To clarify the genetic mechanisms of the ultra-tight sandstone reservoirs in the 3rd member of the Upper Triassic Xujiahe Formation (Xu 3 Member) in the Dayi structure, Western Sichuan Depression, systematically characterized the reservoir petrology, petrophysical properties, reservoir-space types, and diagenetic characteristics, and investigated the main factors controlling reservoir densification. The study integrated multiscale analytical methods, including core observations, cast thin-section examination, scanning electron microscopy, cathodolumine-scence analysis, X-ray diffraction, petrophysical testing, fluid-inclusion microthermometry, carbon and oxygen isotope analysis, and formation-water hydrochemistry. The results showed that the sandstone reservoirs of the Xu 3 Member in the Dayi structure were dominated by feldspathic litharenite and litharenite and had low compositional and textural maturity. Primary intergranular pores had nearly disappeared as a result of intense compaction and cementation. Secondary reservoir spaces consisted mainly of intragranular dissolution pores in feldspars and lithic fragments, together with tectonic microfractures. Porosity values were concentrated below 4%, and permeability was generally lower than 0.5 × 10-3 μm2, indicating strongly heterogeneous ultra-tight reservoirs with ultra-low porosity and ultra-low permeability. The diagenetic sequence showed that the reservoirs underwent intense compaction, multiple episodes of cementation, and dual dissolution. Early rapid deep burial caused deformation of ductile lithic fragments and fracturing of rigid grains. The compaction rate ranged from 27.08% to 90.36%, with an average as high as 76.22%. This process eliminated most of the primary pores and constituted the primary factor in reservoir densification. Subsequently, the diagenetic environment became highly alkaline and was characterized by closed, high-salinity fluids. Strong alkaline diagenesis caused cements to occlude the pores. Within this closed system, feldspar dissolution products could not be effectively removed and instead precipitated nearby and were retained in situ, further occluding pore throats and intensifying the heterogeneous densification of the reservoirs. Organic-acid dissolution and late-stage fracture development improved reservoir properties and formed a composite network of dissolution-enlarged pores and fractures. These processes locally improved reservoir properties and were key to the development of the relatively high-quality reservoirs present today. In summary, intense early compaction and the in situ retention of dissolution products within the closed system are the key mechanisms of reservoir densification, whereas the superimposed effects of late-stage acidic dissolution and fracturing are important factors in improving reservoir properties.
The Baiyun Sag represents a core exploration fairway for natural gas in the Pearl River Mouth Basin. Located on a continental margin lithospheric thinning belt, it is endowed with a distinctive geological framework featuring highly heterogeneous geothermal gradients and widespread multiple-source-rock sequences. Drilling data have verified the development of an integrated Eocene lacustrine-deltaic-marine depositional system across the sag. However, the depositional environments and lithofacies of Eocene source rocks within the Enping and Wenchang formations have not been systematically characterized for individual sub-sag compartments. Meanwhile, the primary gas-source intervals and spatial distribution of gas kitchens that supply commercial gas fields remain poorly constrained. Integrated analysis of elemental geochemistry, paleontological data, molecular biomarkers and hydrocarbon generation potential from core source-rock samples reveal a persistently warm and humid climate throughout the Eocene. During the deposition of the Wenchang Formation, shallow lacustrine facies were widely distributed, while semi-deep-lacustrine facies were restricted to localized depocenters. During the subsequent depositional stage of the Enping Formation, terrigenous deltaic sediment flux from the northwest intensified, coupled with progressive marine transgression from the southeast, which drove continuous shrinkage of lacustrine depocenters. The Baiyun Sag hosts an extensive paleolake basin. Although prolific algal growth in semi-deep-lacustrine and transgressive source rocks improved organic matter quality, inadequate delivery of terrigenous nutrients suppressed aquatic paleoproductivity. By contrast, shallow lacustrine source rocks received abundant terrestrial parent organic matter input, yielding elevated total organic carbon (TOC) contents and constituting high-grade gas-prone source intervals. Two major commercial gas accumulation fairways (northern and eastern Baiyun Main Sag) exhibit consistent ethane carbon isotope signatures ranging from -29.5‰ to -28.0‰ (mixed Type Ⅱ1-Ⅱ2 kerogen precursors). Condensate n-alkane carbon isotopes cluster between -30‰ and -28‰, showing robust geochemical affinity with shallow lacustrine mixed-source rocks of the Wenchang Formation. Methane carbon isotope evolution modeling was performed across multiple stratigraphic units using virtual wells, generating quantitative curves of gas generation intensity and isotopic fractionation. Calibration of these modeled trends against present-day gas reservoir methane isotopic compositions and hydrocarbon charge timelines enables precise delineation of the stratigraphic intervals and spatial extents of dominant gas kitchens. The 4th member of the Wenchang Formation along the eastern slope of the Baiyun Main Sag acts as the primary source rock for commercial gas accumulations in the eastern sag compartment. For the northern Baiyun Main Sag, the 3rd to 4th members of the Wenchang Formation in the sag depocenter and the 4th to 5th members of the Wenchang Formation across the sag slope represent the dominant gas-prone source intervals for all discovered commercial gas fields in this zone.
