Thermal stability and parameter validity of hopane series in mature shales – A case study from Dongying Depression, eastern China

Abstract

Molecular compositions extracted from source rocks offer direct indicator of the organic matter input, depositional redox conditions and thermal maturity levels that source rock has experienced, however, the application of such technique commonly encounters difficulty in highly mature system. A suite of twenty-eight organic-rich lacustrine shale samples from the Eocene Shahejie Formation in the Dongying Depression, East China were geochemically investigated, aiming to obtain a comprehensive thermal stability sequence of hopanes and the validity of various molecular parameters. By means of systematic comparisons on compound ratios and concentrations, the thermal stability of hopanes is presented as follows: [C35 hopane (H) < (C33H, C32H) < C34H < C30H < C31H < C29H < 17α(H)-22,29,30-trisnorhopane (Tm) < 18α (H)-30-norneohopane (C29Ts) < C30 diahopane (D) < 18α(H)-22,29,30-trisnorneohopane (Ts), from the least stable to the most stable]. Limited validity of the paleoenvironment and organic matter input parameters such as C35H/C34H and C31 22R/C30H ratios in mature source rocks is caused by the preferential decomposition of C35H and C30H compounds during thermal maturation. Therefore, application of molecular parameters derived from hopanes need to be exercised with extra caution. Their validity varies with maturity levels and most hopane related parameters will lose their effectiveness at peak oil generation stage. Meanwhile, a novel maturity parameter (Ts/C29Ts ratio) derived from thermal stability difference of hopanes was proposed herein, supported by strong correlations with vitrinite reflectance and other two reliable maturity parameters. Upon further calibration, this novel maturity parameter can be deemed as a reliably alternative indicator and is expected to have wide application in highly mature source-rock/oil maturity assessment, especially for the shale oil system. The findings of this study may help for better understanding of limited validities of molecular indicators during thermal evolution.