Brunei Darussalam: Characteristics of selected petroleums and source rocks

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The development of three Tertiary deltaic complexes has resulted in the deposition of up to 10 km of sandstones and shales comprising the sources and reservoirs for crude oils that occur onshore, near-offshore and, with future exploration efforts, those likely to be encountered in deepwater reservoirs north of the Brunei coastline. We examined a series of offshore oils and onshore rock samples in Brunei Darussalam (a) to delineate oil family groups and their source rock characteristics, and (b) to assess the source potential of the sedimentary sequence with respect to lithology and depositional setting. Twelve offshore oils and 53 shales, coaly shales and coals were examined. The oils contain indicators of allochthonous (e.g. bicadinanes, oleananes) and autochthonous (e.g. cholestanes and methylcholestanes) components in the source organic matter. Predictable geographic variations of this mixed input are clearly evident in the sample set (e.g. allochthonous input appears to increase in offshore Brunei to the northeast). Although this molecular source signature is relatively clear, migration of these oils from deep (and unidentified) source rocks has resulted in extensive migration-contamination with respect to the tetracyclic and pentacyclic hydrocarbons. This contamination has resulted in strong correlations between certain molecular maturity indicators and the present-day temperature of the reservoirs. Liquid hydrocarbon source rock potential is present in the tidal and coastal embayment facies, and is greatest in the Miocene coals. Neither the shales nor coaly shales contain significant oil generative potential. The thermal immaturity of the sample set precludes valid oil–source rock correlations without conducting artificial maturation experiments on the coals.