Geochemical behavior of aliphatic hydrocarbons during primary migration and implication for hydrocarbon expulsion: A case study in Qikou Sag, Bohai Bay Basin, eastern China

Abstract

Lipid biomarker parameters and molecular carbon isotope compositions of n-alkanes were used as tracers to determine geochemical behavior of aliphatic hydrocarbons during their migration from two sequences of source rock-type mudstones (4050.8–4062.2 and 4067.4–4073 m depth, respectively) to the interbedded reservoir sandstones (4062.6–4066.3 m depth) in the Qikou Sag, Bohai Bay Basin, eastern China. Abnormally high values of the production index (0.69–0.90) and total extract/total organic carbon (TOC) ratios (0.53–1.21) in the sandstone samples indicated that the hydrocarbons in the sandstones were not produced in situ but migrated from the adjoining mudstones. A comparison of values in lipid biomarker parameters (oleanane index, gammacerane index, C23 tricyclic terpane/C30 αβ-hopane, and C29/C27 ααα20R sterane ratios) in the sandstone with the mudstone samples indicated steroids and hopanoids in the upper section of the sandstone layer (4062.6–4065.2 m depth) were transported from the overlying mudstones, while those in the lower sandstone layer (4066.3 m depth) were transported from the underlying mudstones. Molecular carbon isotope data suggest that all n-alkanes in the upper section of the sandstone layer were transported from the overlying mudstones. However, in the lower sandstone layer, most of the C15–C17n-alkanes were transported from the overlying mudstones, while the C18–C31n-alkanes were transported mainly from the underlying mudstones. These combined results suggest the short-chain n-alkanes (<C17) were expelled preferentially relative to other compounds (probably via molecule diffusion) while the long-chain n-alkanes (>C18) and steroids/hopanoids may have been expelled in a separate oil phase.