Screening and Ranking of Chad Basin for co2 Sequestration ... · PDF filethere are seven (7) sedimentary basins; Chad basin, Sokoto basin, Bida Basin, Anambra Basin, Dahomey basin,

INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES

Volume 4, No 1, 2013

© Copyright 2010 All rights reserved Integrated Publishing services

Research article ISSN 0976 – 4380

Submitted on June 2013 published on August 2013 16

Screening and Ranking of Chad Basin for co2 Sequestration Potential in

Nigeria

I. Yusuf1* and N.G Obaje

1

Department of Geology and Mining, Ibrahim Badamosi Babangida University, Lapai, Nigeria

[email protected]

ABSTRACT

Sedimentary basins are suitable to different degrees for CO2 geological sequestration as a

result of various intrinsic and extrinsic characteristics. This paper preliminarily screened and

ranked the Chad Basin of Nigeria sector based on Bachu (2003) ranking and screening

criteria adapted which includes factors such as tectonic setting, basin size and depth, geology,

hydrogeology, hydrocarbon potentials, climate, geothermal, existing resources and industry

maturity. For each criterion i (i = 1…5) used for the evaluation of basin suitability,

monotonically – increasing numerical function fi is assigned, which are continuous or discrete,

to describe a value placed on a specific class j for that criterion. The lowest and the highest

functions of this functions characterize the worst and best class in terms of suitability for that

criterion, i.e. fi,1 = min (fi), where and fi,n = max (fi); where ( n = 3, 4 or 5).The criteria relate

to either the containment security, the volume of storage capacity achievable, or considering

the economic or technological feasibility. The results shows that Chad basin has Rk score

value of 0.53 against the fi, n = max (fi) value equal to 1 as highest value of the function

characterize the best in terms of suitability for the criterion in which this ranking are based on.

Regional screening and ranking of the entire basins are recommended while detailed local

site characterisation of the basin is needed to assess its overall suitability for CO2

sequestration potentials, since countries like Cameroon, Central African Republic, Niger,

Chad, and Nigeria shares the basin on regional level

Keywords: Chad Basin, Nigeria, CO2 geological sequestration, Criterion, Ranking and

Screening

1. Introduction

Global search for economic and viable earthly minerals for energy development; Biomass,

Solid minerals, Fossil fuel, Hydrocarbon and the solution to natural and man-made

phenomenal affecting the earth; Earth quake, Greenhouse gases by the Geologist,

Geophysicist and other Scientist is now a global issue to save the planet. Geoscientist have

been looking for way forward for an alternative sources of energy (Renewable Energy) that is

safer for about less to non emission of carbon dioxides (CO2), Methane (CH4), and other

related greenhouse gases into the atmospheres other than Nuclear Energy that will greatly

reduces the measure of continuous raising in temperature of the earth we lived; The Global

Warming! As a result of anthropogenic carbon dioxide (CO2) emissions, atmospheric

concentrations of CO2, a major greenhouse gas, have risen from pre-industrial levels of 280

to 360 ppm, primarily as a consequence of fossil fuel combustion for energy production

(Bryant 1997). Increasing concentrations of CO2 affect the Earth–atmosphere energy balance,

enhancing the natural greenhouse effect and thereby exerting a warming influence at the

Earth’s surface. Because of uncertainties regarding the Earth’s climate system, there is much

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Screening and Ranking of Chad Basin for co2 Sequestration Potential in Nigeria

I. Yusuf

International Journal of Geomatics and Geosciences

Volume 4 Issue 1, 2013 17

public debate over the extent to which increased concentrations of greenhouse gases have

caused or will cause climate change, and over potential actions to limit and/or respond to

climate change. Planetary cooling forces that are intensified by warmer temperatures and by

strengthening of biological processes, which would be enhanced by the same rise in

atmospheric CO2 concentrations, may cancel the predicted climate warming (Idso 2001).

