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Deepwater Hydrocarbon Charge Systems of Offshore NW Sabah, Malaysia

Abstract

The deep offshore of NW Sabah contains a petroleum system that is, in terms of depositional environment, ‘deepwater’ in every sense of the word. Not only is the location in deepwater at present day, but all elements of the petroleum system - source rock, reservoir, seal, overburden - are deepwater deposits. The primary play type is turbidite sands entombed in pelagic shales, structurally trapped in fold and thrust belt anticlines. This play has been proven successful with a number of oil and gas discoveries during the past decade and with exploration still actively chasing this play type. In such a play, it would seem intuitively obvious that the entombing marine shales would represent the main source rock facies. However, this has been shown to be not the case. This paper describes and discusses the charge system that drives this deepwater play. The oil geochemistry itself gives the first indication that marine shales are not the main source facies. The biomarker distribution of the oils are characterised by the presence of higher plant derived biomarkers such as 18(ğ)-oleanane, bicadinanes, a sterane distribution dominated by the C29 homologues and high Pr/Ph ratios. Marine biomarkers such as C30 steranes are, however, not completely absent, suggesting a minor marine organic matter contribution. Source rock studies have been carried out to evaluate and characterise the likely source rock facies of the Sabah oils. This work has focused not only on the penetrated rock sections (which invariably only penetrated immature and/or apparently non-source facies), but also on extensive analogue studies of deepwater successions (albeit older, less confined, and thermally more mature) that outcrop onshore Sabah. The geochemical and organic petrographic work by the current authors has indicated the occurrence of organic-rich slumps and debris flows which possess oil-prone Type III/II kerogen. These deposits are primarily observed in the onshore analogues and are yet to be encountered in drilled sections of the subsurface intervals. Basin modelling studies suggest that the effective source of the oils lies within the deepwater sequences beyond the total depths of the wells drilled to date. And so, as an ironic proviso to the opening remark that all the elements of the petroleum system are deepwater deposits, the actual organic matter that started it all is distinctly non-marine.