Cong Yu

We systematically analyze the geochemical characteristics of source rocks of the Yan Chang Formation in the Ordos Basin to reveal the influence mechanisms of the volcano on lacustrine organic matter enrichment. Organic matter enrichment is not synchronous with the volcanic eruption in time, but it has a “hysteresis” effect. The evidence of volcanic activity mainly includes two aspects: one is a large amount of volcanic ash in black shale, and the other is a negative shift of organic carbon isotopes of source rocks and finding that black shale was affected by climate events caused by the volcano. The climate event caused by the volcano has three effects on organic matter enrichment in black shale: (1) it brought nutrients for the growth of organisms and released a large amount of greenhouse gases, such as CO2, to accelerate photosynthesis and promote the growth of organisms; (2) a large amount of toxic gases, such as SO2 and H2S, were released and O2 was consumed and diluted in the air, forming a severe anoxic environment and accelerating the preservation of organic matter; and (3) it caused extremely hot weather, resulting in long-time surface runoff and other climate events, increasing input of terrestrial organic matter, and forming light components of hydrocarbons.

We did have done thermal simulation experiments on coal-measure source rocks, carbon and oxygen isotope analysis of calcite cement in tight sandstone reservoirs, as well as uniform temperature and composition determination of fluid inclusions to illustrate the relationship between the gas generation of coal-measure source rocks and reservoir densification. Coal-measure source rocks produce a considerable amount of CO2 during the entire thermal simulation experiment, especially in the initial stage with the carbon isotope of CO2 ranging from −21.2‰ to −11.6‰, which indicates organic origin. The carbon isotope value of CO2 becomes heavier as the degree of thermal maturation increases. The carbon isotope of calcite cements in the tight sandstone reservoirs of the lower Shihezi Formation is in the range of −13‰ to −4.7‰, which may have the same origin as the CO2 produced during the gas generation of coal-measure source rocks. CO2 derived from coal-measure source rocks can promote the formation of cement, which may be a key reason for the densification of sandstone reservoirs. The fluid inclusions related to oil and gas charging which are mainly methane and carbon dioxide also can confirm this conclusion. At the same time, the CO2 also will impact the dissolution of potash feldspar, albite, and other minerals as well as the reprecipitation of clay minerals, resulting in a more complicated variation of porosity and permeability. CO2 is of great significance in improving oil recovery and the application of carbon capture, utilization, and storage. Studying the influence of CO2 generated from coal-measure source rocks on the physical properties of reservoirs can provide guidance for the mechanism of CO2 storage in the ground.