Carbon dioxide can be sequestered in geological media by geological (stratigraphic and

structural) trapping in depleted oil and gas reservoirs, solubility trapping in reservoir oil and

formation water, adsorption trapping in uneconomic coal beds, cavern trapping in salt

structures, and by mineral immobilization (Figure 1) (Blunt and others 1993; Gunter and

others 1993, 1997; Hendriks and Blok 1993; Dusseault and others 2002). Use of CO2 in

enhanced oil and gas recovery (EOR and EGR; Holtz and others 2001; Koide and Yamazaki

2001) and in enhanced coalbed methane recovery (ECBMR; Gunter and others 1997; Gale

and Freund 2001), and hydrodynamic trapping in deep aquifers (Bachu and others 1994)

represent actually forms of CO2 geological storage with retention times of a few months to

potentially millions of years, depending on flow path and processes. In all cases of enhanced

recovery of hydrocarbons, CO2 ultimately breaks through at the producing well and has to be

separated and recirculated back into the system, thus reducing the storage and sequestration

capacity and efficiency of the operation, notwithstanding the additional CO2 produced during

the separation and compression stages. However, the economic benefits of incremental oil

and gas production make EOR, EGR, and ECBMR operations most likely to be implemented

first. Only sedimentary basins contain geological media generally suitable for CO2 storage

and/or sequestration: oil and gas reservoirs (geological and solubility trapping), deep

sandstone and carbonate aquifers (solubility, hydrodynamic and mineral trapping), coal beds

(adsorption storage and trapping), and salt beds and domes (cavern trapping). In addition,

these media have both the space (porosity) and injectivity (permeability) necessary for CO2

injection, and, by and large, have the ability to either prevent or delay for geologically

significant periods of time the CO2 return to the atmosphere. Crystalline and metamorphic

rocks, such as granite, on continental shields, are not suitable for CO2 storage and

sequestration because they lack the porosity and permeability needed for CO2 injection, and

because of their fractured nature. Volcanic areas and orogenic belts (mountains) are also

unsuitable mainly because they lack capacity and are unsafe. Fortunately and

serependitously, sedimentary basins are also where fossil energy resources are found,

produced and, by and large, used for power generation (Hitchon and others 1999). In Nigeria,

there are seven (7) sedimentary basins; Chad basin, Sokoto basin, Bida Basin, Anambra

Basin, Dahomey basin, Niger Delta basin and Benue Trough (Upper, Middle and Lower) all

distributed majorly on the inland, while Dahomey and Niger Delta basin are both offshore

(Figure 2). Chad basin has the largest basin size and deepest sedimentary piles in Nigeria

when looking at the inland basins and Niger Delta basin for offshore respectively.

Figure 1: Showing various means of CO2 sequestration or storage in geological media (after

Bachu, 2003)


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1.2 Location and geological setting

The Nigerian sector of the Chad Basin, known locally as the Bornu Basin, is one of Nigeria’s

inland basins occupying the northeastern part of the country. It represents about one-tenth of

the total area extent of the Chad Basin, which is a regional large structural depression

common to five countries, namely, Cameroon, Central African Republic, Niger, Chad, and

Nigeria. The Bornu Basin falls between latitudes 110N and 14

0N and longitudes 9

0E and 14

0

E, covering Borno State and parts of Yobe and Jigawa States of Nigeria, figure 2.

Figure 2: Generalized geological map of Nigeria showing the location of Chad Basin (after

Obaje, 2009)

The Chad Basin belongs to the African Phanerozoic sedimentary basins whose origin is

related to the dynamic process of plate divergence. Notable exceptions, however, are the

deformed basinal sequences of the Paleozoic fold belts of Moroco and Mauritania which

resulted from the Hercynian convergent motion and collision of Africa and North America,

and the Tindouf and Ougarta basins which are Paleozoic successor basins (Burke, 1976;

Petters, 1982). It is an intracratonic inland basin covering a total area of about 2,335,000 km 2

with Niger and Chad Republics sharing more than half of the basin. The basin belongs to a

series of Cretaceous and later rift basins in Central and West Africa whose origin is related to

the opening of the South Atlantic (Obaje et al., 2004). In Nigeria, other inland basins of the

same series include the Anambra Basin, the Benue Trough, the Mid-Niger (or Bida) Basin

and the Sokoto Basin. The Nigerian sector of the Chad Basin, known locally as the Bornu

Basin represents about one-tenth of the whole basin. It constitutes the southeastern sector of

the Chad Basin.

1.3 Lithostratigraphy

Geologic outcrops in the Chad Basin are scarce, being blanketed by Quaternary sediments.

