Tectonophysics, 213 (1992) 169-185

Elsevier Science Publishers B.V., Amsterdam

Regional framework, structural and petroleum aspects of riftbasins in Niger, Chad and the Central African Republic (C.A.R.)

G.J.GenikExxon Company, International, P.O. Box 146, Houston, TX 77001-0146, USA

(Received May 14, 1991; revised version accepted July 18, 1991)

ABSTRACf

Genik, G.J., 1992. Regional framework, structural and petroleum aspects of rift basins in Niger, Chad and the CentralAfrican Republic (CAR.). In: P.A Ziegler (Editor), Geodynamics of Rifting, Volume II. Case History Studies on Rifts:North and South America and Africa. Tectonophysics, 213: 169-185.

ea

This paper overviews the regional framework, tectonic, structural and petroleum aspects of rifts in Niger, Chad and theCA.R. The data base is from mainly proprietary exploration work consisting of some 50,000 kilometres of seismic profiles,50 exploration wells, one million square kilometres of aeromagnetics coverage and extensive gravity surveys. There havebeen 13 oil and two oil and gas discoveries.

A five phased tectonic history dating from the Pan African orogeny (750-550 MY B.P.) to the present suggests that theWestern Central African Rift System (WCAS) with its component West African Rift Subsystem (WAS) and Central AfricanSubsystem (CAS) formed mainly by the mechanical separati& of African crustal blocks during the Early Cretaceous. Amongthe resulting rift basins in Niger, Chad and the CAR., seven are in the WAS-Grein, Kafra, Tenere, Tefidet, Termit,Bongor, and N'Dgel Edgi and three, Doba, Doseo, and Salamat are in the CAS. The WAS basins in Niger and Chad are allextensional and contain more than 14,000 m of continental to marine Early Cretaceous to Recent clastic sediments andminor amounts of volcanics. Medium to light oil (200 API-46° API) and gas have been discovered in the Termit basin inreservoir, source and seal beds of Late Cretaceous and Palaeogene age. The most common structural styles are extensionalnormal fault blocks and transtensional synthetic and antithetic normal fault blocks. The CAS Doba, Doseo and Salamat areextensional to transtensional rift basins containing up to 7500 m of terrestrial mainly Early Cretaceous clastics. Heavy tolight oil (150-390 API) and gas have been discovered in Doba and Doseo basins. Source rocks are Early Cretaceouslacustrine shales, whereas reservoirs and seals are both Early and Late Cretaceous. Dominant structural styles areextensional and transtensional fault blocks, transpressional anticlines and flower structures.

The existence of a total rift basin sediment volume of more than one million cubic kilometres with structured reservoir,source and seal rocks favours the generation, migration and entrapment of additional significant volumes of hydrocarbons inmany of these basins.

c

!,

Introduction

This paper overviews the regional framework,tectonic aspects and structural styles of rifts inNiger, Chad and the Central African Republic(CAR.) (Fig. 1). Petroleum geology is summarilyaddressed. The data base for this contribution is

mainly the proprietary hydrocarbon explorationwork carried out by Exxon and its partners in

Correspondence to: GJ. Genik, Exxon Company, Interna-tional,P.O. Box 146, Houston, TX 77001-0146, USA

these countries from 1969 through 1989. The dataincludes 81,400 line. kilometres (one millionsquare kilometres) of aeromagnetics, 10520 linekilometres of gravity surveys, some 50,000 kilome-tres of multi-coverage reflection seismic data andthe results of 50 exploration wells.

The systematic hydrocarbon exploration ofrifted basins in this region was pioneered byConoco in Chad in 1969. Subsequently, Texaco,Global Energy (now Amax Oil and Gas Inc.),Phipps Petroleum, Shell, Chevron, Exxon and Elfhave explored the area. At present Exxon, Shelland Chevron jointly explore a 104,234 km2 permit

0040-1951/92/$05.00 @ 1992 - Elsevier Science Publishers B.V. All rights reserved

169

170

in Chad and an Exxon-SNEA partnershipprospect a 36,580 km2 permit in Niger.

The first oil was discovered in the area in 1974by Conoco in the Termit basin, Lake Chad areaof Chad and the most recent oil discovery was in1989 in the Doba Basin by Exxon and partnersShell and Chevron. To date 13 oil and two oil andgas discoveries have been made in three of theten rift basins.

Regional framework and tectonic evolution

The Cretaceous-Palaeogene rifts of Niger,Chad and the CAR. make up a large part of ageotectonic continuum termed the West andCentral Mrican rift system (WCAS) (Fairhead,1986) that extends 4000 km from the Gao troughin Mali to the Anza basin in Kenya (Figs. 2 and5d). This rift system is subdivided into two coevalCretaceous genetically related but physically sep-arated rift subsystems (Fig. 2), the West MricanRift Subsystem (WAS) and Central African RiftSubsystem (CAS).

10°

o 1000 2000KM~o 500 1000..

10°

GJ. GENIK

The geologic framework and distribution ofrifted basins (Fig. 2) in the study area evolved infive major tectonic phases:

0) Pan Mrican crustal consolidation (750-550Ma)

(2) Palaeozoic-Jurassic platform development(550-130 Ma)

(3) Early Cretaceous, early rift 030-98 :t Ma)to Late Cretaceous, late rift (98-75 :t Ma)

(4) Maastrichtian-Palaeogene rift and emer.gence (75-30 Ma)

(5) Neogene-Recent mainly emergence (30-0Ma).

