Characteristics of fluid inclusions and natural gas accumulation stages in Majiagou Formation of Ordovician in middle-eastern Ordos Basin

The marine carbonate rock series of the Ordovician Majiagou Formation in the central and eastern part of the Ordos Basin has great natural gas exploration potential. However, the hydrocarbon accumulation mechanism is complex. Studying the hydrocarbon accumulation stages and patterns of this formation series is of great significance for regional oil and gas exploration geological evaluation. In this paper, petrography of inclusions and homogenization temperature analysis were carried out on the carbonate rock samples of the Ordovician Majiagou Formation in the basin, and the PetroMod software was used to restore the thermal evolution history of the Ordovician. The results show that two phases of hydrocarbon fluid inclusions have developed in the carbonate rock series of the Majiagou Formation. The first phase is mainly hosted in the dissolution pores of calcite and dolomite, with a homogenization temperature of 110～130 ℃, and the corresponding natural gas charging period is 210～165 Ma (from the end of the Triassic to the end of the Middle Jurassic). The second phase is mainly hosted in the dolomite veins or late-stage calcite, with a homogenization temperature of 160～170 ℃, and the corresponding natural gas charging period is 123～97 Ma (the end of the Early Cretaceous). The Ordovician in the central and eastern part of the basin has experienced three thermal evolution stages: a slow heating stage before the Late Permian, a rapid heating stage from the end of the Late Permian to the end of the Early Cretaceous, and a cooling stage after the Early Cretaceous. The charging of natural gas in both phases occurred during the rapid heating stage, with an average heating rate of 0.86 ℃/Ma. The Ordovician carbonate rock reservoir is in direct contact with the coal-measure source rocks of the Upper Paleozoic in the upper part, and has a self-generation and self-reservoir input from the sub-salt marine source rocks in the lower part. The natural gas migrates along the pores and fractures and accumulates in the lithologic traps of dolomite. Since the Late Cretaceous, the basin has been uplifted and cooled, with an average cooling rate of 0.63 ℃/Ma, and the hydrocarbon generation of the source rocks has ceased.