Isotopic and molecular characteristics of Miocene-reservoired oils of the Kutei Basin, Indonesia

Thirty-two oils reservoired in Miocene sands of the Kutei Basin of Indonesia have been examined isotopically and molecularly for the purposes of determining source-defined families and estimating the depositional and compositional characteristics of their source rocks. The oil set is geographically distributed within, north, east, and south of the Mahakam River delta, and comprises oils from wells drilled onshore, on the continental shelf and on the continental slope. Isotopic data discriminate a single megafamily of oils dominated by angiosperm debris from two outliers showing evidence for algal lacustrine input to their source rock(s). The megafamily is separable into two sub-families according to the relative concentrations of oleananes, bicadinanes, taraxastane and, especially, certain norlupanes and bisnorlupanes. This is the first observation of widespread norlupane and bisnorlupane occurrence in a low-latitude oil set. The lupanoid ratio [(24,28-bisnorlupanes + 24-norlupane)/hopane] distinguishes the two Kutei Basin sub-families best, distinctly separating onshore and continental shelf oils (low lupanoid ratio) from continental slope oils (high lupanoid ratio). Although this chemical distinction between Onshore/Shelf and Slope oils is likely to be source-defined, migration-induced compositional changes also appear to play a role. Molecular maturity ratios are inversely proportional to the relative concentrations of various oleanenes and bicadinenes in this oil set, and suggest that migration-contamination has compromised maturity assessment of these oils.