ABSTRACT: Assessing the impact of reservoir architecture, faulting and karst on the performance of a Middle Miocene carbonate reservoir, offshore Sarawak using 3D modelling techniques

Vahrenkamp, Volker¹, Ryan Ross², Paul Crevello³, and Mark Sams⁴
(1) Sarawak Shell Bhd, Miri, Malaysia
(2) Sarawak Shell Bhd, N/A
(3) Petroleum Geoscience Consulting, Bandar, SB, BS 8675, Brunei
(4) Jason Geosystems Asia, Singapore

The Jintan reservoir of the Luconia Province, offshore Sarawak is a broad termination on top of a thick Middle Miocene carbonate platform (30x50x1.2km). It formed during a series of third-order sealevel back-steps (TB2.3-2.6) which led to its demise at the end of a second order sealevel fall (TB2). Reservoir thickness (<100 m) is thin compared to an estimated aquifer thickness of more than 1km. Aquifer movement is key for predicting hydrocarbon recovery and field development.

Three-D seismic reveals bank margin collapse and spectacular karst dissolution and collapse features aligned with a deep-seated fault system, which periodically became activated during carbonate growth. A major area of syndepostional karst collapse was subsequently filled by prograding reef-margin sequences. Faulting and karstification has disturbed a 30 m thick transgressive layer which has extremely low porosity and plays a key role in determining the aquifer strength felt in the reservoir.

Anchored by 3 wells a seismic inversion technique was used to construct 3D computerized porosity models in depth. These models were conditioned by seismic horizons, fault polygons and core/log derived permeability and saturation. Analyses using 3D seismic as backdrop in the reservoir modelling stage reveals the preservation of even intricate karst features in the 3D static porosity and permeability models. Seamless upscaling of the detailed static model combined with multiple scenario variation of the geological and petrophysical uncertainty allows during subsequent dynamic reservoir simulation the assessment of the uncertainties critical for reservoir development.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia