The discovery of high-yield industrial gas flows in the limestone layer of the Permian Taiyuan Formation in the Ordos Basin highlights promising prospects for natural gas exploration and has positioned this region as a key exploration area. However, research on the origin and distribution of natural gas in the Taiyuan Limestone Formation, especially its hydrocarbon generation potential and whether these organically enriched limestones can serve as effective source rocks, remains limited. In this study, we aimed to analyse the composition and carbon isotopes of the Upper Palaeozoic Taiyuan Limestone Formation natural gas, including propane-specific isotopes. The isotopic data were compared with coal-type gases from the corresponding strata and oil-type gases from the Lower Palaeozoic carbonate strata. Natural gas in the Upper Palaeozoic Taiyuan Limestone Formation was markedly distinct from the 'self-generated and self-accumulated' oil-type gas in the Lower Palaeozoic subsalt strata, indicating no obvious correlation between the two gases. The results did not support the notion that large-scale natural gas accumulations in the Lower Palaeozoic carbonate formations could be originated from Upper Palaeozoic limestone source rocks. The natural gas in the Upper Palaeozoic Taiyuan Limestone Formation was highly consistent with the typical coal-type gas from the Upper Paleozoic. The geochemical characteristics of the natural gas in the region were consistent with that of conventional natural gas, the position-specific isotopic composition of propane Δ_C−T values had a narrow, positive. This showed that the gas in the Taiyuan Limestone Formation was derived from type Ⅲ kerogen, and propane mainly generated through the n-C₃H₇ free radical pathway. Geochemical analyses of the Taiyuan Limestone source rocks, such as the determination of total organic carbon, kerogen carbon isotopes, organic macerals. Combined with the geochemical analysis of natural gas, it revealed low abundance of organic matter but good kerogen types, predominantly type Ⅱ–Ⅲ, at a late to high-maturity evolution stage. Although the formation had certain hydrocarbon generation potential, it falls short of the hydrocarbon generation capacity of the Carboniferous–Permian coal measure source rocks. At present, there is no large–scale hydrocarbon generation, and it is not enough to provide hydrocarbon for the Lower Paleozoic.

Marine organic-rich marl is not only a high-quality hydrocarbon source of conventional oil and gas, but also a new type and field of unconventional oil and gas exploration. An understanding of its pore structure evolution characteristics during a hydrocarbon generation process is theoretically significant and has application prospects for the exploration and development of this special type of natural gas reservoirs. This study conducted thermal simulation of hydrocarbon generation under near-geological conditions during a whole process for cylinder samples of low mature marine organic-rich marl in the Middle Devonian of Luquan, Yunnan Province, China. During this process, hydrocarbon products at different evolution stages were quantified and corresponding geochemical properties were analyzed. Simultaneously, field emission scanning electron microscopy (FE-SEM) and low-pressure gas adsorption (CO₂, N₂) tests were applied to the corresponding cylinder residue samples to reveal the mechanisms of different types of pore formation and evolution, and clarify the dynamic evolution processes of their pore systems. The results show that with an increase in temperature and pressure, the total oil yield peaks at an equivalent vitrinite reflectance (VR_o) of 1.03% and is at the maximum retention stage of liquid hydrocarbons, which are 367.51 mg/g TOC and 211.67 mg/g TOC, respectively. The hydrocarbon gas yield increases continuously with an increase in maturity. The high retained oil rate at the peak of oil generation provides an abundant material basis for gas formation at high maturity and over-maturity stage. The lower limit of VR_o for organic matter (OM) pore mass development is about 1.6%, and bitumen pores, organic-clay complex pores together with intergranular pores, grain edge seams and dissolution pores constitute a complicated pore-seam-network system, which is the main reservoir space for unconventional carbonate gas. Pore formation and evolution are controlled synergistically by hydrocarbon generation, diagenesis and organic-inorganic interactions, and the pattern of pore structure evolution can be divided into four stages. A pore volume (PV) and a specific surface area (SSA) are at their highest values within the maturity range of 1.9% to 2.5%, which is conducive to exploring unconventional natural gas.