Impact of Maturation on the Validity of Paleoenvironmental Indicators: Implication for Discrimination of Oil Genetic Types in Lacustrine Shale Systems

Organic geochemistry analyses were carried out on twenty-two shale samples from the Eocene Shahejie (E-s) Formation in two wells at the Dongying Depression, East China, aiming to investigate the evolution of the gammacerane index (GI) and oleanane index (OI) and the impact of maturation on the validity of these paleoenvironment indicators. While high GI and OI ratio values in some samples do indicate the occurrence of a hypersaline interval and higher contribution of a terrigenous organic matter input, respectively, a positive correlation between both molecular markers and known maturity level suggests dual controls from the source and maturation. A component concentration profile in a maturation sequence offers mechanism interpretation for effectiveness of these commonly used molecular parameters. The invalidity as geochemical diagnosis for paleoenvironment indicators in some highly mature samples is attributed to faster depletion of C-30 17 alpha(H), 21 beta(H)-hopane against gammacerane and oleanane in the main oil generation window when the vitrinite reflectance value reaches about 0.7%. The present study sheds light on potential risks for discrimination of oil genetic types by those indicators in the complex orogenic region. Hence, with limited validity of these indicators in high maturity regime, caution must be taken while conducting geochemical interpretation on source rocks and crude oils.