Phase 1. (750-550 Ma). The Pan Africancrustal consolidation produced major basementlineaments and faults whose trends (Fig. 2)formed precursor rift directions or the "essentialstructure" (Christie-Blick and Biddle, 1985) forthe future Cretaceous rifts, e.g., the NW-SEtrends in the Niger-Air region (Greigert andPougnet, 1966; Black and Guirod, 1970) for theW.AS. rifts in Niger and Chad; NE-SW trends.(Chukwu-Ike, 1981; Popoff et aI., 1983; Ajakaiye

18°

~ LlBYA

~

. OILDISCOVERY

I!':!!::!':!':!':ISTUDYAREA

o 100 200 300Kms. I I .

Fig. 1. Map of study area showing rifts in Niger, Chad and the C.A.R.

RIFT BASINS IN NIGER, CHAD.

et aI., 1986; Popoff, 19~rift basins in Nigeria, aing faults (Le Marechalnacchia and Oars, 1983rifts of the CAS in CCAR.

Pan Mrican basemer

of mainly mobile belt a<phic rocks west of Ionmeridian are found mOlthe Pan Mrican thermawells were drilled into

(Table 1), four in the Wtwo in the CAS. in th

4) finding gneisses, peland granodiorites. Theiric dates of 434 to 594

O'

Fig. 2. Regional geological mSubsystem (which also inc1u

RIff BASINS IN NIGER, CHAD AND THE CENTRAL AFRICAN REPUBLIC

et aI., 1986; Popoff, 1988a) of the Benue-Bornurift basins in Nigeria, and the ENE-WSW strik-ing faults (Le Marechal and Vincent, 1972; Cor-nacchia and Dars, 1983; Benkhelil, 1988) for therifts of the CAS in Cameroon, Chad and theCAR.

Pan African basement rocks consist in outcropof mainly mobile belt acid igneous and metamor-phic rocks west of longitude 14°E; east of thismeridian are found more stable acid plutonics ofthe Pan African thermal belt (Windley, 1984). Sixwells were drilled into the basement in the rifts

(Table 1), four in the W.AS. in Niger (Fig. 3) andtwo in the C.AS. in the Doba basin Chad, (Fig.4) finding gneisses, pegmatites, granites, schists,and granodiorites. These have yielded radiomet-ric dates of 434 to 594 Ma (Table 1). A granite

O'

171

from outcrop located west of the Doba basin atlatitude 9°40'N and 14°E (Fig. 4) was dated byRb/Sr analyses as 481 :!:23 Ma.

Phase 2. (550-130 Ma). The study area per-sisted as a stable, north plunging, emergent plat-form onto which Palaeozoic to Jurassic continen-

tal and shallow marine cratonic sag sedimentstransgressed from the north to a latitude of about18°N (Fig. 2) (Faure, 1966; Greigert, 1966;Greigert and Pougnet, 1966; Klitzsch, 1984;Guiraud et aI., 1987). Remnants of these pre riftsediments would be caught up in the W.AS. riftsof northern Niger. There was no known rifting inthe region in Phase 2, but some thermal effect ofthe Late Palaeozoic-Early Mesozoic post Tethyantectonics in North Africa may have reached theTermit basin (Table 1) where the Iaguil and Dilia

LEGENDTERTIARYAND CRETACEOUSRIFT BASIN

CRETACEOUS RIFT BASIN

CHAD BASIN LIMITSOUTCROPS

QUATERNARY TERTIARYmmrn[]]IT]D P.

4'

TERTIARY VOLCANICS

CRETACEOUS

JURASSIC-CRETACEOUS

TRIASSIC-PAlEOZOICS

FAULTS OR STRUCTUALLINEAMENTS

500 KMS

12'£ WE 18'E .... '0'£ '2'£ ,...

Fig.2. Regional geological map with rifts superposed upon the regional surface geology. Note the position of the West African Rift

Subsystem (which also includes Gao trough in Mali and Bida basin in Nigeria; Braide, 1990) and Central African Rift Subsystem.

172

Longrin wells (Fig. 3) penetrated basement con-sisting of hornfels, schist and granites whichyielded K-Ar dates of 266 and 190 Ma.

Phase 3. (130-75 Ma). This was the main timeof rift development. The origin of the WCAS isattributed to the breakup of Gondwana and thestart of separation between Africa and SouthAmerica (Burke, 1976; Browne and Fairhead,1983; Fairhead, 1986, 1988; Benkhelil, 1988;Popoff, 1988a; Fairhead and Binks, 1991). Theseauthors relate rift development to the gradualopening of the central and south Atlantic startingaround 130 Ma when wrench fault zones ex-tended from South America onto the Gulf ofGuinea and Africa, whereby their strike-slipmovement dissipated into extensional basins inNiger, Sudan and Kenya. It may equally be ar-gued that the Sudanese and Kenyan rifts (Reeveset aI., 1986), which also opened at this time,evolved in response to India and Madagascarseparating from Africa (Scotese et aI., 1988;Guiraud and Maurin, 1990).

