Regional framework, structural and petroleum aspects of

Tectonophysics, 213 (1992) 169-185

Elsevier Science Publishers B.V., Amsterdam

lb9

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

G.J. Genik

Exxon Company, Internutional, P.O. Box 146, Houston, TX 77001-0146, USA

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

ABSTRACT

Genik, G.J., 1992. Regional framework, structural and petroleum aspects of rift basins in Niger. Chad and the Central

African Republic (C.A.R.). 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.

This paper overviews the regional framework, tectonic, structural and petroleum aspects of rifts in Niger. Chad and the

C.A.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 have

been 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 the

Western Central African Rift System (WCAS) with its component West African Rift Subsystem (WAS) and Centra! African

Subsystem (CAS) formed mainly by the mechanical separation of African crustal blocks during the Early Cretaceous. Among

the resulting rift basins in Niger, Chad and the C.A.R., seven are in the WAS-Grein. Kafra, Tenere. Tefidet. Tcrmit.

Bongor, and N’DgeI Edgi and three, Doba, Doseo, and Salamat are in the CAS. The WAS basins in Niger and Chad are al1

extensional and contain more than 14,000 m of continental to marine Early Cretaceous to Recent elastic sediments and

minor amounts of volcanics. Medium to light oil (20” API-46” API) and gas have been discovered in the Trrmit basin in

reservoir, source and seal beds of Late Cretaceous and Palaeogene age. The most common structural styles are extensional

normal fault blocks and transtensional synthetic and antithetic normal fault blocks. The CAS Doba, Doseo and Salamat are

extensional to transtensional rift basins containing up to 7500 m of terrestrial mainly Early Cretaceous elastics. Heavy to

light oil (15”-39” API) and gas have been discovered in Doba and Doseo basins. Source rocks are Early Cretaccous

lacustrine shales, whereas reservoirs and seals are both Early and Late Cretaceous. Dominant structural styles are

extensional 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 in

many of these basins.

Introduction

This paper overviews the regional framework, tectonic aspects and structural styles of rifts in Niger, Chad and the Central African Republic (C.A.R.) (Fig. 1). Petroleum geology is summarily addressed. The data base for this contribution is mainly the proprietary hydrocarbon exploration work carried out by Exxon and its partners in

Correspondence to: G.J. Genik, Exxon Company, Interna-

tional, P.O. Box 146, Houston, TX 77001-0146, USA.

these countries from 1969 through 1989. The data includes 81,400 line kilometres (one million square kilometres) of aeromagnetics. 10520 line kilometres of gravity surveys, some 50,000 kilometres of multi-coverage reflection seismic data and the results of 50 exploration wells,

The systematic hydrocarbon exploration of rifted basins in this region was pioneered by Conoco in Chad in 1969. Subsequently, Texaco, Global Energy (now Amax Oil and Gas Inc.). Phipps Petroleum, Shell, Chevron, Exxon and Eli have explored the area. At present Exxon, Shell and Chevron jointly explore a 104,234 km’ permit

0040-1951/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved

170 0 J. GENIK

in Chad and an Exxon-SNEA partnership

prospect a 36,580 km* permit in Niger. The first oil was discovered in the area in 1974

by Conoco in the Termit basin, Lake Chad area of Chad and the most recent oil discovery was in 1989 in the Doba Basin by Exxon and partners Shell and Chevron. To date 13 oil and two oil and gas discoveries have been made in three of the ten rift basins.

Regional framework and tectonic evolution

The Cretaceous-Palaeogene rifts of Niger, Chad and the C.A.R. make up a large part of a geotectonic continuum termed the West and Central African rift system (WCAS) (Fairhead, 1986) that extends 4000 km from the Gao trough in Mali to the Anza basin in Kenya (Figs. 2 and 5d). This rift system is subdivided into two coeval Cretaceous genetically related but physically separated rift subsystems (Fig. 21, the West African Rift Subsystem (WAS) and Central African Rift Subsystem (CAS).

