Biomarker 17 alpha(H)-diahopane: A geochemical tool to study the petroleum system of a Tertiary lacustrine basin, Northern South China Sea

Relatively abundant C-30 17 alpha(H)-diahopane has been detected in oils and rock extracts in the Fushan Depression, a Tertiary lacustrine basin of the Northern South China Sea. C-30 17 alpha(H)-diahopane/(C-30 17 alpha(H)-diahopane + C-30 17 alpha(H)-hopane) ratios (DI) increase with increased burial depth of the source rocks. DI values of most oils from the upper unit (E(2)l(3)(1)) of the third member of the Eocene Liushagang Formation (E(2)l(3)) in the Huachang oil and gas field show a positive correlation with other molecular maturity indicators, such as the DNR (dimethylnaphthalene ratio), MDR (methyldibenzothiophene ratio), 4,6-/1,4-DMDBT (4,6-/1,4-dimethyidibenzothiophene), MPR (methylphenanthrene ratios), or the MPI-1 (methylphenanthrene index). The inflection of the DI-depth curve occurring at about 3200 m, corresponds to 0.55 (R-o%). The oils of the Huachang oil and gas field have higher DI values than the extracts of in situ source rocks, indicating the oils might be sourced from deeply-buried source rocks. Two petroleum systems can be distinguished in the Huachang field in terms of the relative concentration of diahopane and other maturity parameters. The oils in the upper petroleum system, consisting of the reservoirs in E(2)l(3)(1), are mainly sourced from the E(2)l source rocks in the Bailian sag, and are characterized by relatively higher maturity, higher oil and gas yields, and lower contents of diahopane and oleananes. The oils in the lower system, consisting of middle (E(2)l(3)(2) and lower units (E(2)l(3)(3) of E(2)l(3), have higher contents of diahopane, higher contents of CO2 gas, and lower maturity and oil and gas yields. The occurrence and geochemical characteristics of the petroleum system are mainly controlled by the Double Fault System in the Huachang uplift. The C-30 17 alpha(H)-diahopane is an effective biomarker for oil-source rock correlation, oil-oil correlation, maturity assessment and petroleum system study in some terrestrial lacustrine basins. (C) 2008 Elsevier Ltd. All rights reserved.