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Hydraulic characteristics of the Maastrichtian sedimentary rocks of the southeastern Bida Basin, central Nigeria
JournalofAfrlcan Earth Sciences, Vol. 29, No. 4, pp. 6 5 9 - 6 6 7 , 1999
Pergamon
PI1:S0899-5362(99)00122-0
© 2000 Elsevier Science Ltd AI~ rights reserved, Printed in Great Britain 0899-5362/00 $- see front matter
Hydraulic characteristics of the Maastrichtian sedimentary rocks of the southeastern Bida Basin, central Nigeria PETR VRBKA, ~'* OLUSOLA JOHNSON OJO 2 and HOLGER GEBHARDT 2.3 ~Geologisch-Pal~iontologisches Institut, Technische Universit~it Darmstadt, Schnittspahnstr. 9, D-64287 Darmstadt, Germany 2Department of Geology and Mineral Sciences, University of Ilorin, PMB 1515, Ilorin, Kwara State, Nigeria 3Present address: Institut for Angewandte Geowissenschaften II, Technische Universit~t Berlin, Sekr. £B 10, Ernst-Reuter-Platz 1, D-10587 Berlin, Germany
ABSTRACT--A set of outcrop samples from the Lokoja and Patti Formations of the southern Bida Basin (Nigeria) was examined for grain size distribution, sedimentary and hydraulic characteristics. Most of the samples are well-sorted with an uniformity coefficient (U) ranging between 1.3 and 6.3. The mean effective grain diameter (dlo) is in the order of 0.11 mm, the mean value of dgo was determined as 0.66 mm (both geometric mean) and the median grain size (dso) as 0.23 mm. Based on these values, the sedimentary sequence can be described as 'fine to medium sized sand', having minor amounts of either silt, or coarse sand and some gravel. The total porosity of the samples was determined by laboratory methods to be in the range of 9-29%. The hydraulic conductivity (K) was determined according to Hazen. and Beyer, and by Shepherd's formula, the last resulting in a geometric mean of 3.3 m d -~ or 3.3 darcy. The results are used to estimate local groundwater potential. The entire pore space (potential groundwater reservoir) for the area under study is estimated to be in the order of 290-430 km 3. Because of higher hydraulic conductivities, it is recommended that the Lokoja Formation is concentrated on as a target for groundwater exploration. © 2000 Elsevier Science Limited. All rights reserved. RL=SUMI~ - Un nombre repr~sentatif d'~chantillons a ~t~ pr~lev6 dans les formations du Lokoja et du Patti dans le sud du bassin du Bida (Nigeria) afin d'effectuer une analyse granulom~trique et de d~terminer les caract~ristiques s6dimentaires et hydrauliques. La plupart des ~chantillons ont ~t6 d0ment classes avec un coefficient d'uniformit~ (U) entre 1.3 et 6.3. Le diam~tre effectif (dto) est en moyen de 0.11 mm et la valeur moyen pour le dgo a 6t6 6valu6e & 0.66 mm (il s'agit dans le deux cas de moyennes g~om6triques), pour le grains de sables le median d~o est de 0.23 mm. Partant de ces r6sultats, le s6ri~ s~dimentaire peut ~tre d6crit comme ~tant de sable fin ~ moyen contenant de petites quantit~s de silt, de sable grossier ou de conglom~rat. Les donn~es concernant la porosit6 totale des pr~l~vements ont ~t6 d~termin~es grace ~ des m~thodes de laboratoire et estim~es entre 9 et 29%. La conductivit6 hydraulique a (~t~ d~termin~e d'apr~s les m~thodes de Hazen, Beyer et la formule de Shepherd, cette derni~re donnait comme r6sultat une moyen g~om~trique de 3.3 m d 1 ou de 3.3 darcy. Ces r~sultats servent ~ ~valuer le potentiel des eaux souterraines de la r~gion 6tudi6e. Le r~servoir potentiel d'eau souterraine, la porosit~ totale pour cette r6gion se situerait entre les 290 et 430 km 3. A cause de ses conductivit~s hydrauliques plus ~lev6es, il est recommand~ de se concentrer sur la Formation du Lokoja pour I "exploration d "eau souterraine. © 2000 Elsevier Science Limited. All rights reserved. (Received 14/9/98: revised version received 17/12/98: accepted 5/1/99)
* Corresponding author [email protected] (P. Vrbka) Journal of African Earth Sciences 659
P. VRBKA et al.
INTRODUCTION AND GEOLOGICAL SETTING The research area presented in this paper is situated at the southeastern margin of the Bida Basin (synonym with Nupe Basin, middle Niger Basin) around the confluence of the River Niger and River Benue at Lokoja, in central Nigeria (Fig. 1 ). The direct distance to the Bight of Benin (Atlantic Ocean) is about 400 km. The study area has a north-south extension of about 80 km, partly overlapping (around Abaji) with the southern part of the Federal Capital Territory (FCT). The booming capital Abuja has an increasing demand for water, hence the evaluation of nearby water resources is of national interest. Along the road from Lokoja to the new capital Abuja, road construction activities gave the opportunity to study the outcropping Upper Cretaceous sedimentary rock sequences, which are covered elsewhere by thick lateritic soils. Altogether, nine localities were studied for vertical profile description of the consolidated, yet brittle, sediments which are cemented mainly by kaolinite. The depositional environment is mainly continental, with a minor marine influence in the upper part (cf. Adeleye, 1989; Braide, 1992a; Ladipo et al., 1994) and varies only a little in the entire basin (Braide, 1992b, c; Olaniyan and Olobaniyi, 1996). Towards the Anambra Basin, more marine influence is recorded for time equivalent formations (Adeniran, 1991 ; Ladipo, 1986, 1988; Nwajide and Reijers, 1996; Gebhardt, 1998). The mean sediment thickness in the Lokoja area is about 500 m, its maximum does not exceed 1000 m (Ojo and Ajakaiye, 1989). The age of the investigated sediments has been determined by pollen and spore analyses as Maastrichtian by Jan du Chine et aL (1978)i The sedimentary succession comprises the Lokoja, Patti and Agbaja Formations (see also Adeleye, 1975, 1989; Braide, 1992b, c; Whiteman, 1982). Locations 1,2, 4, 10, 11 and 19 represent the Lokoja Formation, whilst locations IDU4, GEl and OR1 represent the Patti Formation (see Fig. 1). The ironstones of the Agbaja Formation have not been sampled and are not represented in the lithologs of Fig. 1. The Patti Formation is stratigraphically younger than the Lokoja Formation. The basal Lokoja Formation exposed between Lokoja and Koton-Karifi consists of conglomerates and sandstones. The conglomerates are dominantly matrix supported and contain randomly orientated, rounded to subrounded clasts suggesting a debris flow deposit (Ojo, 1992). The overlying sandstone facies of the Lokoja Formation consists of coarse to medium sized sand. The cross-stratified sandstone subfacies commonly displays tabular as well as trough cross-bedding. In some places, the cross-
660 Journal of African Earth Sciences
bedded units are interbedded with massive sandstone. The sandstones are generally immature (texturally and mineralogically). Kaolinitic matrix is very common, resulting in light brown to whitish colours. The sedimentological features indicate braided channel deposits (Ojo, 1992). The Patti Formation exposed between the towns of Koton-Karifi and Abaji consists of sandstones, siltstones, claystones and shales. The sandstone facies varies from conglomeratic sandstone to cross and parallel stratified, fine- to coarse-grained sandstone. At its basal part, the conglomeratic unit is about 0.30 m thick and is interpreted as a channel lag deposit. The coarse- to fine-grained sandstone units are characterised by moderate sorting as well as high mineralogical maturity. They probably represent a pointbar sedimentation of a high sinuosity river system (Ojo, 1992). Siltstones are commonly parallel stratified and reach thicknesses of about 15 m. The claystones are massive and commonly kaolinitic. The claystones and siltstones are interpreted as overbank deposits. The greyish shales indicate a deposition in a non-marine swamp subenvironment (Braide, 1992b, c).
Regional climatic conditions The regional climatic conditions are influenced by the location of the investigated area within the woodland savanna just north of the humid tropical rain forest belt. Basic information on the regional climate has been given by Ayoade (1974, 1977). Several important parameters have to be taken into account. Wind pattern The wind pattern over Nigeria is dominated by the Harmattan winds coming from the north during the dry season (November-April) and the southerly Monsoon winds from the Atlantic during the rainy season (May-October). Thus, during the Harmattan, rather dry, dusty winds are blowing, whereas during the rainy season, moisture-loaded air masses from the Atlantic Ocean lead to the development of a cloud cover and, eventually, loose their moisture in the form of rain. Air temperature The lowest daily minimum temperatures are found during November-January at about 20°C. The maximum daily temperatures are reached at the end of the dry season, in February-April, at about 36°C. Thus, the annual fluctuations vary mainly between 20-36°C. As a consequence, the mean annual reference temperature in this area is in the order of 28°C (Mands, 1992).
Hydraulic characteristics of the Maastrichtian sedimentary rocks of the Bida Basin, Nigeria
)Abaji
Location 2
LOcallon IDU4 i.:........
