Controls on Graphitization and Nanopore Characteristics of Organic Matter in Marine Overmature Shale

Abstract

The overmature Lower Cambrian shale in southern China typically exhibits underdeveloped organic matter (OM) pores, with low porosity, and it is commonly believed that both of them have a causal linkage. However, there remains a lack of in-depth study on the characteristics of OM pores and their controlling factors for this shale. In this study, a suite of Lower Cambrian shale samples was taken from a well in the Upper Yangtze Platform, and their isolated kerogen was used to represent their OM. The shale samples were subjected to the analysis of TOC (total organic carbon) contents, mineral composition and maturity, and the kerogen samples were measured by low pressure gas absorption and X-ray photoelectron spectroscopy to characterize the OM nanopore structure and heterogeneity, and the degree of graphitization, respectively. These data were jointly used to investigate the influencing factors of OM nanopores. The results show that the shale samples were rich in quartz and clay minerals, mainly belonging to siliceous shale in lithofacies, their OM was overmature, with average equivalent vitrinite reflectance (EqVRo) values of 3.48–3.49% and graphitization degrees of 12.77–18.56%. The development of their OM nanopores varied widely, with total pore volumes of 0.321–0.786 cm³/g and total specific surface areas of 142.27–206.02 m²/g (the data were normalized by the elemental carbon content of kerogen samples). The variable graphitization degree and pore structure parameters of OM in the shale samples are primarily attributable to the differential compaction caused by their differences in TOC content and mineral composition. The shale samples with higher TOC contents tended to have lower ratios of quartz to TOC, with the disadvantage to the formation of an effective rigid framework, which increases the compaction of OM particles in shale, and enhances their graphitization degree as well as the collapse of their larger nanopores (such as mesopores and macropores) to form smaller nanopores (typically micropores). However, these processes are weakened to some extent by the pressure-shielding effect of OM-clay aggregations. In contrast, as the graphitization degree increases, the orderly arrangement of carbon atoms is enhanced, leading to the OM particles are easier to be deformed. Combined with the influence of compaction, the graphitization can promotes the transformation of OM mesopores and macropores into micropores, which also complicates the pore structure to enhance the heterogeneity. Therefore, the OM nanopore characteristics and heterogeneity of the studied overmature shale samples were directly affected by their compositions, and the primary mechanism was the synergistic effect of compaction and graphitization.