The Paleogene strata in the southwestern Tarim Basin record the transgression-regression process of the Tarim Basin and the early Cenozoic orogenesis of the Pamir-West Kunlun orogenic belt. Thus, restoring the tectonic-paleogeographic framework of the Paleogene in the southwestern Tarim Basin is significant for understanding the paleogeographical and paleoenvironmental changes in central Asia. Based on the geological survey on the Qimugan section of the Paleogene in the southwestern Tarim Basin, together with available drilling and outcrop data, the stratigraphic framework and sedimentary sequence of the Paleogene were investigated, the tectonic-lithofacies paleogeography of the Paleogene was mapped, and the basin-range coupling process of the Paleogene was analyzed. The results show that the southwestern Tarim Basin had a higher topography in the east than in the west in the Paleogene. During the Paleocene-Late Eocene, the southwestern Tarim Basin was dominated by marine sediments in the western part, marine-continental transitional sediments in the central part, and delta sediments in the eastern part. During the Late Eocene-Oligocene, the southwestern Tarim Basin witnessed a further uplift in the southern part, together with expanded delta sediments, and a dominance of shallow lake and near-shore submarine fan sediments in the western part and of lakeside sediments in the eastern part. As a whole, the southwestern Tarim Basin fully transformed into a lacustrine depositional environment. Generally, the southwestern Tarim Basin experienced two cycles of transgression-regression during the Paleogene. After the second regression, the sea water completely retreated from the southwestern Tarim Basin. The spatio-temporal coincidence of the regression with the crustal thickening and shortening of Pamir and the global sea level drop suggests that the final regression of the Tarim Basin is probably a result of the combined effect of tectonic and climatic changes.
There are two sets of high-quality source rocks, namely the alkaline lacustrine Fengcheng formation and the saline lacustrine Lucaogou formation, in the Permian of the Junggar Basin. Due to different sedimentary environments, the two sets of source rocks show distinct hydrocarbon-generating parent materials. The hydrocarbon generation characteristics have not been correlated systematically for source rocks of different organic facies, and the gas generation capacity of lacustrine source rocks under oil expulsion conditions has not been investigated. Through the analysis on organic petrology and biomarkers of source rocks, hydrocarbon-generating parent materials such as Dunaliella-like algae, Cyanobacteria, and benthic macroalgae were found in the Fengcheng formation source rocks, and a large amount of Tasmanites were discovered in the Lucaogou formation source rocks. Combined with semi-closed thermal simulation experiments on source rocks and closed thermal simulation experiments on crude oil, it is indicated that source rocks of different organic facies are varying in hydrocarbon generation evolution patterns. Specifically, the Dunaliella-like algae source rocks are characterized by large oil yield, long oil generation window, and delayed oil generation peak, corresponding to the vitrinite reflectance (Ro) at the peak of oil generation up to 1.31%, the maximum oil yield of 836.3 mg/g, and the maximum gas yield of residual organic matter up to 312.0 mg/g. The Dunaliella-like algae + Cyanobacteria source rocks incorporate the hydrocarbon generation characteristics of both Dunaliella-like algae source rocks and Cyanobacteria source rocks, with long oil generation window, corresponding to the Ro at the peak of oil generation up to 1.15% and the maximum gas yield of residual organic matter up to 217.3 mg/g. The Cyanobacteria + benthic macroalgae source rocks exhibit early oil generation and low oil yield, corresponding to the Ro at the peak of oil generation up to 0.91% and the maximum gas yield of residual organic matter up to 292.9 mg/g. The Tasmanian algae source rocks demonstrate large oil yield and high gas-to-oil ratio (GOR), corresponding to the Ro at the peak of oil generation up to 1.09%, the maximum oil yield of 756.1 mg/g and the maximum gas yield of residual organic matter up to 330.2 mg/g. For the Fengcheng formation sapropelic source rocks, the gas yield is close to that of humic source rocks in the Junggar Basin when the Ro is 1.50%, and it increases continuously with increasing thermal maturity, indicating that the sapropelic source rocks of the Fengcheng formation still have strong gas generation capacity and gas exploration potential after oil expulsion.