LAJ2P
iiii!i~iii~i~iiL
iiiiiiiiiiiii}i K.:.:,:.:-:,:.:.:. ~ A J 2 B
Ge g ub e rl T
~ ~ Location G~ GEl ~1
n 19 L;):;I[~:,7, -~OZlg E
, Location 11
i:~;! .~OZ 19C ......
IGE1E
l:i;i:i!i?~',
:::::::::::::::::::::::: .....
Koton-Karifi
Atlantic Ocean
Location OR1
Lateritic ironstone Localion 4
Conglomerate Coarse grained sandstone Medium grained sandstone
":':':':':::':':::i
~:.: .....-...... -.*., ::::::::::::::::i: ii?!!i?!i!iiiii!:!i
i: : ;.:-:- = LocalJorl 10
i:iii:i:iiii
ii?
~--z-,z~-;- ~ t.lOD
Fine grained sandstone Siltstone Shale, mudstone and cl~ Ferrugenized mudstone
Locallon I
".'.4
D :::iL1E
N
15 m
1 tom
Lokoja- / o~
I
10 km
I
Figure 1. Location o f research area and lithological sections. The positions o f the investigated samples are indicated.
Locations 1, 2, 4, 10, 11 and 19 are assigned to the Lokoja Formation; locations IDU4, GEl and ORI represent the Patti Formation. Lokoja is situated at latitude 7 ° 4 9 ' N and longitude 6°44"E.
Journal of African Earth Sciences 661
P. VRBKA et al. Precipitation, evaporation The Atlantic Ocean is the main source of vapour for humid air and clouds being transported across the African continent, which eventually may recharge the Nigerian aquifers as they are moved towards the north and northeast by the Monsoon winds. The mean annual precipitation is in the range of 1000-1500 mm a-~ (e.g. meteorological station Kwali, 8°52'N; 7 °00'E, period 1969-1981 ), the potential evapotranspiration according to Schendel's method results in > 4000 mm a~, whereas the real evapotranspiration is in the range of 900-1100 mm a1 (Mands, 1992). According to this range, between 100-600 mm a-~ of rain are available for overland flow, stream discharge and groundwater recharge.
Relative humidity The relative humidity of the air is high due to the relatively high precipitation amounts, evaporation and the vicinity of the River Niger Valley. At the meteorological stations in Bida and Minna, about 50% of relative humidity of the air were reported on the mean for the period 1969-1981. Whereas August is the wettest, January and February are the driest months (Mands, 1992). In the valleys of the major streams, the humidity values may reach 70-80%.
the pore volume (V w) is determined by saturation with distilled, air-free water. This can be done by allowing the water to soak the rock specimen by capillary rise forces at first, so that air included in the porous space is repelled in the upward movement. The sample is then completely covered with water for at least 24 hours. After weighing the saturated sample, the porosity volume ( = w a t e r volume V W) can be determined directly by assuming a water density of 1 g cm -3. In the next step, the total volume of the (saturated) rock sample (V s) is determined by a water-displacement test. The sample volume (V) is calculated by weighing the repelled water to an accuracy of at least 0.1 g (=0.1 cm3), or read off at the calibrated scale of the beaker. However, if problematic samples are being examined (i.e. which disintegrate rather easily as in this case) the following approach for the estimation of total porosity was applied with satisfying results. Method B The second method is based on the following formula to determine the total porosity (n) of a formation:
n = 100 (1 - d b d l ) , METHODS APPLIED
In areas where the experimental laboratory equipment is limited and funds for sophisticated, but often also sensitive methods are restricted, simple straightforward procedures are to be applied. The hydraulic conductivity (K), the intrinsic permeability (K~)and the total porosity (n) of the sediments were determined using different methods as a check on each other. In the following, the two applied experimental laboratory methods for the study of the total porosity (n) of the Upper Cretaceous sedimentary sequences of the Bida Basin are presented. Method A The total porosity (n) of a rock specimen can be obtained using the pure volumetric approach according to the formula:
n = 1 0 0 V wVs-1,
(1)
where n = total porosity (as a percentage); V w = volume of water filling the voids of the aquifer material (cm3); and V s =volume of the sample of the aquifer material (cm3). At first the dry weight of the sample is determined by oven drying at 105°C for at least 24 hours to constant weight. Then the amount of water entering
662 Journal of African Earth Sciences
(2)
where n = total porosity (as a percentage); d b= total density of the material (g cm-3); and dp=particle density of the material (g cm-3). There are several ways to determine the density values. The total density of the aquifer material is the mass of the sample after oven-drying divided by the total sample volume. According to Ojo (1992), the rock samples are mainly composed of SiO 2 grains. Hence, an average particle density of 2.65 g cm -3 can be assumed (Fetter, 1994). This density value could be increased by Fe cement or if the share of heavy minerals is appreciably high. Both factors are absent in most of the samples. For samples with Fe cement, the method was adapted as follows: the particle density was determined by dividing the oven-dried mass by the volume of the mineral skeleton. The volume of the mineral skeleton was determined by a water-displacement test. This simple method gave reproducible, satisfying results. It can accommodate sandstone samples, which may decompose if left in water for saturation for a long time. The results of both methods are given together with other findings in Table 1. The results of both methods differ only by 1-2%. The total porosity of the samples varies between 9-29%, the geometric mean being 15 or 20%. However, the porosity values
Hydraulic characteristics of the Maastrichtian sedimentary rocks of the Bida Basin, Nigeria Table 1. Effective grain diameters, porosities, hydraulic conductivity (K), intrinsic permeability (K{), and total porosity (n) of Upper Cretaceous, consolidated sediments of the Bida Basin of central Nigeria Sample
dlo
dso
d6o
d9o
mm
mm
mm
mm
U C K K K K~ Porosity n d6o/ Beyer Hazen Beyer Shep. Shep. Meth.A Meth. B dlo x 1 0 .4 m d -1 m d 1 m d -1 darcy % %
A J2 AJ2A AJ2B AJ2G AJ2M AJ2P GE1E GE1G IDU4F L1D L1E L4B L4F L10D LIOF L11G OR IC OZ19C OZ19E
0.17 0.20 0.11 n.d. n.d. 0.07 0.05 0.04 0.09 0.29 0.22 0.04 0.29 0.13 0.16 0.33 0.07 0.03 0.05
0.24 0.26 0.13 n.d. n.d. 0.10 0.14 0.10 0.11 0.44 0.66 0.11 0.71 0.64 0.25 1.00 0.11 0.17 0.18
0.25 0.27 0.14 n.d. n.d. 0.11 0.17 0.12 0.17 0.57 0.87 0.12 0.81 0.67 0.31 1.15 0.12 0.19 0.20
0.55 0.36 0.16 n.d. n.d. 0.14 0.47 0.32 0.55 2.46 2.30 0.25 2.46 0.95 1.52 3.73 0.50 0.70 0.38
1.50 1.40 1.30 n.d. n.d. 1.50 3.40 3.40 2.00 2.00 4.00 3.40 2.80 5.20 1.90 3.50 1.80 6.30 4.00
90 91 92 n,d. n.d. 90 76 76 85 85 73 76 78 68 86 75 87 65 73
42.3 32.8 61.3 48.4 19.0 14.7 n.d. n.d. n.d. n.d. 7.5 5.9 3.9 2.5 1.9 1.2 11.2 8.2 129.6 95.0 74.3 46.7 1.9 1.2 129.6 86.4 25.9 15.6 39.7 29.4 164.2 112.3 7.1 5.4 1.4 0.8 3.9 2.4
3.6 4.1 1.4 n.d. n.d. 1.0 1.6 1.0 1.0 8.9 16.3 1.0 18.2 15.6 3.8 30.5 1.1 2.1 2.3
3.6 4.1 1.5 n.d. n.d. 1.0 1.5 1.0 1.0 8.7 16.2 1.0 18.2 15.2 3.9 30.4 1.1 2.1 2.3
27 n.d. 10 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 12
25 28 9 29 22 n.d. n.d. n.d. n.d. 27 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 12
Max. Min. St. dev. Range MeanA MeanG
0.33 0.03 0.10 0.30 0.14 0.11
1.00 0.10 0.27 0.90 0.31 0.23
1.15 0.11 0.32 1.04 0.37 0.27
3.73 0.14 1.06 3.59 1.05 0.66
6.30 1.30 1.40 5.00 2.90 2.60
92 65 8 27 80 80
164.2 112.3 1.4 0.8 52.2 36.2 162.8 111.5 42.6 29.9 16.5 11.3
30.5 1.0 8.5 29.5 6.7 3.3
30.4 1.0 8.4 29.4 6.6 3.3
27 10 9 17 16 15
29 9 8 20 22 20
Max.: Maximum value; Min.: Minimum value; St. dev.: Standard deviation; Mean A: Arithmetic mean; Mean G: Geometric mean; n.d.: not determined.
are from the Lokoja Formation only. For the samples from the Patti Formation the porosity could not be determined due to decay of the samples after wetting.
