Petroleum Potential of Tertiary Coal-Bearing Heterolithic Successions Within the Sarawak Basin, Malaysia: Multiple Role as Source, Carrier, and Reservoir Rocks

Abdullah, Wan H.; Hakimi, Mohammed H.; Hassan, Meor Hakif A.; Kugler, Ralph L.; Shuib, Mustaffa K.; Ng, Tham F.
Geology, University of Malaya, Kuala Lumpur, Malaysia

The petroleum-producing Balingian Province is situated in northwest Borneo and is a sub basin of the greater Sarawak Basin, Malaysia. The province, straddling both onshore and offshore, consists of a thick clastic succession ranging in age from Oligocene to Plio-Pleistocene. Oil production is limited to the offshore areas but stratigraphic equivalents of the offshore producing sections are exposed onshore. The field observations and laboratory analyses of these onshore successions carried out in this study suggest an almost self contained petroleum system within these coal-bearing thick heterolithic clastic sections.

The onshore section is dominated by the Oligocene-Miocene Nyalau Formation. Within the Nyalau Formation fine-grained lithofacies occur in close association with heterolithic facies of interlayered muddy/sandy sediments within a tidally-influenced succession of delta plain deposits. Oil-prone coaly laminae and dispersed particles are common, particularly associated with carbonaceous shales or carbargillites. Terrigenous organic matter is observed to be not only restricted to the coaly and shaley sediments but is also widely distributed in coarser grained sediments including the sands and silts of the predominantly tidal-influenced heterolithic facies.

Petrographic and organic geochemical characteristics denote good petroleum source potential within these heterogenous facies association as suggested by reasonably high HI in the range of about 200-500mgHC/gTOC (in support of the predominance of type II & III kerogen), rich in extractable organic matter and Py-GC pyrograms that are dominated by n-alkene/alkane doublets.

Onshore field studies show these coaly heterolithic facies that developed within the lower coastal plain setting to be laterally extensive within the Sarawak Basin. Such sequences thicken up in the offshore where they provide an additional source bearing section in addition to the already proven thin (<1m) coal source rocks.

In terms of carrier and/or reservoir bed potential the extensive lateral extent of the heterolithic facies, characterised by close association of fine grained thin shaley intervals and coarser silty and sandy sections, provides an effective source-carrier-reservoir coupling with the necessary continuity preferred for hydrocarbon migration whilst thick shale beds act as the effective seal.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012