MODELOS DE EVALUACION DE RIESGO CREDITICIO

4.5.1 Regional geology

The Makurdi Sandstone forms part of the Turonian Eze-Aku Group. They occur across more than 1000 km^ o f territory extending from the Makurdi area to central Benue State. In the type section at Makurdi they form a thick sequence of felspathic sandstones interbedded with marine carbonaceous mudstone and limestone (Nwajide 1986). Poorly- sorted, feldspathic sandstone alternates rhythmically with clay-silt layers. Several-fining upward cycles o f deposition have been recognised in the Makurdi Sandstone Formation. Away from Makurdi, the sandstones generally become thinner and fine-grained.

4.5.2 Local geology

The Makurdi Sandstone Formation occurs in a band across central Oju/Obi and roughly corresponds to the boundary between the two local government areas. It forms a prominent ridge at Adum East and Ochingini where the thickest and youngest sandstone crops out. The Makurdi Sandstone is a series of fluviatile sandstone units interbedded with black to dark grey carbonaceous mudstones, siltstones and thin muddy limestones. The sandstone bodies become thicker and more competent higher up the sequence (i.e. towards the northwest). In some instances, the sandstones form high ridges, enhanced by the silcretisation of the near surface sands while the softer mudstones are eroded to form valley like depressions. The Makurdi Sandstone outcrop is also characterised by a series of shallow dry valleys that have formed perpendicular to the strike o f the formation.

H yd ro g eo lo g ica l investigation s in Oju and Obi

Because of the shallow water table in these valleys, they form valuable sources of domestic water during the dry season.

The hydrogeological nature of the Makurdi Sandstone was investigated at several locations in Oju and Obi. Six exploratory boreholes were drilled into the Makurdi Sandstone at Ochingini (BGS7, BOSS, BGS9, BGS 10, BGS 11 and BGS 12), two at Odaleko Adiko (BGS 13, BGS 13a) and three at Anyoga Adum East (BGS36, BGS37 and BGS38). Simplified lithological logs of some of these boreholes are shown in Figure 4.7.

The studies confirm that in Oju and Obi, the sandstone mostly comprises fine- to medium-grained sandstone interbedded with mudstone and occasional thin limestone layers. The sandstone layers vary in thickness from several millimeters to several metres. Sandstone layers are commonly cross-bedded. Load cast features and evidence of bioturbation are also present. Much feldspar is present within the sandstone, although quartz remains the dominant mineral. At Adum East the sandstones are muddy and silty

BGS4 BGS5 BGS6 BGS7 BGS8 BGS9 BGS10 BGS11 BGS12 5 - 10 £ g 15 20 2 5

t

■

_I

i

c = ^ First main water strike

Ferruginous soil Clay weathered mudstone r~ - I Mudstone 1‘ ' ■ . I Weathered sandstone Sandstone Limestone Siltstone

Figure 4.7 Lithological logs for exploration boreholes drilled into the Upper Eze-A ku (B G S 4-6) and Makurdi Sandstone Formation (B G S 7-12) at O chingini.

H yd ro g eo lo g ica l in vestigation s in Oju a n d O bi

and contain much mica. The sandstone is generally very hard and well-cemented. Limestone layers, such as those noted at BGS8 are commonly shelly and muddy and contain vertical fractures.

A traverse of six boreholes drilled across the outcrop of the Makurdi Sandstone at Ochingini illustrates the variability o f the sandstone. Several distinct bands of sandstone are present near the top o f the formation which commonly form ridges. Interbedded between these sandstones are thick (possibly several hundreds of metres) sequences where mudstone dominates. Towards the base of the Makurdi Sandstone, sandstone units become much more common, and the interbedded mudstone layers become thinner.

A deep borehole drilled 83 m into the Makurdi Sandstone (BGS 13) provides some detailed information about the upper part o f the formation (Figure 4.9). Fining upwards sequences, 4-7 m thick, of coarse- to fine-grained feldpathic sandstones capped by black carbonaceous mudstone were observed. A 20-m thick layer of soft, friable homogeneous sandstone was penetrated from 60 to 80 mbgl which appeared to have been deposited under deeper fluviatile conditions.