HYDRAULIC PROPERTIES OF THE CRETACEOUS SANDSTONES
In order to make a statement on the groundwater resources of an area (next to the storage capacity for water) the potential of transmitting water from one locality to another or to a well is to be examined. For 17 rock samples, the hydraulic conductivities (K) have been derived from grain size analyses using the dry sieving values of Ojo's (1992) study. Together with the total aquifer thickness, they are used to describe the transmissivity (T). For a better understanding, the intrinsic permeability (K~) was also
calculated, based on the K values, the dynamic viscosity of water (p) circulating through the rock, the density (r) of water and the acceleration of gravity (g). For the calculation of the hydraulic conductivity (K) the methods after Hazen (1892) and Beyer (1964) were applied first, also described in detail by Langguth and Voigt (1980). The empirical Hazen formula is: KH .... =
0.0116 (dlo)2 (0.7 + 0.03 T),
(3)
where dlo=effective grain diameter (in mm); and T=temperature of the water (in °C). The Hazen equation can be applied only to grain size distributions with a uniformity coefficient U < 5. Based on this empirical relation, Beyer (1964) has adjusted Hazen's formula for varying uniformity coefficients (U) of a sediment introducing a factor C:
Journal of African Earth Sciences 663
P. VRBKA et al. KBeye, = C (dlo)2 (0.7 + 0.03 T).
(4)
If the variables d~o and T are as above, the formula is valid for d~o values between 0.06-0.6 mm. Factor C varies according to the uniformity coefficient (U) and the packing of the grains (Table 1 ). For the consolidated rock samples, adapted C values for tight packing according to Langguth and Voigt (1980) were chosen. The above equations are based on empirical relationships, hence they are not dimensionally correct. The hydraulic conductivities result in the unit m s4. For the proper calculation, the effective grain diameter (d~o) and the uniformity coefficient (U = d6o [dlo ]4) must be determined. The hydraulic conductivity values given in Table 1 are calculated for a mean aquifer temperature of 2 8 ° C (mean annual temperature of the area: direct groundwater temperatures were not available). However, the above formulae were developed for non-consolidated material (sand filters) and have restrictions as to uniformity (Hazen: U < 5 ) and the effective grain size (Beyer: 0 . 0 6 < d ~ o < 0 . 6 mm). Shepherd (1989; in Fetter, 1994) analysed results of 18 studies where hydraulic conductivity values had been related to the grain size. He found that all studies could be related to the general formula for hydraulic conductivity: Kshepherd = C " (dho)',
(5)
where C =varying shape factor; d~o= mean grain size (in mm); and t = exponent to be varied according to maturity. In contrast to the above formulae of Hazen and Beyer, the temperature of the flowing medium is not needed. However, C and t depend on the texture of the sedimentary material. The greatest values of C and t are for texturally mature sediments, and smallest for texturally immature and consolidated sediments. They vary between 4 0 , 0 0 0 and 1 O0 for C, and 2 to 1.5 for t. The higher values are valid for glass spheres (representing texturally mature sediments), whereas the lower values are applied to texturally immature, consolidated sediments. A further advantage is the introduction of the mean grain diameter (d~o) instead of the effective grainsize (dlo). Thus, the formula can also be applied for sediments where the above restrictions do not allow the application of the Hazen or Beyer approach. The application of Shepherd's formula, again dimensionally not correct, gives hydraulic conductivities in the unit ft d 1, which were transferred into m d 4 (Table 1 ). For completion and control of the values calculated according to Hazen and Beyer, Shepherd's formula
664 Journal of African Earth Sciences
was also applied to the analysed samples (Table 1 ). The sedimentary sequence, as described earlier, is mainly built up of Upper Cretaceous braided streams and channel deposits. Therefore the following C and t values for consolidated sediments were taken for the calculation of the hydraulic conductivity (K) (C = 100; t = 1.5). In Table 1, the results for K are all given in m d 1. The values calculated by the three methods range from 0.8 to 164 m d 1 . In addition to arithmetic means, the geometric means are also given here in respect to the non-Gaussian distribution of the values and the occurrence of 'outliers'. The geometric mean values are KHazen> KBeyer> Kshepherdand are related by 1.5 : 1.0 : 0.3. Thus, the method of Shepherd gives results about 5 times smaller than the Hazen, and about 3 times smaller than the Beyer method. In the past, the t w o last methods were applied for the determination of K values for older, consolidated sediments (Kheir, 1986; Schneider, 1986). Usually this was due to the lack of pumping tests taking into account the characteristics of the material, or for comparison. However, these two empirical formulae were developed for and tested on non-consolidated sands for filtering purposes. The results show that the Shepherd formula gives more realistic results, if mature and consolidated sediments are examined. For the Cretaceous formations of the Bida Basin a range of 1-30 m d 4 for the hydraulic conductivity (K) is considered to indicate more realistic values, the geometric mean of Shepherd is in the range of 3.0-3.5 m d 1. The approximations, however, need to be interpreted in a conservative way. For example, Vrbka (1996) calculated geometric mean values for the hydraulic conductivity (K) in the order of 1.3-1.7 m d -1 for comparable Lower Cretaceous sandstones in Sudanese sedimentary basins (Kordofan State), based on pumping test analyses. These values from a fractured, doubleporosity sandstone aquifer are still lower by a factor of 2, probably due to compaction. Hence, assuming deep buried sediments in the Bida Basin, a range of 1.5-1.7 m d 4 will be considered for further calculations. Darcy's proportionality constant (K) is a function of properties of both the porous medium and the fluid passing through it. However, there is another proportionality constant used in fluid hydraulic studies, termed the intrinsic permeability (K~), which is a function of the porous media alone: K i = C d 2,
(6)
where d = mean pore diameter as in Shepherd's formula; and C = s h a p e factor. Thus the intrinsic
Hydraulic characteristics of the Maastrichtian sedimentary rocks of the Bida Basin, Nigeria u
i
1 0-5
I
1 0-3
I
t
I
1 0-1
1
1 01
I
I
I
Patti F m ~ . , =
1 0-5
I
1 0-3
I
I
I
Iver, ow i
ow
1 0-1
I
1 03
I ~-~
1
I
1 01
I
I
t
1 05
I
I
Lokoja F m
1 03
I
1 05
I
j
Ki (darcy)
oermeabeI hob Iver, h oh I p000' ermeability
i
Figure 2, Evaluation o f permeability based on the hydraulic conductivity (K) and intrinsic permeability (K). The ranges for the investigated sediment samples o f the Lokoja and Patti Formations are shown.
permeability is used to describe the overall effect of the shape of the pore space. The dimension of K~ is length 2, or area. The relationship between hydraulic conductivity (K) and intrinsic permeability (K~)is given as:
K
=
Ki g/j-l,
(7)
where g =specific weight of the fluid (density [r] times acceleration of gravity [g] = 9.81 m s2); and p=dynamic v i s c o s i t y . For t h e g i v e n mean groundwater temperature of 28°C, the following values can be used: p = 0 . 8 3 6 0 x 10 .3 kg s -1 m l ; r = 996.23 kg m -3. Thus, the intrinsic permeability (K)~ can easily be determined if the hydraulic conductivity and the physical properties of the fluid at a given temperature are known. In the petroleurn industry, the darcy is used as a unit of intrinsic permeability, where 1 darcy is defined as 9 . 8 7 x 1 0 .9 cm 2 ( = 9 . 8 7 x 10 -13 m2).
RESULTS Most of the samples are well-sorted with a uniformity coefficient (U) between 1.3 and 6.3 (Table 1). The mean effective grain diameter (dlo) is 0.11 mm, the mean (dgo) was determined as 0.66 mm and the median grain size (d~o) as 0.23 mm. Based on these values, the sediments can be described as 'fine to medium sized sand', having only minor amounts of either silt, or coarse sand and some gravel.
estimated, giving potential groundwater storage capacities between 120-180 L m -3.
Hydraulic conductivity The hydraulic conductivity (K) determined according to Hazen (1892)is in the range of 1.4-164.2 m d -1, whereas according to Beyer (1964), the range is 0 . 8 - 1 1 2 . 3 m d -1 (both at 28°C). The Shepherd approach gives a range of 1.0-30.5 m d 4. Samples of the Lokoja Formation show higher hydraulic conductivities than those of the Patti Formation, corresponding with their larger grain size. The values are in the range 'permeable' to 'highly permeable' (Fig. 2).
Intrinsic permeability The intrinsic permeability (K~) was calculated based on the K value achieved by the Shepherd formula and is given in darcy units: the values range from 1.0-30.4 darcy. In the present case, for a mean annual temperature of 28°C, the values of hydraulic conductivity (K) in m d -1 correspond to the values of the intrinsic permeability (K~) in darcy. The geometric mean for both aquifer properties is given as 3.3 m d -1 or 3.3 darcy (Table 1).
Transmissivity Based on the above mean value for K and a mean saturated aquifer thickness of about 500 m, the mean total t r a n s m i s s i v i t y of the sedimentary sequence can be determined as 1650 m 2 d 1 or 1650 darcymetres.
