Dynamics of Gas Accumulation in Yinggehai Basin

Based on the integrated analysis of geological and geochemical data, gas accumulation process is discussed in the Yinggehai basin by means of macro- and microapproaches. Interreservoir compositional heterogeneities in shallow gas field at diapiric structures indicate that hydrocarbon gases may undergo the processes of multiple mixing, episodic migration and accumulation. The faults at diapiric structures provide a good pathway for natural gas vertical migration from the deep source rocks into the shallow reservoirs as evidenced from high-resolution seismic images. Natural gases are derived from the Early Miocene Meishan and Middle Miocene Sanya source rocks distributed within overpressured zones. This overpressuring, together with active faulting, results in the episodic expulsion of fluids distributed within overpressured zones as a result of multiple infilling into shallow reservoir or leakage loss of hydrocarbon gases. Therefore, the short-lived rapid accumulation of hydrocarbon gases happens only about 1.2-0.1 Ma before the present time because of a plentiful of gas sources and high hydrocarbon expulsion efficiency.