Dawsonite fixation of mantle CO_2 in the cretaceous Songliao Basin, Northeast China: a natural analogue for CO_2 mineral trapping in oilfields

摘要

Abundant crude oil and CO_2 gas coexist in the fourth member of the Upper Cretaceous Quantou reservoir in the Huazijing Step of the southern Songliao Basin, China. Here, we present results of a petrographic characterization of this reservoir based on polarizing microscope, X-ray diffraction, fluid inclusion, and carbon-oxygen isotopic data. These data were project, and to determine what effects might the presence of CO_2 have on the properties of crude oil in the reservoir. Tused to identify whether CO_2 might be trapped in minerals after the termination of a CO_2-enhanced oil recovery (EOR)he crude oil reservoir in the study area, which coexists with mantle-derived CO_2, is hosted by dawsonite-bearing lithic arkoses and feldspathic litharenites. These sediments are characterized by a paragenetic sequence of clay, quartz overgrowth, first-generation calcite, dawsonite, second-generation calcite, and ankerite. The dawsonite analysed during this study exhibits δ~(13) C (Peedee Belemnite, PDB) values of -4.97%o to 0.67%o, which is indicative for the formation of magmatic-mantle CO_2. The paragenesis and compositions of fluid inclusions in the dawsonite-bearing sandstones record a sequence of two separate filling events, the first involving crude oil and the second involving magmatic-mantle CO_2. The presence of prolate primary hydrocarbon inclusions within the dawsonite indicates that these minerals precipitated from oil-bearing pore fluids at temperatures of 94-97°C, in turn suggesting that CO_2 could be stored as carbonate minerals after the termination of a CO_2-EOR project. In addition, the crude oil in the basin would become less dense after deposition of bitumen by deasphalting the injection of CO_2 gas into the oil pool.