The rare exposures of the older series of Early Cretaceous are mostly found in the Niger

Republic part of the basin. The sedimentary fill in most parts of the basin is made of Late

Cenozoic – middle Eocene continental sediments and Cretaceous and Tertiary series

accumulating preferentially in tectonic rifts. Data gathered from the adjacent basins and


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boreholes indicate that the Bornu Basin is made up of five stratigraphic units that include the

Bima Sandstone at the bottom, the Gongila Formation, the Fika Shale, the Keri–Keri and

Chad Formations. In most cases the Chad Formation lies directly unconformably on the Fika

Shale Figure 3.

Figure 3: Idealized N-S stratigraphic cross-section the Benue Trough and the relationship to

Niger Delta and Chad Basin (vertical scale exaggerated; erosion and uplift not considered)

(after Obaje, 2009)

2.1 Methodology for basin-scale screening and ranking of co2 geological storage

potential

A preliminary local – scale screening and ranking process (Chad basin; Nigeria sector) is

used to establish the potential of the basin for CO2 geological storage potential before

detailed site characterisation will be carried out.

Sedimentary basins can be screened and ranked as to their overall suitability for CO2 storage,

based on geological, geographical and industrial characteristics. This study has adapted

screening and ranking criteria developed by Bachu (2003), which includes factors such as

tectonic setting, basin size and depth, geology, hydrogeology, geothermal regimes,

hydrocarbon potential, maturity, on/off shore, climate and accessibility among others. Table 1

documents the criteria that were used to assess the basin-scale suitability of Chad basin

studied for geological storage of CO2.

For each criterion, the classes are arranged from least favorable to most favorable left-to-right

across the table. The criteria relate to either the containment security, the volume of storage

capacity achievable, or consider the economic or technological feasibility.

The present-day tectonic setting of a basin gives an indication as to the likely tectonic

stability of the region, which is an important consideration for containment risk (i.e.

tectonically-active areas, such as subduction zones, are the least favorable due to their

increased susceptibility to natural earthquake risk and attendant fault seal failure). The basin

size and depth reflects the possible storage capacity achievable, as the larger and deeper the


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basin is, the greater the likelihood of having laterally extensive reservoir and seal pairings,

possibly in more than one stratigraphic interval.

Table 1: Criteria for Sedimentary basins for CO2 geological sequestration (after Bachus, 2003)

The depth of the sedimentary fill of the basin is also relevant to the phase state of the CO2 (i.e.

depths greater than ~800 m result in dense supercritical CO2 and hence significantly increased

storage capacity) and also impacts on the likely economic feasibility, as the greater the depth

to the injection target the larger the associated costs of drilling. The stratigraphy of the area is

reviewed to identify possible geological formation that may provide reservoir and seal pairs.

The reservoir-seal pair criteria (geology) are a qualitative assumption about the likely

abundance, lateral extent, thickness and depth of possible reservoir-seal horizons.

While, faulting intensity as component of geology view both containment and capacity issue.

The more extensively fractured that an area is, the greater the risk for containment breaches,

and the lower the likely storage volume achievable due to the need to inject within individual

fault blocks. The geothermal conditions of the basin has an impact on the storage capacity, as

within colder basins; more CO2 can be contained within the same unit volume of rock due to

the increased density of the CO2, while verse- verse in warm basin.

The hydrocarbon potential of a region gives an indication of the suitability of the area for

CO2 storage, on the assumption that if the rocks are suitable for containing and storing oil

and gas, then it is likely that they are also suitable for storing CO2. Maturity of the extractive

industries in the region reflects the likely database available that is the more developed an

area is the greater amounts of data available for CO2 storage assessment. The climate of the

region affects the likely surface temperatures (and hence the geothermal conditions) and

likewise, accessibility and infrastructure reflect the variability in condition in terms of getting

the captured anthropogenic CO2 from source to point of sequestration.


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2.1.1 Fundamental of Screening and Ranking Sedimentary Basins

According to Bachus, 2003; there are no large-scale operations for the geological

sequestration or storage of CO2, and whatever operations exist, they were driven by other

considerations, such as increasing oil production, avoiding a carbon tax, or complying with

regulations regarding sulfur emissions. However, if CO2 geological sequestration or storages

are to be implemented on a large scale, then there is need for a systematic, quantitative

analysis of sedimentary basins in terms of their suitability to serve as enhanced CO2 sinks.

