Characteristics and dominant controlling factors of organic-rich marine shales with high thermal maturity: A case study of the Lower Cambrian Niutitang Formation in the Cen’gong block, southern China

Abstract

The Lower Cambrian organic-rich marine shale, which is a significant source of China’s shale gas, is widely distributed in southern China. An integrated characterization of the Niutitang shale is provided in this study in terms of organic geochemistry, mineralogy, pore characterization, methane sorption capacity, rock mechanical properties, fractures and gas content based on samples from three wells. The results indicate that the Lower Cambrian Niutitang shale is thermally over-mature and has rich shale gas resources, with a total organic carbon content (TOC) between 0.51% and 10.49% and a high quartz content between 35.3% and 78.5%. Compared to the major gas-producing shales in the U.S. and China, most of the organic matter (OM)-hosted pores in the Niutitang shale are generally smaller than 5 nm, significantly affecting the methane sorption capacity. The inter-particle and intra-particle pores and fractures are the primary sources of storage space, especially for free gas. For samples with TOC values less than 6.5%, TOC is positively correlated with the total porosity, total pore volume, brittleness (Young’s modulus), core fracture density, free gas content and Langmuir pressure; however, for samples with TOC values greater than 6.5%, the positive correlations become negative. These characteristics are due to the ductility and low hydrocarbon generation potential of organic matter in high thermal maturity shales that are vulnerable to compaction. Thus, TOC has a significant impact on the macroscopic (e.g., brittleness) and microscopic (e.g., pore structure and sorption capacity) properties of shale reservoirs, potentially controlling the enrichment and productivity of shale gas. These results can be used to optimize drilling and fracturing stimulation intervals during shale gas exploration and development.