The reservoirs of the Paleozoic Shihezi-Taiyuan formation in the Xunyi area of the Ordos Basin are tight sandstone reservoirs featured with strong diagenesis, low porosity, low permeability, and strong heterogeneity. In this paper, through analyses of rock thin sections, cast thin sections, whole rock X-ray diffraction (XRD), scanning electron microscopy (SEM), and petrophysical properties, the Shihezi-Taiyuan formation reservoirs were systematically investigated for their development characteristics, differential evolution of components/structures during diagenesis, and controlling factors. The Shihezi-Taiyuan formation reservoirs in the Xunyi area are divided into three types: highly-plastic lithic sandstone, quartz-rich and lowly-plastic lithic sandstone, and quartz sandstone + lithic quartz sandstone. The reservoirs of Shihezi, Shanxi and Taiyuan formations exhibit an average porosity of 5.67%, 2.79% and 5.64%, and an average permeability of 1.37 mD, 0.30 mD and 0.27 mD, respectively. The pore types are mainly intergranular dissolved pores and intragranular dissolved pores, followed by primary intergranular pores and clay mineral intercrystalline pores. The interstitial materials are represented by authigenic clay minerals, as well as authigenic quartz and calcite. The research results indicate that the source for the sweet spot was mainly supplied from the southwest provenance of the study area. The dissolution of reservoirs from the Middle Jurassic to the Late Cretaceous resulted in the extensive development of secondary pores, which is a primary controlling factor of the sweet spot reservoir in Shihezi-Taiyuan formation. The reservoir protection and improvement resulted from clay minerals is a secondary factor, and it is mainly manifested by the protection of the primary pores in the reservoir by the authigenic chlorite film in the Shihezi formation, and the improvement of reservoir porosity and permeability by the intercrystalline pores formed during the transformation from smectite to illite in the mixed layer in the Shanxi formation and Taiyuan formation.
The microbial dolomite of the Sinian Dengying formation in the Penglai gas field, central Sichuan Basin, is an important option for achieving additional reserves and production of oil and gas in deep to ultra-deep marine carbonate rocks in the basin. However, it is challenging to identify the impacts of reservoir diagenesis and pore evolution on hydrocarbon charging in this area due to its complex history of reservoir evolution. To objectively understand the potential and direction of petroleum exploration in the Penglai gas field, based on previous research results and exploration practice, together with comprehensive analysis of core thin sections, cathodoluminescence (CL) and trace elements, the relationship between diagenesis and hydrocarbon charging in the Dengying formation in the Penglai gas field was determined. The results show that the reservoirs of the Dengying formation in the Penglai gas field have been reworked by multiple stages of various diagenetic processes, including dissolution, cementation and filling, compaction-pressure-dissolution, recrystallization, tectonic disruption, and silicification. The diagenetic evolution process of the Dengying formation in the Penglai gas field includes five stages: contemporaneous-penecontemporaneous cementation, shallow-burial early diagenesis, uplifted-exposed epidiagenesis, burial diagenesis, and deep-burial late diagenesis. Ancient oil reservoirs were developed in the second member of Dengying formation (Deng 2 member) during the Caledonian period. The diagenetic minerals records three phases of hydrocarbon charging: the first phase is the cementation of fibrous dolomite (FD) cements around grape lace-like lattice pores, which emits no light or dim light; the second phase is the replacement of algal ring and fibrous dolomites by atmospheric freshwater dolomite (AFD) cements, which emits dim light; and the third phase is the filling of silty-fine granular dolomite (GD) cements into intergranular pores, intragranular pores, and lattice pores, which emits dim to dull-red light. The reservoir of the fourth member of Dengying formation (Deng 4 member) is filled with dolomite cements of multiple stages, with the reservoir subjected to multiple phases of hydrocarbon charging, including two phases of asphalt charging: the first phase, the Caledonian, exhibits low abundance, while the second phase, the Yanshanian, shows high abundance.
