Compositional variabilities among crude oils from the southwestern part of the Qaidam Basin, NW China

A suite of crude oils have been analyzed by GC/MS in order to understand compositional heterogeneities in the oilfields located in the southwestern part of the Qaidam Basin, NW China. The oil samples investigated in this study can be grouped into two broad groups, A and B, based on the distributions of pentacyclic triterpanes. Group A oils predominately occurred in the northwestern part of the study area contain relatively high amounts of gammacerane and C35 hopanes with the ratio of gammacerane to C30 hopane (G/H) > 0.7 and the C35/C34 hopane ratio (C35/C34) > 1.2. In contrast, Group B oils mainly occurred in the southeastern part of the study area have relatively low values of G/H (~ < 0.7) and C35/C34 (< 1.2). Furthermore, Group A oils can be subdivided into three subgroups (A1, A2, and A3) by detailed investigation of molecular compositions of steranes, hopanes and aromatic sulfur compounds. Subgroup A1 oils, which mainly occurred in the westernmost corner, contain low amounts of dibenzothiophene (DBT) and methyldibenzothiophenes (MDBT) relative to C30 hopane with the ratio of (DBT + MDBT)/C30 hopane (DBT + MDBT/H) < ~ 0.25 and display high abundances of ααα20R C28 sterane relative to C29 compound with the C28/C29 sterane ratio > ~ 0.90. In contrast, subgroup A3 oils, which mainly occurred in depression areas, have relatively high values of (DBT + MDBT)/H (~ > 0.25) and relatively low ratios of C28/C29 sterane (~ < 0.90). Subgroup A2 oils, occurring in Gasikule and nearby oilfields, seem to have intermediate amounts of aromatic sulfur compounds and C28 steranes relative to A1 and A3 oils, indicating a mixing signature of the two subgroups. The oil groups or subgroups revealed by the compositional heterogeneities and genetic affinities, as well as their regular occurrence in different oilfields, may indicate secondary petroleum systems existing within the Tertiary saline lacustrine petroleum in the southwestern Qaidam basin.