Determination of critical parameters for evaluating coal measure shale gas in China

Abstract

Coal measure shale is one of the most abundant types of marine-continental shales in China. Two sets of typical coal measure shales in China are developed in the Shanxi Formation (Ordos Basin) and in the Longtan Formation (Guizhou Province). In this study, thermal simulation, rock pyrolysis, and X-ray diffraction experiments were used to analyze four coal measure shale samples collected from typical areas of China. Based on the study of hydrocarbon generation process of the samples, the lower limits of thermal maturity (Ro) of favorable and target areas for coal measure shale were ∼1.8% and 2.5%, respectively. The threshold Ro value of oil generation in coal measure shale was preliminarily inferred to be 0.6%. The gaseous hydrocarbon production per unit of S1+S2, after the gas generation threshold, was linearly and positively correlated with Ro. The lower limits of total organic carbon (TOC), corresponding to the favorable and target areas, were ∼2.45 wt% and 2.84 wt%, respectively. Combined with X-ray diffraction experiments, the diagenetic stage and the hydrocarbon generation model of coal measure shale were established and could be divided into four evolutionary stages. The results of this study have direct scientific and practical significance for the evaluation of gas content and selection of areas in China for the extraction of coal measure shale.