A method for such a quantitative analysis based on parametric normalization and ranking, is

proposed here, which can be further developed or adapted to more specific conditions.

For each criterion i ( i = 1,…5) in Table 1 for evaluating a basin suitability, monotonically –

increasing numerical function F i is assigned, which can be continuous or discrete, to describe

a value placed on a specific class j for that criterion. The lowest and the highest functions of

this function characterize the worst and best class in terms of suitability for that criterion, i.e.

Fi,1 = min (Fi), where and Fi,n = max (Fi), where ( n = 3, 4 or 5). If the classes have a

relatively equal importance assigned o them, then a linear function is probably the best for Fi.

If the increasing value (or importance) is placed on increasingly favorable classes, then

geometric or exponential function are probably better. Table 2 presents the numerical values

assigned here to the various classes for the criteria in Table 1.

For any sedimentary basin k that is evaluated in terms of its general suitability for CO2

sequestration or storage, the corresponding class j for each criterion i s identified as in Table

1, resulting in a corresponding score Fi,j as in Table 2, because the function fi has different

ranges of values for each criterion, making comparison and manipulation difficult, the

individual scores Fi,j are normalized according to

Such that Pi = 0 for the least favorable class and Pi = 1 for the most favorable class for all the

criteria i = 1,…15. As a result of this process, each sedimentary basin k being evaluated is

characterized by individual scores Pk

i.

The effect of parameterization and normalization is that it transforms various basin

characteristics, which have differing meanings and importance, into dimensionless variables

that vary between 0 and 1. These can subsequently be added to produce a general score Rk,

used in basin ranking, which is calculated using:

Where Wi are weighting function that satisfy the condition:


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Table 2: Scores and weight assigned to criteria and classes for accessing sedimentary basins

in terms of their suitability for CO2 sequestration in geological media (after Bachus, 2003)

The weights w1 assigned in this study to the various suitability criteria are shown in Table 2.

The number of criteria (currently 15), the functions Fi (i=1,..15) and weights wi the can be

changed and/or adapted to changing conditions and priorities. Using this methodology,

sedimentary basins, or parts thereof, within a given jurisdiction or geographic region can be

assessed and ranked in terms of their suitability for the geological sequestration or storage of

CO2. This ranking can be then used in making decisions for the large-scale implementation of

such operations.

2.1.2 Screening and Ranking Chad Basin (Nigeria sector)

The Chad basin was evaluated against criteria for assessing sedimentary basins for CO2

geological sequestration Table 1 adapted from Bachu (2003). Table 3 shows summarises the

results of the screening criteria for the basin studied. A brief discussion of some of the key

features of each basin is presented below.

3.1 Criterion potential evaluation

Injecting carbon dioxide, generally in supercritical form, directly into underground geological

formations like oil and gas fields, saline formations, unmineable coal seams, and saline-filled

basalt formations have been suggested as storage sites. Various types of physical trapping;

structural: anticline and fault; stratigraphic: unconformity and change in type of rock, or a

particular formation thinning out, highly impermeable rock, geochemical trapping

mechanisms would prevent the CO2 from escaping to the surface.

Sedimentary basins are considered suitable targets for storing large volumes of CO2, having

characteristics that favour effective storage over hundreds of thousands to millions of years

(geological time periods), as demonstrated by the widespread existence of natural CO2

accumulations as occurred in Colorado Plateau and Rocky Mountain region of the USA

( IPCC 2005 ) as well as hydrocarbons trapped in reservoirs.

http://en.wikipedia.org/wiki/Supercritical_fluid

http://en.wikipedia.org/wiki/Oil_field

http://en.wikipedia.org/wiki/Gas_field

http://en.wikipedia.org/wiki/Coal_seam


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The following below nine (9) criterions are evaluated in view of Bachus, 2003 criteria for

assessing sedimentary basins for CO2 geological sequestration (Table 1).

Table 3: Results of ranking of the Nigeria sector Chad sedimentary basin in terms of suitability for

CO2 geological sequestration.

