Ichnofacies and sedimentary structures: A passive relationship with permeability of a sandstone reservoir from NW Borneo

Abstract

Permeability network is influenced by ichnofacies in the surface and subsurface sedimentary media, which modifies textural heterogeneity of reservoir rocks by the filling of burrows with the surrounding sediments. In situ measurements of rock permeability from surface analogues of proven subsurface hydrocarbon traps demonstrate the influences of ichnofacies in primary sedimentary structures and resultant high variability within reservoir zones. This work emphasizes on the evaluation of three ichnofacies (i.e., Psilonichnus, Skolithos and Cruziana) that are interdependent and characterized by dense ichnofabrics with sedimentary structures in the exposed rocks of Sandakan Formation. The sedimentary structures associated with ichnofacies are parallel to subparallel, multidirectional trough cross-stratification with current cross laminae and low-angle, undulatory, hummocky cross-stratification (HCS) and oscillation ripple laminated sand with the alteration of silty or sandy mudstones. Results show that the Sandakan Formation was deposited in a transitional environment from a back to foreshore setting and extends to very distal fringes of a proximal lower shoreface or distal delta front to pro-delta. The effective permeability network in the bioturbated horizons of sandstone and interbedded mudstone is very high (250–950 mD). The integration of ichnofacies and sedimentary structures with high permeability profile suggests that the Sandakan Formation has good reservoir potential with the textural heterogeneity. The implication is that the presence of ichnofacies in sedimentary reservoir horizons can substantially increase secondary porosity and enhance pore connectivity; thus, making them valuable and lucrative subsurface hydrocarbon reservoirs.