Impact of CO2 Generated from Coal Measure Source Rocks on Physical Properties of Reservoirs-a Case Study of Sulige Gas Field in the Ordos Basin, NW China

We did 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 gas generation of coal measure source rocks and reservoir densification. Coal measure source rocks produced considerable amount of CO2 during the entire thermal simulation experiment especially in the initial stage with the carbon isotope of CO2 range from -21.2‰ to -11.6‰, which indicated 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‰~-4.7‰, which may have a 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, may be a key reason for the densification of sandstone reservoirs. The fluid inclusions related to oil and gas charging are mainly methane and carbon dioxide can also confirm this conclusion. At the same time, the CO2 will also impact the dissolution of potash feldspar, albite and other minerals and the reprecipitation of clay minerals, resulting in 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 (CCUS). 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.