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 co mpositions (delta(13)C) of hydrocarbon biomarkers. All of the bitumens cont ain isorenieratane (delta(13)C ca. -17 parts per thousand) indicating the p resence of Chlorobiaceae, and thus periods of euxinic conditions extended i nto the photic zone. Aliphatic hydrocarbon fractions are dominated by prist ane and phytane and contain relatively low amounts of 4 alpha- and 4 beta-m ethylsteranes (C-30, delta(13)C ca. -16 parts per thousand) believed to ori ginate from 4-methylsterols in dinoflagellates. delta(13)C values of gammac erane (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 c iliates that fed on Chlorobiaceae. The bitumens contain low relative amount s of organo-sulphur compounds. One rock contains C-37 and C-38 n-alkanes (d elta(13)C ca. -36 parts per thousand) derived from alkadienes and alkenones of prymnesiophytes. The distributions of hydrocarbon biomarkers suggest th at they were initially sulphurised during early diagenesis and later releas ed during early thermal maturation. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.