川西坳陷上三叠统须家河组天然气分布及成藏特征

Characteristics of gas distribution and reservoiring in Upper Triassic Xujiahe Formation in Western Sichuan depression

川西坳陷自晚三叠世以来 ,经历了印支、燕山、喜马拉雅多期构造运动 ,导致了上三叠统须家河组岩石物性、构造特征、流体性质和地压场的多期次变化 ,油气运移聚集经多次反复而成藏。川西坳陷上三叠统须家河组天然气富集带环绕彭灌生烃凹陷 ,在其东、西、北坡呈马蹄形分布 ,显裂缝系统构成天然气自然渗流的重要通道 ,天然气藏主要聚集在以早期构造为基础的各种复合圈闭中。综合源岩生排烃期 -储层致密化期 -流体充注储层时期 -圈闭形成期的分析 ,川西坳陷上三叠统的原生气藏成藏期为印支中晚期 -燕山中晚期。晚白垩世以后 ,储层整体致密化 ,烃源岩生排烃高峰期结束 ,油气被致密化作用所“封存”。喜马拉雅运动使区内产生强烈形变 ,主要表现为裂缝对早期致密化“封存”油气的活化作用。因此 ,川西坳陷上三叠统须家河组天然气的成藏特征 :早期“古构造”是天然气藏聚集的基础 ;中期“致密化封存”是天然气保存的条件 ;晚期“裂缝活化”是天然气富集的关键。

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.