Total porosity The total porosity of the samples investigated is in the range of 9-29%, the median being 15-20%. However, since the rock samples were taken close to the surface, for the sequence of buried sediments lower effective porosities n e between 12-18% are
DISCUSSION The research area is situated in the Lower Niger Basin, which corresponds to the southeastern part of the (geological) Bida Basin. The ground slopes
Journal of African Earth Sciences 665
P. VRBKA et al.
towards the River Niger from elevations above 300 m asl on both valley sides to less than 100 m where the river flows. The Lower Niger Basin area is drained by minor rivers flowing roughly southwestnortheast and north-south from the Kabba and Jos highlands in the south and the north, respectively. The Patti and Lokoja Formations, overlying the basement rocks in succession, are reported to be equivalents to the Enagi Siltstone, Bida Sandstone or Nupe Formations further to the north (e.g. Adeleye, 1975, 1989; Whiteman, 1982; Offodile, 1992). The Patti Formation has a maximum thickness of about 100 m and is composed of fine to medium sized sandstones, some claystones and carbonaceous siltstones. Oolithic inclusions of ironstones are reported towards the top. The samples have distinctively lower hydraulic conductivities than those of the deeper Lokoja Formation, and may therefore not be a primary target for groundwater exploration. Additionally, intercalated clay and shale layers may reduce the productivity of wells, if vertical recharge plays a major role. The sandstones of the Lokoja Formation rest directly on the granitic to quartzitic basement rocks. The whole sequence consists of pebbly, clayey grits and sandstones with a minimum thickness of 250 m. Due to their higher hydraulic conductivity values, groundwater exploration should focus on rocks of the Lokoja Formation. Mands (1992) reported on three pumping tests situated within the sedimentary series of the Bida Basin, which were performed at Shashi Dama, Gaba and Etsugaie. The transmissivities are in the range 5.5-29.3 m ~ d -1. The resulting hydraulic conductivities were reported as 6 x 106 to 3 x 10 .5 m s-1 (0.5-2.6 m d-l), which are within the range of the results for this study. The work of Kehinde (1990) indicates lower transmissivities for sediments of the Bida Basin (mainly <-50 m 2 d-1) than those calculated here (1.65 m 2 d-lL This discrepancy may partly be due to the fact that mainly sandy samples were examined, resulting in higher hydraulic conductivities. Furthermore, pumping test wells usually are not fully penetrating and the screens may have been positioned in zones with relatively low hydraulic conductivities. Thus, the transmissivity values from pumping tests are based on relatively short screen sections of several metres, whereas this study calculated the bulk transmissivity for a sediment thickness of 500 m. Based on the mean value, for screen lengths of 3-10 m a transmissivity in the range of 10-33 m 2 d-1 would result, to some extent confirming the findings of Kehinde (1990) and Mands (1992). Hence, as a first approach to describe hydraulic conductivities of consolidated sediments and for the approximation of the bulk transmissivity, the appropriate application of the method according
666 Journal of African Earth Sciences
Shepherd will give usable results. Based on the thickness data of Ojo and Ajakaiye (1989: ca 500 m) for the Lokoja-Abaji section and a void space of 12-18%, the entire void ratio in the area (about 60 x 80 km = 4800 km 2, Fig. 1) is estimated to be in the range 290-430 km 3, corresponding to a groundwater reservoir of similar size. However, for a better understanding and evaluation of the groundwater situation in the area, further long-term studies need to be carried out.
ACKNOWLEDGEMENTS The authors are grateful to the DAAD (German Academic Exchange Service) which enabled two of the authors (PV, HG) to spend time doing research at the University of Ilorin, Nigeria. Dr P. Olasehinde, Department of Geology and Mineral Sciences of the University of Ilorin, Nigeria, has provided essential literature and expert knowledge. Special thanks are due to Maria Mendes, Berlin, Germany, for correcting the French r~sum~. Editorial handling - P. Eriksson & G. W. McNeill
REFERENCES Adeleye, D.R., 1975. Nigerian Late Cretaceous stratigraphy and paleogeography. Bulletin American Association Petroleum Geologists 59, 2302-2313. Adeleye, D.R., 1989. The Geology of the Middle Niger Basin. In: Kogbe, C.A. (Ed.), Geology of Nigeria, 2nd edition. Elizabethan Publishing Co., Lagos, pp. 283-287. Adeniran, B.V., 1991. Maastrichtian tidal flat sequences from the northern Anambra Basin, southern Nigeria. Nigerian Association Petroleum Explorationists Bulletin 6, 56-66. Ayoade, J.O., 1974. A statistical analysis of rainfall over Nigeria. Journal Tropical Geography 39, 11-23. Ayoade, J.O., 1977. Evaporation and evapotranspiration in Nigeria. Journal Tropical Geography 42, 9-19. Beyer, W., 1964. Zur Bestimmung der Wasserdurchl~issigkeit von Kiesen und Sanden aus der Kornverteilungskurve. Wasserwirtschaft, Wassertechnik 14, 165-168. Braide, S.K., 1992a. Geological development, origin and energy mineral resources potential of the Lokoja Formation in the southern Bida Basin. Journal Mining Geology 28, 33-44. Braide, S.K., 1992b. Alluvial fan depositional model in the northern Bida Basin. Journal Mining Geology 28, 65-73. Braide, S.K., 1992c. Syntectonic fluvial sedimentation in the central Bida Basin. Journal Mining Geology 28, 55-64. Fetter, C.W., 1994. Applied Hydrogeology, 3rd edition. Prentice Hall, New Jersey, 691 p. Gebhardt, H., 1998. Benthic foraminifera from the Maastrichtian lower Mamu Formation near Leru (southern Nigeria): paleoecology and paleogeographic significance. Journal Foraminiferal Research 28, 76-89. Hazen, A., 1892. Some physical properties of sands and gravels with special reference in their use in filtration. Annual Reports Massachusetts State Board Health 24, 541-555. Jan du Ch6ne, R.E., Adegoke, O.S., Adediran, S.A., Petters, S.W., 1978. Palynology and foraminifera of the Lokoja Sandstone (Maastrichtian), Bida Basin, Nigeria. Revista EspaSola Micropaleontologfa 10, 379-393.
Hydraulic characteristics of the Maastrichtian sedimentary rocks o f the Bida Basin, Nigeria Kehinde, M.O., 1990, Die Grundwasser-Ressourcen des BidaBeckens, ZentraI-Nigeria. Ph.D. Thesis, University of MLinster, Germany, 150p. Kheir, O.M., 1986. Hydrogeology of Dongola Area, Northern Sudan. Berliner Geowissenschaftliche Abhandlungen, Reihe A 74, 1-81. Ladipo, K.O., 1986. Tidal shelf depositional model for the Ajali Sandstone, Anambra Basin, southern Nigeria. Journal African Earth Sciences 5, 177-185. Ladipo, K.O., 1988. Paleogeography, sedimentation and tectonics of the Upper Cretaceous Anambra Basin, southeastern Nigeria. Journal African Earth Sciences 7, 865871. Ladipo, K.O., Akande, S.O., M~cke, A., 1994. Genesis of oolithic ironstones from the Middle Niger Sedimentary Basin: evidence from the sedimentological, ore microscopic and geochemical studies. Journal Mining Geology 30, 161-168. Langguth, H.-R., Voigt, R., 1980. Hydrogeologische Methoden. Springer-Verlag, Berlin, 486p. Mands, E., 1992. Kritische Betrachtung der Wasserbilanzparameter und hydrologische Untersuchungen der Einzugsgebiete River Gbako und River Gurara (Mittelnigeria). Giessener Geologische Schriften 47, 1-174. Nwajide, C.S., Reijers, T.J.A., 1996. Sequence architecture in outcrops: examples from the Anambra Basin, Nigeria.
Nigerian Association Petroleum Explorationists Bulletin 11, 23-32. Offodile, M.E., 1992. An Approach to Ground Water Study and Development in Nigeria. Mecon Services Ltd., Jos, 247p. Ojo, S.B., Ajakaiye, D.E., 1989. Preliminary interpretation of gravity measurements in the Middle Niger Basin area, Nigeria. In: Kogbe, C.A. (Ed.), Geology of Nigeria, 2nd edition. Elizabethan Publishing Co., Lagos, pp. 347-358. Ojo, O.J., 1992. Petroleum geology and sedimentology of Patti Formation, Bida Basin, Nigeria. M.Sc. Thesis, University of Ibadan, Ibadan, Nigeria, 149po Olaniyan, O., Olobaniyi, S.B., 1996. Facies analysis of the Bida Sandstone Formation around Kajita, Nupe Basin, Nigeria. Journal African Earth Sciences 23, 253-256. Schneider, M., 1986. Hydrogeologie des Nubischen Aquifersystems am SL~drand des Dakhla-Beckens, SL~d~igypten/Nordsudan. Berliner Geowissenschaftliche Abhandlungen, Reihe A 71, 1-66. Vrbka, P., 1996. Hydrogeologische und isotopenhydrologische Untersuchungen zu regionalen Problemen der GWNeubildung, der GW-Zirkulation und des Wasserhaushaltes im Nordsudan. Berliner Geowissenschaftliche Abhandlungen, Reihe A 186, 1-158. Whiteman, A., 1982. Nigeria: Its petroleum geology, resources and potential. Graham and Trotman, London, 394p.
