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Hydrological Sciences-Journal-des Sciences Hydrologiques, 42(2) April 1997 141

Hydrodynamic flow and formation pressures inthe Anambra basin, southern NigeriaK. O. UMA & K. MOSTO ONUOHADepartment of Geology, University ofNigeria, Nsukka, NigeriaAbstract Pressure measurements and records of drill-stem tests (DST) from deeppetroleum exploration wells in the Anambra basin have been analysed together withfluid potential data obtained from over 500 water wells drilled in the basin. Theanalyses indicate the existence of three distinct hydraulic systems in the basin, viz.:an upper system with hydrostatic formation pressures; a middle system in whichpressures are just moderately higher than hydrostatic; and a relatively deep system ofabnormally high formation pressures. The three hydraulic systems correspondapproximately to three hydrostratigraphic units that are clearly discernible from thelithologie logs of boreholes and oil wells drilled in the basin. The main fluid in theuppermost hydraulic system is circulating meteoric water, and the fluid potentialdistribution is largely governed by the local topography at the surface. Within themiddle hydrostratigraphic unit, hydraulic heads and fluid energies are highest at mebasin edge to the east where the major aquifer of the unit is exposed, and muchlower in the basin centre to the southwest where the aquifer is confined. Themagnitude and distribution of fluid potentials in the two top hydraulic systemssuggest that the general hydrodynamics of the basin are, to a large extent,responsible for the generation of the fluid pressures. In the third and deepesthydraulic system, however, the situation is different. The hydraulic heads andformation pressures are very high indeed, and cannot be explained purely in terms ofcirculating meteoric waters. Some other fluid energy sources must also be active inthis part of the basin. The fluid potentials and pressures fluctuate very rapidly bothlaterally and vertically, suggesting the existence of distinct flow units within theentire system. Each flow unit appears to be hydraulically closed, sealed bothvertically and horizontally, and characterized by a unique fluid energy distribution.The existence of both vertical and horizontal potential gradients at depth, especiallyat the basin centre, indicates that the fluids are not static, but mobile, and that thecomplex movement of fluids could be through deep-seated drains. Such fluidmovements obviously affect the temperatures of the sedimentary layers and couldalso be significant in the migration and accumulation of hydrocarbons in the basin.Flux hydrodynamiques et pressions des formations dans le bassind'Anambra (Nigeria mridional)Rsum Les mesures de pression et les enregistrements des tests de forageprovenant de puits d'exploration ptrolire profonds du bassin d'Anambra ont tanalyss conjointement avec les donnes recueillies sur plus de 500 puits hydrauliques fors dans le bassin. Ces analyses montrent l'existence de trois systmeshydrauliques : un systme suprieur o les pressions sont hydrostatiques, un systmemoyen o les pressions sont lgrement suprieures des pressions hydrostatiques etun systme relativement profond o les pressions sont anormalement leves. Cestrois systmes hydrauliques correspondent sensiblement aux trois units hydro-stratigraphiques que l'on peut clairement discerner sur la lithologie des logs dessondages et des puits fors dans le bassin. Le principal fluide circulant dans lesystme suprieur est de l'eau mtorique et la distribution des charges estessentiellement gouverne par la topographie locale. A l'intrieur de l'unit hydro-stratigraphique moyenne, les charges hydrauliques et l'nergie des fluides sont pluslevs l'extrmit orientale du bassin, l o l'aquifre le plus important est dcouvert, qu'au centre et au sud-ouest du bassin o il est confin. L'amplitude et larpartition des charges hydauliques dans les deux systmes suprieurs suggrent quec'est bien l'hydrodynamique propre du bassin qui est dans une large mesure

Open for discussion until I October 1997

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142 K. O. Uma & K. Mosto Onuoha

l'origine de la pression des fluides. La situation est diffrente dans le troisimesystme situ plus grande profondeur. Les charges hydrauliques et la pression desformations sont particulirement leves et ne peuvent s'expliquer seulement entermes de circulation d'eaux mtoriques. D'autre sources d'nergie doiventvraisemblablement tre actives dans cette partie du bassin. Les charges et lespressions varient trs rapidement, aussi bien verticalement qu'horizontalement,suggrant la prsence de flux distincts au sein du systme. L'existence de gradientshorizontaux et verticaux en profondeur, surtout au centre du bassin, indique que lesfluides sont en mouvement et que ces mouvements complexes peuvent se faire travers des drains profonds. De tels mouvements de fluides affectent videmment latemprature des couches sdimentaires et peuvent jouer un rle dans la migration etl'accumulation des hydrocarbures dans le bassin.

INTRODUCTIONFluids in porous media move from regions of high energy (high potential) to regionsof low energy. The energy gradient to which the fluids react consists of an aggregateof potentials resulting from elevation and pressure (hydraulic potential), thermal,electro-osmotic and chemico-osmotic forces within the system. The intensity of anyof these sources varies considerably and depends on the specific attributes andgeological history of the basin. The overall effect of the various fluid potential sourcesis measured in a well as the formation pressure, which is equivalent to the height towhich the formation fluid (water, oil or gas) will rise in a well drilled to a given depth.In a perfectly unconfined system (one in which there is no upper seal), the fluidlevel in a well is usually equivalent to the elevation of the water table. In this casethe formation pressure is hydrostatic. Formation pressures above or below thehydrostatic pressure are often referred to as abnormal pressures. In a sedimentarybasin, it is logical to expect that fluid flow patterns will be closely related to theformation pressure distribution. Several studies of basin hydrodynamics with relationto formation pressures have been made in the past, e.g. by Hitchon (1969a,b),Carstens & Dpvik (1981), and Belitz & Bredehoeft (1983, 1988). These studies haveconfirmed that understanding the fluid flow pattern is essential in studies dealing withformation pressures in a sedimentary basin.

This paper presents the results of an investigation into the fluid potentialdistribution and pattern of fluid flow in the Anambra basin of Nigeria. The mainobjectives were to delineate the flow systems and outline the various sources of fluidenergy for each system. This paper throws light on the distribution of formationpressures and on the energy characteristics of the overpressured zones in the basin.

DESCR IPTION OF THE ANAMBRA BASINThe Anambra basin is situated at the southwestern extremity of the Benue Trough ofNigeria (Fig. 1). It is bounded on the west by the Precambrian basement complex rocksof western Nigeria and on the east by the Abakaliki Anticlinorium. The boundaries tothe north and south are not very well defined. For this study, the northern boundarywas taken to be the limit of exposure of the Maestrichtian sediments, while thesouthern boundary was set at Onitsha, the northernmost limit of the present day NigerDelta basin. The stratigraphie succession in the basin has been described by several

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Hydrodynamic flow and formation pressures in the Anambra basin, Nigeria 143

0 50 100kmI r ~ ^ ^0 20 40 60mi

Fsm ando i f f ePoo gCi

Rfrst Tert iary DslaicS*^imntt(Eocnt-R#c?rt5Traiwgrsssiv LowsrT w t i o r y S t d i m s n t t(Pateoc*n-EoeB)Regressive Uppr f>*ooySediments (Csrapef iiea-

Folded Cra'fscs3". S&mmtU(ApUm-Samimkm )Mtamorphic B

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144 K. O. Uma & K. Mosto O nuoha

-A N A M B R AO T U O C H A

A S i NA K A M A N G W O

O V E

fO 15Km.

l O n tl y S h a l e U n i t sP e r m e a b i e U n i t sA M u v i a i D e p o s i t sB o r e h o l e P o i n tA m e k i F o r m a t i o n! m o F o r m a t i o nN s u k k a F o r m a t i o nA j a i i S a n d s t o n eM a m u F o r m a t ) onN k p o r o an d A w gu F o r m a t i o n

L Eea

s\ y

S E N DDeep Weils Drilled for OiWater Borehole

Topographic contour (ft)

Exploration ^ RiverRood

a Stream

Fig. 2 (a) Schematic geological section across the Anambra basin; and (b) locationof some wells discussed in the text.

