Lithofacies analysis and paleosedimentary evolution of Taiyuan Formation in Southern North China Basin

The Permian Taiyuan Formation contains alternate shale and coal lithologies which are vital for the exploration of shale gas reservoirs. However, previous studies have neglected the influence of paleosedimentary environment on the formation and distribution of marine-continental shale. A better understanding of the evolution characteristics of the paleosedimentary environment of the marine-continental strata is particularly critical to the accurate prediction of high-quality shale reservoirs. In this study, the geochemical and mineralogical characteristics of Taiyuan Formation, marine-continental strata in the Southern North China Basin, were carried out using lithology identification, element analysis, and X-ray diffraction analysis. These interpretations were helpful to develop a sedimentary depositional model by principles of sedimentology and sequence stratigraphy. The evolution of the paleosedimentary environment and the control of the paleosedimentary environment on the organic matter abundance, mineral composition, lithofacies, and gas enrichment of shale were analyzed. The results show that the vertical variation of the paleosedimentary environment had obvious stages during the Taiyuan Stage of the Early Permian. Based on sequence identification and division, a multi-stage superimposed marine-continental sedimentary model of transgressive and regressive assemblages was established. From tidal flat facies to lagoon facies and then to restricted platform facies, the contents of felsic and clay minerals decreased, while the contents of TOC and carbonate minerals increased sequentially. The lithofacies types of limited platform shale and tidal flat shale were different and relatively single, while the lagoon contained the shale lithofacies types of the above two sedimentary microfacies due to its sedimentary microfacies development location. The lagoon facies shale of highstand system tract had high-quality paleo-productivity, and was distributed in a stretch on the plane and superimposed in multiple stages vertically of the basin, which could be selected as a favorable facies belt of marine-continental facies shale gas.