G.J. GENIK

Considering these ideas, we suggest that theWCAS formed primarily as a result of simultane-ous, lateral, mainly mechanical divergence be-tween discrete crustal blocks W, C, Sand Eduring the Early Cretaceous 130-98 Ma (Fig. Sa).Sinistral movement of block W relative to blocksC, Sand E caused transtensional opening of theWAS in the Benue- Bornu region. This was ac-companied by the extensional subsidence of theNiger-Chad basins. Block C moving dextrally rel-ative to blocks Sand E obliquely opened the CASbasins of southern Chad and c.A.R. and causedthe extensional subsidence of the Sudan and

Kenya rifts (Fairhead, 1986; Schull, 1988).During Phase 1, up to 126 Ma the boundaries

between the blocks corresponded to major frac.tures representing reactivated Pan African crustaldiscontinuities; only minor actual rifting occurred(Fig. Sa) during this time. The period from 126to98 Ma is the early rift stage (equal to Popoff,1988b, syn-rift Phase I). The rifts became fullydeveloped across middle Africa at about 108 Ma

(R) = Radiometrically dated.

(S) = Age estimated from associated stratigraphy and mode of occurrence.* = Locations on Figs. 3 and 4.** = Age of post emplacement thermal events, true basement age is very likely Pan African.a.-h. = Named wells for Fig. 3.i.-n. = Named wells for Fig. 4.

RIff BASINS IN NIGER. CHAD

(Figs. Sa and band 6:tireiy continental sedinsupported by paleontdrilled in the Tenere,basins in Niger (J.P. (and Y.Y. Chen, pers.Muglad basin in Sudathe Anza basin, Kenyaof Kenya, 1990), as welthe Benue basin (POINiger rifts (Faure, 191

20"

16'~

Fig. 3. Simplified isopacha = Seguedine #1; b = Gos

TABLE 1

Drilled igneous and basement rocks *-Niger, Chad, Rifts

Basin Well Depth range Rock type Mode of Age MY B.P.(meters) occurrence

a. Grein/Kafra Seguedine-l 3143 Biotite gneiss, pegmatite Basement 434-489 (R)

b. Termit Gosso Lorom outcrop Basalts, dolerites, tuffs Extrusives < 1 to 10 (R)

c. Termit Iaguil-l 2486 Hornfels, schist Basement 266 + and < 116 (R) ..c. Termit Iaguil-l 1250 Alkalic diabase Sill 8.6 :t 0.5 (R)

d. Termit Dilia Longrin-l 1987 Granite Basement 190:t 7 (R)e. Termit Sedigi-l 2095 Rhyolite, basalt Dike ::;;85 (S)

f. Termit Sedigi-2 2103 Rhyolite Dike ::;;85 (S)

g. Termit Kumia-l 4100 Dolerite Sill ::;;95 (S)

h. N'Dgel Edgi N'Dgel Edgi-l 2776 Metamorphics quartzite,micaschist, phyllite Basement Pan African (?)

i. Bongor Naramay-l 1256 Dolerite Sill 52-56 (R)

j. Doba Kassi-l 2766 Granodiorite Basement 568 :t 26 (R)

k. Doba Beboni-l 3496 Gneiss Basement 474:t 17 (R)

I. Doseo Tega-l 1875 Basalt Sill 97 :t 1.2 (R)m. Doseo Keita-l 2395 Altered andesitic basalt Sill ::;;110 (S)n. Doseo Kikwey-l 1100 Altered basalt Sill 101.1:tl.l (R)

RIFT BASINS IN NIGER, CHAD AND THE CENTRAL AFRICAN REPUBLIC

(Figs. 5a and band 6) and were filled with en-tirely continental sediments. This age of rifting issupported by paleontological data from wellsdrilled in the Tenere, Bongor and N'Dgel Edjibasins in Niger (J.P. Colin, pers. commun., 1990and Y.Y. Chen, pers. commun., 1991), from theMuglad basin in Sudan (Schull, 1988) and fromthe Anza basin, Kenya (National Oil Corporationof Kenya, 1990), as well as from outcrops flankingthe Benue basin (Popoff, 1988a,b) and easternNiger rifts (Faure, 1966; Greigert and Pougnet,

173

1966). This rift period clearly correlates with theearly rift phase in West Africa and Brazil (Popoff,1988a).

During the late stage of Phase 4, at about 98to 75 Ma (Fig. 5c) a late rift to sag stage followed[equal to Popoffs (1988a,b) syn-rift Phase II] andwas accompanied by a marine transgression whichreached the study region from the Tethys to thenorth via Mali and Algeria and from the SouthAtlantic to the south through the Benue trough,as indicated from the respective microfauna

NIGERIA

LEGEND

@] QUATERNARY.TERTIARY

[!E] TERTIARY.CRETACEOUS

[!] CRETACEOUS

~ PERMO.TRIASSIC

~ PALEOZOIC

~ PRECAMBRIAN

[Y] CENOZOICVOLCANICS

Fig.3. Simplified isopach of the WAS with surrounding generalized surface geology. Named well locations and outcrop are:a= Seguedine #1; b = Gosso Lorom outcrop; c = Iaguil-I; d = Dilia Longrin-I; e = Sedigi-I; f = Sedigi-2; g = Kumia-I; h = N'dgel

Edgi-l.

