Abstract

A paradox is a condition with seemingly contradictory qualities. Such a condition appears to exist in the organic geochemical data from Central Sumatra. Crude oil data, including biomarker distributions and whole-oil carbon isotopic composition, have been interpreted to suggest the presence of four distinct crude oil families, each having a distinct source. In contrast, source rock data such as organic richness, pyrolytic yield, kerogen type, and level of thermal maturity have indicated that only one effective oil source exists within the region, the Pematang Brown Shale. Therein lies the paradox, only one source unit but four oil families.

Numerous investigators have attempted to explain this apparent contradiction by suggesting that the oils may be partially sourced from regionally well-developed coals and/or unidentified source rock units within the region. Recent studies on other lacustrine source rock systems suggest that facies variations within the Brown Shale itself could explain the observed differences in the geochemical attributes of the crude oils. Climatically induced changes in temperature and lake level, as have been observed in the tropical regions of Africa during the Quaternary, would be sufficient to alter the nature of the organic input and water chemistry. These climatic changes appear to strongly influence the stratigraphic distribution of oilprone source material in the Pematang Brown Shale. The differences in the oils, however, appear to reflect the different depositional and environmental histories of the various individual Pematang lakes.

Therefore, it may be concluded that the four crude oil families which exist within the region could have been derived from a "single" source, the Pematang Brown Shale. Each family represents a slightly different organic facies within the Brown Shale. These facies probably represent deposition within different sub-basins with different water chemistries.