A new strategy to study geochemical characteristics and migration of crude oils in Zhu I Depression, Pearl River Mouth Basin, South China Sea

Abstract

Oxygen- and sulfur-containing compounds were proved to be good alternatives to pyrrolic nitrogen compounds in the study of oil migration. To promote the detectability of complex and trace analytes, a simple and efficient device, gas purge microsyringe extraction (GP-MSE), was developed and combined with comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry (GC × GC-TOFMS) for the simultaneous analysis of target compounds. Oil samples were collected from the Zhu I Depression of the Pearl River Mouth Basin in the South China Sea and classified into three groups in terms of geochemical distribution characteristics, parameters and concentrations of carbon isotopes, alkanes, diamondoids, terpanes and steranes. Based on nitrogen-, sulfur- and oxygen-containing compounds, the general oil filling into the Zhu Ⅰ Depression was determined from the central sags southward to the Dongsha Uplift. Faults provide pathways for vertical migration and delta sandstones served as the main lateral and long-distance regional migration pathways. The molecular dynamics (MD) simulation provides a nanoscale perspective into geochromatographic fractionation, which is generated mainly by the interaction between polar compounds and clay minerals. In MD simulations, the binding affinity of carbazole, dibenzothiophene and dibenzofuran on clay minerals is higher than that on non-clay minerals due to the available active sites on the surface of clay minerals for the formation of electrostatic and van der Waals interaction. Carbazole, dibenzothiophene and dibenzofuran achieve adsorption stability in a parallel manner on chlorite and kaolinite, whereas on illite, adsorption stability is achieved in tilted manner. Based on the adsorption energies, kaolinite shows the highest adsorption affinity, whereas calcite exhibits the lowest adsorption affinity. Microcline even demonstrates a noticeable repulsive effect. The geochromatographic fractionation effect seems to be not controlled only by the affinity between oil composition and clay mineral during oil migration.