By Anigbogu, Emeka Cornelius
The drive for successful hydrocarbon exploration and production in the Anambra basin of
Nigeria necessitates a critical reservoir quality assessment of the outcropping Mamu
Formation in Enugu and its environs. Lithofacies analysis, petrography, x-ray diffraction,
granulometric evaluation and structural studies were employed in this study. Sedimentary
facies were grouped into four facies associations as follow: horizontal planar laminated
sandstone and carbonaceous shale, bedded very fine sandstone, wave ripple laminated
sandstone and swaley cross-stratified sandstone. Depositional environments of the facies are
sub-environments of delta plain, lower to upper shoreface, wave dominated upper to lower
shoreface and storm dominated shoreface respectively. Linear discriminant factors for
depositional environment indicate shallow marine environment for the reservoir sands. Sand
grains are fine to very fine, moderately sorted to moderately well sorted, fine skewed to
symmetrical, mesokurtic to leptokurtic. Thin section studies of the sandstone reservoirs show
that the sandstones are mainly feldspathic arenites. Diagenetic processes in the sandstone
reservoirs are dominated by mechanical and chemical compaction, alteration of framework
grains, precipitation of authigenic minerals, and dissolution of grains and cement which lead
to alteration of porosity and permeability. Porosity varies from 14 to 11.1%. Reservoir quality
of the study is poor to fair, as it is controlled by diagenesis, grain sizes, sorting, grain contacts
and grain shapes. Inversely, grain and cement dissolution, and interconnecting fractures
associated with the area as observed in thin section studies and outcrop scales respectively increase the reservior quality