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Integrated Reservoir Quality Study of Deep-Water Clastic Deposits, Offshore Niger Delta

Abstract

The study aims at accessing the reservoir quality and heterogeneity in a selected Deep-water wells by objectively evaluating the reservoir depositional facies, environment of deposition and sandbody geometries, stratigraphic and structural positions, detrital composition, sediment source, pore architecture and pore-throat characteristics. Cores, thin-sections and SEM photomicrographs, XRD data, porosity and permeability data, and well log (gamma ray, sonic, resistivity and density) data were integrated at reservoir scale. Seven deep-water depositional facies (Coarse to pebbly, massive, poorly sorted, matrix supported conglomeratic facies {ScM}; medium, moderately to well sorted, normally graded facies {SmP}; fine to very fine, planar laminated, moderately to well sorted facies {SfP}; fine to very fine, cross, wavy to ripple laminated, well sorted facies {SfC}; sand-rich heterolithics with fine to silty mud intercalations and slump facies {HsP}; mud-rich heterolithics with diffuse parallel laminated silt, rip-up clasts, contorted and convolute bedding facies {HmP} and massive, laminated and brecciated mudstone facies {M}) were described from the cores using lithofacies scheme of “Davies et al 1997” integrated with “Bouma Sequences” and the Shell deepwater “Grand Unified Slope Theory”. These were grouped into four facies associations (coarse debris flow, sandy debris flow, mixed sand / mud slides & slumps and hemipelagic mud. The thin-section photomicrographs show that the studied reservoirs have undergone minor to insignificant mechanical compaction and intergranular pressure dissolution; have open pores that are well interconnected, loose to point-grain contacts; have suffered suppressed cementation by quartz overgrowths, replacive pyrite, siderite, kaolinite and replacive authigenic opaques. The framework mineralogy is dominated by monocrystalline quartz arenite as interpreted from modal analysis. SEM photomicrographs show dissolution pits of framework feldspars, presence of inter- / intra-granular macropores and evidence of leaching by dissolution and kaolinization. Whole rock mineralogy (XRD) indicates that quartz, alkali feldspar, kaolinite, plagioclase feldspar, illite-smectite, pyrite, siderite and barite are the major minerals. The results of this study are expected to have important implication for ranking of hydrocarbon exploration prospects and optimisation of future development well locations in deep-water settings.