Reservoir Characterization of Low Permeability Lower Shoreface Facies in the Baram Delta Province, Brunei Darussalam

Guilpain, Pierre¹, Howard D. Johnson², Abdul Razak Damit³ (1) Norsk Hydro, Bergen, Norway (2) Imperial College London, United Kingdom (3) Brunei Shell Petroleum,

Brunei Darussalam (NW Borneo) contains the prolific hydrocarbon province of the paleo-Baram and paleo-Champion deltas. The onshore Baram Delta Province provides spectacular outcrop examples of Middle-Late Miocene lower shoreface successions from the Belait Formation.

Lower shoreface deposits outcropping in onshore Brunei Darussalam include sharp-based tabular sandstone beds, interbedded shale layers and erosional scour features such as sandstone-filled gutter casts. Gutter-cast sandbodies are either attached to the base of an overlying tabular sandstone bed, or isolated within shale. Interpreted shoreface environments range from distal to proximal lower shoreface (i.e. between fairweather- and storm-wave bases), with shallower environments characterized by higher sandstone proportions.

Dimensions of more than a hundred gutter-cast sandbodies have been measured from exposures along the Berakas Syncline, onshore Brunei. The sandbodies display dm-scale thicknesses and dm- to m-scale widths. These dimensions are larger than those noted in published datasets for gutter casts measured in other areas. The internal architectures of the gutter-cast sandbodies show evidence for storm processes as the main control on formation and infilling of the gutter casts. The gutter casts formed by localized erosion of the mud substrate and were later filled laminasets that represent several successive storm events.

Sandbody connectivity within thin-bedded successions in onshore Brunei is strongly influenced by the presence of gutter-cast sandbodies. These features are also likely to control reservoir architecture in lower shoreface intervals of shallow-marine reservoirs, offshore Brunei. Incorporating their effects into reservoir characterization and modelling workflows requires quantitative databases of gutter-cast sandbody dimensions, such as that presented here.