Coupled accumulation mechanisms and quantitative evaluation of tight and shale gases: A case study of the Cretaceous Shahezi Formation, Xujiaweizi Fault Depression, Songliao Basin

The deep whole petroleum system in the Xujiaweizi Fault Depression of the Songliao Basin consists primarily of three parts: (1) shale gas retained in the Cretaceous Shahezi Formation source rocks; (2) tight gas reservoirs formed through long-distance migration; and (3) fault-transported gas accumulations in both basement metamorphic rocks and volcanic rocks of the Yingcheng Formation. Within this system, tight and shale gases tend to be interrelated rather than occurring independently. However, previous studies largely investigated them separately, failing to systematically analyze their paragenetic characteristics and interrelations. This study examines the stratigraphic interval consisting of shales interbedded with sandy conglomerates in the Shahezi Formation, Xujiaweizi Fault Depression. The shale gas retained and the tight gas that migrates over a short distance are analyzed together as tight sequence gas. Based on the clarification of gas generation and expulsion capabilities of source rocks and the gas storage capacity of reservoirs within the tight sequence system, we establish the three- and four-fold criteria for grading evaluation of source rocks and reservoirs, respectively. Considering the sand-to-mud ratios, we develop numerical models to elucidate the partitioning and distribution patterns of gas resources in the tight sequence system with varying source rock-reservoir configurations during the coupled reservoir formation of tight and shale gases. Accordingly, we determine the complementary or co-enrichment relationships between tight and shale gases. The results indicate that the tight sequence system containing coupled tight and shale gases can be categorized into 24 distinct types. Among these, four types featuring grade Ⅰ or Ⅱ source rocks, grade Ⅰ or Ⅱ tight reservoirs, and sand-to-mud ratios equal to or below their optimal value represent the co-enrichment areas of tight and shale gases. Four types characterized by grade Ⅰ or Ⅱ source rocks, grade Ⅰ or Ⅱ tight reservoirs, and sand-to-mud ratios exceeding their optimal value denote areas with shale gas enrichment, and tight gas enrichment immediately adjacent to source rocks. Eight types exhibiting grade Ⅰ or Ⅱ source rocks, grade Ⅲ or Ⅳ reservoirs, and any sand-to-mud ratios represent areas enriched in shale gas enrichment but depleted in tight gas. The remaining eight types featuring grade Ⅲ source rocks, any grades of reservoirs, and any sand-to-mud ratios represent the co-depletion areas of tight and shale gases. Generally, the Shahezi Formation exhibits overall co-enrichment, together with local occurrences of complementation. Major co-enrichment areas are located in the Anda-Songzhan area in the north. However, the sweet spots of the two gases do not completely overlap in these areas. It is recommended that joint exploitation of both gases should be attempted in well block Dashen-24 and the Songzhan area, while highlighting tight gas recovery in other areas.