Recognizing Oil Mixing in Terms of Commercial Evaluation and Application to Petroleum Systems in Superimposed Petroliferous Basins
State Key Lab of Organic Geochemistry, Guangzhou Institute of Geochemistry, Guangzhou, China
Oil mixing frequently occurs in superimposed basins because of its multiple source rock distributions. Biomarker assemblages in oil-source rock pairs are the most frequently used to identify oil mixing. Indeed petroleum geochemists can cite a large suit of regional-accepted oil-source rock pairs to recognize oil mixing and calculate mixing ratio. However, it is still ambiguous and usually unacceptable for petroleum geologists. One key point arise from this approach is how to assess the representative of these biomarkers that normally have an amount of <0.1% in crude oils. Moreover, if multiple end members have drastically different absolute concentrations for certain compound class, biomarker distribution in the mixed oils may be severely biased towards one of the end members in which their concentrations are significantly high. Therefore, conclusions from biomarker distributions only will easily result in a misleading on regional exploration strategy-making. In such a way, emphasis on the representative of mixing signatures is strongly recommended, including the amount level of compounds used in crude oil and a widely molecular weight range parameters. Following this consideration, the uses of distribution, absolute concentration, and stable carbon isotopic composition of n-alkanes as main components in crude oil, together with biomarker assemblages and specific source and/or environmental-dependent compounds within geological context, to commercial evaluation of oil mixing are advocated. A case study from one typical superimposed petroliferous basin, Tarim basin, NW China, was presented.
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