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Table 1 Generalized stratigraphie section in the Anambra basin.Recent Marine deltaic deposits, alluviumNeogeneMoocene-Pleistocene Benin FormationOligocne? - Miocene Ogwashi-Asaba FormationPalaeoceneBartonian Possibly, upper part of Ameki FormationLutetian Am eki FormationYpresian Possibly part of Ameki Formation Nanka SandPalaeogene Imo ShaleUpper CretaceousDanian Nsukka FormationAjali SandstoneMaestrichtian Mamu FormationCampaian Enugu Shale Nkporo ShaleConiacian-Santonian Awgu ShaleTuronian Eze-Aku ShaleCenomanian Odukpani FormationLower CretaceousAlbian Abakaliki Shale "Asu River Group"(Based on Reyment, 1965; Whiteman, 1982).

The three hydrostratigraphic units identified in the Anambra basin are separated bythick (>100 m ) clay-Shale units which act as confining beds and provide effectivevertical seals against the escape of fluid pressure. The middle hydrostratigraphic unit isthe most prolific and its surface outcrop forms the hydrological boundary of the basinin the north and east. Analyses of lithological and geophysical logs indicate that thereis a permeability continuity throughout much of the middle unit. On the other hand,there are rapid lateral facis changes and interfmgering between sandy and Shaley unitsin both the upper and the middle hydrostratigraphic units.DATA PREPARATIONTwo types of fluid potential data were used for this study. The first comprises waterwell data, while the second is made up of pressure measurements in deep wells drilledfor oil exploration purposes. More than 500 water wells have been drilled in theAnambra basin by various federal and state government agencies and by privateinstitutions and organizations. The boreholes range in depth from less than 60 m toabout 300 m below the surface. During a field inventory by the authors, theseboreholes were located on a topographic map on a scale of 1:50 000. Information onstatic water levels at the time of drilling were obtained from the agencies that drilledand completed the boreholes, while elevations at borehole locations were estimatedfrom topographic maps. Hydraulic potential values were obtained as the differencebetween the elevation and the static water level.

The pressure data were obtained from bottom-hole pressure measurements andrecords of drill stem tests contained in various wire-line logs that were ran in the deep

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146 K. O. Uma & K. M osto Onuoha

wells drilled for petroleum exploration. Pressure values in pounds per square inch (psi)were converted to fluid potential using the relationship:h = (1)

where p = formation pressure (M L"1 T 2); 5 = fluid density (M L~3); g = accelerationdue to gravity (L T 2); and h = pressure head (L).The density of fresh water was taken as 1000 kg m"3 throughout the study. Thepressure head values were used as total hydraulic potential at the depth ofmeasurement, once adjustments were made for elevation differences. The fluid potential data were then grouped and analysed by hydrostratigraphic units.The three hydrostratigraphic units were isolated and traced laterally at depth on thebasis of lithological and geophysical logs. The fluid potential data for each unit werethen contoured to produce isopotential maps. A pressure profile of fluids (Fig. 3) in theentire basin was also prepared for a critical assessment of the fluid pressures withrespect to the hydrodynamics of the basin.

9- Fluid Pressure (p si )2000 3000

c( ! ) Zona l(2) Zone U :

(3) Zone m

u m m o r yPressurePressures80 psi) oPressures

of Pressure Distributionis hydrostaticar ebovear e

Un I d a 200 -psi above

just marginally ( teas thanhydrostatic ;abnormally high1200-, l b 19 0 0- 2 30 0hydrostatic) '/.%.,.

Fig . 3 Fluid pressure profile in the Anambra basin.

DETAILED ANALYSES OF THE RESULTSThe pressure profile in Fig. 3 shows three distinct hydraulic units: zone I - fluids withhydrostatic pressures at the near surface; zone II - those with slightly higher formationpressures at moderate depths (600-1600 m; zones Ilia and Mb - fluids with high andvery high pressures respectively at the deeper levels of the basin. The differenthydraulic units correspond to, and therefore justify, the existence of thehydrostatigraphic units already outlined earlier.