The formation test and production test in the Xujiahe formation in the Puguang area of the Sichuan Basin are closely related to the development of natural fractures. Based on field geological outcrop, core and imaging logging data, the types and parameters of natural fractures in the Xujiahe formation in the Puguang area are statistically analyzed. Combining with regional tectonic evolution, the formation mechanism and controlling factors of the natural fractures are evaluated. The results show that the natural fractures in the Xujiahe formation are highly efficient. Two groups of effective fractures in NW-SE and NE-SW trending are developed in the formation. The NW-SE fractures are predominant, and dominated by low-angle oblique fractures with the aperture of 10-20 μm and the permeability of 10-50 mD, acting as important flow pathways in the reservoir. The natural fractures in the Xujiahe formation formed in three periods: (1) the late Yanshanian, when conjugate shear fractures were formed in nearly N-S and NW-SE trending under the action of NW-SE compression and then highly filled; (2) the early Himalayan, when a series of NE-SW natural fractures were formed under the NW-SE compression, and these fractures are effective; and (3) the late Himalayan, when conjugate shear fractures in NE-SW and nearly N-S trending were formed due to the intense NE-SW compression, associated with NW-SE structural fractures locally. The natural fractures in the Xujiahe formation are controlled by structure, lithology, sedimentary microfacies, and rock mechanical layer thickness, among which structure is the dominant factor.
Numerous strike-slip faults with small throws, complex planar configurations, and insufficient quantitative constraints on multi-phase activities are developed on the northern slope of the Leshan-Longnvsi paleouplift in the Sichuan Basin. Based on detailed interpretation of key horizons and faults using newly acquired contiguous 3D seismic data, this study investigates the changes in stratigraphic extension and thickness-domain subsidence,and analyzes the deformation characteristcs and evolutionary stages of the strike-slip faults, and their influences on hydrocarbon accumulation. The research results reveal that the strike-slip faults on the northern slope primarily trend in nearly EW and NW-SE. In cross-section, they exhibit vertical, Y-shaped, and flower structure geometries. On plan view, they display combinations such as miniature pull-apart faults, linear faults, en echelon faults, and horsetail faults. The strike-slip faults are characterized by layered and segmented structural deformation. Considering the regional tectonic setting, the strike-slip faults on the northern slope are interpreted to have undergone multistage and inherited development, with the Late Sinian-Early Cambrian and Middle-Late Permian being the primary active periods, which respectively correspond to the developmental timing of the Mianyang-Changning extensional trough and Pengxi-Wusheng subsag within the Sichuan Basin. The formation and evolution of the strike-slip faults in the study area took place across hydrocarbon generation, migration, accumulation, and destruction process, playing a controlling role in the formation of the Permian reservoirs.
In the Jimsar sag, petroleum exploration mainly focuses on unconventional oil reservoirs. In recent years, breakthroughs have been made in multiple strata above and below the source rocks of the Lucaogou formation and across the sag, revealing its good exploration potential and the characteristics of a whole petroleum system. Based on seismic, drilling, logging, and organic geochemistry data, the formation conditions of the whole petroleum system, and the hydrocarbon accumulation model are investigated by thoroughly dissecting known oil reservoirs. The results show that there is an orderly symbiosis between conventional and unconventional reservoirs in the Jimsar sag. Horizontally, shale oil, tight oil, and conventional sandy conglomerate oil reservoirs are developed successively from the sag area through the slope area to the structural high. Vertically, tight oil, shale oil, and conventional oil reservoirs are found successively in the Jingjingzigou formation-Lucaogou formation-Wutonggou formation sequence. The source rocks of Lucaogou formation undergone mass hydrocarbon generation and expulsion in low maturity stage, laying a material foundation for the whole petroleum system. The Lucaogou formation contains sandstone/conglomerate, mixed-rock, and shale reservoirs successively from basin margin to basin interior horizontally, and full grain sequence reservoirs of overlying Wutonggou formation and underlying Jingjingzigou formation are found vertically. A three-dimensional hydrocarbon accumulation model consisting of three horizontal zones and three vertical floors is established. Controlled by the hydrocarbon generation evolution of source rocks of the Lucaogou formation and the presence of multi-type reservoirs, the Permian strata in the Jimsar sag show characteristics of a whole petroleum system with orderly symbiosis between unconventional and conventional reservoirs. Based on the theory of the whole petroleum system and the exploration practice in the Jimsar sag, the Permian petroleum exploration in eastern Junggar Basin should focus on the sags such as Shishugou and Jinan, especially for finding tight oil and gas reservoirs in the slope or sub-source areas, structural-lithologic oil and gas reservoirs in the above-source fault-step belts and high structural positions, and shale oil reservoirs in the inner-source zones.
