Genetic Combination of the Sandbody in the Fourth Members of Xujiahe Formation (Upper Triassic) and Its Influence on Oil and Gas Reservoirs

The sandbody of Upper Triassic Xu 4 member in central Sichuan basin is thick,continuous sedimentation,and characterized by shore-shallow lacustrine-delta front-delta plain which is proximal.During sedimentation,there were different accommodation space /sedimentary supply values,the Xu 4 member sandbody consists of three vertical superposed types which is regressive,channel and transgressive from bottom to top;and the channel sandbody composition on sequence boundary is the best genetic sandbody.Regressive sandbody mainly includes high energy truncation style channel,truncation style mouth bar and far bar sands;channel model sandbody mainly includes eroded style channel,high energy truncation style channel;transgressive sandbody mainly includes low-energy channel,low-energy truncation channel,complete mouth bar and lower part deletion mouth bar.Because the distribution of the different sand bodies combination,the different contributing sandbody formed different diagenetic facies and reservoir facies,the diagenesis of the channel sandbody is corrosion facies and lining chlorite facies,high-porosity,so it is good reservoir;the diagenesis of the transgressive sandbody is compaction and binder facies,poor physical property;the diagenesis of the transgressive sandbody is corosion and binder facies,better physical property,so it is medium reservoir.Because the interface between channel model and regressive model is unconformity which constitutes the pathways for gas migration.In addition,the channel model sandbody is wide-ranging distribution,high energy truncation style channel sandbody stable depoition,and providing sufficient space for enrichment of oil-gas.The transgressive model sandbody reservoir on the top becoming tight as favorable caprock,and promotes the channel model sandbody to become the favorable position of central Sichuan basin.