The deep Carboniferous-Permian strata of the Ordos Basin contain abundant coalbed methane resources, and an in-depth understanding of the development characteristics of pore and fracture systems in coal reservoirs is one of the keys to their efficient development. Focusing on typical coal facies in the basin, this study systematically collected different types of samples and applied multiscale reservoir characterization techniques to characterize in detail the macroscopic and microscopic pore structures and fracture-development characteristics of Carboniferous-Permian reservoirs of typical coal facies, thereby revealing the intrinsic coupling between coal-forming environments and pore and fracture characteristics. As indicated by the research results, coal facies are one of the core geological factors controlling the development of pore and fracture systems in deep coal reservoirs.Three coal facies are predominantly developed in the Carboniferous-Permian coal measures: low-lying swamp (reed) facies, wetland herbaceous swamp facies, and dry/wet forest swamp facies. The low-lying swamp (reed) facies contains abundant micropores, and its fracture system is characterized by major bedding fractures with long trace lengths and good connectivity. The pore system of the wetland herbaceous swamp facies is dominated by micropores, while its fracture network is characterized by the preferential development of vertical butt cleats and relatively good connectivity. In the dry/wet forest swamp facies, as the degree of water inundation decreases, the pore structure gradually changes from a bimodal distribution comprising micropores and mesopores to a unimodal distribution dominated by micropores, while the fracture network exhibits strong heterogeneity. Under deep-burial conditions characterized by high in-situ stress and intense compaction, coal facies constrain the original material composition, thereby influencing the mechanical properties of coal and the basis for fracture-network development, and controlling the preservation state of effective fractures and pore-throat connectivity.
As a key driver for paleoclimate evolution, astronomical orbital parameters effectively modulate sedimentary-filling processes and differential organic-matter enrichment within continental lacustrine basins. Taking well DW7 penetrating the Chang 7 oil-reservoir set of the Triassic Yanchang Formation in the Xunyi area of the Ordos Basin as the research object, this study established a high-precision cyclostratigraphic framework and an organic-matter enrichment model. The workflow integrated natural-gamma-ray logging data with total-organic-carbon (TOC) and elemental-geochemical datasets, and adopted multiple approaches including spectral analysis, eCOCO-based sedimentation-rate estimation, astronomical tuning and filtering analysis. The results revealed well-preserved Milankovitch cyclic signals within the Chang 7 oil-reservoir set of the Xunyi area. Astronomical tuning anchored to the most stable 405-kyr long-eccentricity cycle constrained the total depositional duration of the Chang 7 oil-reservoir set to approximately 1.29 Ma. Combined with high-precision age anchor points, the basal and top boundary ages of this set were determined as (241.56 ± 0.22) Ma and (240.27 ± 0.22) Ma, respectively. Sedimentation rates derived from the eCOCO method ranged from 1.09 to 13.60 cm/ka. Four long-eccentricity cycles (E1-E4) and fourteen short-eccentricity cycles (e1-e14) were vertically identified via Gaussian filtering, on which basis the cyclostratigraphic framework was constructed. Reconstruction of the paleo-depositional environment indicated that the Chang 7 oil-reservoir set was predominantly deposited under warm-humid, fresh-to-brackish, anoxic semi-deep-to-deep lacustrine conditions. Both depositional settings and organic-matter enrichment varied synchronously with eccentricity cycles. This demonstrated that the paleoenvironmental evolution and organic-matter accumulation of the Chang 7 oil-reservoir set were governed by the 405-kyr long-eccentricity cycle, with superimposed effects from tectonism, volcanic-hydrothermal activity, water-column stratification and gravity-flow events. During the long-eccentricity maxima, amplified seasonal contrasts intensified East-Asian monsoon precipitation, enhanced terrigenous-nutrient influx and strengthened water-column stratification, giving rise to an enrichment regime characterized by "high primary productivity + favorable preservation". By contrast, the long-eccentricity minima corresponded to an arid-cold climate that suppressed productivity and degraded preservation conditions. Meanwhile, frequent gravity-flow events occurred during such periods, lowering organic-matter accumulation through clastic dilution and perturbation of the anoxic bottom water.
