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

Immature organic matter in lacustrine source rocks from the Jianghan Basin, eastern China, was studied for distributions and stable carbon isotopic compositions (delta(13)C) of hydrocarbon biomarkers. All of the bitumens contain isorenieratane (delta(13)C ca. -17 parts per thousand) 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 alpha- and 4 beta-methylsteranes (C-30, delta(13)C ca. -16 parts per thousand) believed to originate from 4-methylsterols in dinoflagellates. delta(13)C values of gammacerane (ca. -24 parts per thousand) and C-30 17 alpha,21 beta(H)-hopane (ca. -23 parts per thousand) 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 C-37 and C-38 n-alkanes (delta(13)C ca. -36 parts per thousand) 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. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.