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Quick Evaluation of Present‐Day Low‐Total Organic Carbon Carbonate Source Rocks from Rock‐Eval Data: Middle–Upper Ordovician in the Tabei Uplift, Tarim Basin

Previous studies have postulated the contribution of present‐day low‐total organic carbon (TOC) marine carbonate source rocks to oil accumulations in the Tabei Uplift, Tarim Basin, China. However, not all present‐day low‐TOC carbonates have generated and expelled hydrocarbons; therefore, to distinguish the source rocks that have already expelled sufficient hydrocarbons from those not expelled hydrocarbons, is crucial in source rock evaluation and resource assessment in the Tabei Uplift. Mass balance can be used to identify modern low‐TOC carbonates resulting from hydrocarbon expulsion. However, the process is quite complicated, requiring many parameters and coefficients and thus also a massive data source. In this paper, we provide a quick and cost effective method for identifying carbonate source rock with present‐day low TOC, using widely available Rock‐Eval data. First, we identify present‐day low‐TOC carbonate source rocks in typical wells according to the mass balance approach. Second, we build an optimal model to evaluate source rocks from the analysis of the rocks' characteristics and their influencing factors, reported as positive or negative values of a dimensionless index of Rock‐Eval data (IR). Positive IR corresponds to those samples which have expelled hydrocarbons. The optimal model optimizes complicated calculations and simulation processes; thus it could be widely applicable and competitive in the evaluation of present‐day low TOC carbonates. By applying the model to the Rock‐Eval dataset of the Tabei Uplift, we identify present‐day low‐TOC carbonate source rocks and primarily evaluate the contribution equivalent of 11.87×109 t oil.