Molecular isotopic characterisation of hydrocarbon biomarkers in Palaeocene-Eocene evaporitic, lacustrine source rocks from the Jianghan Basin, China

Abstract

Immature organic matter in lacustrine source rocks from the Jianghan Basin, eastern China, was studied for distributions and stable carbon isotopic compositions (13C) of hydrocarbon biomarkers. All of the bitumens contain isorenieratane (13C ca. −17 ) indicating the presence of Chlorobiaceae, and thus periods of euxinic conditions extended into the photic zone. Aliphatic hydrocarbon fractions are dominated by pristane and phytane and contain relatively low amounts of 4- and 4-methylsteranes (C30, 13C ca. −16 ) believed to originate from 4-methylsterols in dinoflagellates. 13C values of gammacerane (ca. −24 ) and C30 17,21(H)-hopane (ca. −23 ) are consistent with an origin from bacterivorous ciliates that fed on Chlorobiaceae. The bitumens contain low relative amounts of organo-sulphur compounds. One rock contains C37 and C38 n-alkanes (13C ca. −36 ) derived from alkadienes and alkenones of prymnesiophytes. The distributions of hydrocarbon biomarkers suggest that they were initially sulphurised during early diagenesis and later released during early thermal maturation.