Characteristics of Organic Matter Types and Organic Matter Pore Development in Marine–Continental Transitional Shale

The types of organic matter (OM) significantly impact the hydrocarbon generation potential, reservoir capacity and mechanical properties of shale. Unlike the relatively homogeneous OM in high-maturity marine shale (predominantly solid bitumen), marine–continental transitional shale exhibits a more complex OM assemblage, comprising a mixture of marine and terrestrial kerogen (vitrinite, inertinite and liptinite) and solid bitumen. Accurate differentiation of OM types and quantification of their associated pores are critical for evaluating the reservoir properties of transitional shale formations. This study investigates the Upper Carboniferous Taiyuan Formation shale in the Ningwu Basin, a representative marine–continental transitional system. A correlative microscopy approach integrating scanning electron microscopy (SEM) and optical microscopy was employed to classify OM types, which was further augmented by image segmentation processing techniques to elucidate their pore development characteristics. It reveals disparities in pore development associated with different OM types and proposes an SEM–based identification method for distinguishing them. Results indicated that vitrinite has thick cell walls with vertical fractures, a low grayscale value and a carbon content ranging from 80% to 95%; inertinite has thinner walls with distinct protrusions, a slightly higher grayscale value than vitrinite and carbon content between 75% and 90%; liptinite often retains the morphology of residual vegetation, such as serrated cutinite, flat ring-like sporinite and lens-like structured alginite. Solid bitumen has the highest grayscale value and is amorphous in shape. The development of OM pores is significantly related to the OM type, with the most developed pores found in solid bitumen, which is secondary in origin. This is followed by vitrinite and inertinite, with both secondary and primary pores present. The development of pores in OM is also related to the degree of thermal evolution and the types of surrounding minerals and contact relationships. The Taiyuan Formation transitional shale in the Ningwu Basin exhibits a wide range of total organic carbon (TOC) contents, with an average of 2.14%. Its microscopic components are primarily vitrinite and inertinite (over 50%), with a low degree of thermal maturity (average 1.17%) and low content of rigid minerals (average 42.3%), resulting in poorly developed OM pores and primarily intergranular pores in the storage spaces. However, the high organic carbon content and the strong adsorption capacity of vitrinite still endow the reservoir with the ability to enrich shale gas. The transitional shale possesses unique geological features, including interspersed thin coal seams and sandstone layers. The coal generates hydrocarbons and supplies gas in conjunction with shale, whereas thin sand layers enhance the reservoir capacity of the Taiyuan Formation shale, harbouring the potential for large-scale gas reserves. Thus, an integrated exploration and exploitation model involving coal, shale and tight sandstone is recommended to maximise the resource potential of marine–continental transitional shale.