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

The upper Paleozoic strata in the western part of theOrdos Basinhave rich oil and gas resources but low exploration levels. Thesestrata were subjected to multiple tectonic events, such as the Caledonian,Hercynian, Indosinian, and Himalayan movements, which led to a relativelycomplex process of hydrocarbon accumulation in the study area. Thesestrata also have obvious structural segmentation in the north-southdirection. However, the accumulation periods of the upper Paleozoicstrata in different structural sections of the western Ordos Basinand their differences are poorly understood. A total of 65 sandstonesamples from the upper Paleozoic reservoirs in 16 representative wellswere selected for fluid inclusion analyses. The results of fluid inclusionanalyses, combined with the burial-thermal histories of representativewells, were used to determine the hydrocarbon accumulation periodsof the main layers and summarize their patterns in different structuralregions and layers. The results show that the formation of fluid inclusionsin 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, andminor nonhydrocarbon gas inclusions. The hydrocarbon components aremostly CH4 and minor asphaltene, and the nonhydrocarbongases are dominantly CO2 and minor SO2. Thehomogenization temperatures of the brine inclusions associated withhydrocarbon inclusions in major layers in the study area have a widedistribution and multiple peaks; the main peaks in the central partof a given tectonic area are slightly lower than those in the easternpart, and the main peaks in a given location tend to increase withdecreasing burial depth. Hydrocarbon accumulation in the upper Paleozoicstrata in the study area mainly occurred in the Early and Middle Jurassicand Early Cretaceous. The Early and Middle Jurassic were the matureoil and gas accumulation periods, and the Early Cretaceous was thehigh-maturity natural gas accumulation period and the most criticalaccumulation period. The accumulation period in the central part ofa given structural region occurred earlier than that in the easternpart, and the accumulation period in different layers in a given locationgradually shifted at a later time from deep to shallow.

Automated summary

The upper Paleozoic strata in the western part of the Ordos Basin have rich oil and gas resources but low exploration levels. The strata have undergone multiple tectonic events, resulting in a complex process of hydrocarbon accumulation. The accumulation periods of the strata in different structural sections of the western Ordos Basin and their differences are poorly understood.