Lithological typing, depositional environment, and reservoir quality characterization of the “M-Field,” offshore Douala Basin, Cameroon

Abstract

Sedimentary features such as channel sands and submarine fan sequences are important hydrocarbon reservoirs throughout the world and may be interpreted from bed thickness trends and log motifs. The log motifs of four well logs were employed during this study to delineate reservoir architectural depositional features and environment, determine their lithological attributes, and evaluate reservoir properties. Gamma ray log signatures were utilized to infer depositional features, discriminate litho-units, and define depositional environment; resistivity log was employed to identify reservoir zones and formation fluids, while the combined density and neutron logs aided in the estimation of reservoir porosity and lithological typing. Results of analysis show seven laterally discontinuous reservoirs: one in Well X (X2), three in Well Y (Y1, Y2, Y3), and three in Well W (W1, W2, and W3) with thicknesses of 6.2 m, 19.2 m, 7.6 m, 78.7 m, 100.9 m, 38.1 m, and 41.6 m, respectively. Reservoirs architecture reveal distributary channel sands, submarine fan lobes, and deep tidal channel-fills deposited within the deep marine mud-rich N’kapa Formation. Average porosity, water saturation, and hydrocarbon saturation values of Well X and Well Y reservoirs are 20.8%, 30.8%, 69.2%, and 40.2%, 18.3%, 81.7%, respectively, indicating moderate-to-good reservoir quality. Also, the permeability values are moderate to good (74.89–12,584 md), except for Y3 and Well W reservoirs whose well log-derived permeabilities were unreliable due to their non-irreducible nature. Density-neutron crossplots indicate heterogeneous reservoir lithologies comprising of limestone, dolomite, and sandstones, with predominant calcite and subordinate dolomite as mineral matrix. Crossplot of water saturation and porosity revealed grain-size variation ranging from fine-grained to silty sediments with bulk volume water values suggesting Well X and Y reservoirs to be homogeneous and will produce water-free hydrocarbons, while Well W reservoirs are heterogeneous water-prone reservoirs.