174

(Furon, 1963; Faure, 1966; Murat, 1972; Petters,1978, 1979; Allix et aI., 1981; Petters and Ekweo-zor, 1982; Benkhelil and Robineau, 1983; Du-faure et aI., 1984; Reyment and Dingle, 1987).This transgression is coincident with the world-wide Late Cretaceous eustatic sea level highstand(Haq et aI., 1987). Micropalaeontological datafrom exploration wells tends to corroborate thisbi-directional marine flooding. The epeiric seasattained their maximum eastern extent at about80 Ma when they reached the western most endof the Doba basin (Fig. 5c). Well data shows thatseveral thousand metres of Late Cretaceous ma-

rine sediments filled the WAS, whereas the CASreceived similar thicknesses of purely continentalsediments.

Regional rifting slowed to the point of being asag and essentially peaked during the Santonian.This rifting slowdown culminated with a marked

1'6'

Q BONGORBASIN

Q

SEDIMENTTHICKNESS'

(111-3 KM

IffiillI 3-5 KMIiliiliJ

~ 5-6 KM.>6 KM

LEGEND@]OUATERNARY-TERTIARY

IEill UPPERCRETACEOUS

[Eg LOWERCRETACEOUS

~ PRECAMBRIAN

/' MAJORFAULTS

G.J. GENIK

tectonic pulse at around 85 Ma when the regionalstress regime apparently changed abruptly. Thetectonic pulse may have resulted from variationsin the spreading rate and direction between theequatorial and southern Atlantic plates (Fairheadand Binks, 1991) and/or by a N-S compressionbetween the African and European plate(Guiraud, 1987, 1992, this volume). This tectonicpulse, which seems to have been chiefly trans-pressional in the region, lasted some 5 Ma asseen on seismic records. It caused folding andbasin inversion in the Benue, Yola and Bornubasins (Popoff et aI., 1983; Benkhelil, 1988;Avbovbo et aI., 1986), as well as similar structur-ing in the southern Termit and Bongor basins. Atthe same time, the Agadez lineament (Figs. 2 and3) (Guiraud et aI., 1983) was reactivated, separat-ing the Termit from the northern Niger rifts,Moreover, the Doba, Doseo and Salamat basins,

2"0' 2'2'

CHAD

SCALEo 50 100 150km

"* OILANDGAS

. OIL-<>- DRY

F- F' - STRUCTURALCROSSSECTIONS

1.3.5 SEISMICSECTIONS

i.,J. K... NAMED WELLS TABLE 3

NIGER! :

( CHAD (.-;

~

. __~' SUDA'

$, AP~E '~I '-- "-.r" CAR \,IA"'I".""~v.r"-'-.

-'1<>1'

~GA8OH)I. ZAIRE

..7j..-v

Fig. 4. Simplified isopach of the CAS with surrounding surface geology. Named well locations are: i = Naramay-l; j = Kassi-l;k = Beboni-l; 1= Tega-l; m = Kikwey-l; n = Keita-l.

RIFT BASINS IN NIGER, CHA;

which up to this time 1basin, were separatedby dextral movement!Additionally, anticline:of the Muglad basin ithe Anza basin of Kel

Phase 4.(75-30 Matectonics a new rift

Niger rifts and the Cwith the accumulatior

trichtian - Paleogenewhere, the WCAS bagent (Fig. 5d).

Phase 5. (30-0 Ma,tectonic phase, the ,

SOUTH AMERICA

126MYBPNEOCDMIAN- CRUSTALBLOCKBOur

mm ESEDiMEI

Fig. 5. Palaeographic m!

directions, half arrows are

relative to 126 Ma. (c) Ma

arrows show transpressio

system shown by dotted Iiifrom literatI

RIFf BASINS IN NIGER, CHAD AND THE CENTRAL AFRICAN REPUBLIC

whichup to this time had effectively been a singlebasin, were separated into three discrete basinsby dextral movements along the Borogop fault.AdditionalIy, anticlinal folds developed in partsof the Muglad basin in Sudan (Schull, 1988) andthe Auza basin of Kenya.

Phase 4. (75-30 Ma). FolIowing the preceedingtectonics a new rift cycle started in the WASNiger rifts and the CAS, Muglad and Anza riftswiththe accumulation of up to 4000 m of Maas-trichtian-Paleogene continental clastics. Else-where, the WCAS basins were essentially emer-gent(Fig. 5d).

Phase5. (30-0 Ma). During this the last majortectonic phase, the WCAS remained effectively

126 MYBPNEOCOMIAN- CRUSTAlBlOCKBOUNDARY _CONTINENTAlSEDIMENTRIFTI.FILL

8.

,.,,.~

_MARINESEDIMENTS _CONTINENTALSEDIMENTRIFT INFILl

c.

emergent. Areas east of the Cameroon volcanicline (Fitton, 1980) in the Adamawa area wereuplifted (Fig. 2) and eroded. Geochemical dataindicates that up to 2500 m of sediments wereeroded from the western part of Doba basin andthe Bongor basin. At the same time, the GossoLorom volcanics developed in the northwest Ter-mit basin along the Agadez Lineament (Fig. 2).Elsewhere in this basin a 350-500 m thick blan-

ket of continental sediments was deposited in thearea centered on the present Lake Chad(Mothersill, 1975).

