Accumulation conditions of tight gas in the Upper Paleozoic of Linxing Block

Tight gas reservoir at Linxin area in the east margin of Ordos basin had similar characteristics with its neighboring area,which were with flat tectonic background and widely distributed sand-mud interbred. Because of the reservoir in different tectonic positions,its forming conditions are significantly different. Based on the analysis of hydrocarbon source conditions,tight sandstone reservoir characteristics and tight gas reservoir forming conditions,the study shows that the main source rocks are the coal seams of Taiyuan formation ( No． 8+9 coal) and Shanxi forma- tion ( No． 4+5 coal) . Affected by the Zijinshan tectonic magmatic,the main source rocks had entered into dry gas phase which provided sufficient supply for tight gas reservoir in some areas. The development and distribution of high quality reservoir was controlled by lithofacies paleogeographic and diagenetic. Multi-period and multi-layer reservoirs were formed which was resulted in by ancient geographical environment changes from sea to land and the water shocking frequently from the Benxi formation to the Shiqianfeng group. The advantaged tight reservoirs were the coarse grained sand bodies,which mainly included the tidal bar and underwater distributary channel sandstones,and decreased from north to south gradually. The vertical combination of coal bed and sand which were controlled by depositional environment determines the accumulation modes of the tight sandstone gas reservoirs. Porosity and per- meability of sandstone reservoir was poor,but significant dissolution was effectively reconstructed on deep reservoir. Based on the vertical combination of coal bed and sand which controlled the migration and accumulation methods of sandstone gas reservoir,the tight sandstone gas reservoirs were divided into two types,one is in-situ matching gas reservoir in which source and reservoir were contacted,the other is remote marching gas reservoir which dual con- trolled by reservoir and migration channels. Micro-fractures were the main vertical migration and formed in the influ- ence of faults and collapse column.