The Lower Permian Fengcheng formation in the Mahu sag of the Junggar Basin hosts alkaline ore which represents an important type of solid mineral resources. The alkaline ore has been insufficiently studied with respect to genetic mechanism and sedimentary evolution process. This paper restores the spatial distribution of alkaline ore in the Fengcheng formation in the Mahu sag through detailed core description, whole-rock X-ray diffraction(XRD) analysis, rock thin section identification, scanning electron microscopy(SEM) analysis, and geochemical analysis, and considering the geophysical characteristics, and then establishes the development model of the alkaline ore. It is found that the alkaline ore in the Fengcheng formation is predominantly composed of carbonate minerals, including trona, nahcolite, northupite, eitelite, and shortite, and it was formed with the source supply by deep volcanic hydrothermal activities. The symbiotic combination of Na-carbonate minerals are constrained by formation water salinity, and the logging responses to the minerals are characterized by high CAL, high AC, high CNL, low RT, low DEN, and low GR values, with obviously opposite trends for RLLD and RLLS. The alkaline ore is distributed in both slope and depression zones, with significant differences in macroscopic occurrence states. Based on the analysis of sedimentary facies and sequence of core samples, a model of multi-source alkaline-ore development under the alternating effects of climate fluctuation and episodic volcanic activity was established. The study results provide valuable reference for the exploration and comprehensive utilization of alkaline ore in similar lake basins.
The genetic differences of zeolite cements lead to diverse pore space types, complex composition, and strong heterogeneity of reservoirs. To investigate their impacts on reservoir space, this study systematically compares and analyzes the types and formation mechanisms of zeolite cements, and reservoir space in the Lower-Middle Permian strata of the Mahu and Shawan sags by integrating macroscopic and microscopic approaches such as core observation, thin-section analysis, scanning electron microscopy (SEM), whole-rock X-ray diffraction (XRD), and energy-dispersive spectroscopy (EDS). The results indicate that variations in detrital composition control the types and genesis of zeolite cements: the zeolite cements in the Fengcheng and Xiazijie formations of the Mahu and Shawan sags were originated from the hydration of volcanic glass in tuff, while the zeolite cements in the Jiamuhe formation of the Zhongguai and Chepaizi bulges from the albitization of plagioclase. These genetic differences resulted in distinct reservoir space: in the Fengcheng and Xiazijie formations of the Mahu and Shawan sags, the evolution of zeolite cements involved changes in cement density and release of crystalline water, facilitating the creation of grain-edge fractures; while in the Jiamuhe formation of the Zhongguai and Chepaizi bulges, the dissolution of laumontite and calcite resulted in reservoir space dominated by dissolution pores.
The Lower-Middle Jurassic Shuixigou group in the Turpan-Hami Basin develops a fluvial-deltaic-lacustrine sedimentary system as a whole. However, the basin has undergone multiple tectonic movements, and the paleogeomorphological framework during the deposition of the Shuixigou group is believed to be the main factor controlling the evolution of this sedimentary system. Based on drilling, seismic, core and outcrop data, and combined with previous research, the sedimentary evolution of the Shuixigou group was comprehensively analyzed. The study shows that the Shuixigou group is generally thick in the north and thin in the south, and many small separated depocenters from the early stage of Early Jurassic to the Middle Jurassic migrated to the Taibei sag, forming a large and unified depocenter. The paleogeomorphology and sedimentation-provenance pattern of the Shuixigou group were shaped by the tectonic uplifting of the basin-margin mountains and the main bulges in the basin. The deposition of the Shuixigou group was mainly governed by both northern and southern provenances, as well as the sediment supply from the intra-basinal bulges. The early-formed paleo-bulges in the southern part of the basin served as the primary provenance, mainly owing to the distal deltaic system. The Bogda uplift and Buerga bulge, which rose continuously during the deposition of the Shuixigou group, were secondary provenances. The frequent fluctuation of lake level is another contributor to the distribution of the Shuixigou group sedimentary system. The early stage of lake transgression (the depositional period of the Badaowan formation) and the late stage of lake regression (the early depositional period of the Xishanyao formation), when delta plain subfacies was widely distributed within the basin, are critical coal-accumulation stages. The Sangonghe formation records the maximum lake transgression during the Early-Middle Jurassic, giving rise to the high-quality lacustrine source rocks of the Lower-Middle Jurassic Shuixigou group.
