Geochemical Characteristics of Three Oil Families and Their Possible Source Rocks in the Sub-Sag A of Weixinan Depression, Beibuwan Basin, Offshore South China Sea

Three oil families from the sub-sag A of the Weixinan Depression are identified by integrated analysis of physical properties, stable carbon isotopes, and gas chromatography-mass spectrometry (GC-MS). Their similarities and differences in relative thermal maturities, depositional environments, and biological sources of organic matter (OM) are investigated. A possible oil-source correlation of this area is established. Group A1 oils, defined as low-maturity oils, are characterized by high density and high viscosity. They contain more terrigenous OM deposited in a freshwater environment with unstratified water columns reflected by a relatively high terrestrial/aquatic ratio and Pr/Ph values, low abundance of C₃₀ 4-methlysteranes, and low δ¹³C values. They are derived from the upper hydrocarbon supply combination. Group A2 oils are characterized by moderate density and viscosity and medium stable carbon isotope values. This group of oils has lower terrestrial/aquatic ratios and Pr/Ph values and a medium concentration of C₃₀ 4-methlysteranes and δ¹³C values, suggesting that the oils are derived from the shales which have more contribution from the algal input and are formed in a weakly oxidizing environment. They are a mixture generated from the source rocks in the middle and lower hydrocarbon supply combination. Groups A3 oils, defined as light oils, have low density and viscosity. The geochemical data of the A3 oils, including a less-negative stable carbon isotope, high abundance of C₃₀ 4-methylsteranes, low Pr/Ph values, and highest Ts/(Ts + Tm) ratios (Ts represents C₂₇ 18α(H)-22,29,30-trisnorneohopane and Tm represents C₂₇ 17α(H)-22,29,30-trisnorhopane), suggest that they are generated from the source rocks deposited in a subanoxic environment with the large input of dinoflagellates. The A3 oils are generated from the shales from the lower hydrocarbon supply combination. The oil-source correlation results can be further supported by the distribution of faults and structural ridge as the migration channel of petroleum developed around the sub-sag A.