ACCUMULATION PROCESS OF THE UPPER GAS ASSEMBLAGE ON THE SOUTHWEST MARGIN OF UPPER YANGTZE BASIN

Three typical gas reservoirs, the Pingluoba, Baimamiao and Guanyinsi, selected from the upper gas reservoir assemblage on the southwest margin of the Upper Yangtze Basin, are studied for gas distribution in the Middle Jurassic Shaximiao Formation with the research results from gas transportation system, fluid inclusion accumulation period and tectonic evolution. The study has provided a solid geological basis for the determination and evaluation of favorable exploration target areas. The gas reservoirs, dominated by structural-lithologic and lithologic traps, are mainly accumulated in the structural highs of Sha I, Sha Ⅱ and Sha Ⅲ Members and deposited in the time span of 93-72 Ma, by two stages of hydrocarbon charging. Yanshan movement is the main period for the formation of gas reservoirs, active faults originated in the Yanshan movement provided channels connecting the source rocks and the gas accumulations. Himalayan movement is the main period for the adjustment of natural gas, the reverse fault activity and tectonic peak adjustment in the Himalayan movement led to the readjustment of gas reservoirs. The accumulation processes are different in different tectonic provinces. Fault-sand matching, structures and physical properties of sand bodies are the main controlling factors in the western fold belt, while structural high and fault sealing the main factors controlling the gas abundance; fault-sand matching, structures and sealing fault are the main controlling factors in the eastern slope belt, and the structural high and physical properties of sand bodies are the factors controlling the gas abundance.