Reservoir Characteristics and Hydrocarbon Accumulation of Basement Play in the Dongping Area, Qaidam Basin

ABSTRACT Based on drilling core observation, polarising microscope identification, physical property analysis, capillary pressure test, scanning electron microscopy, seismic interpretation and hydrocarbon accumulation model characterisation, the characteristics and main controlling factors of hydrocarbon accumulation of the bedrock gas reservoir in Dongping area of Qaidam Basin are systematically analysed. The results show that: (1) The reservoir space of bedrock is dominated by matrix micropores, dissolution pores and micro‐fractures. Dissolution pores are mainly of hydrothermal origin, which can be verified by their vertical distribution in bedrock profile, abundant hydrothermal minerals in fractures, Nb/Ta ratio comparable to or lower than mantle‐derived rocks, Nd/Th ratio ranging between crust‐derived rocks and mantle‐derived rocks and the well‐developed of deep and large seismic faults. (2) The local enrichment of gypsum caprocks is in good agreement with the distribution of large gas reservoirs. (3) The thrusting nappes dominate the source–reservoir configuration, meanwhile, the thrust faults generated by thrusting nappe are migration pathways for oil and gas, good source‐reservoir‐capping conditions are the main controlling factors for sustained high and stable production. In conclusion, the effectiveness of gypsum caprock deposited in salinised lacustrine basin, the reservoir scale formed by hydrothermal dissolution and the optimised source‐reservoir combination by thrusting nappe are the main controlling factors for hydrocarbon accumulation in the bedrocks of the study area. The results can be used for reference in the exploration of targets with similar geological conditions.