Distribution of high-quality reservoir of the second member of Xujiahe Formation in western Yuanba area, northeastern Sichuan Basin

The potential for tight sandstone gas resources in the second member of the Xujiahe Formation in the Yuanba area of northeast Sichuan is enormous, but the proven reserves are low and the testing results vary widely, mainly due to strong heterogeneity of the tight sandstone reservoirs and unclear distribution pattern of high-quality reservoirs. This research makes full use of well logging, core samples, and related analysis and testing data to characterize the sedimentary microfacies distribution in the Second Member of the Xujiahe Formation based on sedimentary facies division, and carries out investigations into the sedimentary evolution process, reservoir characteristics, and distribution of the reservoirs.The results show that: (1) The second member of Xujiahe Formation in the west of Yuanba area has undergone an evolutionary process from braided delta to shallow lacustrine to braided delta, with sedimentation controlling the spatial variation in sand body size, grain size, and mineral composition; (2) The second member of Xujiahe Formation contains diverse types of sandstone, which are tight in the overall, and are low-porosity and low-permeability reservoirs. The physical properties of reservoirs are closely related to factors such as sandstone grain size, feldspar, quartz, and calcite cement content, which are mainly controlled by sedimentation and later diagenesis; (3) There are two favorable lithofacies in the second member of the Xujiahe Formation: massive medium-grain feldspathic lithic sandstone and cross-bedded medium-grain quartz sandstone. The former gradually thins from the northwest to the southeast and spreads in strips, while the latter is discontinuous and potato-shaped, primarily distributed in the southwestern region.