On the eastern side of Mrica, rifting of theRed Sea area started about 40 Ma (LowelI andGenik, 1972) and became fulIy developed during

.,...,~ (:"('/

., ,F~""-'\'r- -,-.~-. .-,,,,J ('--," { -~ '..-, \~, "" ." ~('... l t:\

108 MYBPTOPAPTIAN

_ CONTINENTAlSEDIMENTRIFTINFILL

b.

ATLANTIC

311-0 MYBP.RECENTOLIGOCENE

~ ~:~~E~~~cJ::,~::iAlRIFTBASINS_CRETACEOUS.TERTIARYCONTINENTALRIFT BASINS

175

Fig.5. Palaeographic maps. (a) 126 Ma, note major faults separating crustal blocks A,B,C,D, open arrows are extensional

directions, half arrows are relative direction of wrench fault movements. (b) Note major development of rift basins at 108 MY B.P.

relative to 126 Ma. (c) Major extent of Late Cretaceous marine transgression 90-84 Ma; open arrows show extension direction, half

arrows show transpressional direction (Santonian squeeze). (d) Rift configuration at 30 Ma. Note position of East African rift

systemshown by dotted lines. Marine areas mapped in Nigeria, western most Niger, Mali and Western Algeria have been compiledfrom literature (Reyment, 1972; Petters, 1981; Petters and Ekweozor, 1982; Reyment and Dingle, 1987).

d.

176 G.J. GENIK

..............-

RIFT BASINS IN NIGER, CHA

the Neogene as did the East African rift system(Rosendahl et aI., 1986). The Red Sea-EastAfrican rift system is genetically completely inde-

pendent of the WCAS African rift system(Fairhead and Green, 1989), over prints the for-mer and transects it at the Anza rift (Fig. 5d) in

the border zone bel'Kenya.

Rift basin descriptionA A'

w E WAS basins in Nigera

o

8 . . 8'

The Grein, Kafra,extensional asymmetrcated at the north endbasins range in lengthwidth from 40 to 70contain 3500-6000 mand marine clastics

Jurassic pre-rift non-The basins' structural

tilted, normally faultlwere active during 1Palaeogene. Phase 4,

<J)ffi 5000tu::;10.000

8.

5000

w E

<J)a;wtu10.000::;

15,000

b. WESTAI

BONGOR BASINC'

N

DOOBA BASIN

D'NNW

KMo

GREINKAFRA

o 5 KMS.~o 5MI.

. o 5 KMS.~o 5MI.

SEo

~ 2500t;;sooo2

7500

NW

1

C

"'2500

~500027500

SEo

~ 2500

t;;sooo27500

c. d.

E E' SALAMA T BASIN

2

SEo

~ 2500

~50007500

o 5 KMS.~o 5MI.

F

3

e. f. 4

LEGEND'5

IRM I RECENTMIOCENE

~ PALEOGENE

IKUI UPPERCRETACEOUS

IKLI LOWERCRETACEOUS

IKPZICRETACEOUSPALEOZOIC

IiPe 1 PRECAMBRIAN

Fig. 6. Seismicallyderivedbasin profiles, locationsof a and b are on Fig.3; C,d, e and f are on Fig. 4. (a) Tenere, Grein, Kafrabasins are well defined asymmetric half grabens. (b) Termit basin showing Late Cretaceous Phase 3 and Palaeogene Phase 4normal faulting. (c) Bongor basin with normally faulted asymmetric flanks and Late Cretaceous structural inversion. (d) Doba basinwith Borogop boundary wrench fault and Late Cretaceous flexing. (e) Doseo basin with Early Cretaceous Phase 3 and LateCretaceous late Phase 3 ductile structuring. (f) Salamat basin with brittle transtensional Borogop fault zone and extensional fault

blocks along basin margins.

Fig. 7. Generalized stratigra

RIFf BASINS IN NIGER, CHAD AND THE CENTRAL AFRICAN REPUBLIC 177

the border zone between Sudan, Ethopia andKenya.

marked and tend to obscure the early rift Phase 3faulting.

The Termit basin, located further to the south,is the largest of the WAS basins (Figs. 2 and 3). Itis separated from the aforementioned basins bythe Agadez Lineament (Guiraud et aI., 1985),across which the regional Bouguer gravity pat-terns (Louis, 1970; Fairhead, 1988) as well as thestructural styles change. The Termit basin is anextensional asymmetric rift, 575 km long and150-200 km wide (Figs. 3 and 6b). Palinspasticfault reconstruction suggests extension of about40 km (SNEA, pers. commun., 1990). Preliminaryinterpretation of deep reflection seismic suggestsa minimum Moho depth in the order of 26 km(O.D. Baldwin and l.R. Gaither, pers. commun.,1991). Other geometric reconstructions using asimilar Moho depth suggest basin extension ofabout twice this amount. The Termit basin con-

Rift basin description and structure

WASbasins in Niger and Chad

The Grein, Kafra, Tenere and Tefidet areextensional asymmetric rift basins which are lo-cated at the north end of the WAS (Fig. 2). Thesebasinsrange in length from 250 to 500 km and inwidth from 40 to 70 km (Figs. 3 and 6a). Theycontain 3500-6000 m of Cretaceous non-marineand marine clastics and minor Palaeozoic to

Jurassic pre-rift non-marine sediments (Fig. 7).Thebasins' structural style is dominated by largetilted, normally faulted blocks (Fig. 8-1) whichwere active during the Early Cretaceous andPalaeogene. Phase 4, Palaeogene movements are

IKM

oREv.MIOe.

