Oil origin and migration direction in Wushi sag, Beibuwan Basin, South China Sea

Abstract

The Wushi sag is one of the most important petroleum-bearing units located in the Beibuwan Basin, South China Sea. Insufficient understanding of oil origin and migration direction, and lack of characterizing oil differential accumulation distribution patterns in Wushi sag greatly restrict further hydrocarbon exploration. In this study, oil-source rock genetic relationships and migration directions were analyzed in detail by combining geochemical analysis of 45 source rock samples, 23 crude oil samples, 12 oil sandstone samples, and basin modeling results. The geochemical data show there is distinguishable heterogeneity in Liushagang Formation (Fm) source rock in Wushi sag, which is divided into two types. The first type (including the Member (Mbr) 23 and Mbr 3 of Liushagang Fm source rocks) was deposited in a weak reducing to suboxic and fresh to brackish water environment with a predominance of aquatic algal plankton input. The second type (including the Mbr 1 and Mbr 21 of Liushagang Fm source rocks) was deposited in a suboxic and fresh to brackish water environment with mixed inputs of aquatic algal plankton and terrestrial higher plants. Three groups of crude oils (A, B, and C) are identified using their physical properties and biomarker data. The group A oil is characterized by a higher abundance of oleanane, lower abundance of C30 4-methylsterane, higher values of Pr/Ph, and lighter δ13C values, and is mainly derived from the Mbr 21 of Liushagang Fm source rock. Group B and group C oils are characterized by a lower abundance of oleanane and higher abundance of C30 4-methylsterane, but present some differences in Pr/Ph and δ13C values. Both groups of oils are sourced from the Mbr 23 of Liushagang Fm source rock, and their differences in geochemical characteristics indicate the lateral heterogeneity of source rock. Oil migration directions are determined by combining maturity parameters, pyrrolic nitrogen compounds, and also basin modeling results. The preferential oil migration directions in Wushi sag are from the central to the northeast and south of the sag. The accumulation distribution of different groups of oils is mainly controlled by the vertical and lateral heterogeneity of Mbr 2 of Liushagang Fm source rock.