Impact of CO2 generated from coal-measure source rocks on physical properties of reservoirs - A case study of the Sulige gas field in the Ordos Basin, Northwest China

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.