25ac

joe.~~~LEOMAAS.

RM

1P

2

3 KU

C/):::)oUJC,,)«I-UJa:C,,)

4

KL5KL

BASEMENTIGN-MTMPHC

--'.oILTESTED~ OIL/GAS

. TESTED OIL

Fig.7, Generalized stratigraphic columns WAS; Grein, Kafra, Tenere, Termit and Bongor basins. Note absence of marine strata in

Bongor.

178

tains estimated maximum sediment thicknesses ofaround 14,000 m (Fig. 7). These sediments arecomposed of 1000-3000 m of Early Cretaceousterrigenous clastics, 5000-7000 m of Late Creta-ceous shallow marine shales, sands, silts and mi-nor carbonates and up to 4000-5000 m of Ceno-zoic continental sands, and shaly sediments.

Volcanics occur within the basin in the Lake

Chad region where rhyolite and basalt dikes, anddolerite sills were penetrated at Sedigi and Ku-mia, respectively (Fig. 3; Table 1). These vol-canics have been dated stratigraphically to be noolder than 85-95 Ma. Rhyolite outcrops to the

(1)

W.....

(3)

W

G.J. GENIK

south of Lake Chad are probably of Cenozoic age(Wolff, 1964). In the northwest Termit basin theoutcroping of Gosso Lorom basalts, dole rites andtuffs (Fig. 3), range in age from Recent to 10 Ma(SNEA, internal commun., 1990; Wilson andGuiraud, 1992, this volume).

The two main structural styles over the major-ity of the Termit basin reflect predominantlyPhase 3 extension (Fig. 6b) and late Phase 3, andPhase 4 sinistral wrench tectonics. Phase 3 NW-

SW trending synthetic normal fault blocks (Figs.8-2 and 9) are common on the Iaguil platformand in the central and western Termit area. Phase

E

(2)

W E

E(4)NW ".S SE

Fig. 8. Seismic section structural examples in WAS, scales-horizontal as shown, vertical in seconds, locations on Fig. 3. (1) Termit

basin normal synthetic tilted fault block. (2) Termit basin, basement horst and Late Cretaceous draped anticline. (3) antithetic,

tilted fault block with a discovery. (4) Termit basin transpressional anticline with a discovery.

RIFT BASINS IN NIGER. CHAt

50 Kms. I

12'

Fig. 9. Termit basin fault trl

tiary extensional faults, NN

mal faults developed in latl

WI

4 and 5 NNW-SSW

thetic, normal and Idominate the easternthe Termit basin in8-3 and 9). AssociatlNE-SW trending trail3 and 8-4). These PIconsistent with the c<lateral wrench induciBornu basin (AvbovbBenue- Yola basins (I

In the Termit basirhave been discoveredanticlines in marine t

voir sands, which wecoeval marine to estu20°-36° API has alsorable structures in Ereservoirs which are :

gene shales and sourshallow marine and E

N'Dgel Edgi grabenrift on the southwest

(Figs. 2 and 3). It is 1~contains approximateceous chiefly contineand tilted blocks have

(Fig. 3).

RIFT BASINS IN NIGER, CHAD AND THE CENTRAL AFRICAN REPUBLIC

15"

1~ 1~ 1~

Fig, 9, Termit basin fault trends, NE-SW are mainly pre-Ter-

tiary extensional faults, NNE-SSW are mainly antithetic nor-

mal faults developed in late Phase 4 and 5 probably sinistral

wrenching.

it

4 and 5 NNW-SSW trending principally anti-thetic, normal and transtensional fault blocksdominate the eastern and southeastern parts ofthe Termit basin in Niger and Chad (Figs. 6b,8-3 and 9). Associated with this wrenching areNE-SW trending transpressional anticlines (Figs.3 and 8-4). These Phase 4 and 5 structures areconsistent with the contemporaneously aged leftlateral wrench induced folds in the contiguousBornu basin (Avbovbo et aI., 1986) and in theBenue-Yola basins (Benkhelil, 1988).

In the Termit basin, oil (43°-46° API) and gashavebeen discovered in fault blocks and faultedanticlines in marine tidal Late Cretaceous reser-

voir sands, which were sourced and sealed bycoevalmarine to estuarine shales (Fig. 7). Oil of20°-36°API has also been discovered in compa-rable structures in Eocene lacustrine sandstone

reservoirswhich are sealed by overlying Palaeo-gene shales and sourced from Late Cretaceousshallowmarine and Eocene lacustrine shales.

N'Dgel Edgi graben is a small extensional WASrift on the southwest flank of the Termit basin

(Figs.2 and 3). It is 150 km long, 40 km wide andcontains approximately 3000 m of Early Creta-ceouschiefly continental coarse clastics. Horstsandtilted blocks have been mapped in this basin(Fig.3).

c,

179

The Bongor basin (Figs. 2, 4 and 6c)is inter-preted to represent a partly exhumed, rotated,wrench modified, rift. This asymmetric basin is300 km long, 75 km wide and contains an esti-mated maximum 7000 m of Early Cretaceouscontinental siltstones, mudstones, shales andsandstones (Fig. 7). Minor volcanism is noted inthe form of a dolerite sill radiometrically dated at52-56 Ma in the Naramay well (Fig. 3; Table 1).The main structural styles in Bongor are exten-sional, rotated, large normal faults blocks of earlyPhase 3 age, and transpressional inverted anti-clines of late Phase 3, probably Santonian, age(Fig. 6c).

