the study area has strong heterogeneity and complex pore-throat structure, generally developing medium-porosity, low to ultra-low permeability reservoirs, with "sweet spot" reservoirs of high porosity and high permeability existing in some areas. (2) The difference in reservoir physical properties is controlled by the sedimentary-diagenetic coupling effect. Sedimentation lays the material foundation for the Lingshui Member 3 reservoir, and diagenesis is the main controlling factor affecting the reservoir type. Compaction, affected by burial depth, is the main cause of reservoir differentiation. Cementation intensifies the differentiation of reservoir physical properties, and the differences in the type, content and occurrence of cements lead to the differentiation between low-permeability and ultra-low-permeability reservoirs. Dissolution controlled by oil and gas charging plays a constructive role in reservoir porosity, and the formation of a large number of mold pores results in maintaining medium porosity while low permeability in the deeply buried reservoir. (3) The shallow, weak-diagenetic zone in the south is a favorable reservoir distribution area, where thick underwater distributary channel sand bodies exhibit high porosity and permeability characteristics; the deep reservoirs in the north require special attention to zones with developed dissolution pores and weak cementation.
Focusing on the core issue of unclear natural gas genesis and the dominant controlling factors of hydrocarbon accumulation in the Sinian Dengying Formation in southwest Sichuan Basin, this study utilizes drilling and outcrop data to investigate the development characteristics of source rocks, as well as the components, carbon and hydrogen isotopic compositions, gas-source correlation of natural gas in Dengying Formation, and further establish a hydrocarbon accumulation pattern. The results show that: (1) Three sets of source rocks with good hydrocarbon generation potential and large-scale development are present in southwest Sichuan Basin, including the Lower Cambrian Qiongzhusi Formation, the argillaceous shale of the Sinian Doushantuo Formation, and the carbonaceous slate of the Middle Proterozoic Ebian Member 3. (2) Natural gas in the Dengying Formation is mainly composed of hydrocarbon gases, with a high dryness coefficient and characteristics of low sulfur and high helium content. The δ13C1 value ranges from -31.5 ‰ to -30.9 ‰, which are heavier than that in Gaoshiti-Moxi area of central Sichuan Basin. The δ 2HCH4 value is -153‰ to -149‰, which are lighter than that in Gaoshiti-Moxi area. (3) Based on the analysis of δ13C1, δ2HCH4, and 40Ar/36Ar geological dating, it is believed that the natural gas in the Dengying Formation is contributed by source rocks from the Lower Cambrian Qiongzhusi Formation and the Sinian-pre-Sinian systems, with a higher contribution from the Sinian-pre-Sinian source rocks compared to the Gaoshi-Moxi area. (4) The three sets of source rocks have favorable spatial relationships with high-quality reservoirs of platform margin in the Dengying Formation, where underlying basement-involved faults provide pathways for hydrocarbon migration, and the overlying salt-gypsum rocks of the Triassic Leikoupo Formation serve as effective cap rocks. This study proposes a hydrocarbon accumulation pattern for the Dengying Formation characterized by "multi-source bidirectionally supplying hydrocarbon and dual-structure controlling accumulation". These findings provide geological support for hydrocarbon exploration in the Dengying Formation in southwest Sichuan Basin.
In order to clarify the unconventional oil and gas exploration plays in the Sichuan Basin, a comprehensive analysis is conducted to study the characteristics of mixed sedimentation of the Jurassic Da'anzhai Member and their significance for oil and gas accumulation by integrating data from outcrops, drilling, seismic and others. The priority plays for the next phase of unconventional oil and gas exploration are also identified. This study reveals that: (1) The Da'anzhai Member is characterized by high-frequency alternation of lacustrine carbonate rocks and clastic rocks. High-quality source rocks (shales) are closely associated with favorable reservoirs. This constitutes a superior hydrocarbon accumulation assemblage characterized by "source reservoir integration" or "source reservoir proximity". (2) The Da'anzhai Member has great potential for stereoscopic exploration of tight oil/gas and shale oil/gas. The thick shale in the second sub-member of Da'anzhai is not only served as source rock, but also has the potential to form reservoirs for shale oil and gas. The thick shell limestone in the first and third sub-member of Da'anzhai has the potential to form reservoirs for tight oil and gas. (3) The Yilong-Zhongjiang-Shehong-Suining-Guang'an area, which is closely adjacent to the source rocks of the second sub-member on the plane and develops thick shell limestones, features a "source-reservoir interbedding" and "upper source and lower reservoir" type source-reservoir configuration, making it the most favorable area for integrated exploration of unconventional oil and gas in the mixed rocks of the Da'anzhai Member.
In response to the "one well, one reservoir" characteristics of the Taiyang shallow shale gas fields in the the Zhaotong Demonstration Zone, the main controlling factors for gas enrichment and high production have been clarified by focusing on analyzing the structural deformation features, the pore evolution process of shale reservoirs, and the coupling relationship between hydrocarbon supply processes and sealing capacity of the key target layers (Wufeng Formation-Longmaxi Formation). The main three understandings are as follows: (1) Based on the pattern of regional "north-south zoning, east-west blocking" as well as characteristics such as faults, folds, and fractures, the gas field is divided into eight shale gas occurrence and development units. (2) Gas reservoirs have typical characteristics of self-sourcing, self-reservoiring, and self-sealing, with continuous and stable distribution. The gas drive mechanism is elastic gas drive without edge or bottom water. (3) After undergoing multiple stages of burial-uplift cycles and multiple episodes of hydrocarbon generation, the current gas-rich and high-production potential of shale reservoirs is mainly controlled by the combined effects of later structural modification intensity and sealing preservation conditions. The overall performance of the gas field shows that the wide and gentle inclined fold zone is conducive to shale gas enrichment, while the narrow and steep anticline fold zone is relatively poor in shale gas. The new progress clarifies the controlling effect of the hydrocarbon supply-sealing coupling relationship on high production. It also establishes a development unit classification scheme based on structural characteristics, providing a geological basis for subsequent directional drilling and engineering optimization.
The fracture development characteristics and fluid distribution in shale reservoirs are key factors for evaluating shale gas exploration and development. However, the influence of pore shape on reservoir elasticity and physical properties is often neglected by existing methods, resulting in limited prediction accuracy. To address this issue, this study proposes an improved method centered on considering the effect of pore shape. Firstly, based on the cross-plot analysis of logging petrophysical parameters, the Gassmann fluid term is selected as the fluid identification factor for gas-bearing shale in the target area. Secondly, shale reservoirs with high-angle fractures are approximated as horizontal transverse isotropy (HTI) media. Combined with petrophysical modeling, an anisotropic reflection coefficient equation for HTI media (considering the effect of pore shape) is derived and established. On this basis, a two-step pre-stack anisotropic inversion method based on the Bayesian framework is constructed to realize the direct inversion of fluid identification factors and fracture parameters. Synthetic seismogram tests show that the inversion results of this method have high consistency with model values and strong noise resistance. Field test results of the shale gas reservoirs of the Wufeng-Longmaxi Formations in the Daozhen syncline, Northern Guizhou area indicate that the inversion results have high consistency with logging interpretation, and can effectively characterize the development characteristics of high-angle fractures and the distribution of gas-bearing shale. The research results provide new theoretical and technical support for fracture prediction and fluid identification in shale reservoirs, and have important practical application value.
