Abstract:

Since Late Triassic,Western Sichuan depression has experienced Indosinian,Yanshanian and Himalayan movements,leading to the multi-stage changes of poroperm characteristics,structural features,fluid properties and geopressure fields,as well as reservoiring through repeated migration and accumulation of hydrocarbons.Gas in Upper Triassic Xujiahe Formation in Western Sichuan depression has been accumulated around the Pengguan hydrocarbon-generating sag,and distributed in the eastern,western and northern slopes of the sag.Distinct fracture systems have acted as the key pathways of natural seeping of gas.Gas has mainly accumulated in various composite traps formed on the basis of early structures.Comprehensive analysis of hydrocarbon generation and expulsion stages,reservoir compaction stage,fluid charging stage and trap forming stage shows that the original gas reservoirs in the Upper Triassic in Western Sichuan depression have been formed in middle-late Indosinian to middle-late Yanshanian. Since Late Cretaceous,the whole reservoirs have been compacted,peak of hydrocarbon generation and expulsion has finished,and hydrocarbons have been "sealed up" as a result of the compaction.This area has been strongly deformed by Himalayan movement and the hydrocarbons "sealed up" during the earlier compaction have been reactivated by the fractures.Therefore,the reservoiring of gas in Upper Triassic Xujiahe Formation of in Western Sichuan depression are characterized by the early "palaeo-structures" to be the basis of gas accumulation,intermediate "compaction seal-up" to be the condition of gas preservation,and the late "fracture activation" to be the key to gas enrichment.

Key words: Western Sichuan, Upper Triassic, natural gas, reservoiring characterisitcs