Porosity and permeability of Mauddud reservoirs in Raudhatain and Sabiriyah oil fields, Kuwait: Interpretation of controlling processes

ABSTRACT The middle Cretaceous (Albian) Mauddud Formation is a complexly layered limestone reservoir of approximately 117-m thickness in the Raudhatain and Sabiriyah fields. Stratigraphic trends in reservoir quality are strongly related to smaller (fourth-order) depositional sequences, and there is a large-scale upward-increase in quality through the third-order highstand systems tract that makes up most of the formation. Depositional environments and textures show only subtle correlation with porosity and permeability within the proximal, inner-ramp facies, but the more distal, muddier deposits have distinctly lower reservoir quality. Petrography reveals overall volumetric dominance of micropores over macropores. Karst surfaces are not observed in the studied cores, but meteoric water may well have been important during early diagenesis. Heterogeneity of reservoir quality results in large degree from heavily calcite-cemented nodules and thicker cemented zones, with the heavily cemented masses distinguished from surrounding porous limestone by the absence of oil staining. The nodular cementation was probably localized by burrowing and was partly supplied by early dissolution of locally concentrated aragonitic green algae. Field-wide compilations of core and log data reveal trends of upward porosity increase from the flanks to the crest of each field that are similar to the porosity trends of giant domal oil fields in Abu Dhabi and the North Sea, where late calcite cementation was inhibited by oil accumulation. The maximum crest-flank porosity difference due to late cementation is approximately 10 vol. %. This study was performed to provide input for building reservoir models, but has broader significance for understanding the impact on reservoir quality of depositional environments, cemented nodules, sequence stratigraphy, and the preservation of crestal porosity by oil in many other microporous limestone reservoirs.