Journalof AfricanEarthSciences667
Pergamon
PI1:S0899-5362(99)00122-0
© 2000 Elsevier Science Ltd AI~ rights reserved, Printed in Great Britain 0899-5362/00 $- see front matter
Hydraulic characteristics of the Maastrichtian sedimentary rocks of the southeastern Bida Basin, central Nigeria PETR VRBKA, ~'* OLUSOLA JOHNSON OJO 2 and HOLGER GEBHARDT 2.3 ~Geologisch-Pal~iontologisches Institut, Technische Universit~it Darmstadt, Schnittspahnstr. 9, D-64287 Darmstadt, Germany 2Department of Geology and Mineral Sciences, University of Ilorin, PMB 1515, Ilorin, Kwara State, Nigeria 3Present address: Institut for Angewandte Geowissenschaften II, Technische Universit~t Berlin, Sekr. £B 10, Ernst-Reuter-Platz 1, D-10587 Berlin, Germany
ABSTRACT--A set of outcrop samples from the Lokoja and Patti Formations of the southern Bida Basin (Nigeria) was examined for grain size distribution, sedimentary and hydraulic characteristics. Most of the samples are well-sorted with an uniformity coefficient (U) ranging between 1.3 and 6.3. The mean effective grain diameter (dlo) is in the order of 0.11 mm, the mean value of dgo was determined as 0.66 mm (both geometric mean) and the median grain size (dso) as 0.23 mm. Based on these values, the sedimentary sequence can be described as 'fine to medium sized sand', having minor amounts of either silt, or coarse sand and some gravel. The total porosity of the samples was determined by laboratory methods to be in the range of 9-29%. The hydraulic conductivity (K) was determined according to Hazen. and Beyer, and by Shepherd's formula, the last resulting in a geometric mean of 3.3 m d -~ or 3.3 darcy. The results are used to estimate local groundwater potential. The entire pore space (potential groundwater reservoir) for the area under study is estimated to be in the order of 290-430 km 3. Because of higher hydraulic conductivities, it is recommended that the Lokoja Formation is concentrated on as a target for groundwater exploration. © 2000 Elsevier Science Limited. All rights reserved. RL=SUMI~ - Un nombre repr~sentatif d'~chantillons a ~t~ pr~lev6 dans les formations du Lokoja et du Patti dans le sud du bassin du Bida (Nigeria) afin d'effectuer une analyse granulom~trique et de d~terminer les caract~ristiques s6dimentaires et hydrauliques. La plupart des ~chantillons ont ~t6 d0ment classes avec un coefficient d'uniformit~ (U) entre 1.3 et 6.3. Le diam~tre effectif (dto) est en moyen de 0.11 mm et la valeur moyen pour le dgo a 6t6 6valu6e & 0.66 mm (il s'agit dans le deux cas de moyennes g~om6triques), pour le grains de sables le median d~o est de 0.23 mm. Partant de ces r6sultats, le s6ri~ s~dimentaire peut ~tre d6crit comme ~tant de sable fin ~ moyen contenant de petites quantit~s de silt, de sable grossier ou de conglom~rat. Les donn~es concernant la porosit6 totale des pr~l~vements ont ~t6 d~termin~es grace ~ des m~thodes de laboratoire et estim~es entre 9 et 29%. La conductivit6 hydraulique a (~t~ d~termin~e d'apr~s les m~thodes de Hazen, Beyer et la formule de Shepherd, cette derni~re donnait comme r6sultat une moyen g~om~trique de 3.3 m d 1 ou de 3.3 darcy. Ces r~sultats servent ~ ~valuer le potentiel des eaux souterraines de la r~gion 6tudi6e. Le r~servoir potentiel d'eau souterraine, la porosit~ totale pour cette r6gion se situerait entre les 290 et 430 km 3. A cause de ses conductivit~s hydrauliques plus ~lev6es, il est recommand~ de se concentrer sur la Formation du Lokoja pour I "exploration d "eau souterraine. © 2000 Elsevier Science Limited. All rights reserved. (Received 14/9/98: revised version received 17/12/98: accepted 5/1/99)
* Corresponding author [email protected] (P. Vrbka) Journal of African Earth Sciences 659
P. VRBKA et al.
INTRODUCTION AND GEOLOGICAL SETTING The research area presented in this paper is situated at the southeastern margin of the Bida Basin (synonym with Nupe Basin, middle Niger Basin) around the confluence of the River Niger and River Benue at Lokoja, in central Nigeria (Fig. 1 ). The direct distance to the Bight of Benin (Atlantic Ocean) is about 400 km. The study area has a north-south extension of about 80 km, partly overlapping (around Abaji) with the southern part of the Federal Capital Territory (FCT). The booming capital Abuja has an increasing demand for water, hence the evaluation of nearby water resources is of national interest. Along the road from Lokoja to the new capital Abuja, road construction activities gave the opportunity to study the outcropping Upper Cretaceous sedimentary rock sequences, which are covered elsewhere by thick lateritic soils. Altogether, nine localities were studied for vertical profile description of the consolidated, yet brittle, sediments which are cemented mainly by kaolinite. The depositional environment is mainly continental, with a minor marine influence in the upper part (cf. Adeleye, 1989; Braide, 1992a; Ladipo et al., 1994) and varies only a little in the entire basin (Braide, 1992b, c; Olaniyan and Olobaniyi, 1996). Towards the Anambra Basin, more marine influence is recorded for time equivalent formations (Adeniran, 1991 ; Ladipo, 1986, 1988; Nwajide and Reijers, 1996; Gebhardt, 1998). The mean sediment thickness in the Lokoja area is about 500 m, its maximum does not exceed 1000 m (Ojo and Ajakaiye, 1989). The age of the investigated sediments has been determined by pollen and spore analyses as Maastrichtian by Jan du Chine et aL (1978)i The sedimentary succession comprises the Lokoja, Patti and Agbaja Formations (see also Adeleye, 1975, 1989; Braide, 1992b, c; Whiteman, 1982). Locations 1,2, 4, 10, 11 and 19 represent the Lokoja Formation, whilst locations IDU4, GEl and OR1 represent the Patti Formation (see Fig. 1). The ironstones of the Agbaja Formation have not been sampled and are not represented in the lithologs of Fig. 1. The Patti Formation is stratigraphically younger than the Lokoja Formation. The basal Lokoja Formation exposed between Lokoja and Koton-Karifi consists of conglomerates and sandstones. The conglomerates are dominantly matrix supported and contain randomly orientated, rounded to subrounded clasts suggesting a debris flow deposit (Ojo, 1992). The overlying sandstone facies of the Lokoja Formation consists of coarse to medium sized sand. The cross-stratified sandstone subfacies commonly displays tabular as well as trough cross-bedding. In some places, the cross-
660 Journal of African Earth Sciences
bedded units are interbedded with massive sandstone. The sandstones are generally immature (texturally and mineralogically). Kaolinitic matrix is very common, resulting in light brown to whitish colours. The sedimentological features indicate braided channel deposits (Ojo, 1992). The Patti Formation exposed between the towns of Koton-Karifi and Abaji consists of sandstones, siltstones, claystones and shales. The sandstone facies varies from conglomeratic sandstone to cross and parallel stratified, fine- to coarse-grained sandstone. At its basal part, the conglomeratic unit is about 0.30 m thick and is interpreted as a channel lag deposit. The coarse- to fine-grained sandstone units are characterised by moderate sorting as well as high mineralogical maturity. They probably represent a pointbar sedimentation of a high sinuosity river system (Ojo, 1992). Siltstones are commonly parallel stratified and reach thicknesses of about 15 m. The claystones are massive and commonly kaolinitic. The claystones and siltstones are interpreted as overbank deposits. The greyish shales indicate a deposition in a non-marine swamp subenvironment (Braide, 1992b, c).
Regional climatic conditions The regional climatic conditions are influenced by the location of the investigated area within the woodland savanna just north of the humid tropical rain forest belt. Basic information on the regional climate has been given by Ayoade (1974, 1977). Several important parameters have to be taken into account. Wind pattern The wind pattern over Nigeria is dominated by the Harmattan winds coming from the north during the dry season (November-April) and the southerly Monsoon winds from the Atlantic during the rainy season (May-October). Thus, during the Harmattan, rather dry, dusty winds are blowing, whereas during the rainy season, moisture-loaded air masses from the Atlantic Ocean lead to the development of a cloud cover and, eventually, loose their moisture in the form of rain. Air temperature The lowest daily minimum temperatures are found during November-January at about 20°C. The maximum daily temperatures are reached at the end of the dry season, in February-April, at about 36°C. Thus, the annual fluctuations vary mainly between 20-36°C. As a consequence, the mean annual reference temperature in this area is in the order of 28°C (Mands, 1992).
Hydraulic characteristics of the Maastrichtian sedimentary rocks of the Bida Basin, Nigeria
)Abaji
Location 2
LOcallon IDU4 i.:........