CAS basins in Chad and CAR

The Doba basin is regarded as a rotated,wrench modified, rift (Figs. 4 and 6d). This asym-metric basin is some 300 km long, 150 km wideand contains up to an estimated 7500 m of Neo-comian to Recent, predominantly non marinesandstones, shales and mudstones (Fig. 10). TheEarly Cretaceous series is more than 3000 mthick. Similar but smaller types of rift basins havebeen mapped in Cameroon 50 km west of theDoba Basin (Maurin and Guiraud, 1990).

Structures in the Doba basin are primarilyextensional rotated fault blocks (Figs. 6d and11-1) of Early Cretaceous Phase 3 age. Trans-pressional anticlines (Fig. 11-2) of Santonian ageare also present and form good hydrocarbon traps.On the west side of the basin, numerous easterlyplunging half grabens, steep sided to the north(Fig. 11-1) are separated by an accommodationzone (Rosendahl et aI., 1986; Fig. 4) from halfgrabens steep to the south on the east side of thebasin. The individual fault blocks trend NW-SE

subparallel to the basin margins (Fig. 12a), con-

sistent with their early extensional origin.. Theyounger anticlines are aligned in a northeasterlydirection reflecting their origin from the Santo-nian right lateral wrench movements.

High gravity, average 34° API waxy oil istrapped in anticlines, reservoired in Early Creta-ceous fluvial sandstones, and sourced and sealedby associated lacustrine shales. Low-mediumgravity oil 15°-25° API was tested in Late Creta-

180

ceous anticlines and faulted roll-over closures.

This oil, which tends to be biodegraded, was alsogenerated from the Early Cretaceous lacustrineshales. It is reservoired in extremely porous andpermeable (up to 10 Darcies) sandstones and issealed by younger Cretaceous lacustrine and floodplain shales.

The Doseo basin is a transtensional assymmet-ric complex rift in a "restraining bend" position(Christie-Blick and Biddle, 1985; Harding et aI.,1985), which is located along the south side of theBorogop wrench fault zone (Figs. 3, 4 and 11-3).Dextral displacement in basement outcrops alongthe Borogop fault has been estimated at about40-50 km (Cornacchia and Dars, 1983). Recent

G.J. GENIK

in house work suggests that Early Cretaceousdextral displacement of about 35 km (N.W.McAllister, pers. commun., 1991) along the Boro-gop fault zone has caused transtensional subsi.dence of the Doseo basin. The Doseo rift is 480

km long, 90 km wide and contains up to 7700 mof mainly Cretaceous continental shales, sand.stones and mudstones (Fig. 10). The Early Creta-ceous sediments make up to 5000 m of this inter-val, the Late Cretaceous clastics are about 2500m thick and Tertiary sediments barely exceed 200m. In this basin, the wells Keita, Kikwey andTega drilled 5-25 m thick sills consisting of al.tered basalt and andesitic basalt radiometricallydated at 97-101 Ma (Table 1; Fig. 4).

CENTRALAFRICANRIFTSUBSYSTEMDOBA DOSED SALAMAT I I AGEKM

o

1

2

3."---

KL--------.~..........._._-.

4..::.. '.':.~"'" :',.

5

-,..-/.-

6KL?

