The digital rock analysis of biogenically induced reservoir heterogeneities in Cretaceous reservoirs of Saudi Arabia

Abstract

The characterization of the fluid-flow properties in biogenically altered formations is a key for successful exploration campaigns. This study assessed reservoir quality and evaluated permeability of the biogenically-modified Cretaceous carbonate section in Saudi Arabia. When dealing with these bioturbated carbonates, characterization of sedimentary heterogeneities is often overprinted by the complex spatial geometries of burrows. The high-resolution 3D X-ray microscopy imaging and analysis of these bioturbated sections lead to a better understanding of the interconnectivity between permeable burrows and tight matrix. The analysis deploys multiscale imaging from whole core to sub-micron scale. The coarse imaging at 20-50 microns resolution helped to identify bioturbated sections to select samples for higher resolution tomography. The 3D sample tomograms were segmented to define burrows and matrix distributions, where samples were extracted for thin-sections, scanning electronic microscopy (SEM) and mercury injection capillary pressure (MICP) analyses to refine the pore sizes and rock types. The analysis showed an intricate, highly connected, mixed horizontal and inclined burrow system dominated by Thalassinoides. Intergranular porosity, associated with the fill of Thalassinoides, constitutes a mechanism for permeability enhancement in a tight matrix. Increased permeability is associated with higher dolomite content that might be used as a sweet spot identifier from wireline logs.