Microfacies analysis of marine shale: A case study of the shales of the Wufeng–Longmaxi formation in the western Chongqing, Sichuan Basin, China

Abstract

It is a great challenge for sedimentologists to perform the facies analysis of shales due to the lack of macro-structures and high heterogeneity in shale, and thus, conventional research methods are poorly applied in the field of shale sedimentology. To establish a typical depositional model for marine shales, a new method adopted from marine carbonate rocks was employed to perform the microfacies analysis of marine shales of the Wufeng and Longmaxi Formation in Sichuan Basin. This method emphasizes the fact that many skeletal constituents in marine shales have specific bathymetric distributions, allowing for a better understanding of the depositional process of shales. With this approach, a total of nine microfacies were identified and two depositional models for marine shales were proposed. The depositional models show that sea levels were high during the Middle to Late Wufeng period, which was followed by a rapid regression that led to a significant sea-level drop by over 50 m at the topmost of the Wufeng Formation, and consequently, widespread fine-grained sandy shales with large amounts of benthic bioclasts were deposited in the study region. The eustatic sea-level changes recovered by using the method of microfacies analysis are in line with the global trend, indicating that the method has promising applications in the field of marine shale sedimentology.

Depositional Model and Controlling Factors of High-Quality Shales of the Wufeng and Longmaxi Formations in Western Chongqing, Sichuan Basin, China

“…Nine microfacies were identified in this study based on their petrographic features and fossils ( Table 3 ) and data sourced from Chen et al 20 The results of microfacies analysis enable a high-resolution reconstruction of eustatic sea level changes, a crucial factor governing the enrichment of organic matter. These nine microfacies can be grouped into sedimentary environments corresponding to the inner shelf, middle shelf, and outer shelf.…”

“…The Wufeng and Longmaxi Formation shale in the western Chongqing region is rich in drilling, coring, and logging data and is one of the most studied regions in the Sichuan Basin. Based on these data, the upper Wufeng Formation and the lower Longmaxi Formation sedimentary paleogeography was reconstructed by Chen et al 20 Microfacies analysis showed that in the late period of the Wufeng Formation, a significant regression occurred in the western Chongqing area, resulting in the exposure of shallow waters in the northwest and southeast of the study area and fine sandy shales containing benthic bioclasts widely developed in the middle and deep water regions. In the early period of the Longmaxi Formation, a rapid transgression occurred and a set of homogeneous siliceous shales were widely deposited in the study area.…”