3.1.1 Tectonic Stability

The tectonic regime as reviewed in this paper was probably dominated by tensional

movement as indicated by the preponderance of high – angled normal faults and the scarcity

of reverse faults. Majority of the faults in the basin are basement-involved faults; movements

along these faults led to high angled faults in the overlying strata. Tectonic pulse was a basin

– modifying event, which caused folding and basin inversion in the Bornu basins. Folds

within the basin are said to be simple and symmetrical with low fold frequencies and

amplitudes which increase towards the centre of the basin. Major basement lineaments and

faults were produced within the basin during the Pan African crustal consolidation.

These revealed truly that the Chad basin of the Nigeria sector is on a stable intracratonic

basement rock and not on seismically active region of the continent. The faulting intensity is

ranked not extensive and that capacity, containment for CO2 would be in substantial volume

after detail characterisation on the basin.

3.1.2 Basin Size and Depth

Basin size and depth give an estimate of the overall storage volume achievable. The

sedimentary basin needs to be deep enough to store CO2 in a supercritical phase (a depth of

approximately 800m is needed for this), while Nigerian sector of the Chad Basin (Borno

Basin) covering a total area of 233,500km2

and over 3,600 m of sediments have been


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deposited. The basin can be termed “very large” in terms of size and “deep” when

considering the depth as a criteria to estimate the storage volume of the basin.

3.1.3 Geology

As a result of the tectonic tensional movement resulting into the high – angled normal faults;

low frequencies, amplitudes simple - symmetrical fold within basin, which increases towards

the centre of the basin.

The basin can be said to be moderately faulted, such that only detail site characterisation

would give the extended from the faulted basement rock to extent on overlying sedimentary

formations within the basin.

Many studies reveal that the natural underground geological formations can provide adequate

CO2 storage for a very long period of time, considering the nature of geologic storage

potential. The sedimentary piles in the basin, as related to CO2 sequestration potentials are

reviewed as follows:

3.1.3a Geological Formation: Bima Sandstone

The Albian Bima Sandstone lies unconformably on the Precambian Basement. This

formation was deposited under continental conditions (fluvial, deltaic, lacustrine) and is made

up of coarse to medium grained sandstones, intercalated with carbonaceous clays, shales, and

mudstones. It is the deeper part of the aquifer series in the Nigerian sector of the basin and

rests unconformably on the basement. The thickness ranges from 300 to 2,000 m and the

depth between 2,700 and 4,600 m.

This geological formation can be categorize as ‘intermediate’ with depth range greater than

3500m according to Bachus,2003 basin-scale suitability criteria. At this depth CO2 can be

store at greater depth in its superficial phase, and large volume of the CO2 would be stored

considering the basinal size of the basin. This stable large basinal size and volume, point the

basin as having the potential to store large volume of CO2 over hundreds of thousands to

millions of years (geological time).

3.1.3b Reservoir – Seal: Fika Shale

Turonian–Maastrichtian in age, and fully marine blue–black Fika shale locally gypsiferous

with intercalation of limestones with thicknesses of 430 m, 0–900 m, 890 m and 840–1,453 m

recorded from exploratory wells by Carter et al. (1963), Avbovbo et al. (1986), Okosun

(1995) and Olugbemiro et al. (1997), respectively would serves as reservoir seal considering

the shale thickness as reviewed for underlying Bima sand stone (Reservoir) to prevent the

CO2 from horizontal and vertical migration at its superficial phase within the basin and on

regional base.

This can be termed “intermediate” according to Bachus, 2003 basin-scale suitability criteria

and “excellent” when later proven by detail geophysical site characterisation analyses.

3.1.4 Hydrogeology

Maduabuchi et al. (2006) undertook some groundwater investigations in the Nigerian sector

of the Chad Basin and in the process gave some brief descriptions of the geologic and


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hydrogeologic settings of the Chad Basin. The Precambrian Basement Complex constitutes

the bedrock on which sediments ranging in age from Palaeozoic to the Quaternary have been

deposited.

The Chad Formation is essentially an argillaceous sequence in which minor arenaceous

horizons occur. Barber and Jones (1965) named three clearly defined arenaceous horizons in

the NE Nigeria of Chad Basin consisting of the upper aquifer and two confined middle and

lower aquifers. The upper aquifer consists of Quaternary (lower Pleistocene) alluvial deposits

of lake margin origin, alluvial fans or deltaic sediments related to sedimentation around lake

Chad covered in many locations by recent sand dunes. The thickness increases considerably

from 15 to 100 m north of the lake.

