ObjectiveShales in the Lower Cambrian Qiongzhusi Formation within the Upper Yangtze region preserve critical information about primitive oce
ObjectiveShales in the Lower Cambrian Qiongzhusi Formation within the Upper Yangtze region preserve critical information about primitive oceans. Furthermore, these shales serve as vital source rocks and reservoirs, having demonstrated considerable potential for shale gas exploration. Methods Using seismic, borehole, and outcrop data, this study investigated the shale strata and their distribution in the Qiongzhusi Formation through comparative analysis. Accordingly, the sweet-spot interval was determined. Results and conclusions The results indicate that the Lower Cambrian Qiongzhusi Formation in the Upper Yangtze region can be divided into the first and second members (also referred to as the Qiong 1 and 2 members, respectively), with the former consisting of Q1-1 to Q1-4 sub-members and the latter comprising Q2-1 to Q2-4 sub-members. The Maidiping Formation and the Qiong 1 Member predominantly occur within the Deyang-Anyue intracratonic rift, while the Qiong 2 Member is extensively distributed both inside and outside the rift. Within the rift, the west and east sides of the Maidiping Formation - Qiong 1 Member terminates at the limestone of the Dengying Formation, with the west side gradually thinning while the east side pinching out rapidly. In the north-south direction, the Qiong 1 Member thins southward, while the Qiong 2 Member is thin in the central part but thick in the northern and southern parts. Outside the rift, the Qiong 2 Member is distributed throughout the region, with the Q2-1 sub-member directly overlapping either the Qiong 1 Member or the limestone of the Dengying Formation. Shales in the Q2-1 sub-member enjoy superior hydrocarbon generation and reservoir conditions, as well as vertical/lateral sealing performance, emerging as a shale gas enrichment interval. For instance, in the northern part of the intracratonic rift, shales in the Q2-1 sub-member in well Tianxing-1 contain well-developed micropores and microfractures, with logging-derived porosity ranging from 4.93% to 6.57% (average: 5.08%). In the central part of the Deyang-Anyue intracratonic rift trough, shales in the Q2-1 sub-member in well Weiye-1 well exhibit organic matter-hosted pores, with logging-derived porosity ranging from 4.20% to 4.70% (average: 4.51%). In the Qujing area, Yunnan Province, within the southern part of the rift, black shales in the Qiongzhusi Formation in wells Qudi-1 and Quye-1 reveal well-developed black shales with high-pressure mercury injection-derived porosity spanning 1.59% to 11.33% (average: 5.0%). Although shales in the Qiong 1 Member exhibit favorable source-reservoir properties, the shale gas generated is prone to escape along karst zones or faults, unconducive to in-situ accumulation. For instance, in the northern part of the intracratonic rift, the Q1-3 submember in well Chuanshen-1 exhibits a thickness of about 40 m, logging-derived average TOC of 3.5%, and logging-derived porosity of 6.4%, suggesting high-quality source rocks and reservoirs but unfavorable conditions for in-situ accumulation. Overall, shales in the Q2-1 sub-member are identified as a sweet-spot interval for shale gas exploration due to their high porosity and great sealing performance. This study can serve as a critical basis and guide for future shale gas exploration in the Upper Yangtze region.
