Abstract: Shallow Subsurface Lithologic Model of Central Anambra Basin Nigeria, using Resistivity and Well Data

EBOKOSIA, EMMANUEL E., Nnamdi Azikiwe University Dept. of Geology, Awka, Nigeria

The study area covers about 3,6OO km{2} in central Anambra basin, Nigeria. In this area geologic information is limited, particularly, little information is available on subsurface lithostratigraphy. This study used field resistivity data and laboratory measurements of selected samples with varying water saturation to interactively model the geoelectric sections. The sections were then interpreted in terms of lithology using available water well data and statistical, control. Each well used is also a sounding center and models were constrained to correspond with well data.

Three Formations occupy this area, the Imo Shale (oldest), predominantly a thick sequence of shale, the Nanka sands, an alternating sequence of loose sand, claystone and siltstone bands, and the Ogwashi-Asaba Formation (youngest), consisting mostly of sand, clay and typically, lignite. All the formations are overlain by laterite.

Vertically the top of the Imo shale is characterized by a sharp drop in the resistivity curve, up to 450. The Nanka Sands is identified by greater resistivity variability and steep positive gradients. The Ogwashi-Asaba formation has similar response pattern as the Nanka sands but may be identified by extremely high resistivities where lignite is intercepted.

The lateral boundary between the Imo shale and the Nanka Sands is marked by an initial flat response followed by a sharp drop. Usually, resistivity values for laterites are observably distinct from those of the bedrock. This was used to determine weathered layer thickness.

AAPG Search and Discovery Article #90931©1998 AAPG Foundation Grants-in-Aid