Abstract
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 C 35 hopanes with the ratio of gammacerane to C 30 hopane (G/H) > 0.7 and the C 35/C 34 hopane ratio (C 35/C 34) > 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 C 35/C 34 (< 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 C 30 hopane with the ratio of (DBT + MDBT)/C 30 hopane (DBT + MDBT/H) < ~ 0.25 and display high abundances of ααα20R C 28 sterane relative to C 29 compound with the C 28/C 29 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 C 28/C 29 sterane (~ < 0.90). Subgroup A2 oils, occurring in Gasikule and nearby oilfields, seem to have intermediate amounts of aromatic sulfur compounds and C 28 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.
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