Abstract

Deposition and preservation of organic matter in the offshore NW Sabah Basin occurred in three broad palaeoenvironments: 1) coastal-lower coastal plain environments comprising inter-channel swamps, back-mangrove swamps, lagoonal and inter-distributary areas, 2) fluviomarine environments, and 3) continental shelf-deep marine areas. Success in the preservation of organic matter in the NW Sabah Basin seems to be related to the high primary terrigenous productivity, high sediment accumulation rates and the resistant nature of the Type III higher land plant waxes to oxidation and biodegradation. Interestingly, as much of the Sabah shelf tended to be oxic during Middle Miocene, it appears that anoxic depositional conditions were not essential for organic matter preservation.

Although the source rocks were deposited in three different depositional environments, the oils discovered in each of these settings bear unmistakably similar biomarker fingerprints. They are all characterised by a large oleanane peak, considerable amounts of bicadinanes, and C₂₄-tetracyclic terpane. However, only a small portion of the potential source extracts correlates positively with the Sabah oils. The rather arduous correlation of the oils with potential source extracts suggests the likelihood of a different organic matter assemblage in the kitchen areas compared to those in the penetrated shale sequence.