Sedimentology, diagenesis, and reservoir quality assessment of the Upper Cretaceous sedimentary succession (Anambra Basin) in Inyi and environs, southeastern Nigeria

The Upper Cretaceous sedimentary facies in areas around Inyi and environs, southeastern Nigeria comprises Ajali and Nsukka formations. A total of 19 outcrops were mapped and logged. Interpretation was based on field facies analysis and laboratory tests which include granulometric and X-ray diffraction (XRD) analyses of the rock samples. The objective of the study was to reconstruct the environment of deposition as well as understand the textural attributes of the sand size sediments for reservoir quality assessment. Eleven lithofacies: conglomeritic sandstone facies (Gc), cross-bedded sandstone facies (Scb), pebbly sandstone facies (Sp), bioturbated sandstone facies (Sb), heterolithic facies (Sht), claystone facies (Cl), coal facies (Co), bioturbated shale facies (Shb), laminated siltstone facies (Stl), muddy sandstone facies (Sm), and black carbonaceous shale facies (Shcb), were identified with four sub-facies occurring within low- to high-energy environment. These lithofacies were grouped into four facies association (FA1, FA2, FA3, and FA4). FA1 is dominantly conglomerates and pebbly sandstone deposited as channel/alluvial fan deposits. FA2 are deposits in a swamp or lagoon setting consisting of coal and black carbonaceous shale. FA3 is characterized by bioturbated sandstone, muddy sandstone, and heteroliths deposited in a lower shoreface setting. The cross-bedded sandstones and the heterolithic facies belong to the FA4 deposited in an upper shoreface setting. Sieve analysis results show sediments that are medium to coarse grained in sizes, poorly to moderately sorted, dominantly very negatively skewed to symmetrical and mainly mesokurtic. These granulometric attributes are typical of shallow marine deposits influenced by fluvial processes under low- to high-energy conditions. Result of XRD analysis show the presence of clay and non-clay minerals in the sediments, with quartz as the most dominant. Diagenetic processes which include compaction, cementation, precipitation, and dissolution influenced the reservoir quality of the sands. Compaction significantly reduced the primary porosity, and the sand-size grains (quartz) created a self-supporting framework while the clay minerals, mainly kaolinite and dickite, occluded the pores spaces. Dissolution removed some detrital grains leaving obvious indications of few interconnecting and independent pores and moderately to poorly sorted sands. These clay minerals bridge the pore throat connectivity and cause reduction in pore radii, and consequently reducing the reservoir potentials of the sands. However, the sediments could serve as potential hydrocarbon reservoirs especially for gas. The coal seams and the black carbonaceous shale facies can serve as the source or seal rocks whereas the sandstone, siltstone, and heteroliths are possible reservoir rocks of the petroleum system in the study area.