Hydrocarbon Accumulation Periods in the Upper Paleozoic Strata of the Western Ordos Basin, China, Based on Fluid Inclusions and Basin Modeling

[Image: see text] The upper Paleozoic strata in the western part of the Ordos Basin have rich oil and gas resources but low exploration levels. These strata were subjected to multiple tectonic events, such as the Caledonian, Hercynian, Indosinian, and Himalayan movements, which led to a relatively complex process of hydrocarbon accumulation in the study area. These strata also have obvious structural segmentation in the north–south direction. However, the accumulation periods of the upper Paleozoic strata in different structural sections of the western Ordos Basin and their differences are poorly understood. A total of 65 sandstone samples from the upper Paleozoic reservoirs in 16 representative wells were selected for fluid inclusion analyses. The results of fluid inclusion analyses, combined with the burial–thermal histories of representative wells, were used to determine the hydrocarbon accumulation periods of the main layers and summarize their patterns in different structural regions and layers. The results show that the formation of fluid inclusions in the main upper Paleozoic strata can be divided into two stages. The first-stage inclusions mainly occur in secondary quartz edges, and the second-stage inclusions mainly occur in healed microcracks. The inclusion types are dominantly hydrocarbon-bearing, brine, and minor nonhydrocarbon gas inclusions. The hydrocarbon components are mostly CH(4) and minor asphaltene, and the nonhydrocarbon gases are dominantly CO(2) and minor SO(2). The homogenization temperatures of the brine inclusions associated with hydrocarbon inclusions in major layers in the study area have a wide distribution and multiple peaks; the main peaks in the central part of a given tectonic area are slightly lower than those in the eastern part, and the main peaks in a given location tend to increase with decreasing burial depth. Hydrocarbon accumulation in the upper Paleozoic strata in the study area mainly occurred in the Early and Middle Jurassic and Early Cretaceous. The Early and Middle Jurassic were the mature oil and gas accumulation periods, and the Early Cretaceous was the high-maturity natural gas accumulation period and the most critical accumulation period. The accumulation period in the central part of a given structural region occurred earlier than that in the eastern part, and the accumulation period in different layers in a given location gradually shifted at a later time from deep to shallow.