The paleo-sedimentary environment of the Permian Fengcheng formation in the Mahu sag of the Junggar Basin controlled the development of shale oil sweet spots. However, such controlling mechanism in alkaline lake environment is unclear, which restricts the efficient exploration and development for shale oil. Based on the core, geochemical and logging data of the Fengcheng formation in Well MY1, the paleo-sedimentary environment parameters (e.g. paleo-water depth, paleo-climate, paleo-salinity, and paleo-redox conditions) of different qualities of source rocks, reservoirs, and source-reservoir assemblages were analyzed, and the influences of paleo-sedimentary environment on the development of shale oil sweet spots were dissected. The results show that the Class Ⅰ high quality source-reservoir assemblages of the Fengcheng formation in Well MY1 were formed in the saltwater environment with a relatively small range of paleo-water depth, relatively warm and humid paleo-climate, relatively high contents of carbonate and terrigenous clastic sediments, and relatively low paleo-salinity, and also in the environment with stronger paleo-reduction condition. An appropriate sedimentary environment provides a possibility for the flocculation and enrichment of algae organic matters featuring a high hydrocarbon yield, promotes the formation of primary productivity, and contributes organic matter preservation conditions, allowing for extensive hydrocarbon generation from organic matters with relatively low maturity. The interlayered, laminar, and pure shale-type source-reservoir assemblages respectively correspond to the Class Ⅰ, Class Ⅱ and Class Ⅲ source-reservoir assemblages in a descending order of oil expulsion efficiency. It is determined that the paleo-sedimentary environment controls the development of shale oil sweet spots in the Fengcheng formation through a three-factor (source-storage-preservation) mechanism. This research insight provides a theoretical support for shale oil exploration.
The Permian Lucaogou formation in the Jimsar sag, Junggar Basin, represents one of the most significant continental shale oil plays in China. However, uncertainties remain regarding the primary geological controls of “sweet spots”, incomplete evaluation frameworks, and discontinuous distribution of productive intervals. Clarifying the formation mechanisms and identification criteria of the sweet spots is critical for advancing shale oil exploration theory and guiding efficient development. Based on core, well logging, and experimental data, this study systematically investigates the controlling factors of shale oil sweet spots in the Lucaogou formation from four dimensions (reservoir capacity, oil-bearing capacity, mobility, and fracability). The results indicate that the Lucaogou formation can be divided into upper, middle, and lower sweet-spot intervals, among which the middle interval remains largely undeveloped yet holds substantial potential. Siltstone and calcareous-felsic shale which are characterized by abundant macropores, high free hydrocarbon content, and strong mobility constitute the most favorable lithofacies. Intervals with moderate total organic carbon (TOC) content, moderate maturity, and high brittle-mineral content are more conducive to forming fracable sweet spots. A four-property coupling evaluation highlights the middle interval as a prime development target which is further validated by the high-yield performance of Well JHW85-71. This study provides a scientific foundation for sweet spot identification and development planning of shale oil in the Lucaogou formation in the Jimsar sag.
The shale oil sweet spot reservoirs of the Permian Lucaogou formation in the Jimsar sag of Junggar Basin are characterized by complex lithology and greatly varying physical properties, and diagenesis has played an important role in the pore evolution of these reservoirs. Using rock slice, cast thin section, scanning electron microscopy (SEM) and X-ray diffraction (XRD), this paper studies the microscopic characteristics of the pores in the reservoirs to reveal pore genesis, and variations and controlling factors of reservoir physical properties. The results indicate two types of reservoirs, i.e. sandstone reservoir and dolomite reservoir are developed in the sweet spots of Lucaogou formation in the Jimsar sag. Both reservoirs are dominated by secondary pores such as intergranular dissolution pores, intragranular dissolution pores, moldic pores and dissolution fractures, and contain the macropores with diameter >50 μm accounting for more than 50%. The sandstone reservoir and dolomite reservoir have similar physical properties, belonging to medium porosity and low-ultra-low permeability reservoirs. The average porosity and permeability of the sandstone reservoir are 13.51% and 0.81mD, respectively. The dolomite reservoir exhibits an average porosity of 12.86% and an average permeability of 2.38 mD. Both reservoirs have undergone compaction, weathering/leaching dissolution, cementation, and organic acid dissolution, and are in phase A of the middle diagenetic stage. The weathering/leaching dissolution in the epidiagenetic stage accelerated the formation of large pores like capillary pores and supercapillary pores, which are the main contributors to reservoir space. The dissolution of organic acid in the middle diagenetic stage promoted the formation of nanoscale microcapillary pores, which exerted a limited improvement on reservoir physical properties.
