Oil content in argillaceous dolomite from the Jianghan Basin, China: Application of new grading evaluation criteria to study shale oil potential

Abstract

Shale oil resources are found in the argillaceous dolomite in the Xingouzui Formation of the Jianghan Basin. The grading evaluation criteria based on the oil content was established to discuss the shale oil potential. The parameter of hydrocarbon potential index, (S1+S2)/TOC×100, suggests that the hydrocarbon potential of the argillaceous dolomite becomes lower significantly with the type of kerogen changing from I to III. However, a corresponding decrease in the amount of residual hydrocarbon (S1/TOC) is within a small range, because the argillaceous dolomite with I and II1 kerogen (especially I kerogen) have relatively larger hydrocarbon expulsion amount. For shale oil exploration, besides type I source rocks, source rocks with II1 and II2 kerogen should also be focused. However, source rocks with III kerogen bear no exploration significance owing to the low hydrocarbon generation capacity and unfavorable molecular configuration. According to the relationship between S1 and TOC, combined with parameter of the relative oil content, the shale oil resources can be divided into four classifications: enriched resources (S1>4mg/g and S1/TOC>1), moderately enriched resources (1mg/g<S1<4mg/g and S1/TOC>1), less efficient resources (1mg/g<S1<4mg/g and S1/TOC<1), and ineffective resources (S1<1mg/g and S1/TOC<1). The continuous distribution of the oil content of the argillaceous dolomite in the Lower Xingouzui Formation can be obtained using the method of Δlog R. The enriched or moderately enriched resources are primarily distributed in the argillaceous dolomite with kerogen types I and II1. In addition, moderately enriched resources also exist in the argillaceous dolomite with kerogen type II2 in the middle and bottom of the Lower Xingouzui Formation, but the distribution is more dispersed.