Semi-closed hydrous pyrolysis of type I source rock using formation water: Implications for biomarker behaviour under supercritical conditions

Abstract

Hydrous pyrolysis is a useful artificial method to simulate hydrocarbon generation process and investigate the geochemical behaviour of biomarkers with the presence of water, which has great importance for conventional and unconventional oil-gas exploration. Formation water is ubiquitous saline water in oilfields, whereas not commonly utilized in hydrous pyrolysis. To determine the influence of formation water on the expulsion/retention behaviour of commonly utilized oil-source correlation indictors (Pr/Ph, Pr/n-C17, Ph/n-C18, and gammacerane index, C22/C21, C24/C23 and C26/C25 tricyclic terpane ratios, C31R/C30hopane, C31(22S/22S + 22R) hopane, moretane/hopane and Ts/Ts + Tm), two series of semi-closed hydrous pyrolysis experiments were conducted in 300–550 °C for the typical type I Green River Shale source rocks using distilled water and formation water. Such comparative study can also improve the understanding of whether supercritical fluids affect the biomarker behaviour because the temperature for major hydrocarbon generation in hydrous pyrolysis approaches the critical temperature for distilled water. The studied oil-source correlation indictors of immiscible oils generated in both experiments using distilled water and formation water exhibit similar patterns with the increase of thermal maturity levels. Both retained and expelled immiscible oils generated in the experiments using formation water and distilled water are situated in similar area of empirical genetic diagrams (e.g. Pr/n-C17vs Ph/n-C18, C31R/C30 hopanes vs C26/C25 tricyclic terpanes, and ternary plot of C27–C28–C29 regular steranes), suggesting that the presence of formation water/supercritical water does not significantly compromise the utility of these diagrams for oil-source correlation. Outcomes of this study also demonstrate that semi-closed hydrous pyrolysis is a useful artificial method for characterizing biomarker compositions of oils generated by type I source rocks and understanding geochemical behaviour of biomarkers under supercritical conditions.