Application of SEM and Petrographic Studies to Reservoir Characterisation of the Eze-Aku Sandstone (Afikpo Sub-basin), Southeastern Nigeria

Abstract

The reservoir characteristics of the Pre-Santonian Eze-Aku sandstone wereassessed using an integrated thin section petrography and SEM Back-ScatteredElectron (BSE) imaging methods. Fresh outcrop data were collected in the Afikpoarea (SE Nigeria). Twenty-eight representative samples from the different localitieswere analysed to obtain mineralogical and petrographical datasets germane forreservoir characterisation. Thin section petrography indicates that the sandstones aremedium-grained, have an average Q90F10L0 modal composition, and are classified asmainly sub-arkose. The sandstones on SEM reveal the presence of cement in theform of quartz overgrowths, authigenic clays and feldspar. From epoxy-stained thinsections and/or SEM BSE image analysis, primary, secondary (inferred from partialto total feldspar dissolution/alteration), and micro- (in authigenic clays) porositytypes were identified. Although the development of quartz overgrowths, authigenicclay precipitation, and feldspar dissolution often affect reservoir properties ofsandstone negatively, the Eze-Aku sandstones still retained very good reservoirproperties (porosity, average 22.7 %; permeability, average 745.43 mD). This can beattributed to the abundance of framework grains that have been stabilized by quartzcement and relatively limited feldspar dissolution. The stabilized framework grainshelp preserve reservoir characteristics and resist further compaction beyondeogenetic depths. The feldspar dissolution produced silica and aluminium that wasprecipitated to form the quartz overgrowths and the authigenic clays (e.g., kaolinite),respectively. The relatively concurrent precipitation of silica and aluminium fromdissolution products reduces the quartz overgrowth quantity and authigenic clays thatcould have adversely affected the reservoir characteristics.