7

REv.-I

R

MIOC. a: M

2 LI ~a:

JOC. ~I p

~~~LEOMAAS.

I I

KUI .

84 (/)

SENON. 596 ~ALBIAN ~08 5

APTIAN I KL'BARRENEOC.126

1

131SO°ie-

- CHAD

LITHOLOGY

MARINE

~SHALE

NON-MARINE~~

SH-CLST.

EdSILTSTONE

EBSILTV/SHALVSANDSTONE

~611BSANDSTONE

VOLCANICS

BASEMENTIGN-MTMPHG~ TESTEDTOIL/GAS. TESTEDOIL

Fig. 10. Generalized stratigraphic columns CAS, note thick development of Early Cretaceous shales in Doseo relative to Doba andSalamat and near absence of Late Cretaceous sediments in Salam at.

~

RIFT BASINS IN NIGER, CHAD

(1)

sw

RIFf BASINS IN NIGER, CHAD AND THE CENTRAL AFRICAN REPUBLIC 181

(1)

sw NE

(2)

S. N

ad

182 G.J. GENlK

(8)

S

(7)

S NN.

-!_YO_O 00;::-. ~

... ...

Fig. 11. Seismicsection structural examplesCAS, scales-horizontalas shown, vertical in seconds, locationson Fig. 4. (1) Doba

basin, tilted normal fault blocks. (2) Doba basin transpressed Santonian anticline. (3) Borogop wrench fault zone with major

positive flower structures, Doba basin is relatively "toward" and Doseo is "away" to plane of section. (4) Doseo basin with EarlyCretaceous fault blocks and Late Cretaceous roll-over on downside of listric blocks. (5) Doseo basin positive flower structure in

ductile section, south (5) part of section is "away", north (N) is "toward" plane of section. (6) Doseo basin, transpressional

anticline with discovery, note Early Cretaceous inversion and Late Cretaceous thinning and truncation. (7) Doseo basin wrenched

basement high block on S end of section with transtensional negative flower structure towards E side of section. (8) Salamat basintraf!stensional normal fault block with 2 second Early Cretaceous down to south (5) throw.

Structures in Doseo basin are greater in num-ber, more varied, complex and more severelyfaulted than in Doba basin. This is due to the

more pronounced wrench effects upon a moreductile stratigraphic sequence. Transtensional(Harding, 1985) and transpressional (Harding andTuminas, 1988; Harding, 1990) structural stylesdominate. Examples are: transtensional and ex-tensional listric normal fault blocks (Fig. 11-4),

transpressional positive flower structures (Fig.11-5) and anticlines (Fig. 11-6), transpressionalbasement-high fault blocks (Fig. 11-7) andtranstensional negative flower structures. Re.gional Phase 3 faults strike WNW-ESE (Fig.12b) and trend at an oblique angle to the basins'ENE- WSW directed axis. Mapped northeastaligned en echelon anticlines reflect the Santo.nian transpressional deformations discussed ear.

RIFT BASINS IN NJGER, CHA

lier (Fig. 4). This stmthe dextral transtenslthis basin.

The Early Cretamedium to high gravbased oils and gas. Tthe reservoirs are flubiditic sandstones. Thlacustrine shales inter

The Salamat basinChad and western

transtensional comple"release bend" (Chri~location between maeastern end of the BIto the Doseo and Dol

shows the greatest rthrows on major nonmay reflect more brithe deformed sequen(by 60 km wide; it contlargely Early Cretaceand minor shales (Fi~vial sandstones, lacustpotential and floodcapable of being sealSalamat basin. The SI

very large, W- E treblocks (Fig. 11-8), wiway time (Fig. 6{).

8'15'

Fig. 12. (a) Early Cretaceouorigin of faults and basin.normal faults relative to I

IK RIFT BASINS IN NIGER, CHAD AND THE CENTRAL AFRICAN REPUBUC

lier (Fig. 4). This structural pattern characterizesthe dextral transtensional wrench formation ofthis basin.

The Early Cretaceous sediments containmedium to high gravity 24°-39° API, paraffinicbased oils and gas. The traps are anticlines andthe reservoirs are fluvial, fluvio-deltaic and tur-biditic sandstones. The source and seals are thicklacustrine shales interbedded with the reservoirs.

The Salamat basin (Fig. 2, 4 and 60 in easternChad and western CA.R. is classified as a

transtensional complex rift which developed in a"release bend" (Christie-Blick and Biddle, 1985)location between major horsetail splays at theeastern end of the Borogop fault zone. Relativeto the Doseo and Doba basins, the Salamat basinshows the greatest number of and the largestthrows on major normal intra basinal faults; thismay reflect more brittle lithologic properties ofthe deformed sequence. The basin is 300 km longby60km wide; it contains an estimated 6000 m oflargely Early Cretaceous terrigenous sandstonesand minor shales (Fig. 10). Reservoir quality flu-vialsandstones, lacustro-deltaic shales with sourcepotential and flood plain-lacustrine mudstonescapable of being seals have been drilled in theSalamat basin. The structure is characterized byvery large, W-E trending, transtensional faultblocks(Fig. 11-8), with throws up to 2 s of twowaytime (Fig. 60.)oba

aajorEarlyre in.ionalIchedbasin

(Fig.lonal

andRe-

(Fig.lsins'heastanto-

a.

183

Conclusions

The polyphased geologic evolution of the Cre-taceous- Tertiary rift basins in Niger, Chad andthe CA.R. has resulted in the formation of nu-merous traps for oil and gas that are typified bydiverse extensional, transtensional and transpres-sional structural styles. The sediments in thesebasins contain continental to marine sandstonereservoirs, kerogenous lacustrine source shalesand lacustrine to flood plain sealing shales. Todate, 15 oil and gas discoveries have been madein the Termit, Doba and Doseo basins. The totalprospective sediment volume exceeds 1 X 106km3. Geologic conditions favor finding additionalsignificant hydrocarbon volumes in many of theten studied WCAS rift basins.

Acknowledgements

I thank Exxon Company, International for theopportunity to prepare and publish this paper. Ithank also Exxon's present partners, ChevronOverseas (CO.P.I.), Shell In ternationale(S.I.P.M.) and Elf Aquitaine (SNEA(P)), as wellas former partners, Texaco, Conoco and AmaxOil and Gas for their permission to use jointlyacquired proprietary data.

I am grateful to the many colleagues whoworked the area over the years for their many

19' 20'

b.

Fig, 12. (a) Early Cretaceous fault trend map, Doba basin-note general fault subparallelism to basin margins suggesting extensional

origin of faults and basin. (b) Early Cretaceous fault trend map, Doseo basin-note obliquity of WNW-ESE trending pattern of

normal faults relative to ENE-WSW trend of Borogop fault zone and basin's long axis, patterns are consistent with dextral

transtensional movement along Borogop fault zone., ear-

184

valued technical interpretations and I particularlywant to recognize W.E. Sparks, E.B. Nelson, N.W.McAllister, E.E. Clark, the late J.T. Woodman,M.L. Taylor, R.E. Metter, J.P. Colin, Ian Nortonand Yow Yuh Chen.

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