A research into the crude oil geochemistry and oil-source correlation by means of gas chromatography and gas chromatography–mass spectrometry: a case study of the Laizhou Bay Sag, Bohai Bay Basin, China

Laizhou Bay Sag in the south of Bohai Bay Basin is a petroliferous province and hot study area for oil explorers and geologists. Oil-source correlation is of great importance in the study of the petroleum system, for it contributes to the selection of favorable exploration sites. Previously, much attention has been paid to the study of the third member of the Shahejie Formation (E2s3 Fm.). The contribution of the fourth member of the Shahejie Formation (E2s4 Fm.) to the petroleum accumulation often failed to get adequate attention, which resulted in some difficulties in exploiting this area. In view of this, in our research, in the light of the theory of molecular organic geochemistry, the geochemical characteristics of 19 crude oil and 8 source rock samples (cuttings) from the study area are analyzed by means of gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS). On the basis of the analysis of oil composition and 16 biomarker parameters of saturated fractions, the following findings are generated: (a) the crude oil of Laizhou Bay Sag can be classified into three oil families: oil family A, oil family B, and oil family C. (b) the E2s4 source rocks were deposited in a low-alkalinity, hypersaline lacustrine environment under heavily reducing condition in which carbonates or evaporites were being precipitated and the organic matter is planktonic-organism-sourced (e.g., dinoflagellate-sourced); (c) oil family A, with medium maturity, is generated from the E2s3 FM. Oil family B, with low maturity, is generated from the E2s4 FM. Oil family C is the mixtures of oil family A and B oil. (d) A set of specific system has been established for the effective evaluation of the source and properties of the oil from the study area.