The reservoir is composed of interbedded sands, clays, silts and discontinuous sandy clay

lenses which give aquifer characteristic ranging from unconfined, semi-confined to confined

type. The transmissivity ranges from 0.6 to 8.3 m2

/day and the aquifer which recharges from

rainfall and run-off is mainly used for domestic water supply (hand dug wells and shallow

boreholes), vegetable growing and livestock water in (Maduabuchi et al., 2006).

The lower Pliocene sequence composed of grey to bluish grey clays varying in thickness

from few tens of meters to over 350 m at the edge of the lake separates the middle aquifer

from the upper aquifer. The middle aquifer is the most extensively encountered aquifer in the

Nigerian sector of the Chad Basin. It lies at a depth between 240 and 380 m and consists of

10–40 m thick sand beds with interbedded clays and diatomites of Early Pliocene age. The

sand fraction consists of moderately coarse to coarser grains of quartz, feldspar, mica and Fe-

oxides. The aquifer geometry has a gentle northeast dip and does not outcrop in the Nigeria

sector of the Chad Basin. The average transmissivity is 360 m2 /day and the hydraulic

gradient is 0.015% in the NE direction (Maduabuchi et al., 2006).

3.1.5 Geothermal Regime

Geothermal regime of sedimentary basin is one of the most important elements to be

considered as criteria for suitability assessment of a geological storage; as result of phase

behavior and variation of CO2 properties with temperature, pressure and depth.

At normal atmospheric conditions; CO2 is thermodynamically very stable gas heavier than air

and also at temperature greater tan Tc = 31.1oC and pressures greater than Pc=7.38 MPa

(critical point), CO2 is in a supercritical state. Bachus, 2003 indicated that basin < 30oC/km is

termed as cold basin and while in Chad Basin, the geothermal gradient falls between 7.6 and

5.90oC/100m by Kurowska, 2010 reflecting that Chad Basin can be termed one of

intracratonic cold basin around the world. In these regards, the basin can said to be favorable

for CO2 storage potential in the area. As of the time of compilation these preliminary results,

information regarding hydrostatic variation of pressure is not available.

3.1.6 Hydrocarbon Potential/Maturity

Rocks that are suitable for containing and producing oil and gas are likely to be suitable for

storing CO2. The potential for storing CO2 will be dependent on the timing of possible

hydrocarbon production. If there is a mature oil/gas industry in the area, there will be a larger

amount of available information about the site. Most of the hydrocarbon and coal would have


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been discovered and there are likely to be depleted oil and gas reservoirs. Such areas are

likely to have good infrastructure such as roads, pipelines and wells.

Twenty three wells were drilled. The first well TUMA-1 was spudded on July 27, 1984, of

the 23 wells drilled, only two encountered non-commercial gas while the remaining 21 were

abandoned as dry wells which led to the temporary suspension of exploration activities before

the recent renewed search. On this note, the hydrocarbon potential and maturity can be

considered “medium” for now pending further discoveries are made in the basin.

3.1.7 Climate

Climate affects the surface temperatures, the depth of the water table and the ease of

development of storage facilities. The Chad Basin embraces a great range of tropical climates

from north to south, although most of these climates tend to be dry. Apart from the far north,

most regions are characterized by a cycle of alternating rainy and dry seasons. And this

according to Bachus, 2003 screening and ranking; tropical climatic condition of this area is

next to the most favorable temperate condition for CO2 sequestration potential.

4.1 Discussions

The results in Table 3 shows that Rk score value of 0.53 against the fi, n = max (fi) value equal

to 1 as highest value of the function characterize the best in terms of suitability for the

criterion in which this ranking is based upon.

These score values characterize Chad basin to have tropical climatic condition having

deposited on a tectonically stable cratonic divergent plates, with very large basin size of

233,500 km2 and more than 3600 m deep sedimentary piles. The basin is tectonically

moderately faulted and possibly fractured by the events, with a moderate geothermal gradient

of about 5.9 and 7.6oC / 100 m, shallow and short flow Hydrogeological systems.

The hydrocarbon potential in the basin is ranked for now medium, and the basin is mature on

that about 23 wells were drilled into the basin and gas discovered. Ongoing, the basin is

under 3D seismic detailed survey prospectivity for oil/gas in commercial quantity.

