The genesis of the Triassic Xu 4 Member sand body in central Sichuan and its influence on oil and gas migration

Abstract：As a result of changes in the strength of tectonic activity, the sand bodies of Upper Triassic Xu 4 Member in central Sichuan basin had different accommodation space /sedimentary supply values in the process of sedimentation. The Xu 4 Member sand bodies consist from bottom to top of three vertical superimposed types, namely regression, aggradation and transgression． The regressive sand body mainly includes high energy truncation style channel,truncation style mouth bar and far-bar sand groups. The aggradation sand body mainly comprises erosion style channel and high energy truncation style channel, and the transgressive sand body is mainly composed of low-energy channel, low-energy truncation channel,complete mouth bar and lower breached mouth bar． Different sand body combinations have different styles of sand bodies, particles are of different sizes, and strong and weak hydrodynamic activities have different features. Hence sand bodies of different geneses formed different diagenetic facies and reservoir facies. The aggradation sand body is of corrosion facies and lining chlorite facies with high porosity, so it constitutes a good reservoir; the transgressive sand body is of compaction and binder facies with unsatisfactory physical properties; and the transgressive sand body is of corosion and binder facies with relatively good physical properties, so it serves as a medium-sized reservoir. Of these sand groups of three different geneses,the aggradation sand body has developed the high energy truncation style channel and formed widely distributed high-energy water channels in the plane,the sand body is thick, the extension is long with the development of favorable diagenetic facies,the physical properties are relatively good, and hence it can provide sufficient space for oil-gas enrichment． In addition, the sedimentary contact between the channel model and the regressive model is unconformable and shows a mosaic relation, so it can act as the channel way for gas migration．The transgressive sand body reservoir at the top tends to become tight and form a favorable cap rock, thus constituting a favorable position for oil and gas enrichment in central Sichuan basin．