Maturity and depositional controls on compound-specific sulfur isotope values of saline lacustrine source rocks in the north Dongpu Depression, Bohai Bay Basin

Abstract

Source rocks and oil sands from the northern Dongpu Depression have been analyzed by gas chromatography multicollector inductively coupled plasma mass spectrometry (GC-MC-ICP MS) to investigate the compound-specific sulfur isotope (δ34S) values of saline lacustrine source rocks. The samples were also subject to more traditional organic geochemical analyses including hydrocarbon biomarker detection by gas chromatography/mass spectrometry (GC/MS), carbon and oxygen isotopic analysis by isotope ratio mass spectrometer, and major and trace inorganic element analysis. δ34SOSCs values measured for a limited number of organic sulfur compounds (OSCs) showed a quite large range (5.16‰–44.73‰). The source rocks and oil sands could be separated into two groups with δ34S values above or below 25.00‰. The first group with δ34S > 25.00‰, were primarily the Weicheng source rocks, which also typically showed an increase in δ34S values with increasing alkylation. The second group had δ34S < 25.00, the δ34S of dibenzothiophenes showed a positive correlation with the concentration of the saturated hydrocarbon fraction of rocks, and a negative correlation with the polar (resin + asphaltene) fraction. The δ34SOSCs show obvious positive correlation with gammacerane/C31 hopane and C35/C34 hopanes and excellent correlation with contents of minerals and ratios of trace elements, reflecting a 34S enrichment in closed evaporative environment (conducive to the sulfurisation of organic matter) and relative 34S depletion in terrestrial environments. These trends identify the depositional environment of the source rocks as one of controls on δ34SOSCs values. Thermal maturity was also observed to influence δ34SOSCs values, becoming heavier with increasing maturity. There was a good result of oil-source rock correlation by δ34SOSC in this study. This study demonstrates the contribution δ34SOSCs data can make to oil-oil and oil-source rock correlations as well as providing further evidence of their sensitivity to depositional environments and thermal maturity.