Chemometric differentiation of oil families in the Mahu sag, Junggar Basin, NW China

Abstract

Most oil reservoirs that were found in the Junggar Basin are located in the Mahu sag and neighboring areas. Oil sources and classifications remain unresolved in this region. Oil source assessment can be partially inconsistent on the basis of different molecular and isotopic parameters. In the present study, classifications for the 92 studied oils from the Mahu sag and neighboring areas were performed using chemometric analysis, e.g., hierarchical cluster analysis (HCA) and principal component analysis (PCA) on the basis of integration of sixteen facies parameters. These parameters consist of isotope reversal index (RI), δ¹³C of n-C₂₅, Ph/n-C₁₈, β-carotane/n-C₂₁, six terpane ratios of Ts/C₂₃ tricyclic terpanes, Ts/(C₂₈+C₂₉ tricyclic terpanes), C₂₉ Ts/C₂₃ tricyclic terpanes, C₂₉Ts/(C₂₈+C₂₉ tricyclic terpanes), C₃₀ diahopane/C₂₃ tricyclic terpane and C₃₀ diahopane/(C₂₈+C₂₉ tricyclic terpanes), and six ratios of polynuclear aromatic hydrocarbons (PAH) including trimethylnaphthalenes (TMNs)/(TMNs + phenanthrene (Phen)), tetramethylnaphthalenes (TeMNs)/(TeMNs + Phen), TMNs/(TMNs + methylphenanthrenes (MPs)), TeMNs/(TeMNs + MPs), TMNs/(TMNs + chrysene (Ch)) and TeMNs/(TeMNs + Ch). These sixteen parameters are mainly influenced by source facies and less influenced by maturity as demonstrated in the crossplots of these sixteen parameters versus concentrations of C₃₀ hopane. Oil classifications are more reliable and convenient using chemometric analysis (HCA and PCA) integrating the sixteen facies parameters, compared with using crossplots of two parameters or star charts of several parameters. The 92 oils are classified into three groups using HCA and PCA, i.e., Group I, II and III. Group I and II oils are derived from source rocks within the Lower Permian Fengcheng Formation (P₁f) and Middle Permian Lower Wuerhe Formation (P₂w), respectively. Group III oils are mixtures of Group I and II oils. Group I consists of fifty oils mainly located at the northeastern and central areas of the Mahu sag with only three oils at the southwestern area of the Mahu sag. Group II consists of fourteen oils at the southwestern area of the Mahu sag. Group III consists of twenty-eight oils located at the southwestern and central areas of the Mahu sag. Locations of Group I, II and III oils reflect the distributions of effective source rocks containing oil-prone Type I/II kerogen within the Fengcheng (P₁f) and Lower Wuerhe formations (P₂w).