The use of GOI and MCI of oil inclusions in reconstructing oil charge history of gas-condensate reservoirs: example from the Mobei area, central Junggar Basin (NW China)

摘要

Gas-condensate has been currently explored in Jurassic reservoirs of the Mobei area (central Junggar Basin, NW China). Based on the carbon isotopic values of these gas-condensate (δ~(13)C_1=-36.81～-34.69 per thousand, δ~(13)C_2=-29.74～-25.91 per thousand, Xiang et al., 2003), they show a humus source, which correlate well to the Middle Permian Lower Wuerhe (P_2w) Formation source rock in the area. In addition, condensate has been found in the Jurassic reservoirs here. According to Wang and Kang (2001), these hydrocarbons are mainly sourced from the Lower Wuerhe sources. Thereafter, it is widely acknowledged that the Lower Wuerhe Formation generated oil and gas over a considerable maturity range over geological time. However, Pan et al. (2003) suggested that in the central Junggar Basin, the Lower Permian Fengcheng (P_1f) source rock is also present besides the Lower Wuerhe Formation. As different oils in a petroleum reservoir tend to form a homogeneous mixture (England et al., 1987), and early oil charges may be displaced by subsequent gas charges or mixed with subsequent oil charges, or may be altered by biodegradation or water-washing prior to later oil charge, these blur geochemical differences and often make it difficult to reconstruct the detailed charge history of a petroleum reservoir based on the analysis of recovered oils or gas-condensate alone.