Non-overmature equivalents confirmed a high initial hydrocarbon generation potential of the Permian Longtan Shale in southern China

Abstract

This study presents the first organic and inorganic geochemical characterization of a non-over mature section of the Permian Longtan Shale as it can be found in the southwestern Guizhou Province, China. Compared to previous studies, the here analyzed interval lies in the oil window, which allows a more detailed and precise interpretation of the original kerogen quality, the depositional conditions as well as its initial petroleum generation potential. The chemical and mineralogical index of alteration consistently reveal a warm to hot and humid climate with moderate to strong weathering during the Late Permian. Terrigenous input contributes the main components of the Longtan Shale. The overall organic matter contents are high with an average total organic carbon (TOC) content of 5.9 wt%, and maximum values reaching up to 36.4 wt%. Organic matter is mainly comprised of type III with a small contribution of type II kerogen bearing an intense petroleum generation and expulsion potential based on petrographic and open-system pyrolytic analysis. Organic matter pores are clearly visible (20 to 200 nm) in these samples although they are less developed than in the gas window. Our results suggest that the Longtan Shale had a high initial hydrocarbon generation potential. High mud gas content in a vertical profile correlates with occurrence of organic-rich black shales, which suggests that shale gas exploration in the Longtan Formation should focus on sweet spots in or close to organic matter rich intervals.