LAJ2P
iiii!i~iii~i~iiL
iiiiiiiiiiiii}i K.:.:,:.:-:,:.:.:. ~ A J 2 B
Ge g ub e rl T
~ ~ Location G~ GEl ~1
n 19 L;):;I[~:,7, -~OZlg E
, Location 11
i:~;! .~OZ 19C ......
IGE1E
l:i;i:i!i?~',
:::::::::::::::::::::::: .....
Koton-Karifi
Atlantic Ocean
Location OR1
Lateritic ironstone Localion 4
Conglomerate Coarse grained sandstone Medium grained sandstone
":':':':':::':':::i
~:.: .....-...... -.*., ::::::::::::::::i: ii?!!i?!i!iiiii!:!i
i: : ;.:-:- = LocalJorl 10
i:iii:i:iiii
ii?
~--z-,z~-;- ~ t.lOD
Fine grained sandstone Siltstone Shale, mudstone and cl~ Ferrugenized mudstone
Locallon I
".'.4
D :::iL1E
N
15 m
1 tom
Lokoja- / o~
I
10 km
I
Figure 1. Location o f research area and lithological sections. The positions o f the investigated samples are indicated.
Locations 1, 2, 4, 10, 11 and 19 are assigned to the Lokoja Formation; locations IDU4, GEl and ORI represent the Patti Formation. Lokoja is situated at latitude 7 ° 4 9 ' N and longitude 6°44"E.
Journal of African Earth Sciences 661
P. VRBKA et al. Precipitation, evaporation The Atlantic Ocean is the main source of vapour for humid air and clouds being transported across the African continent, which eventually may recharge the Nigerian aquifers as they are moved towards the north and northeast by the Monsoon winds. The mean annual precipitation is in the range of 1000-1500 mm a-~ (e.g. meteorological station Kwali, 8°52'N; 7 °00'E, period 1969-1981 ), the potential evapotranspiration according to Schendel's method results in > 4000 mm a~, whereas the real evapotranspiration is in the range of 900-1100 mm a1 (Mands, 1992). According to this range, between 100-600 mm a-~ of rain are available for overland flow, stream discharge and groundwater recharge.
Relative humidity The relative humidity of the air is high due to the relatively high precipitation amounts, evaporation and the vicinity of the River Niger Valley. At the meteorological stations in Bida and Minna, about 50% of relative humidity of the air were reported on the mean for the period 1969-1981. Whereas August is the wettest, January and February are the driest months (Mands, 1992). In the valleys of the major streams, the humidity values may reach 70-80%.
the pore volume (V w) is determined by saturation with distilled, air-free water. This can be done by allowing the water to soak the rock specimen by capillary rise forces at first, so that air included in the porous space is repelled in the upward movement. The sample is then completely covered with water for at least 24 hours. After weighing the saturated sample, the porosity volume ( = w a t e r volume V W) can be determined directly by assuming a water density of 1 g cm -3. In the next step, the total volume of the (saturated) rock sample (V s) is determined by a water-displacement test. The sample volume (V) is calculated by weighing the repelled water to an accuracy of at least 0.1 g (=0.1 cm3), or read off at the calibrated scale of the beaker. However, if problematic samples are being examined (i.e. which disintegrate rather easily as in this case) the following approach for the estimation of total porosity was applied with satisfying results. Method B The second method is based on the following formula to determine the total porosity (n) of a formation:
n = 100 (1 - d b d l ) , METHODS APPLIED
In areas where the experimental laboratory equipment is limited and funds for sophisticated, but often also sensitive methods are restricted, simple straightforward procedures are to be applied. The hydraulic conductivity (K), the intrinsic permeability (K~)and the total porosity (n) of the sediments were determined using different methods as a check on each other. In the following, the two applied experimental laboratory methods for the study of the total porosity (n) of the Upper Cretaceous sedimentary sequences of the Bida Basin are presented. Method A The total porosity (n) of a rock specimen can be obtained using the pure volumetric approach according to the formula:
n = 1 0 0 V wVs-1,
(1)
where n = total porosity (as a percentage); V w = volume of water filling the voids of the aquifer material (cm3); and V s =volume of the sample of the aquifer material (cm3). At first the dry weight of the sample is determined by oven drying at 105°C for at least 24 hours to constant weight. Then the amount of water entering
662 Journal of African Earth Sciences
(2)
where n = total porosity (as a percentage); d b= total density of the material (g cm-3); and dp=particle density of the material (g cm-3). There are several ways to determine the density values. The total density of the aquifer material is the mass of the sample after oven-drying divided by the total sample volume. According to Ojo (1992), the rock samples are mainly composed of SiO 2 grains. Hence, an average particle density of 2.65 g cm -3 can be assumed (Fetter, 1994). This density value could be increased by Fe cement or if the share of heavy minerals is appreciably high. Both factors are absent in most of the samples. For samples with Fe cement, the method was adapted as follows: the particle density was determined by dividing the oven-dried mass by the volume of the mineral skeleton. The volume of the mineral skeleton was determined by a water-displacement test. This simple method gave reproducible, satisfying results. It can accommodate sandstone samples, which may decompose if left in water for saturation for a long time. The results of both methods are given together with other findings in Table 1. The results of both methods differ only by 1-2%. The total porosity of the samples varies between 9-29%, the geometric mean being 15 or 20%. However, the porosity values
Hydraulic characteristics of the Maastrichtian sedimentary rocks of the Bida Basin, Nigeria Table 1. Effective grain diameters, porosities, hydraulic conductivity (K), intrinsic permeability (K{), and total porosity (n) of Upper Cretaceous, consolidated sediments of the Bida Basin of central Nigeria Sample
dlo
dso
d6o
d9o
mm
mm
mm
mm
U C K K K K~ Porosity n d6o/ Beyer Hazen Beyer Shep. Shep. Meth.A Meth. B dlo x 1 0 .4 m d -1 m d 1 m d -1 darcy % %
A J2 AJ2A AJ2B AJ2G AJ2M AJ2P GE1E GE1G IDU4F L1D L1E L4B L4F L10D LIOF L11G OR IC OZ19C OZ19E
0.17 0.20 0.11 n.d. n.d. 0.07 0.05 0.04 0.09 0.29 0.22 0.04 0.29 0.13 0.16 0.33 0.07 0.03 0.05
0.24 0.26 0.13 n.d. n.d. 0.10 0.14 0.10 0.11 0.44 0.66 0.11 0.71 0.64 0.25 1.00 0.11 0.17 0.18
0.25 0.27 0.14 n.d. n.d. 0.11 0.17 0.12 0.17 0.57 0.87 0.12 0.81 0.67 0.31 1.15 0.12 0.19 0.20
0.55 0.36 0.16 n.d. n.d. 0.14 0.47 0.32 0.55 2.46 2.30 0.25 2.46 0.95 1.52 3.73 0.50 0.70 0.38
1.50 1.40 1.30 n.d. n.d. 1.50 3.40 3.40 2.00 2.00 4.00 3.40 2.80 5.20 1.90 3.50 1.80 6.30 4.00
90 91 92 n,d. n.d. 90 76 76 85 85 73 76 78 68 86 75 87 65 73
42.3 32.8 61.3 48.4 19.0 14.7 n.d. n.d. n.d. n.d. 7.5 5.9 3.9 2.5 1.9 1.2 11.2 8.2 129.6 95.0 74.3 46.7 1.9 1.2 129.6 86.4 25.9 15.6 39.7 29.4 164.2 112.3 7.1 5.4 1.4 0.8 3.9 2.4
3.6 4.1 1.4 n.d. n.d. 1.0 1.6 1.0 1.0 8.9 16.3 1.0 18.2 15.6 3.8 30.5 1.1 2.1 2.3
3.6 4.1 1.5 n.d. n.d. 1.0 1.5 1.0 1.0 8.7 16.2 1.0 18.2 15.2 3.9 30.4 1.1 2.1 2.3
27 n.d. 10 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 12
25 28 9 29 22 n.d. n.d. n.d. n.d. 27 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 12
Max. Min. St. dev. Range MeanA MeanG
0.33 0.03 0.10 0.30 0.14 0.11
1.00 0.10 0.27 0.90 0.31 0.23
1.15 0.11 0.32 1.04 0.37 0.27
3.73 0.14 1.06 3.59 1.05 0.66
6.30 1.30 1.40 5.00 2.90 2.60
92 65 8 27 80 80
164.2 112.3 1.4 0.8 52.2 36.2 162.8 111.5 42.6 29.9 16.5 11.3
30.5 1.0 8.5 29.5 6.7 3.3
30.4 1.0 8.4 29.4 6.6 3.3
27 10 9 17 16 15
29 9 8 20 22 20
Max.: Maximum value; Min.: Minimum value; St. dev.: Standard deviation; Mean A: Arithmetic mean; Mean G: Geometric mean; n.d.: not determined.
are from the Lokoja Formation only. For the samples from the Patti Formation the porosity could not be determined due to decay of the samples after wetting.