Significant breakthroughs have been made in the exploration of continental shale oil in China. However, the strong heterogeneity and complex source-reservoir coupling in these shales have hindered the understanding of sweet spot formation mechanism. In this paper, taking the Permian Fengcheng formation in the Junggar Basin as an example, the characteristics of shale laminae and their controls on shale oil sweet spots were systematically investigated using multiple techniques such as large-area thin-section scanning, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), confocal laser scanning microscopy (CLSM), and organic geochemical analysis. The results indicate that the lacustrine shales of the Fengcheng formation are well-laminated. The laminae can be classified into six types: silt-grade felsic lamina (SFL), argillaceous-grade felsic lamina (AFL), sparry dolomitic lamina (SDL), sparry calcite lamina (SCL), spherulitic siliceous lamina (SSL), and alkaline mineral lamina (AML). Two predominant laminated shale combinations are identified, i.e., SFL + AFL, and SCL/SDL + AFL. These lamina types exhibit significant variations in source-reservoir characteristics. AFL and SSL, characterized by high organic matter (OM) contents and the presence of high-quality hydrocarbon precursors such as laminated algae and rhodophyta spores, serve as the primary hydrocarbon-generating laminae. In contrast, SFL exhibits well-developed micropores and nanopores, including quartz/feldspar intercrystallline pores and feldspar intragranular dissolved pores, with a high proportion of free oil, rendering it favorable reservoir lamina. The superimposition of multiple lamina types governs organic-inorganic interactions, reservoir space characteristics, and hydrocarbon micro-migration processes, ultimately leading to differential enrichment of shale oil across various intervals. It is noted that the SFL + AFL combination represents the optimal source-reservoir configuration, demonstrating excellent overall oil content and forming an enrichment model characterized by oil generation in argillaceous lamina and accumulation in silty lamina. This combination is identified as a favorable target for shale oil exploration and development.
Complex microscopic occurrence and unclear production dynamics of shale oil in continental saline lacustrine basins challenge the study of shale oil enrichment theory and development law. Taking the Lucaogou formation in the Jimsar sag of Junggar Basin as an example, this paper characterizes the fluid occurrence state and mobility in the shale reservoirs using the techniques such as nuclear magnetic resonance, confocal laser scanning microscopy, argon ion polishing, and scanning electron microscopy, and validates against the monitoring results of produced fluid from individual wells in the pilot test area. The results show that the microscopic occurrence of the shale oil is characterized by oil fully saturating nanopores and both oil and water coexisting in sub-micron to micron-sized pores, with light and heavy components in hydrocarbons arranged in an onionskin pattern. The natural depletion of shale oil prefers the light hydrocarbon components stored in sub-micron to micron-sized pores, where free water is involved in the fluid flow. Changes in crude oil properties and produced water during well production represent effective responses to the microscopic fluid occurrence state. Shale oil well production is featured with by long-term water production and sequential producing of light and heavy components.
The main factors controlling the enrichment and high production of alkaline lacustrine shale oil in the Permian Fengcheng formation in the Mahu sag of Junggar Basin remain unclear, which limits theoretical understanding and efficient development. Based on the petrological, organic geochemical and physical characteristics of the Fengcheng shale oil reservoirs, the effects of source rock and reservoir lithology on shale oil enrichment in the Fengcheng formation were analyzed. Combined with the production profiles obtained from the key wells, the influences of shale oil enrichment, fractures and formation overpressure on the production of shale oil were identified. The findings are obtained in five aspects. First, in the Fengcheng shale oil reservoirs, the free hydrocarbon content increases with the increase of total organic carbon content (TOC), and the oil saturation index increases with the burial depth, indicating that the abundance, type and maturity of organic matter are the key factors determining the enrichment of shale oil. Second, siltstone exhibits the best storage space, followed by mudstone and endogenous rock, suggesting that the reservoir lithology controls the storage space and thus affects the enrichment of shale oil. Third, the enrichment is fundamental to the high production of shale oil. The shale oil production per meter from the Fengcheng reservoirs increases with the increase of TOC and feldspar mineral content. Fourth, the relationship between fracture orientation and present-day maximum horizontal principal stress direction affects the flow and production of the shale oil. Fifth, formation overpressure plays a role in retaining porosity, increasing permeability, reducing viscosity, and enhancing production/recovery of shale oil. The study clarifies the main factors controlling the enrichment and high production of shale oil in the Fengcheng formation, providing a scientific basis for further exploration and development.
