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Abstract Tight sandstone gas is an important unconventional hydrocarbon resource, but its exploration is made difficult by the compactness and heterogeneity of the sandstone. Fluid–rock interaction and subsequent cementation play key roles in developing this heterogeneity. We conducted a petrographic, trace element, and isotopic study of the Upper Triassic Xujiahe sandstones from the Sichuan Basin, to investigate fluid–rock interactions and associated diagenetic effects. Carbonate fragments and cements are widespread in these sandstones, and are the primary control on the nature of the Xujiahe sandstone reservoirs. The carbonate cements primarily comprise of calcite and are typically distributed around the margins of sandstone bodies and along fine-grained beds. Carbonate minerals in the centre or at the margin of sandstone units have heavy δ13C carb values of ca. 0‰, whereas δ13C carb is markedly depleted in the transition zone with values as low as −10‰, reflecting a contribution from organic carbon. Manganese, Strontium and Barium generally become enriched in the carbonate cements with diagenesis, and their concentrations correlate with porosity and δ13C carb. Redox sensitive Mn, and fluid-mobile Sr and Ba were analysed to reveal the diagenetic processes and environment of carbonate precipitation. We propose that fluids flowed preferentially along the central parts of sandstone units, associated with mineral dissolution, whereas cementations took place near the margins of the units. Fluid–rock interaction, especially that involving late hydrocarbon-related fluids, greatly enhances the heterogeneity of sandstones and can potentially create high-quality reservoirs in the central parts of tight sandstone units. [ABSTRACT FROM AUTHOR]

Heterogeneity is an essential feature of tight sandstones because it is directly related to reservoir quality. The Upper Triassic Xujiahe Formation in the Sichuan Basin was used as a case study to analyse the diagenetic constraints on the heterogeneity of tight sandstone reservoirs. The three main diagenetic processes of interest are mechanical compaction, cementation, and dissolution. A dissolution facies, quartz cement facies, and carbonate cement facies present sequentially from the centre to the margin of sand bodies. Abundant feldspars and rock fragments were dissolved to form secondary porosity or filled by carbonate cement. Authigenic chlorite coatings, generated from eogenesis to the deep burial stage, are thickest in high-porosity-samples. The δ 13 C of carbonate cement ranges from −10.56‰ to 1.15‰ (PDB), suggesting multiple sources of carbon, including carbonate fragments, early inorganic carbonate cement, and organically derived CO 2 . Based on δ 18 O, carbonate cementation occurred from 33 °C to 110 °C. Microthermometric data of aqueous inclusions indicate that quartz cement precipitated at a temperature range of 120 °C–140 °C. A summary model of the diagenetic constraints on the heterogeneity of sandstones was constructed. The spatial distribution of digenetic facies is controlled by differences in fluid flow velocity and pH. Fluids, especially hydrocarbons, migrated preferentially within the centre of sand bodies, and caused dissolution. Dissolution generated secondary porosity and provided materials for authigenic mineral precipitation. Carbonate fragments were better preserved and superimposed with later carbonate cement along the sandstone-mudstone boundary. Quartz overgrowths formed after carbonate cementation, and developed in the transition zone between the intervals of carbonate cementation and dissolution. Consequently, the reservoir quality of the Xujiahe sandstones decreases gradually moving from the centre to the margin of the sand bodies. [ABSTRACT FROM AUTHOR]

The Sichuan Basin is overpressured and has experienced a reduction in temperature since the Late Cretaceous due to uplift and denudation. The temperature and pressure of the central Sichuan Basin are investigated utilising borehole data. Pressure positively correlates with temperature in each overpressured system, with a coefficient of 0.851 MPa/°C for the Cambrian pressure system and 1.064 MPa/°C for the Upper Triassic pressure system. Physical simulation experiments carried out with absolute sealing conditions demonstrate that the temperature–pressure relationship in a water-filled system is linear with a gradient of 1.076 MPa/°C, similar to the observed temperature–pressure gradients in the study area. The integration of thermal indicators, including vitrinite reflectance and fluid inclusions, is used to reconstruct the palaeotemperature of the basin. Formation temperatures have decreased since 90 Ma, with a significant decrease since 20 Ma. The amount of cooling varies between hydrocarbon fields as it is dependent on the local extent of exhumation during basin uplift. The Weiyuan field exhibits the greatest amount of cooling, followed by the Moxi field, with the Bajiaochang field experiencing the smallest reduction in temperature. The maximum palaeopressure is calculated to be > 200 MPa in the Cambrian strata and ∼150 MPa in the Upper Triassic strata. The existence of gas within the hydrocarbon fields can change the temperature–pressure relationship, lowering the maximum pressure. However, we propose that temperature reduction is the primary cause of the pressure decrease since 90 Ma and that it controls the present-day overpressure distribution in the basin. [ABSTRACT FROM AUTHOR]

