The lower Cambrian organic-rich shales are widely distributed in the middle-upper Yangtze platform, which are also the main target layers for shale gas exploration and extraction. Revealing the enrichment mechanisms of organic matter in these shales is of great significance for understanding the relationship between ocean bio-environmental systems and Earth resources. Through systematical geochemical analyses, this study investigated the lower Cambrian Niutitang Formation slope facies shales at the western Xuefeng Uplift, to elucidate the paleoenvironmental characteristics and their main controlling factors. The results show that the Niutitang Formation shales generally have high TOC contents, with higher TOC contents in the lower part than those in the upper part. Sedimentological geochemistry analyses indicate that the Niutitang Formation formed under high paleo-productivity and anoxic-anaerobic even sulfurized conditions. The formation generally has a lower degree of reduction in the late period of sedimentation than at earlier times, and was deposited in a moderately hydrologically restricted setting, with weak terrestrial input and hydrothermal activities. The enrichment of organic matter in the Niutitang Formation slope facies shales was mainly controlled by the redox conditions, however upwelling currents also contributed to the primary input of organic matter. During the early deposition stage of the Niutitang Formation, the nutrients brought by upwelling currents promoted primary paleo-productivity, resulting in the booming of algal organisms and subsequent bacterial sulfate reduction (BSR) and anoxic even sulfurized environments. During the late deposition stage of the Niutitang Formation, a drop in sea level led to a weaker degree of reduction and lower contents of TOC. Our results are of great significance to the enrichment mechanisms of marine organic-rich shales in petroliferous basins.

The Paleozoic source rocks in southern China contain mainly Type Ⅰ-Ⅱ kerogen and are currently overmature with vitrinite reflectance (R_o) greater than 2.0 %. Accurate evaluation of the gas generation potential of Type Ⅰ-Ⅱ kerogen after oil generation and expulsion and geochemical characteristics of over-mature gases are of great significance to natural gas exploration of deep and ultra-deep Paleozoic gas pools in southern China. In view of this, the mature Devonian source rock samples (R_o≈1.1 %) collected from the Guanwushan Formation in Northwestern Sichuan Basin are applied for gold tube pyrolysis experiments, and the gas generation potential of Type Ⅰ-Ⅱ kerogen at the highto-over mature stage and the geochemical characteristics of over-mature gas generated are investigated in combination with existing documents. The results show that the residual oil content of the studied samples is 140 mg/gTOC and has a gas generation potential of 220 mL/gTOC at present, indicating high gas-generation potential. Furthermore, the amount of gas from kerogen cracking is at least 140 mL/gTOC and that of gas from residual oil cracking is at best 80 mL/gTOC, indicating that kerogen-cracking gas is the main source of late gas generated from the sapropelic source rocks under high oil expulsion efficiency. Meanwhile, the geochemical characteristics of the late gases generated from the sapropelic source rocks are obviously affected by oil expulsion efficiency. For example, the dryness coefficient of the gases is high, and the methane carbon isotope grows heavier rapidly with increasing methane yield. At R_o≈3.50 %, the carbon isotope of methane is close to that of the original kerogen (with a mere carbon isotopic fractionation difference of 0.5 ‰). The results obtained above will be of scientific basis to the evaluation of natural gas exploration potential and the identification of natural gas sources in the deep and ultra-deep Paleozoic reservoirs in southern China.