Impact of CO2 influx on sandstone reservoir quality: A case study of the Quantou Formation, southern Songliao Basin, China

The CO2-gas reservoirs have been recorded in many petroliferous basins worldwide. However, the impact of deep inorganic CO2 influx on reservoir quality has received little attention. Here, a new set of mineralogical and geochemical data collected from the Lower Cretaceous Quantou Formation sandstones in the southern Songliao Basin are presented to address this issue. The sandstones were broadly subdivided into two zones based on their mineralogical compositions: (1) a normal zone with higher porosity (average 13.7%) and permeability (average 3.27 md) that is located >10 km from the Gudian fault (composed of ferrocalcite, ankerite, quartz, mixed-layer illite/smectite (I/S), kaolinite, illite, and chlorite); and (2) a dawsonite-bearing zone with relatively poor reservoir quality (average 10.1% and 0.4 md) adjacent to the Gudian fault (consisting of dawsonite, ankerite, quartz, I/S, and illite). The carbon sources for dawsonite and ankerite in the dawsonite-bearing zone (δ13C = −5.7‰ to −0.8‰ and δ18O = −20.6‰ to −17.1‰, and Sr = 0.710216–0.712472) are mostly a mix of mantle magmatic CO2 and crustal CO2, with a small amount of organic CO2, which is the opposite of that for the ferrocalcite and ankerite in the normal zone (δ13C = −10.5‰ to −2.3‰, δ18O = −19.3‰ to −14.9‰, and Sr = 0.712060–0.714030). Observations of the dawsonite-bearing zone demonstrate higher contents of carbonate and quartz cements, specific clay mineral types (mixed-layer I/S with Reichweite order of R = 3 and illite), and poor reservoir quality and oil productivity due to the influx of deep inorganic CO2 dating to circa 65–44 Ma.