Effects of deep CO2 on petroleum and thermal alteration: The case of the Huangqiao oil and gas field

Abstract

The CO2-rich natural gas from the Huangqiao oil and gas field of the Subei Basin is produced along with crude oil. The CO2 content, ranging from 80.56% to 99.30% and the 3He/4He ratio of 0.94×10−6 to 4.67×10−6, indicates that this includes a substantial fraction of mantle-derived CO2 in the Huangqiao region. Although the positive carbon isotopic series of alkane gases (i.e., δ13C1<δ13C2<δ13C3) supports that the gaseous alkanes at the Huangqiao region are biogenic gas, some of the abiogenic CH4 is contributed to the natural gas, including the mantle-derived CH4, and the water-rock interaction generated CH4 by based on the relationships of CO2/3He vs R/Ra and CH4/3He vs R/Ra of the components of crust and mantle end members.The crude oil of the Huangqiao region is light or condensate oil, with low densities of 0.7933–0.8255g/cm3. Saturated hydrocarbons comprise 90.06%–97.37% the crude oil, while the aromatic, non-hydrocarbon and asphaltene fractions are very small. The n-alkanes range from C13 to C34, with highest abundances at C16 and C17. The gas chromatogram of the crude oil shows an unresolved hump or unresolved complex mixture (UCM) distribution of a complete n-alkane series, non-odd/even predominance, and major n-alkane peaks on the highest positions of the UCM distribution. This may indicate that deep mantle-derived CO2 fluid extracted and accumulated the oil now found in the reservoirs or source rocks.