The Characteristic Of Micro Structure About Tight Gas Reservoir The 5th Member Of Xin Chang Xujiahe Formation Of The Western Sichuan Depression

According to outcrop,well cores and logging data, characteristics of micro structure of the 5th member of Xujiahe Formation tight gas reservoir are studied in Xinchang area, Western Sichuan Depression. the lithology are tight sandstone、Siltstone and mudstone. There are three types of the stacking pattern of T3X5 sandstone and mudstone, namely sandstone is interbedded with thin mudstone, mudstone are alternated. The formation of the 5th member of Xujiahe Formation is characterized by sand and mud interbed and continental coal bearing strata. Through polarizing microscope,scanning electron microscopy(SEM), laser confocal, nanometer technology such as CT scans,this paper depicts microstructure characteristics of sandstone and mudstone derived from samples of different sedimentary facies, and combined with the means of the conventional test, the physical property of the 5th member of Xujiahe Formation sandstone and mudstone /shale is tested, It comes to the conclusion that the reservoir is super dense and the permeability is ultra-low.According to the lithological difference,sandstone reservoir space of the 5th member of Xujiahe Formation are dissolution pore, intergranular pore, organic pore and a little micro cracks, while the sandstone reservoir mainly develops mineral dissolution pores, Interlayer pore of clay minerals, intergranular pore and Organic nanopores. Combined with CT scans, it showed that the pore connectivity is better where the Organic matter developed, while the isolated pores can` t connect effectively. The porosity of the reservoir is better in the upper and lower segments, the middle segment is unsatisfactory. The diagenesis concludes compaction, cementation, dissolution and fracture. The diagenesis stage is mainly A stage.Combined with sedimentary facies and the micro structure characteristics, distributary channel is the most favorable reservoir face zone of the 5th member of Xujiahe Formation, sand layer and high content of TOC are the advantageous condition of high production. the lithological combination which is rich of sand provides major reservoir space, brittleness mineral and interbedding sand and mudstone determine the reservoir fracturing property, the above four aspects are the major controlling factors. Meanwhile the instable particles such as feldspar, calcite and fragment, to a certain extent, prevent the compaction and pressolution in the early and middle diagenesis, dissolution is the important factor for favorable reservoir which forms a variety of types of secondary dissolution pores.Based on the above research, It can be divided into three kinds of reservoir facies belt, namely I class area, which is the high-energy advantage facies with thick high quality sand body, microfracture, high organic matter content and high gas bearing, underwater distributary channel sand bodies is the skeleton sand body, constructive diagenesis develops. Class II area where high-energy advantage facies belt develops, has a certain of hydrocarbon content. Class III area where the sand body is thick, fracture develops, beach bar sand body is the major layer where reservoir develops.