Depositional environment and thermal maturity of the coal-bearing Longtan Shale in southwest Guizhou, China: Implications for shale gas resource potential

Abstract

Significant knowledge has been assembled on the marine Longmaxi Shale in the Sichuan Basin over the past ten years. However, the distribution of shale gas resources on the periphery of the Sichuan Basin and in other source rock formations are still not sufficiently understood. To improve our understanding on the depositional environment and thermal maturity, and their relevance to the source rock quality and shale gas potential of the thick, coal-bearing Longtan Shale, 36 organic-rich shale samples were selected from a single borehole located in the southwestern Guizhou Province for geochemical and petrographic analysis.Total organic carbon (TOC) contents are high with 7.7 wt% on average, reaching partly up to 28.21 wt%, while carbonate contents are low. Vitrinite reflectance (VRr) values and Rock-Eval Tmax temperatures range from 1.48 to 1.84% and 482 to 583 °C, respectively, indicating a high thermal maturity which is in accordance with aromatic molecular parameters. The remaining, present day hydrocarbon generation potential (Rock-Eval S2) is rather low due to the high maturity. Microscopic investigations further show that the organic matter in the rocks is mainly composed of higher land plant particles, present as large and poorly rounded vitrinite and inertinite particles indicating a rather short transport distance.TOC vs. total sulfur (TS) relationships as well as specific hydrocarbon ratios (e.g. pristane/phytane) and framboidal pyrite size distributions indicate a variable either marine or freshwater environment for the shales in the coal-bearing sequence and suggest that original kerogen was either type III or type II-III. Bottom water during deposition was probably oxygen depleted but not anoxic. High sedimentation rates can be assumed for the now 700 m thick, compacted sequence, favorable for the preservation of organic matter.Overall, the present results are in accordance with previous studies of the Longtan Shale which initially had a high hydrocarbon (mainly gas) generation potential; furthermore the thickness of the formation is much larger in the here studied southwestern part of the Guizhou Province than in the previously studied northern parts.