Evaluation of Fractured Shale Formations as Aquifers: A Case Study of the Albian–Cenomanian Asu River Group, Southern Benue Trough, Nigeria.

Abstract

Groundwater is stored within subsurface pores and fractures. Weathering and diagenesis play critical roles in either increasing or reducing rock porosity. In this context, argillaceous rocks (typically regarded as impermeable) can develop fracture porosity, which enhances permeability and allows substantial water storage through interconnected matrix pathways. Alex Ekwueme University Ndufu-Alike and Gregory University Uturu are expanding institutions facing challenges in securing adequate groundwater for domestic and other uses. The study began with preliminary geophysical investigations using electrical resistivity methods across various locations within both campuses. Based on the findings, five drilling points were recommended: three at Ndufu-Alike and two at Uturu. Drilling results indicated that boreholes equipped with small motorized pumps were feasible at Ndufu-Alike, whereas higher-capacity pumps could be utilized at Gregory University if boreholes were sited at Marist Brothers Uturu, approximately one kilometer east of the campus. Very low resistivity values obtained from the survey confirmed that the lithology is predominantly shale with moderate to high plasticity. Pumping tests were conducted following the Cooper–Jacob (1946) method. Transmissivity (T) at Ndufu-Alike ranged from 18.23 m²/day to 37.44 m²/day, while at Uturu it ranged from 22.85 m²/day to 23.04 m²/day. Storativity (S) values ranged between 0.22 and 0.32, indicating an intermediate class associated with confined aquifers. The researchers observed that at Uturu, sandy shales occur within the borehole depth range, resulting in uniform transmissivity. At Ndufu-Alike, however, fractured shales appear sandwiched between two impermeable layers, creating a confined condition. The pumping tests were closely monitored to establish sustainable pumping rates. The study demonstrates that fractured shale aquifers hold promise for domestic water supply and that shales can be considered viable groundwater sources when developed following careful, site specific geophysical investigations.