Genetic type and pattern of reservoir sand bodies of Xujiahe Formation in Jieshichang-Huangjiachang area of southern Sichuan Basin

Abstract：Generally,Xujiahe Formation in Jieshichang-Huangjiachang area is composed of delt→lake sediments. Sand bodies in the study area are well developed,but the heterogeneity of the sand body is severe and its genesis is complex. This paper tried to explore its genesis and patterns. Under the guidance of sedimentary theories and methods and on the basis of observing three field outcrops and twelve drilling cores,analyzing eighty-three thin sections,SEM and analytical data of physical property, in combination with new information and new achievements obtained recently, the authors made a detailed analysis of sedimentary facies and dissection of section structure of the reservoir sand body,and found that the study area of Xujiahe is composed mainly of single underwater distributary channel sand,single-mouth bar sand,lake sand bar sand,shallow lake sand bar sand,superimposed distributary channel sand,superimposed mouth bar sand,and combination type of underwater distributary channel and mouth bar（including the " river-over-bar " and " bar-over-river "）. Combined with physical property data,the authors systematically studied the accumulation performance of the sand bodies developed in different parts of the river, which are of different genetic types with different lithologic structural characteristics. On such a basis,this paper established two genetic models for sand bodies in the study area： one is the sand body prograding model and the other is sand body retrograding model. The results show that in the study area of Xujiahe, the most favorable places for the reservoir development are the middle-lower reaches of the river and the middle and upper parts of the mouth bar. The most favorable reservoir sand bodies for reservoir accumulation are the superimposed distributary channel and the “river-over-bar”,and they also serve as the main exploration targets in the study area in the future.