Assessing source and maturity of oils in the Mahu sag, Junggar Basin: Molecular concentrations, compositions and carbon isotopes

Abstract

Molecular parameters are both facies and maturity dependent. Source and maturity assessment remains a difficult task for oils which were derived from multiple source rocks at a wide range of maturities from the early to late oil generation window. In this paper, a new approach was presented to assess oil source on the basis of molecular facies parameters under maturity constraint using crossplots of one source parameter versus one maturity parameter. Concentrations of ΣC29 steranes and C30 hopane were tentatively used as maturity indicators. Quantitative GC, GC-MS and GC-IRMS analyses were performed on 52 oils from the southern and central Mahu sag of northwestern Junggar Basin, NW China. The 52 oils are first classified into three groups, i.e., groups I, II and III which were derived from source beds within the Lower Permian Fengcheng Formation (P1f), Middle Permian Lower Wuerhe Formation (P2w) and both, respectively on the basis of carbon isotopes of individual n-alkanes in combination with reservoir geological settings. These three oil groups, in particular groups I and II from different sources were further distinguished sufficiently using crossplots of ratios of pristane (Pr)/n-C17 and phytane (Ph)/n-C18, β-carotane/n-C21 and concentrations of n-alkanes, Pr, Ph, β-carotane, Ts, C29 Ts, C30 diahopane, C20, C21 and C23 tricyclic terpanes versus concentrations of ΣC29 steranes and C30 hopane, respectively. Variation trends of isotopic reversal index (RI) and δ13C value of n-C25, the lowest among those of n-alkanes (n-C12–34), with decreasing concentrations of ΣC29 steranes and C30 hopane demonstrate that carbon isotopes of n-alkanes were unlikely influenced significantly by oil maturities within oil generation window. Concentration of n-alkanes increases while concentrations of Pr, Ph, crotanes, terpanes and steranes generally decrease with decreasing concentrations of ΣC29 steranes and C30 hopane. Application of this approach provides numerous choices and opportunities to distinguish oil groups which were derived from different sources in combination with isotopic data and reservoir geological settings.