The rocks are highly weathered over the uppermost 10-12 m. In the shallowest 2- 3 m a red ferruginous soil is generally developed. Weathering in the mudstone produces thick clay sequences, with illite/smectite clay to 8-10 mbgl (e.g. BGS8). During the dry season the illite/smectite clay layer tends to crack on drying. Weathering in the sandstone produces a thin clay layer (kaolinite/illite) and discoloured, kaolinised sandstone is present beneath the clay (Kemp et a l 1998). At Odaleko and Adum East, silcretised bands are present at depths from between 5-15 mbgl. These thin bands are products of weathering and are extremely hard. Fractures are often present at the base of the weathered zone (from 8-15 mbgl). These fractures are iron-stained and generally allow groundwater seepage. Leaching o f feldspar minerals within the weathered zone enhances the porosity of the sandstone.

4.5.3 Hydrogeology

Prior to this study, the Makurdi sandstone was believed to be the best aquifer in the Oju/Obi area. However, drilling at Adum East, Odaleko and along the traverse line south east of Ochingini showed that the sandstone is complex, highly variable and interlayered with thick mudstones. The best targets for groundwater are fractures at the base of the weathered zone (8-15 m deep) and fractured limestone layers where present.

H yd ro g eo lo g ica l in vestigation s in Oju a n d O bi

The sandstone has moderate porosity. Core samples taken from seven boreholes gave measurements o f porosity varying from 9% to 34 % (median value 16%). Sandstone porosity is enhanced within the weathered zone by dissolution of feldspar crystals leading to the formation of intergranular voids. Unfortunately, these voids may not be well interconnected so that although porosity is high, hydraulic conductivity is often low. Hydraulic conductivity (measured using a gas permeameter) from core samples from the seven boreholes varied from less than 10"^ m.d'^ to 0.7 m.d'^ (median 0.001 m.d‘*). These measurements indicate a very large range in sandstone hydraulic conductivity. Most samples had insufficient intergranular permeability to provide significant flow in the aquifer. .

Eight pumping tests were carried out in the Makurdi sandstone. Transmissivity estimates ranged from 0.07 m^.d'^ to 1.5 m^.d'% with a median value of 0.4 m^.d‘*. Higher transmissivity values were found in most exploratory boreholes that penetrated sandstone to a depth of more than 12 m. These measurements are much higher than the intergranular permeability would predict. Hence the importance of fractures increasing the transmissivity o f the sandstone. Poor yields were measured at BGS 10 and BGS38 - both boreholes penetrated thick mudstone layers. Poor yields encountered in BGS7 were due to fine sand clogging the borehole.

4.5.4 Geophysical investigations

Many geophysical surveys were undertaken at Ochingini, Odaleko and Adum East. In total approximnately 10 km of EM34 surveys, 5 km o f magnetic profiling and 7 resistivity VES were conducted over the Makurdi Sandstone Formation. These surveys have been correlated with lithology from the exploratory boreholes. Typical EM34 measurements from the different Ethologies are shown in Figure 4.8.

1. When the underlying rocks are primarily sandstones the apparent electrical conductivity using a 20-m coil separation is between 10 and 20 m S.m '\ Vertical and horizontal coil measurements are similar.

2. Apparent conductivity values in excess o f 30 mS.m'* are indicative of the predominance o f mudstones. Measurements taken with the vertical coil are often greater than with the horizontal coil.

H yd ro g eo lo g ica l investigations in Oju a n d Obi D is ta n c e fro m v illa g e c e n tr e (m) 1000 2000 3000 4000 5000 6000 -3000 -2000 -1000 100 vertical coil (20 m) horizontal coil (20 m) B G S4 B G S5 B G S6 B G S7 B G S8 B G S9 B G S10 BGS11 B G S 1 2 40 m L im estone C layey w eath ered z o n e

M udstone

F ractu re z o n e S a n d s to n e

Figure 4.8 Apparent conductivity (from E M 34 with 20-m intercoil spacing) for the Upper Eze-A ku (-3000 to -500 m) and Makurdi Sandstone (-5 0 0 to 5000 m) at O chingini. A schem atic interpretation is given from the results o f drilling and the geophysical surveys.

4. From the investigations carried out, none of the geophysical methods could identify if the sandstone contained fractures.

5. Magnetic profiling is of little use in the Makurdi Sandstone since there is little difference in the magnetic properties of the various Ethologies.

6. Resistivity surveys carried out over sandstone and mudstone showed similar profiles: resistive surface layer, conductive middle layer (about 5-10 m thick) and resistive (>80 Q.m) bedrock. Across the sandstones the middle layer had a resistivity of about 30 Q.m; across the mudstones this fell to less than 10 Q.m.