HYDRAULIC PROPERTIES OF THE CRETACEOUS SANDSTONES
In order to make a statement on the groundwater resources of an area (next to the storage capacity for water) the potential of transmitting water from one locality to another or to a well is to be examined. For 17 rock samples, the hydraulic conductivities (K) have been derived from grain size analyses using the dry sieving values of Ojo's (1992) study. Together with the total aquifer thickness, they are used to describe the transmissivity (T). For a better understanding, the intrinsic permeability (K~) was also
calculated, based on the K values, the dynamic viscosity of water (p) circulating through the rock, the density (r) of water and the acceleration of gravity (g). For the calculation of the hydraulic conductivity (K) the methods after Hazen (1892) and Beyer (1964) were applied first, also described in detail by Langguth and Voigt (1980). The empirical Hazen formula is: KH .... =
0.0116 (dlo)2 (0.7 + 0.03 T),
(3)
where dlo=effective grain diameter (in mm); and T=temperature of the water (in °C). The Hazen equation can be applied only to grain size distributions with a uniformity coefficient U < 5. Based on this empirical relation, Beyer (1964) has adjusted Hazen's formula for varying uniformity coefficients (U) of a sediment introducing a factor C:
Journal of African Earth Sciences 663
P. VRBKA et al. KBeye, = C (dlo)2 (0.7 + 0.03 T).
(4)
If the variables d~o and T are as above, the formula is valid for d~o values between 0.06-0.6 mm. Factor C varies according to the uniformity coefficient (U) and the packing of the grains (Table 1 ). For the consolidated rock samples, adapted C values for tight packing according to Langguth and Voigt (1980) were chosen. The above equations are based on empirical relationships, hence they are not dimensionally correct. The hydraulic conductivities result in the unit m s4. For the proper calculation, the effective grain diameter (d~o) and the uniformity coefficient (U = d6o [dlo ]4) must be determined. The hydraulic conductivity values given in Table 1 are calculated for a mean aquifer temperature of 2 8 ° C (mean annual temperature of the area: direct groundwater temperatures were not available). However, the above formulae were developed for non-consolidated material (sand filters) and have restrictions as to uniformity (Hazen: U < 5 ) and the effective grain size (Beyer: 0 . 0 6 < d ~ o < 0 . 6 mm). Shepherd (1989; in Fetter, 1994) analysed results of 18 studies where hydraulic conductivity values had been related to the grain size. He found that all studies could be related to the general formula for hydraulic conductivity: Kshepherd = C " (dho)',
(5)
where C =varying shape factor; d~o= mean grain size (in mm); and t = exponent to be varied according to maturity. In contrast to the above formulae of Hazen and Beyer, the temperature of the flowing medium is not needed. However, C and t depend on the texture of the sedimentary material. The greatest values of C and t are for texturally mature sediments, and smallest for texturally immature and consolidated sediments. They vary between 4 0 , 0 0 0 and 1 O0 for C, and 2 to 1.5 for t. The higher values are valid for glass spheres (representing texturally mature sediments), whereas the lower values are applied to texturally immature, consolidated sediments. A further advantage is the introduction of the mean grain diameter (d~o) instead of the effective grainsize (dlo). Thus, the formula can also be applied for sediments where the above restrictions do not allow the application of the Hazen or Beyer approach. The application of Shepherd's formula, again dimensionally not correct, gives hydraulic conductivities in the unit ft d 1, which were transferred into m d 4 (Table 1 ). For completion and control of the values calculated according to Hazen and Beyer, Shepherd's formula
664 Journal of African Earth Sciences
was also applied to the analysed samples (Table 1 ). The sedimentary sequence, as described earlier, is mainly built up of Upper Cretaceous braided streams and channel deposits. Therefore the following C and t values for consolidated sediments were taken for the calculation of the hydraulic conductivity (K) (C = 100; t = 1.5). In Table 1, the results for K are all given in m d 1. The values calculated by the three methods range from 0.8 to 164 m d 1 . In addition to arithmetic means, the geometric means are also given here in respect to the non-Gaussian distribution of the values and the occurrence of 'outliers'. The geometric mean values are KHazen> KBeyer> Kshepherdand are related by 1.5 : 1.0 : 0.3. Thus, the method of Shepherd gives results about 5 times smaller than the Hazen, and about 3 times smaller than the Beyer method. In the past, the t w o last methods were applied for the determination of K values for older, consolidated sediments (Kheir, 1986; Schneider, 1986). Usually this was due to the lack of pumping tests taking into account the characteristics of the material, or for comparison. However, these two empirical formulae were developed for and tested on non-consolidated sands for filtering purposes. The results show that the Shepherd formula gives more realistic results, if mature and consolidated sediments are examined. For the Cretaceous formations of the Bida Basin a range of 1-30 m d 4 for the hydraulic conductivity (K) is considered to indicate more realistic values, the geometric mean of Shepherd is in the range of 3.0-3.5 m d 1. The approximations, however, need to be interpreted in a conservative way. For example, Vrbka (1996) calculated geometric mean values for the hydraulic conductivity (K) in the order of 1.3-1.7 m d -1 for comparable Lower Cretaceous sandstones in Sudanese sedimentary basins (Kordofan State), based on pumping test analyses. These values from a fractured, doubleporosity sandstone aquifer are still lower by a factor of 2, probably due to compaction. Hence, assuming deep buried sediments in the Bida Basin, a range of 1.5-1.7 m d 4 will be considered for further calculations. Darcy's proportionality constant (K) is a function of properties of both the porous medium and the fluid passing through it. However, there is another proportionality constant used in fluid hydraulic studies, termed the intrinsic permeability (K~), which is a function of the porous media alone: K i = C d 2,
(6)
where d = mean pore diameter as in Shepherd's formula; and C = s h a p e factor. Thus the intrinsic
Hydraulic characteristics of the Maastrichtian sedimentary rocks of the Bida Basin, Nigeria u
i
1 0-5
I
1 0-3
I
t
I
1 0-1
1
1 01
I
I
I
Patti F m ~ . , =
1 0-5
I
1 0-3
I
I
I
Iver, ow i
ow
1 0-1
I
1 03
I ~-~
1
I
1 01
I
I
t
1 05
I
I
Lokoja F m
1 03
I
1 05
I
j
Ki (darcy)
oermeabeI hob Iver, h oh I p000' ermeability
i
Figure 2, Evaluation o f permeability based on the hydraulic conductivity (K) and intrinsic permeability (K). The ranges for the investigated sediment samples o f the Lokoja and Patti Formations are shown.
permeability is used to describe the overall effect of the shape of the pore space. The dimension of K~ is length 2, or area. The relationship between hydraulic conductivity (K) and intrinsic permeability (K~)is given as:
K
=
Ki g/j-l,
(7)
where g =specific weight of the fluid (density [r] times acceleration of gravity [g] = 9.81 m s2); and p=dynamic v i s c o s i t y . For t h e g i v e n mean groundwater temperature of 28°C, the following values can be used: p = 0 . 8 3 6 0 x 10 .3 kg s -1 m l ; r = 996.23 kg m -3. Thus, the intrinsic permeability (K)~ can easily be determined if the hydraulic conductivity and the physical properties of the fluid at a given temperature are known. In the petroleurn industry, the darcy is used as a unit of intrinsic permeability, where 1 darcy is defined as 9 . 8 7 x 1 0 .9 cm 2 ( = 9 . 8 7 x 10 -13 m2).
RESULTS Most of the samples are well-sorted with a uniformity coefficient (U) between 1.3 and 6.3 (Table 1). The mean effective grain diameter (dlo) is 0.11 mm, the mean (dgo) was determined as 0.66 mm and the median grain size (d~o) as 0.23 mm. Based on these values, the sediments can be described as 'fine to medium sized sand', having only minor amounts of either silt, or coarse sand and some gravel.
estimated, giving potential groundwater storage capacities between 120-180 L m -3.
Hydraulic conductivity The hydraulic conductivity (K) determined according to Hazen (1892)is in the range of 1.4-164.2 m d -1, whereas according to Beyer (1964), the range is 0 . 8 - 1 1 2 . 3 m d -1 (both at 28°C). The Shepherd approach gives a range of 1.0-30.5 m d 4. Samples of the Lokoja Formation show higher hydraulic conductivities than those of the Patti Formation, corresponding with their larger grain size. The values are in the range 'permeable' to 'highly permeable' (Fig. 2).
Intrinsic permeability The intrinsic permeability (K~) was calculated based on the K value achieved by the Shepherd formula and is given in darcy units: the values range from 1.0-30.4 darcy. In the present case, for a mean annual temperature of 28°C, the values of hydraulic conductivity (K) in m d -1 correspond to the values of the intrinsic permeability (K~) in darcy. The geometric mean for both aquifer properties is given as 3.3 m d -1 or 3.3 darcy (Table 1).
Transmissivity Based on the above mean value for K and a mean saturated aquifer thickness of about 500 m, the mean total t r a n s m i s s i v i t y of the sedimentary sequence can be determined as 1650 m 2 d 1 or 1650 darcymetres.