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At the near surface (i.e. depths < 500 m), fluid pressures are essentially hydrostaticand water levels in wells completed within this zone are approximately equal to theregional water table. At moderate depths of about 600-1600 m, hydraulic heads are40-85 m above the ground surface at the borehole locations. Hydraulic heads at deeperlevels are generally 150-1000 m above the ground surface.Table 2 displays the fluid pressure and fluid potential vs depth relationships atsome specific well locations. It is evident from the data and from Fig. 3, that at leasttwo overpressured zones occur in the Anambra basin. The first and upper zone is only

Table 2 Fluid potentials and pressures in the third (deepest) hydrostratigraphic unit.Depth (m)

6428301005107014101566163217981800201362263165517552174217622247237688879491285135514181482169918071257144516432267155122302243233821112185

Fluid pressure(m)*690.50875.851049.611116.341469.492109.472210.732257.142249.412574.26651.72658.12681.381974.472345.042340.822350.86772.14817.07934.80996.101329.11140914801537243826211272146516522636159435483578394821222350

838511560302384285770038941366

Fluid potential(m.a.g.s.)48.5045.8544.6146.3459.99544.47578.73459.14449.41561.2629.3327.1826.06219.43171.52164.55127.1449.6449474744546255

07801011838511740.06814.0315.0220.389.42370.3543.471318.001335.381511.1411.3016 5.66

Fluid potential(m.a.s.l.)73.5070.8569.6171.3481.99569.47603.73484.14474.41586.2152.7150.5649.44242.81194.90187.93150.5274.1473.5772.3071.6068.6179.3387.3579.61764.56838.5339.0244.3833.42394.3568.471343.001360.381636.1461.13215.66

GeologicalformationAjaliAjaliAjaliU. MamuU. MamuLower MamuLower MamuNkporoNkporoNkporoAjaliAjaliAjaliNkporoAwguAwguAwguAjaliAjaliAjaliAjaliUpper MamuUpper MamuUpper MamuUpper MamuLower MamuNkporoAjaliAjaliUpper MamuNkporoUpper MamuLower MamuLower MamuNkporoAjaliLower Mamu

Hydn>strat.unitIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII

m.a.g.s. = metres above ground surface; m.a.s.l. = metres above sea level.

AR3

AL1

AR2

OK I

OR1

IJ1

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very m oderately overpressured and is situated between the depths of 600-1600 m belowthe surface. The second and major zone of overpressuring occurs at depths greater than1600 m. The spatial distribution of fluid potentials is given in Figs 4 and 5 for theupper and middle hydrostratigraphic units.Figure 4 shows that the potential distribution in the upper unit is generallyinfluenced by the local topography of the surface. Higher potentials and thus therecharge areas are at the topographic highs, while low potentials, corresponding todischarge zones occur at the lowlands and plains. The main fluid in this zone iscirculating meteoric water. The fluid flow in the middle unit (Fig. 5) is influenced bythe regional topography of the basin. Highest potentials are found at the topographichighs at the periphery of the basin where the rocks are exposed, while lowest potentialsoccur at the basin centre where the unit is confined by thick clay-Shales of the Imoformation. Continuity of fluid flow throughout the middle hydrostratigraphic unit isindicated by the absence of lateral seals. The rocks are continuously permeable fromthe exposed zone at the basin edge to the confined zone at the centre.Figure 5 also indicates two types of groundwater fluxes within the middlehydrostratigraphic unit. The first is a high fluxing zone in the area where the unit isexposed and unconfmed. This is indicated by high potential gradients (closely spacedisopotential lines) towards the eastern edge of the basin. The second zone is one of

6**> & - r W f c

O 10 20 30 KmL E S E H 0

3

V _ > ^D e e p W e i ls D r i l l a d f o r O i l E x p l o r a t i o nWater Boreho leFlu id Pot ent i al Contour Un Metre s) :Contour interval*50 m.

LJA River a S t reamR o a dFig. 4 Fluid potential in the upper hydrostratigraphic unit.

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Deep We i ls Dr i l l ed fo r O i l Ex p lo ra t ion jJZs Rive r a S t reoW a t e r 8 o r e h o l e f~ R o a dFluid Poten tial Contour (in Me tresfC ontour Interval

Fig. 5 Fluid potential in the middle hydrostratigraphic unit.

relatively low fluxing and occurs at the basin centre. The potential gradient here isgenerally less than 0.002, an order of magnitude less than the gradient in the upperzone. The demarcation line occurs where the hydrostratigraphic unit becomes confinedand is marked by the existence of a continuous line of springs (Fig. 2(a)). A schematicillustration of the groundwater flow pattern in the middle hydrostratigraphic unit isshown in Fig. 6. The high fluxing upper system supports the several springs lining thecontact between the unit and the clay Shales of the Imo formation. Deep outflow asdepicted in the diagram is supported by the regional distribution of potential heads inthe basin (Fig. 5).At specific points within the middle hydraulic system, flows have both horizontaland vertical components. Studies carried out by Nwankwor et al. (1988), Uma (1992),and Uma & Onuoha (1988), at the unconfmed section where the aquifer is rechargedindicate that vertical flows are dominant with potential gradients of 0.20-0.25 relativeto horizontal flows with gradients of 0.01-0.05. Vertical flows in this zone aregenerally downward as expected for recharge zones. At the basin centre, flows aregreatly reduced compared to those at the basin edge, but vertical gradients are stilldominant over horizontal and regional gradients. For instance, vertical gradients at theAR3 well (see Fig. 5) vary from 0.014 at 640-830 m below the surface to 0.027 atabout 1000-1070 m below the surface, while the average regional horizontal gradient is0.0015.

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K. O. Uma & K. M osto OnuohaUNCONFINED FLOW

Niger R.

L E S E N DEH

Confining bed\~&4 Water iab ie1' H Unconfined groundwaterf iowl ine.1-VI Deep outf low of grou nd-i J w a t s r

O Borehole po in t.

Fi g. 6 Schem atic section showing fluid flow in the middle hydrostratigraphic unit.

60 70 80 90- > F i u i d P o t e n t i a l ( m . a . m . s . H

60 70 60 90 30 40 50 40 50 60P o

w e l l A R - 3 W e M A R - 2 W e n O K - I W e l l A L - iL E G E N D

Q ] Sand / SandstoneE Z Shale! In fe r red

m.a.m.f i - i = metresabove mean se o level

3

Fig. 7 Fluid potentials at selected locations in the basin centre.

Vertical potential gradients at the basin centre are both downward and upward.Figure 7 is a vertical fluid potential profile in some of the deep wells at the basincentre. The vertical lithological columns at the well locations are also given to aidappreciation of the profiles. It is evident from the data that the fluid potential fluctuateswith depth, and the magnitude of the fluctuation varies from place to place. While thismay reflect some minor stratigraphie variations with depth, it also suggests thepresence of both up-fluxing and down-fluxing groundwaters, in addition to the regionalflows which are essentially horizontal.

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Fluid pressures within the middle system are hydrostatic at the exposed zone wherethe unit is unconfined, but higher within the confined area. The deviation of fluidpressures from the hydrostatic appears to depend on the depth of burial (confinement)of the unit. For instance at Aguobu-Owa and Akpugo where the unit is overlain byabout 100-150 m of argillaceous materials, the fluid overpressure is less than 3.5 m ofwater whereas beneath the AR2 well at the basin centre, with about 700 m of burial,the overpressure is greater than 35 m of water. While the groundwater completedwithin the aquifer at Aguobu-Owa and Akpugo rose to 1.0-1.5 m above the groundsurface, those at the basin centre would rise to over 45 m above the surface if allowedto flow freely. However, if the fluid pressure is referred to a standard datum(e.g. mean sea level), the pressures at Aguobu-Owa and Akpugo would be higher thanthose at the basin centre. The high fluid pressures at the basin centre are thus relative tothe reference elevation.

Figure 8 gives a clearer picture of the fluid energy in the middle hydrostratigraphicunit. At the basin edge, potential heads and thus fluid energies are highest, but lessthan the local surface elevation; fluid pressures are thus hydrostatic since the aquifer isalso unconfined in this region. Towards the basin centre, potential heads are lower(relative to the edge), but the aquifer is deeply confined and the local surface elevationis very low. The fluid potential is above the local surface elevation resulting inlocalized overpressures. The absolute value of the overpressure depends on thedifference between the total potential head and the surface elevation at each specificpoint of drilling.For instance, at Otuocha (Fig. 8), where surface elevation is a little higher than itsimmediate environs, the overpressure value would be much less than that at the westernbank of the Anambra River where the value would be much higher. The conclusionthat is to be drawn from the above discussion is that fluid energies, including those ofthe overpressured formations within the middle hydrostratigraphic unit, are controlledby the regional hydrodynamics existing across the basin, and overpressures which arelocalized are purely hydrogeological in origin.

A N A M B R A B A S I NE N u G u

Fig. 8 Vertical section of fluid potentials across the middle hydrostratigraphic unit.

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The formation pressures and fluid potentials in the deepest hydrostratigraphic unit(i.e. the third hydraulic system) are given in Table 2. These values are very high evenwhen checked against surface elevations in any part of the basin. For instance the totalfluid potential is equivalent to 474-1500 m a.m .s.l. , whereas the highest surfaceelevations in the basin do not exceed 500 m a.m.s.l. The surface elevation in theexposed area of the geological formations containing the aquifer is less than 200 ma.m.s.l. In addition, an examination of lithological and geophysical logs indicates thatthe sandy horizons within the third hydrostratigraphic unit are not continuoushorizontally. Rather, these beds grade laterally into argillaceous units suggesting thatthe sandy horizons occur as lenses enclosed in the argillaceous beds. In addition, theenergy characteristics within each sandy horizon appear distinct and do not show anycontinuity with laterally adjacent units. The fluid potentials and pressures are alsocharacterized by distinctive rapid fluctuations both laterally and vertically (Figs 5, 7and 9). It is reasonable, therefore, to predict the existence of isolated flow systems inthe third hydrostratigraphic unit. Each flow unit is probably closed hydraulically andsealed both laterally and vertically. That the fluid within each of such flow units isprobably not static, but mobile, is supported by the existence of significant potentialvariations within limited depth zones.

At the AL1 well for instance, the fluid potential at a depth of 2174 m is equivalentto 194.9 m a.m.s.l., while the value at 2 m deeper (2176.0 m) is 187.93 m a.m.s.l.,giving a potential drop of 6.97 m (which is equivalent to a vertical gradient of 3.49).All the deep wells drilled at the basin centre displayed similar vertical variations as theAL1 well. These vertical gradients suggest vertical fluxes of the fluid and are evidentin the wells that penetrated both the middle and lower hydrostratigraphic units.

> F l u i d P o t en t ia l ( m . a . m . s . l )2 0 0 4 0 0 6 0 0 8 0 0 1 0 00

1 AR - 3 Well2 AR - 2 Wall

0 3 Al_ - I We ll4 OK - I Well

m.a.m.s. l = metres above mean sea level

Fig. 9 Variation of fluid potential with depth within the third hydrostratigraphic unitfor selected locations.

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Hydrodynamic lowand formation pressures in the. Anambra basin, Nigeria 153

Vertically downward flow at the eastern edge of the basin is consistent withgroundwater recharge which occurs in this area (Nwankwor et al., 1988; Egboka &Uma, 1986). The flux at the basin centre suggests some kind of discharge. However,the geometry, pathways or outlets of such highly mobile fluids are not clearly known.The Cretaceous rocks in many parts of the basin are folded and fractured, andseveral such fractures have been mapped at the outcrop of the formations. They havealso been encountered in drilled boreholes and mine tunnels (Uma & Onuoha, 1989).These fractures are interformational and could form a continuous drain across severalisolated sandy horizons. The magnitude and distribution of fluid energies observed inthe lower hydrostratigraphic unit are not possible under pure gravity flow of meteoricwater. Other energy generating sources must be operational in this part of the Anambrabasin. These sources may be related to the thermal, chemico-osmotic and electro-osmotic forces acting in the basin. The analysis of such factors is beyond the scope ofthe present report.

SUMMARY AND CONCLUSIONAn analysis of the distribution of fluid potentials and pressure depth profiles in theAnambra basin reveals the existence of three distinct hydraulic systems:- an upper system with hydrostatic formation pressures;- a middle system in which fluid pressures are moderately above hydrostatic; and- a third and relatively deep system of very high formation pressures.The three hydraulic systems correspond approximately to three hydrostratigraphic unitswhich are discernible from the lithological logs of boreholes drilled in the basin.Fluid pressures in the basin have two sources; one purely "hydro(geo)-dynam ic",and the other extraneous. The hydrodynamic sources are influential only at therelatively shallower depths in the basin (

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154 K. O, Uma & K. M osto Onuoha

Belitz, K. & Bredehoeft, J, D. (1983) Hydrodynamics of the Denver basin. AAPG Bulletin no. 67, 422.Belitz, K. & Bredehoeft, J. D. (1988) Hydrodynamics of the Denver basin: explanation of sub-normal fluid pressures.AAPG Bulletin no . 72, 1334-1359.Carstens, H. & Dypvik, H. (1981) Abnormal formation pressure and Shale porosity. AAPG Bulletin no . 65 , 344-350.Egboka, B. C. E. & Uma, K. O. (1986) Comparative analysis of transmissivity and hydraulic conductivity values from

the Ajali Sandstone aquifer system of Nigeria. J. Hydrol. 83, 185-196.Hitchon, B. (1969a) Fluid flow in the western Canadian sedimentary basins: 1. Effect of topography. Wat. Resour. Res.5, 186-195.Hitchon, B. (1969b) Fluid flow in the western Canadian sedimentary basins: 2. Effect of geology. Wat. Resour. Res. 5, .460-469.Nwankwor, G. E., Egboka, B. C. E. & Orajaka, I. P. (1988) Groundwater occurrences and flow patterns in the Enugucoal mine area, Anambra State, Nigeria. Hydrol. Sci. J. 35 , 465-481.

Onuoha, K. M. (1986) Basin subsidence, sediment decompaction, and burial history modelling techniques: Applicabilityto the Anambra Basin. In: Nigeria Assoc. Petroleum Expl. Proc, vol. 2, 6-17.

Reyment, R. A. (1965) Aspects of the Geology of Nigeria. University of Ibadan Press, Ibadan, Nigeria.Short, K. C. & Stauble, A. J. (1967) Outline geology of the Niger Delta. AAPG Bulletin, no. 51, 761-779.Uma, K. O. & Onuoha, K. M. (1988) Groundwater fluxes and gully development in southeastern Nigeria. In

Groundwater and Mineral Resources of Nigeria, ed. by C. O. Ofoegbu, 39-60. Earth Evolution ScienceMonograph Series, Vieweg, Wiesbaden, Germany.Uma, K. O. & Onuoha, K. M. (1989) Groundwater resources of the Lower Benue Trough, Nigeria. In: Structure an dEvolution of the Benue Trough an d Adjacent Regions, ed. by C. O. Ofoegbu, 77-92. Earth Evolution SciencesMonograph Series, Vieweg, Wiesbaden, Germany.Uma, K. O. (1992) Origin of acid mine water in Enugu area. J. Environm. Geol. Water Sci. 20(3), 181-194.Whiteman, A. (1982) Nigeria: Its Petroleum Geology, Resources an d Potential, Graham & Trotham, London, U K.Received 10 May 1994; accepted 2 July 1996

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