Izuchukwu Mike Akaegbobi¹, Mutiu A. Adeleye¹

(1) Department of Geology University of Ibadan, Ibadan, N/A, Nigeria

ABSTRACT: Diagenesis and its Implications on the Reservoir Quality of parts of Ajalli Sandstone in the Cretaceous Anambra Basin, Southeastern Nigeria

Exposed sections of Ajalli sandstone around Uturu area in the Anambra basin were mapped and samples collected for diverse investigations aimed at assessing the maturity, and the diagenetic control on the reservoir quality. The sandstones are off-white, yellow, grayish to reddish brown in colour, and friable to compacted, ferruginized and with intercalation of white clayey materials. The grains are also subangular to subrounded. Structures associated with the sandstones are laminated cross beddings (trough and planar), herringbone and biogenic structures e.g. Ophiomorpha and its escape burrows. Granulometric analysis reveals that the sandstones are poorly sorted,medium to coarse grained, negatively skewed and platykurtic. Bivariate plots of grain size parameters and ichnofacies attributes show that the environment of deposition is continental (fluviatile). Mineralogically, Ajalli sandstone is made up of 83.7% quartz, 3.3% feldspars, 2.73% rock fragments, 4.43% cement and 5.84% matrix. However, ultra stable heavy minerals such as zircon, tourmaline, rutile and other opaque minerals are present. Based on framework composition of sandstones, the samples are classified as quartz arenite. Thin section studies and heavy mineral analysis reveal that the sandstones are mineralogically mature and texturally immature. The sandstones show varying degrees of diagenetic changes such as compaction, cementation and clay authigenesis, which are responsible for the difference in the porosity and permeability of the sandstones when compared to their freshly deposited equivalents. A potential reservoir is established for Ajalli sandstones based on the measured reservoir properties despite the diagenetic effects, ferruginzation and the presence of clay drapes.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado