Abstract: Sterane and Triterpane Biomarker Characteristics from Oils and Sediment Extracts of the Middle-Late Miocene Sequences, Northern Sabah Basin, Malaysia

Azlina Anuar, Robert Kinghorn

Biomarker studies using oil samples and sediment extracts from wells located in the northern part of the Sabah Basin indicate the organic matter source for the basin's hydrocarbons to be dominantly terrigenous, indicated by high triterpane/sterane ratios and the presence of compounds diagnostic of land-derived organic matter such as oleanane and resins W, T, and R. The consistently similar fragmentograms suggest that the oils and sediment extracts originated either from the same source rock or from different source rocks with similar chemical compositions. Petrographic studies of kerogen concentrates confirm the dominance of land-plant contribution and the general absence of marine organic matter even in the marine sediments. For the oils, the biomarker maturity ratios were lower than their equilibrium values of 0.55 and 0.60 for the 20S/[20S+20R]-sterane and 22S/[22S+22R]-hopane, respectively. This supports the apparent retardation of the isomerization process previously reported to occur in Far Eastern Tertiary oils. The majority of the extracted sediments are shown to be immature by pyrolysis Tmax, vitrinite reflectance, and kerogen elemental composition, indicating that the extracted hydrocarbons have migrated into the sediments from elsewhere. Therefore, biomarker ratios from the extracts cannot be used to determine the true maturity levels of the sediments as the in situ biomarkers will have been mixed with those already present in the migrating hydrocarbons. The variations of biomarker maturity ratios with depth resemble those of the production index in all the studied wells, thus confirming the influence of the nonindigenous hydrocarbons on the biomarker maturity ratios.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994