Total porosity The total porosity of the samples investigated is in the range of 9-29%, the median being 15-20%. However, since the rock samples were taken close to the surface, for the sequence of buried sediments lower effective porosities n e between 12-18% are
DISCUSSION The research area is situated in the Lower Niger Basin, which corresponds to the southeastern part of the (geological) Bida Basin. The ground slopes
Journal of African Earth Sciences 665
P. VRBKA et al.
towards the River Niger from elevations above 300 m asl on both valley sides to less than 100 m where the river flows. The Lower Niger Basin area is drained by minor rivers flowing roughly southwestnortheast and north-south from the Kabba and Jos highlands in the south and the north, respectively. The Patti and Lokoja Formations, overlying the basement rocks in succession, are reported to be equivalents to the Enagi Siltstone, Bida Sandstone or Nupe Formations further to the north (e.g. Adeleye, 1975, 1989; Whiteman, 1982; Offodile, 1992). The Patti Formation has a maximum thickness of about 100 m and is composed of fine to medium sized sandstones, some claystones and carbonaceous siltstones. Oolithic inclusions of ironstones are reported towards the top. The samples have distinctively lower hydraulic conductivities than those of the deeper Lokoja Formation, and may therefore not be a primary target for groundwater exploration. Additionally, intercalated clay and shale layers may reduce the productivity of wells, if vertical recharge plays a major role. The sandstones of the Lokoja Formation rest directly on the granitic to quartzitic basement rocks. The whole sequence consists of pebbly, clayey grits and sandstones with a minimum thickness of 250 m. Due to their higher hydraulic conductivity values, groundwater exploration should focus on rocks of the Lokoja Formation. Mands (1992) reported on three pumping tests situated within the sedimentary series of the Bida Basin, which were performed at Shashi Dama, Gaba and Etsugaie. The transmissivities are in the range 5.5-29.3 m ~ d -1. The resulting hydraulic conductivities were reported as 6 x 106 to 3 x 10 .5 m s-1 (0.5-2.6 m d-l), which are within the range of the results for this study. The work of Kehinde (1990) indicates lower transmissivities for sediments of the Bida Basin (mainly <-50 m 2 d-1) than those calculated here (1.65 m 2 d-lL This discrepancy may partly be due to the fact that mainly sandy samples were examined, resulting in higher hydraulic conductivities. Furthermore, pumping test wells usually are not fully penetrating and the screens may have been positioned in zones with relatively low hydraulic conductivities. Thus, the transmissivity values from pumping tests are based on relatively short screen sections of several metres, whereas this study calculated the bulk transmissivity for a sediment thickness of 500 m. Based on the mean value, for screen lengths of 3-10 m a transmissivity in the range of 10-33 m 2 d-1 would result, to some extent confirming the findings of Kehinde (1990) and Mands (1992). Hence, as a first approach to describe hydraulic conductivities of consolidated sediments and for the approximation of the bulk transmissivity, the appropriate application of the method according
666 Journal of African Earth Sciences
Shepherd will give usable results. Based on the thickness data of Ojo and Ajakaiye (1989: ca 500 m) for the Lokoja-Abaji section and a void space of 12-18%, the entire void ratio in the area (about 60 x 80 km = 4800 km 2, Fig. 1) is estimated to be in the range 290-430 km 3, corresponding to a groundwater reservoir of similar size. However, for a better understanding and evaluation of the groundwater situation in the area, further long-term studies need to be carried out.
ACKNOWLEDGEMENTS The authors are grateful to the DAAD (German Academic Exchange Service) which enabled two of the authors (PV, HG) to spend time doing research at the University of Ilorin, Nigeria. Dr P. Olasehinde, Department of Geology and Mineral Sciences of the University of Ilorin, Nigeria, has provided essential literature and expert knowledge. Special thanks are due to Maria Mendes, Berlin, Germany, for correcting the French r~sum~. Editorial handling - P. Eriksson & G. W. McNeill
REFERENCES Adeleye, D.R., 1975. Nigerian Late Cretaceous stratigraphy and paleogeography. Bulletin American Association Petroleum Geologists 59, 2302-2313. Adeleye, D.R., 1989. The Geology of the Middle Niger Basin. In: Kogbe, C.A. (Ed.), Geology of Nigeria, 2nd edition. Elizabethan Publishing Co., Lagos, pp. 283-287. Adeniran, B.V., 1991. Maastrichtian tidal flat sequences from the northern Anambra Basin, southern Nigeria. Nigerian Association Petroleum Explorationists Bulletin 6, 56-66. Ayoade, J.O., 1974. A statistical analysis of rainfall over Nigeria. Journal Tropical Geography 39, 11-23. Ayoade, J.O., 1977. Evaporation and evapotranspiration in Nigeria. Journal Tropical Geography 42, 9-19. Beyer, W., 1964. Zur Bestimmung der Wasserdurchl~issigkeit von Kiesen und Sanden aus der Kornverteilungskurve. Wasserwirtschaft, Wassertechnik 14, 165-168. Braide, S.K., 1992a. Geological development, origin and energy mineral resources potential of the Lokoja Formation in the southern Bida Basin. Journal Mining Geology 28, 33-44. Braide, S.K., 1992b. Alluvial fan depositional model in the northern Bida Basin. Journal Mining Geology 28, 65-73. Braide, S.K., 1992c. Syntectonic fluvial sedimentation in the central Bida Basin. Journal Mining Geology 28, 55-64. Fetter, C.W., 1994. Applied Hydrogeology, 3rd edition. Prentice Hall, New Jersey, 691 p. Gebhardt, H., 1998. Benthic foraminifera from the Maastrichtian lower Mamu Formation near Leru (southern Nigeria): paleoecology and paleogeographic significance. Journal Foraminiferal Research 28, 76-89. Hazen, A., 1892. Some physical properties of sands and gravels with special reference in their use in filtration. Annual Reports Massachusetts State Board Health 24, 541-555. Jan du Ch6ne, R.E., Adegoke, O.S., Adediran, S.A., Petters, S.W., 1978. Palynology and foraminifera of the Lokoja Sandstone (Maastrichtian), Bida Basin, Nigeria. Revista EspaSola Micropaleontologfa 10, 379-393.
Hydraulic characteristics of the Maastrichtian sedimentary rocks o f the Bida Basin, Nigeria Kehinde, M.O., 1990, Die Grundwasser-Ressourcen des BidaBeckens, ZentraI-Nigeria. Ph.D. Thesis, University of MLinster, Germany, 150p. Kheir, O.M., 1986. Hydrogeology of Dongola Area, Northern Sudan. Berliner Geowissenschaftliche Abhandlungen, Reihe A 74, 1-81. Ladipo, K.O., 1986. Tidal shelf depositional model for the Ajali Sandstone, Anambra Basin, southern Nigeria. Journal African Earth Sciences 5, 177-185. Ladipo, K.O., 1988. Paleogeography, sedimentation and tectonics of the Upper Cretaceous Anambra Basin, southeastern Nigeria. Journal African Earth Sciences 7, 865871. Ladipo, K.O., Akande, S.O., M~cke, A., 1994. Genesis of oolithic ironstones from the Middle Niger Sedimentary Basin: evidence from the sedimentological, ore microscopic and geochemical studies. Journal Mining Geology 30, 161-168. Langguth, H.-R., Voigt, R., 1980. Hydrogeologische Methoden. Springer-Verlag, Berlin, 486p. Mands, E., 1992. Kritische Betrachtung der Wasserbilanzparameter und hydrologische Untersuchungen der Einzugsgebiete River Gbako und River Gurara (Mittelnigeria). Giessener Geologische Schriften 47, 1-174. Nwajide, C.S., Reijers, T.J.A., 1996. Sequence architecture in outcrops: examples from the Anambra Basin, Nigeria.
Nigerian Association Petroleum Explorationists Bulletin 11, 23-32. Offodile, M.E., 1992. An Approach to Ground Water Study and Development in Nigeria. Mecon Services Ltd., Jos, 247p. Ojo, S.B., Ajakaiye, D.E., 1989. Preliminary interpretation of gravity measurements in the Middle Niger Basin area, Nigeria. In: Kogbe, C.A. (Ed.), Geology of Nigeria, 2nd edition. Elizabethan Publishing Co., Lagos, pp. 347-358. Ojo, O.J., 1992. Petroleum geology and sedimentology of Patti Formation, Bida Basin, Nigeria. M.Sc. Thesis, University of Ibadan, Ibadan, Nigeria, 149po Olaniyan, O., Olobaniyi, S.B., 1996. Facies analysis of the Bida Sandstone Formation around Kajita, Nupe Basin, Nigeria. Journal African Earth Sciences 23, 253-256. Schneider, M., 1986. Hydrogeologie des Nubischen Aquifersystems am SL~drand des Dakhla-Beckens, SL~d~igypten/Nordsudan. Berliner Geowissenschaftliche Abhandlungen, Reihe A 71, 1-66. Vrbka, P., 1996. Hydrogeologische und isotopenhydrologische Untersuchungen zu regionalen Problemen der GWNeubildung, der GW-Zirkulation und des Wasserhaushaltes im Nordsudan. Berliner Geowissenschaftliche Abhandlungen, Reihe A 186, 1-158. Whiteman, A., 1982. Nigeria: Its petroleum geology, resources and potential. Graham and Trotman, London, 394p.
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