The enrichment law of tight sandstone gas in the Upper Triassic Xujiahe Formation in the Sichuan Basin has not been fully revealed. In this study, the controlling effect of source-reservoir assemblage on gas accumulation in the Xujiahe Formation was systematically investigated. The results show that pores and fractures are developed in the tight sandstone reservoirs of the Xujiahe Formation. The main pore types are intragranular dissolved and intergranular pores. The quality of sandstone reservoirs in the Xujiahe Formation is controlled by sedimentation, diagenesis and tectonic processes. Underwater distributary river channels and estuary bars are favorable microfacies for reservoir development, and chlorite cemented facies and dissolution-kaolinite cemented facies are the most favorable diagenetic facies. In addition, the natural gas composition and carbon isotope characteristics of the Xujiahe Formation are significantly different in different intervals of the same gas field and the same interval of adjacent gas fields. This shows that the natural gas has no obvious vertical mixing and lateral migration, and it has the distribution characteristics of “local enrichment”. Then, the natural gas will be preferentially charged in the feldspar lithic sandstone and feldspar quartz sandstone with large thickness and good physical properties by means of short-range migration. According to the research, the hydrocarbon supply capacity of the single layer in each interval is weak, and it leads to the low filling degree of the gas reservoir and the insufficient separation of gas and water. On the whole, four sets of extra-source “lower generation and upper storage” assemblages and two sets of “self-generation and self-storage” source-reservoir assemblages are developed in the Xujiahe Formation. The development scale of natural gas is mainly controlled by the type of source-reservoir combination, and the areas with close source-reservoir contact and high hydrocarbon generation intensity are high-quality reservoir development areas.
In recent years, the geochemical characteristics, genesis and sources of natural gas in the Upper Triassic Xujiahe Formation in the Sichuan Basin have received extensive attention, but their genesis and sources are still controversial. In this study, taking the natural gas from the Xujiahe Formation in the Sichuan Basin as an example, the source and genesis of the natural gas have been systematically analyzed. The results show that the natural gas of the Xujiahe Formation in the Sichuan Basin is dominated by methane, followed by a small amount of CO2 and N2; only the southern Sichuan area contains a small amount of H2S, which comes from the supply of the underlying carbonate source rocks. Except for the western Sichuan Basin, the drying coefficient of the natural gas is generally less than 0.95 (wet gas). Furthermore, the composition of the natural gas is mainly controlled by the maturity of source rocks. The carbon isotope of ethane in natural gas ranges from −33.9 to −21.5‰, and the hydrogen isotope of methane ranges from −188‰ to −151‰. The carbon and hydrogen isotope values are higher in the western Sichuan Basin than in the central, northeastern and southern Sichuan Basin. The identification of the origin of natural gas and the comparison of gas sources show that the natural gas in the Xujiahe Formation is mainly coal-derived gas from its own coal-measure source rocks; the natural gas in the northern part of the southern Sichuan Basin is oil-derived gas originating from the Changxing Formation and the Silurian marine source rocks; however, the natural gas in the northeastern Sichuan Basin is a mixture of coal-derived and oil-derived gases. In addition, the carbon and hydrogen isotopes in some natural gas samples from the Xujiahe Formation have inversions of δ13C1 > δ13C2, δ13C2 > δ13C3, δ13C3 > δ13C4, and δD2 > δD3, and the magnitude of the inversions is small. It is considered to be caused by the mixing of gases from the same source, as well as the mixing of coal-derived and oil-derived gases.
In recent years, with the fine exploration of carbonate reservoirs in the Qixia Formation, central Sichuan Basin, researchers have made important oil and gas discoveries in the dolomite reservoirs. However, the characteristics, genesis and controlling factors of dolomite reservoirs in the Qixia Formation in this area are still unclear. In this paper, the petrology, pore structures, physical properties and geochemical characteristics of the dolomite reservoirs in the Permian Qixia Formation in the central Sichuan Basin have been systematically studied based on a large number of cores, thin sections, physical property and geochemical tests. Furthermore, the genesis and main controlling factors of dolomite reservoirs are clarified. The study shows that the dolomites have the characteristics of small single-layer thickness and multi-layer development, and they are usually interbeded with the limestones. The reservoir types are mainly fine crystal and fine medium crystal dolomite, and the reservoir spaces include intercrystalline pores and dissolution pores (or caves). Moreover, the reservoir physical properties are characterized by medium porosity and medium to high permeability. Quasi-syngenetic dolomitization is the main origin of dolomite in the target layer, and the dolomite was slightly modified by hydrothermal solution at the end of the Maokou Formation period. The development of dolomite reservoirs in the Qixia Formation in the study area is affected by palaeogeomorphology, sedimentation and diagenesis. The granular shoal facies developed in the high parts of the paleogeomorphology provides the material basis for the formation of dolomite reservoirs. The high frequency sequence interface controls the development of dolomite reservoirs in the highstand systems tract. In addition, the quasi-syngenetic dissolution promots the development of the early secondary pores and provids an effective channel for the migration of the diagenetic fluids in the later stage. In the direction of orthogonal horizontal deposition, the dolomites have the characteristics of thin monolayer thickness, multiple sequences and are interbedded with limestone. The dolomite shoals in the study area are distributed as clumps in the plane, which has great potential for hydrocarbon exploration.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.