Contrasting organic matter enrichment mechanisms in the Wufeng-Longmaxi shales of the Weiyuan Block: insights from depositional environments

Introduction Global shale gas exploration and development are shifting toward shallow, normally pressured areas, which are increasingly regarded as important successor targets. However, the organic matter enrichment mechanisms and resource potential of shale in normally pressured settings remain insufficiently understood. This study investigates the Wufeng–Longmaxi shales in the Weiyuan Block of the Sichuan Basin to clarify the differences in shale gas enrichment between normally pressured and overpressured systems and to identify their depositional environmental controls. Methods A comparative study was conducted on Well W207, located in a normally pressured area adjacent to a paleo-uplift, and Well W202, situated in an overpressured depression. Petrological and geochemical analyses were integrated with paleoclimatic and sedimentary facies interpretations to reconstruct depositional environmental evolution and evaluate the respective roles of primary productivity and preservation conditions in organic matter enrichment. Results The shales in both pressure systems are dominated by siliceous, mixed, and argillaceous lithofacies, with quartz and clay minerals as the principal mineral components. Well W207 is characterized by a higher clay mineral content, whereas Well W202 contains a greater proportion of biogenic quartz. Geochemical proxies indicate that the depositional environment of the Long 1-1 sub-member evolved from oxic to reducing and subsequently to suboxic conditions. Organic matter enrichment was jointly controlled by primary productivity and preservation conditions, although their relative contributions varied spatially because of paleogeomorphic differentiation. Accordingly, two distinct enrichment models were identified: a productivity-driven model in the depression represented by Well W202 and a preservation-driven model near the paleo-uplift represented by Well W207. Discussion The results indicate that favorable organic matter enrichment in the Weiyuan area was not restricted to the overpressured depression. Shallow, normally pressured areas adjacent to paleo-uplifts could also develop high-quality source rocks where suitable preservation conditions compensated for relatively limited primary productivity. These findings improve the understanding of differential shale gas enrichment across contrasting pressure systems and provide a geological basis for three-dimensional shale gas exploration and successor-play selection in the Sichuan Basin.