Developmental models of organic-rich shales in the Cambrian Qiongzhusi Formation in the piedmont zone of northern Sichuan Basin

Investigating the distributions，dominant controlling factors，and sedimentary patterns of organic-rich shales in the Cambrian Qiongzhusi Formation in the Micang-Daba piedmont zone of the northern Sichuan Basin holds great significance for shale gas exploration within. Based on data from field geological surveys，drilling，and core observation，as well as analytical and test data derived from rock thin section observations，whole-rock X-ray diffraction （XRD） pattern，and total organic carbon （TOC） content determination，we explore the sedimentary patterns，distributions，and dominant controlling factors of organic-rich shales in the Qiongzhusi Formation，piedmont zone of the northern Sichuan Basin. The results indicate that shales in the Qiongzhusi Formation consist primarily of highly brittle siliceous shales and moderately brittle silica-clay mixed shales. In terms of their TOC content，sedimentary structures，and mineral components，these shales can be categorized into 16 lithofacies types，with the organic-rich laminated siliceous shale facies of high brittleness，organic-rich bedded siliceous shale facies of high brittleness，and the organic matter-bearing bedded siliceous shale facies of high brittleness dominating. The former two dominant shale facies are identified as the primary targets for the exploration and production of shale gas. The organic-rich shales，with TOC content typically exceeding 2 %，are thicker in the south and west but thinner in the north and east，corresponding to a higher organic matter abundance in the south and west and lower abundance in the north and east；The intracratonic sag and the paleogeographic patterns exhibiting alternating uplifts and sags within are identified as the primary causes of the differential distribution of organic-rich shales in the Qiongzhusi Formation. The imbalanced structural evolution in the zone enhances the heterogeneous distribution of shales. The flourishing lower plankton and bacteria serve as substantial sources of organic matter. The anoxic deep-water environment under rapid transgression in the early stage creates favorable conditions for the preservation of organic matter，whereas the continuous uplift of the Motianling and Hannan oldlands in the late stage inhibits the organic matter enrichment. The eustatic movements and paleoclimate evolution govern the vertical cyclic distribution of organic-rich shales. © 2024 Editorial Department of Oil and Gas Geology. All rights reserved.