Sedimentary Characteristics and Their Relationship with Tight Sandstone Gas within the Source Rock of Tight Gas:A case study from Third Member of Xujiahe Formation in Yuanba area, NE Sichuan Basin

Key words: 
• Yuanba area /
• tight sandstone gas reservoir /
• sedimentary facies /
• sediment evolution /
• configuration of source and reservoir

Abstract: The third member of Xujiahe Formation has been considered the source rocks. However, the well obtained 120.8×10⁸ m³/d yield in gas production of the member, showing the exploration prospect being bright within the source rock of tight gas, especially in the third member of Xujiahe Formation. This article aims to use the multi-information integrated research method, to elaborately analyze sedimentary microfacies, features and sediment evolution of different sand groups, and characterize their relation with tight gas sandstone reservoir, based on the data of drilling cores, loggings and analysis assay. The result shows that braided fluvial delta was developed in the west of the study area and meandering fluvial delta was developed in the east. Among those different microfacies, braided channel, subaqueous braided main channel and subaqueous distributary channel are relatively favorable reservoirs. Meanwhile, the difference of east and west sedimentary facies configuration are obvious. In the east, thick set of subaqueous distributary channel resevoirs are distributed at the bottom, and thin set of sandstone and coal bed superimposed each other at the top. In the west, the distribution of braided channel and subaqueous braided channel sandstone are widely and stacked. And carbonaceous mudstone and coal bed with sandstone superimposed each other, appearing as a "thousand layers cake" like stacking pattern. Comparing to the east, the source rock and reservoir are closely proximity, so the west has favorable accumulation condition. The present study suggested that the future exploration would be focused on the west.