Characteristics and significance of the Tertiary oils in the Wangjiagang oilfi eld in the South Slope of the Dongying Depression,Bohai Bay Basin

Detail investigation of the chemical compositions for the oils fro m Shahejie Formation of Tertiary in the Wangjiagang Oilfield in the lower position of the South Slope of the Niuzhuang Sag, the Bohai Bay Basin show similarity of the oils with those of the oils in the Bamianhe Oilfield in the higher position of the slope. The oils bear common characteristics of Tertiary brackish lacustr ine oils in eastern China, including high concentration of steranes, gammacerane and methyltrimethyl tridecylchromans (MTTCs), Pr/Ph 1, “V”shape of C27, C28 , C29-αααsterane and low isomerization of steroids (the levels of C29 steran e ααα20S/(S+R) ranging in 0.30～0.35), etc. The similar characteristics of t he oils in the two oilfields suggest that their genetic kinetics are coincident and they originated from the same set of source rock. Particularly, the results suggest that it is likely possible that the oils of the Bamianhe oilfield are mo stly derived from the Niuzhuang Sag rather than only contributed by the immature source rock in the South Slope of the Niuzhaung Sag. Oils of Kongdian Formation in Dingjiawuzi of the Wangjiagang region have features of both Es4 rock extract s and related Shahejie oils, i.e. high content of gammacerane and Ek rocks extra cts such as obvious predominance of C29-regular sterane among C27, C28, C29 reg ular steranes and high content of dibenzofurane, suggesting mixed resources of K ongdian oils. Much higher mature maturity of Kongdian oils than Shahejie ones sh ows the former originated from deep Es4 and Ek and are of good petroleum potenti al in deep position in the area studied. It is suggested that mixing of oils wit h different maturity and/or different source rocks during oil migration and accu mulation is probably a common phenomenon in composite basins.