Compound specific sulfur isotopes of saline lacustrine oils from the Dongpu Depression, Bohai Bay Basin, NE China

摘要

Lacustrine oils from type I-II kerogen from the Dongpu Depression of Bohai Bay Basin have been characterised by compound specific sulfur isotope and hydrocarbon biomarker analyses. The sample suite included sixteen saline lacustrine oils from the northern part of the Dongpu Depression, most of which (i.e., all but two interbedded samples) could be separated into four tectonically aligned oil families (OF1-4) on the basis of differences in their hydrocarbon composition (e.g. high MW n-alkanes, isoprenoids, polycyclic terpenoids). The other two oils studied were freshwater lacustrine oils from the south of the Dongpu Depression, the hydrocarbon composition of which indicated a fifth oil family (OF5). The delta S-34 values of organic sulfur compounds (delta(SOSCs)-S-34) in the oils showed several general differences consistent with the five hydrocarbon biomarker defined oil families, including the relative magnitude of their delta S-34 values: Ave delta S-34(OSCs) values: OF1 = 19 parts per thousand (relatively depleted delta S-34); OF2 = 28 parts per thousand (enriched delta S-34); OF3 = 24 parts per thousand (intermediate delta S-34); OF4 = 25 delta S-34 (intermediate delta S-34); and OF5 = 18 parts per thousand (depleted delta S-34); and the respective delta(34)S(OSCs )profiles of the oils, e.g., the intermediate delta S-34 values of the LT oils (OF3) showed little variance with alkylation ( 4 parts per thousand), whereas the intermediate delta S-34 values of the higher maturity Wenliu oils (OF4) showed S-34 enriched alkylated DBT5 compared to base DBT (Delta delta S-34(a1kylDBT-DBT) = +7 parts per thousand). This demonstrates the potential of delta S-34(OSCs) data assisting the genetic correlation of S-rich saline lacustrine oils. However, separate correlations between some of the delta S-34(OSCs) data and hydrocarbon indicators of paleo-environmental (C-35-/C-34-hopanes), thermal maturity (C-29 sterane and MDBT parameters) and TSR (Delta delta S-34(BTs-DBTs) = 6-12 parts per thousand) impacts suggest each of these factors also influenced the delta S-34 character of the Dongpu oils. Therefore, delta S-34(OSCs) data should be used cautiously in correlation of oils from complex petroleum systems and where possible integrated with other complimentary analytical information.