The Sichuan Basin is a prime petroliferous basin in China, and multiple overpressure systems exist vertically in the central portion of the basin. Understanding the mechanisms and evolution of an overpressure system is significant for petroleum migration and accumulation. DSTs and well logs data reveal that the Upper Triassic overpressures start at a depth of about 1300 m, and the pressure coefficient ( r ) is greater in the northwestern part of the study area ( r >1.8) and relatively lower in the south ( r ≈1.3). Sonic transit times of shales in the Upper Triassic Formation are abnormally high, and velocity-density plots deviate from the normal compaction trend, which demonstrate that disequilibrium compaction should be the primary mechanism of overpressures. Evolution history of overpressure was reconstructed based on numerical modeling method. The results from 1D and 2D modeling indicate that the Upper Triassic overpressures began developing at 195 Ma because of rapid sedimentation in this period. The maximum overpressure appeared at 90 Ma, which is 38 MPa in the Bajiaochang field and 50 MPa in the Anyue field, respectively. After that, pore pressure reduced to current value due to uplift and erosion that began in the Late Cretaceous. Overpressures are higher in the greater depth area and relatively lower at the structural crest. Gas generation from the Upper Triassic source rocks occurred during 160–90 Ma which is coincident with disequilibrium compaction. The contribution of gas generation to the Upper Triassic overpressures is not important, because gases just created small volume increase and likely to be expelled out by the high disequilibrium compaction overpressure in the corresponding period. Permeabilities of 10 −22 –10 −23 m 2 are required for barriers to maintain the overpressure in geological timescale. Overpressure systems in the central portion of the Sichuan Basin appear to be explained by static paradigms. For that, temperature has significant impact on the sealed systems. Greater pressures are preserved in the area with less temperature reduction. [ABSTRACT FROM AUTHOR]

Reconstructing the evolution of paleofluid pressure in carbonate reservoirs is a challenging problem, particularly when no oil-bearing fluid inclusions are available to provide barometric constraints on the fluid system. To recover the paleofluid pressure in the Sinian Dengying Formation (Z 2 dn) and the Cambrian Longwangmiao Formation (Є 1 l) of the central paleo-uplift of the Sichuan Basin, we conducted a comprehensive analysis combining the inclusion pressure-volume-temperature simulation, inclusions in situ Raman shifts, and basin modeling. The results showed that the evolution of the pore pressure could be divided into four stages. Prior to the end of the late Triassic, the whole central paleo-uplift was in a normal pressure state. During the Jurassic, a moderate overpressure prevailed in the dolomitic reservoirs of the Gaoshiti-Moxi area and decreased in the adjacent units. At the end of the early Cretaceous, the overpressure reached a maximum in the whole region and was strongest in the Longnüsi area, decreasing to the west. From the late Cretaceous to the present day, the central area of the basin has uplifted rapidly with the overpressure significantly decreasing. Z 2 dn has been restored to normal pressure during the Neogene, while Є 1 l remains moderately overpressurized. Based on an analysis of the structural background and preservation conditions, the types of sedimentary facies, the preservation condition of the traps, and the differential weathering process are the primary reasons for the different pressure states in Z 2 dn and Є 1 l during the last stages. [ABSTRACT FROM AUTHOR]

Recently, trillions of cubic meters gas resources have been found in Sinian reservoirs of Gaoshiti-Moxi region in the central uplift of Sichuan Basin, which is the first commercial discovery of primary gas field in Precambrian anywhere in the world. To obtain a great breakthrough in Precambrian hydrocarbon exploration, it is necessary to find out the evolution process of temperature and pressure. Reconstructing the thermal history and pore pressure evolution in carbonate reservoirs are challenging problems, especially in deep-old layers without available samples and methods. To study the temperature and pressure system, we conducted a comprehensive analysis combining the paleo-thermal indicators and inclusions Pressure-Volume-Temperature (PVT) simulation with basin modeling. The results showed that the thermal history of the Gaoshiti-Moxi area could be divided into three stages. From the Late Sinian to Late Paleozoic, the thermal regime was stable with low heat flow value (<65 mW/m 2 ). During the Early Paleozoic, the heat flow increased rapidly to the peak value (75–100 mW/m 2 ). From Mesozoic to present, the thermal subsidence occurred in the basin with the heat flow decreasing to the present value (60–70 mW/m 2 ). With the boundary condition of above thermal history, the simulated pressure of Dengying Formation in Gaoshiti-Moxi area could be divided into four stages. The excess pressure began to form during the Triassic. From the end of Triassic to Middle Jurassic, the excess pressure increased steadily and slowly. From Middle Jurassic to the end of Early Cretaceous, the strata subsided rapidly while the excess pressure increased significantly. Since the Late Cretaceous, the Dengying Formation suffered from rapid uplifting and gas lateral migrating with the excess pressure decreasing, until the Late Neogene, the excess pressure in reservoirs declined to zero. [ABSTRACT FROM AUTHOR]