Hydrocarbon generation-expulsion-retention mechanisms in marine organic-rich marl: Insights from semi-closed hydrous pyrolysis simulation experiments

Marine organic matter-rich marl with dual characteristics of hydrocarbon source rock and in-situ reservoir, being a source of conventional hydrocarbons and an important area for unconventional hydrocarbon exploration. However, the understanding of the discharge mechanisms and quantitative and geochemical responses during the hydrocarbon evolution is still unclear, which restricts the conventional-unconventional oil and gas resource evaluation as well as exploration and development of the whole oil and gas system. In this study, a temperature-pressure-liquid combined controlled hydrocarbon generation-expulsion-retention experiment was carried out on low-mature marine organic-rich marl using the semi-closed hydrous pyrolysis simulation system. Chemical and isotopic compositions of the expelled and retained hydrocarbons were quantified and analyzed, revealing the mechanisms of hydrocarbon generation-expulsion-retention and a dynamic evolution model, especially the systematic changes of hydrocarbon discharge and retention ratios in different evolutionary stages. The results showed that the retained hydrocarbon yield was greater than that of the expelled hydrocarbon during the entire oil generation window, and the percentage of retained oil was as high as 82.64 % at the peak of oil generation (VR_o=1.03 %). Previous conventional studies have somewhat overestimated the hydrocarbon expulsion ratio of organic-rich source rock systems and underestimated their ability to retain hydrocarbons. Carbon isotope values of residual kerogen and chloroform bitumen “A” extracts at the maturity stage, with fractionation less than 1 ‰, and this parameter could be used for oil source comparison and hydrocarbon source tracing. Marine organic matter-rich marl showed a four-stage dynamic evolutionary pattern of generation-expulsion-retention hydrocarbons, and the favorable shale oil exploration maturity interval was between 0.76 % and 1.6 %. The research results are of great significance to broaden the field of unconventional oil and gas exploration.