The Ordovician carbonate reservoirs are the main development targets in the Hetianhe gas field. Taking the Ma 4 block as an example, the paleokarst characteristics were investigated based on core, thin section, logging, drilling and fluid data. The relationship between fractures and paleokarstifcaiton or filling was analyzed, the main factors controlling gas well production were evaluated, and the favorable targets for tapping the potential of the karst reservoirs were clarified. The research results show that the characteristics of fracture development in the vertical flow zone are not only related to tectonic characteristics, but also to surface karstification and filling processes. The activity of bottom water in the gas reservoir is related to the burial dissolution. Karst zonation is the main cause for the dual structure of karst reservoirs. The fracture zone is not the active water zone. The production effect of a single well mainly depends on two factors, namely burial dissolution and fracture development in the vertical flow zone.
The distribution of fault-controlled karst reservoirs in the Permian Maokou formation in the Jingyan area of southwestern Sichuan Basin remains unclear. A seismic identification model for faults and fault-karst bodies in the Maokou formation was established using drilling, geological, seismic and other data. The differences in seismic reflections between faults and fault-karst bodies were analyzed through forward modelling. On this basis, multi-stage superimposed faults were identified using seismic structural attributes such as coherence, maximum likelihood, dip angle and gradient structure tensor etc., and interlayer fault-karst bodies were recognized from seismic texture attributes such as entropy and energy. Then the distribution of Maokou formation fault-controlled karst reservoirs was accurately determined, and a geological development model was established. The results indicate that the study area mainly develops two types of fault-controlled karst reservoirs: multi-stage superimposed fault-karst bodies, which are observed in the southern part of the study area, and interlayer fault-karst bodies, which are developed in the eastern part of the study area. The seismic structural attributes can be used to accurately recognize high, steep and upright multi-stage superimposed fault-karst bodies that exhibit significant differences in the continuity of seismic waveforms, while the seismic texture attributes can be used to accurately represent gentle and low-angle interbedded fault-karst bodies with obvious changes in reflection amplitude energy of seismic waveforms. The predicted results are consistent with actual drilling results, and the research results can guide the future exploration deployment.
Pore structure affects gas storage performance of shale and is an important parameter for evaluating shale gas resource potential. Taking the coal-measure shale of Upper Permian Longtan formation in western Guizhou as an example, micro-pores and micro-fractures were qualitatively observed using scanning electron microscopy (SEM) and classified, and the microscopic pore structure and pore size distribution were quantitatively characterized through high-pressure mercury injection and low-temperature nitrogen adsorption experiments. Combining with organic geochemical parameters and mineral composition distribution characteristics, the factors controlling the pore structures of coal-measure shale reservoirs were identified. The results show that the matrix pores in coal-measure shale of the Longtan formation can be divided into six occurrence types: residual primary intergranular pores, mineral moldic pores, clay mineral intergranular pores, intergranular pores, intragranular dissolution pores, and organic pores, and the micro-fractures are mainly extensional, shear, bedding, and diagenetic shrinkage micro-fractures. Micro-pores (especially those with diameter <5 nm) and transitional pores provide the main pore space. The pore space types are dominated by ink bottle holes and V-shaped holes, with a certain amount of parallel slits, and the connectivity between pores is relatively good. Total organic carbon content (TOC), maturity of organic matter, and mineral composition are the main factors controlling the pore structure of the coal-measure shale reservoirs of Longtan formation in western Guizhou. The single-point pore volume and specific surface area of the shale increase with the increase of TOC. The degree of thermal evolution contributes positively to the increase of micro-pore and transitional pore volume. Clay minerals have complex impacts on the pore structure. High brittleness index has a positive effect on the development of meso-pores, macro-pores and micro-fractures, being conducive to shale gas flow.