There are no reports or record of neither coal nor salt discovery in the basin this past recent

years; and the entire basin can be easily accessed, minor infrastructures are available and with

no CO2 source in the region.

By this ranking and screening criteria, the basin can be compared with SW Ontario basin in

the Canada’s sedimentary basin in terms of its suitability for CO2 geological storage with Rk

value of 0.52 as assessed by Bachu, 2003. It is not yet possible to predict with confidence

storage volumes, sequestration integrity, and fate of injected CO2 over long periods of time at

this local ranking and screening without carrying out regional characterisation of the entire

2,335,000Km2 basins in other bordered countries.

5.1 Conclusions

The geology of the sedimentary basins of Chad basin in the Nigerian sector has been

preliminarily screened and ranked to view its potentials for CO2 sequestration opportunities

in the basin. The ranking and screening criteria adapted were developed by Bachu (2003),

which includes factors such as tectonic setting, basin size and depth, geology, hydrogeology,


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hydrocarbon potentials, climate, geothermal regimes, existing resources and industry maturity.

For each criterion i ( i = 1,…5) in Table 1 for evaluating a basin suitability, monotonically –

increasing numerical function F i is assigned, which can be continuous or discrete, to describe

a value placed on a specific class j for that criterion. The lowest and the highest functions of

this function characterize the worst and best class in terms of suitability for that criterion, i.e.

Fi,1 = min (Fi), where and Fi,n = max (Fi), where ( n = 3, 4 or 5).The criteria relate to either

the containment security, the volume of storage capacity achievable, or consider the

economic or technological feasibility.

The results shows that Chad basin has Rk score value of 0.53 against the Fi, n = max (Fi) value

equal to 1 as highest value of the function characterize the best in terms of suitability for the

criterion in which this ranking is based upon. The criterion individual scores Pk

i for

geothermal, geology; fault intensity, and hydrocarbon potentials can in future be subjected to

amendment as when further discoveries are made to score the criterion more favorable or

better than present ranking score Rk value of 0.53.Furthermore, detail site characterisation of

the geological storage for it overall suitability for CO2 sequestration potentials in the basin are

needed accentuate its general suitability.

6.1 Recommendations

A detailed regional and local site characterisation of geological storages are needed to fully

screen and rank the basins for its overall suitability for CO2 storage potentials in the region,

since countries like Cameroon, Central African Republic, Niger, Chad, and Nigeria shares the

basin on regional level.

The work flow stages for detail of geological storage characterisation on regional base would

continue with basin-scale assessment, since this paper have addressed the basin on country/

state – scale screening. Geosciences characterisation would be carried out to assess the

structural and stratigraphic models; injectivity, containment and capacity of the basin, while

the engineering characterisation would look at parameters like; optimum injectivity rate, well

design e.t.c; long- term migration, dynamic flow behavior; geochemical reactive transport,

geomechanics flow simulations e.t.c

Finally, the socio economics characterisation would look into the capital and operating cost

of compression, transport and injection cost per tonne of CO2, quantitative risk assessment,

monitoring and verification.

6.2 References

1. Avbovbo A.A, Ayoola EO, Osahon GA (1986) Depositional and structural styles in

the Chad Basin of northeastern Nigeria. AAPG Bull 70:1787–1798.

2. Barber W, Jones DC (1965) The Geology and Hydrogeology of the Maiduguri, Borno

Province. REC Geological Survey of Nigeria. Pp.3-11.

3. Burke K (1976) The Chad Basin: an active intra-continental basin. Tectonophysic

36:197–206

4. Bryant E (1997) Climate process and change. Cambridge University Press,

Cambridge


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5. Blunt M, Fayers FJ, Orr FM (1993) Carbon dioxide in enhanced oil recovery. Energy

Convers Manage 34:1197–1204

6. Bachu S, Gunter WD, Perkins EH (1994) Aquifer disposal of CO2 hydrodynamic and

Mineral trapping. Energy Convers Manage 35:269–279

7. Bachu S (2003) Screening and ranking of sedimentary basins for sequestration of

CO2 in Geological media in response to Climate Change. Int’l J Environmental

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Documents

Screening and Ranking of Chad Basin for co2 Sequestration ... · PDF filethere are seven (7) sedimentary basins; Chad basin, Sokoto basin, Bida Basin, Anambra Basin, Dahomey basin,