Impact of Bioturbation on reservoir quality: A case study of biogenically reduced permeabilities in reservoir sandstones of the Baram Delta, offshore Sarawak.

Baram Delta is one of seven geologic provinces in the Sarawak basin and is the most prolific. Bioturbation is an important source of reservoir heterogeneity and has an impact on porosity and permeability of reservoir sandstones. There is still a lack of knowledge on the impact of bioturbation on porosity-permeability characteristics of reservoir rocks in the Baram Delta therefore this study is aimed at evaluating the impact of bioturbation on porosity and permeability of reservoir rocks of the Baram Delta. Reservoir sandstones were analyzed using thin sections, spot permeability, scanning electron microscopy and energy dispersive x-ray. Sample, S1, from the studied core interval is dominated by Diplocraterion ichnofabric. S1 is highly to intensely bioturbated with bioturbation index between 60-99%. Spot permeability values range between 158-381mD in the host sediment and 33.8-176mD in the burrow and burrow lining respectively. This represents a permeability decrease of 78% between host sandstone and burrow. Sediment packing activity was observed in S1. Sediment packers incorporate iron oxides, clays and organic matter from the host sandstone into burrow fills and linings, thereby decreasing isotropy and sorting of the sediments resulting in a reduction of porosity and permeability locally in the burrow relative to the host sandstone.