Accumulation and maturation of organic matter in shales: The coal-bearing Permian Longtan Formation at Tucheng Syncline, Southwestern Guizhou, China

Abstract

Compared with fully marine and lacustrine shales, coal-bearing analogues have been rarely investigated. This study attempts to deepen our insights into the provenance, deposition, maturity and organic matter pores (OMPs) of coal-bearing shales based on detailed organic geochemical and petrographic analysis of a profile of the Longtan Formation (LTF) at Tucheng Syncline, Southwestern Guizhou, China.

Our results showed that the overall total organic carbon (TOC) contents are high (15.51 wt% on average), generally larger than 2 wt% with some coal samples reaching up to 69.5 wt%. Macro- and microscopic observations suggested that terrestrial higher plant-derived macerals make up a predominant proportion to total organic matter, while quantitative analysis based on maceral counting and organic geochemistry revealed a large percentage of aquatic-derived organic matter in the LTF. The aquatic-derived organic matter is expected to affect the empirical relation between vitrinite reflectance (VR_r) and Rock-Eval T_max by showing a divergent regression line compared to that of the type III kerogen. Based on the pore morphology and the outline of dispersed organic matter, OMPs either show sharp edges in discernible terrestrial organic matter or bubble shapes in indistinct aquatic-derived organic matter. VR_r and T_max values range from 1.19 to 1.84 %VR_r (the mean value is 1.48%VR_r) and from 455 to 497 °C (469 °C on average precluding one anomalously high value of 555 °C), respectively. At this level of maturity, hydrocarbons have been generated in large quantities given the high organic matter content of the studied samples. Compared to the successful shale gas plays, e.g., the Barnett Shale and the Longmaxi Shale, the Longtan Shale (LTS) also shows good potential given its much larger thickness and TOC, appropriate thermal maturity. Nevertheless, cautious should be taken considering its smaller porosity and quartz content, which affect the storage capacity and the fracability of the LTS. This study contributes to a better understanding of the accumulation and maturation conditions of organic matter in coal-bearing shales.