Exploration history and petroleum systems of the onshore Baram Delta, northern Sarawak, Malaysia

The onshore portion of Baram Delta petroleum province in northern Sarawak is largely covered by the Block SK333 exploration permit, most recently operated by JX Nippon. It contains a complete sedimentary succession ranging in age from Mid Eocene to Holocene. A sequence-stratigraphic investigation of the area, based on 2009 2D seismic, integrated with recent biostratigraphic analyses conducted in 2010-2011, suggests that the sedimentary section has been affected by three major episodes of deformation which are: (1) Late Cretaceous to Eocene (79.5-36Ma) block faulting, (2) Late Oligocene to Mid Miocene (30-20.5Ma) wrench movement and related folding, followed by (3) Mid Pliocene to Holocene (4.0-0Ma) uplift and compressional folding. These tectonic episodes have resulted in a subdivision of the Block SK333 area into two major anticlinal trends: the Engkabang-Karap Anticline in the south, and separated by the large Badas Syncline, the northern Miri-Asam Paya Anticline.This configuration resulted in two distinct petroleum systems and respective hydrocarbon zones: (i) A southern overmature gas system sourced probably from deeply buried and carbonaceous Eo-Oligocene basinal shales containing reworked terrestrial organic matter, which charged wrench induced traps such as at the Engkabang-Karap Anticline that were later overprinted by compressional folding. The surface expression of this petroleum system is manifested by an active mud volcano on the western Engkabang-Karap Anticline axis, which emits thermogenic C1 gas. Burial history modelling indicates that an earlier oil charge probably occurred during deep Oligocene burial, preceding basin reversal during the Pliocene-Holocene inversion episode, with the wrench-induced anticlinal closure which subsequently has been charged by late gas. (ii) A block-wide oil and gas system sourced from peak mature Mid-Late Miocene carbonaceous shales and coals in the synclines, charging inversion and compressional fold structures along the northern Miri-Asam Paya anticlinal trend, and also the Miocene section at the Engkabang-Karap Anticline. Expulsion and charge to traps commenced during the Late Miocene and is continuing to the present-day. Although the exploration results of the southern Eo-Oligocene carbonate play have been disappointing to date, the onshore Baram Delta still contains a number of attractive, both untested and partially tested plays that are yet to be fully explored. Lowstand delta and turbidite plays, a highstand delta shoreface play in the Miri-Asam Paya anticlinal area and a moundform stratigraphic play in the southern limb of the Badas Syncline are among the untested play identified in the study area.