RECONSIDERATION OF DEEP BASIN GAS IN THE WESTERN SICHUAN DEPRESSION OF THE SICHUAN BASIN

The Western Sichuan Depression of the Sichuan Basin is qualified for deep basin gas accumulation due to basin types,hydrocarbon source rock characteristics,reservoir-source combination and reservoir tightness.However,the Western Sichuan Depression is a combined basin influenced by stages of tectonic movements.Mass charges of hydrocarbon of the Xujiahe Formation of the Upper Triassic take place before reservoir tightening.Natural gas migrates under hydrodynamic and buoyancy effects,which is different from deep basin gas in accumulation mechanisms.Inversion of gas and water cannot be found in the same reservoir of the Upper Triassic.Natural gas is zoned obviously,mainly controlled by the Yanshanian and Indosinian uplifts.Distribution of gas and water is also different from deep basin gas.Water product of single well may reach 1000m³/d.Present accumulation types of natural gas of the Xujiahe Formation are mainly combination traps.Gas content and water distribution are controlled by fractures in traps and diagenetic reservoir-leaking bodies.The Xu4 reservoir is probably a residual reservoir,partially filled with water.The Xujiahe Formation of the Upper Triassic in the Western Sichuan Depre-ssion is accumulated conventionally,yet deformed by stages of tectonic movements.Though it is characterized by abnormal gas and water distribution,it does not belong to deep basin gas.