Differential fluid overpressure evolution and hydrocarbon accumulation mechanisms of deep Ordovician reservoirs in areas with and without strike-slip faults, Tarim Basin

The deep Ordovician reservoirs in areas with and without strike-slip faults in the Tarim Basin exhibit significant difference in fluid pressure evolution characteristics. Existing exploration practices have shown areas without strike-slip faults hold significant potential for deep hydrocarbon exploration and exploitation. However, there is a lack of systematic studies on the hydrocarbon accumulation and overpressure evolution processes in these areas. In this study, we examine these processes in areas with and without strike-slip faults within the Fuman oilfield based on log data, reservoir characteristics, paleo-pressure reconstruction from fluid inclusions, and the calcite U-Pb isotopic dating. The results are as follows. First, the areas without strike-slip faults in the Tarim Basin underwent three critical pressure evolution stages, namely normal pressure, weak overpressure, and strong overpressure in sequence. On the other hand, the areas with strike-slip faults in the basin also experienced three critical pressure evolution stages: normal pressure, weak overpressure, and normal pressure in sequence. Second, during the Middle-to-Late Caledonian, crude oil charging occurred for reservoirs in these areas, resulting in weak overpressure. In the late stage, numerous pores and fractures are formed in carbonate reservoirs in these areas under tectonic fracturing combined with the dissolution of fluids migrating along faults. The interconnected reservoirs facilitate the redistribution of overpressure within the connected fault systems. As a result, the formation pressure in these areas was gradually reduced to be normal. Third, in the areas without strike-slip faults, the formation pressure increased rapidly to strong overpressure during the Early Yanshanian due to two factors: the lateral migration of deep gas from oil cracking to the Ordovician reservoirs along faults and unconformities and the tectonic compression. The overpressure has been maintained up to now. Fourth, hydrocarbon accumulation in the areas without strike-slip faults followed a sequence characterized by early-stage oil generation, late-stage gas invasion, and lateral adjustment.