Studies on differential tectono-burial diagenesis and pore evolution of carbonate rocks constitute a critical approach to decipher the origin of carbonate reservoirs. Based on boreholes including Chuanshen 1 in central Sichuan and Jinshi 1 in southwestern Sichuan, as well as field outcrops including Gaojiashan at Hujiaba in Ningqiang County and Yangba in Nanjiang County, this study integrates petrography, geochemistry and comparative analysis to clarify diagenetic evolution and dolostone reservoir formation across diverse diagenetic facies tracts of the Sinian Dengying Formation. Basinal diagenetic facies tract and compaction diagenetic facies tract exhibit broadly comparable features, with pervasive marine cementation, siliceous cementation and pore-filling, multi-stage dolomitization and mechanical compaction. The platform-margin microbial reef-shoal facies is well developed within meteoric diagenetic facies tract driven by early meteoric freshwater flow and karstification, with outcrop successions further overprinted by late-stage karst processes. Widespread early dedolomitization occurs in dolo-stones interbedded with evaporite sequences. Tectono-burial diagenetic facies tract is defined by episodic, dominantly stratabound fluid flow with localized cross-stratal migration. Pore-filling diagenetic minerals, such as dolomite, quartz, fluorite and calcite, are ubiquitous within tectono-burial diagenetic facies tract intervals. Liquid hydrocarbons and solid reservoir bitumen occur in three forms: crystal lattice, free phase in intergranular pores or migration of channel form(Darcy flow). Such occurrences record key geological signals of hydrocarbon charging, fluid migration pathways, water washing and phase transformation. Distinctive diagenetic signatures exist across tectonic-burial diagenetic facies tract settings: strata buried at moderate to great depths and exposed outcrop successions show relatively low thermal maturity, retaining CO2-rich low-temperature immiscible fluid inclusions and low-maturity hydrocarbon inclusions (this signature is erased where intense overprinting by fault-associated diagenetic facies tract takes place). Compared with intraplatform mound-shoal facies, platform-margin microbial reef-shoal facies preferentially develop framework pores, dissolution vugs, fractures and micropores, alongside abundant reservoir bitumen, and the two facies also differ substantially in diagenetic mineral assemblages and homogenization temperatures. In fault-associated diagenetic facies tract, tectonic fracturing enlarges reservoir space. Mineral parageneses consisting of saddle dolomite, calcite, quartz, pyrite, fluorite, metal sulfides and solid reservoir bitumen record fault-fluid-rock interactions and post-magmatic low-to-moderate temperature hydrothermal alteration. Marine cementation, siliceous pore filling, compaction, dedolomitization and later mineral cementation collectively diminish primary intergranular porosity. In contrast, meteoric freshwater leaching, karstification, burial dissolution, hydrocarbon charging and tectonic fracturing jointly preserve and improve reservoir porosity and permeability. Diagenetic facies tracts and differential burial diagenesis constrained by heterogeneous tectonic-sedimentary facies represent the primary controls on pore evolution. The evolutionary pattern can be generalized as: disparate initial depositional fabrics, divergent diagenetic pathways, and ultimately distinct reservoir pore architectures.