22'

18'

14'

100

The geologic framework and distribution of

rifted basins (Fig. 2) in the study area evolved in

five major tectonic phases: (1) Pan African crustal consolidation (750-550

Ma) (2) Palaeozoic-Jurassic platform development

(550-130 Ma) (3) Early Cretaceous, early rift (130-98 t_ Ma)

to Late Cretaceous, late rift (98-75 + Ma) (4) Maastrichtian-Palaeogene rift and emer-

gence (75-30 Ma) (5) Neogene-Recent mainly emergence (30-O

Ma). Phase 1. (750-5.50 Ma). The Pan African

crustal consolidation produced major basement lineaments and faults whose trends (Fig. 2) formed precursor rift directions or the “essential structure” (Christie-Blick and Biddle, 1985) for the future Cretaceous rifts, e.g., the NW-SE trends in the Niger-Air region (Greigert and Pougnet, 1966; Black and Guirod, 1970) for the W.A.S. rifts in Niger and Chad; NE-SW trends (Chukwu-Ike, 1981; Popoff et al., 1983; Ajakaiye

OIL DISCOVERY

m STUDY AREA

$y J 0 100 200 300Kms

IL CHAD

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

RIFT BASINS IN NIGER, CHAD AND THE CENTRAL AFRICAN REPUBLIC 171

et al., 1986; Popoff, 1988a) of the Benue-Bornu rift basins in Nigeria, and the ENE-WSW strik- ing faults (Le Marechal and Vincent, 1972; Cor-

nacchia and Dars, 1983; Benkhelil, 1988) for the rifts of the CAS in Cameroon, Chad and the C.A.R.

Pan African basement rocks consist in outcrop of mainly mobile belt acid igneous and metamor- phic rocks west of longitude 14”E; east of this meridian are found more stable acid plutonics of the Pan African thermal belt (Windley, 1984). Six wells were drilled into the basement in the rifts (Table l>, four in the W.A.S. in Niger (Fig. 3) and two in the C.A.S. 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 (TabIe 1). A granite

from outcrop located west of the Doba basin at latitude 9”40’N and 14”E (Fig. 4) was dated by

Rb/Sr analyses as 481 It: 23 Ma.

Phase 2. (550-130 Ma). The study area per- sisted as a stabie, north plunging, emergent platform onto which Palaeozoic to Jurassic continental and shallow marine cratonic sag sediments transgressed from the north to a latitude of about 18”N (Fig. 2) (Faure, 1966; Greigert, 1966; Greigert and Pougnet, 1966; Kiitzsch, 1984; Guiraud et al., 1987). Remnants of these pre rift sediments would be caught up in the W.A.S. rifts of northern Niger. There was no known rifting in the region in Phase 2, but some thermal effect of the Late Palaeozoic-Early Mesozoic post Tethyan tectonics in North Africa may have reached the Termit basin (Table 1) where the Iaguil and Dilia

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 which yielded K-Ar dates of 266 and 190 Ma.

Phase 3. (130-75 Ma). This was the main time of rift development. The origin of the WCAS is attributed to the breakup of Gondwana and the start of separation between Africa and South

America (Burke, 1976; Browne and Fairhead, 1983; Fairhead, 1986, 1988; Benkhelil, 1988; Popoff, 1988a; Fairhead and Binks, 1991). These authors relate rift development to the gradual opening of the central and south Atlantic starting around 130 Ma when wrench fault zones ex- tended from South America onto the Gulf of Guinea and Africa, whereby their strike-slip movement dissipated into extensional basins in Niger, Sudan and Kenya. It may equally be ar- gued that the Sudanese and Kenyan rifts (Reeves et al., 19861, which also opened at this time, evolved in response to India and Madagascar separating from Africa (Scotese et al., 1988; Guiraud and Maurin, 1990).

G.J. GENIK

Considering these ideas, we suggest that the

WCAS formed primarily as a result of simultane-

ous, lateral, mainly mechanical divergence be-

tween discrete crustal blocks W, C, S and E during the Early Cretaceous 130-98 Ma (Fig. 5a). Sinistral movement of block W relative to blocks C, S and E caused transtensional opening of the WAS in the Benue-Bornu region. This was accompanied by the extensional subsidence of the Niger-Chad basins. Block C moving dextrally relative to blocks S and E obliquely opened the CAS

basins of southern Chad and C.A.R. and caused the extensional subsidence of the Sudan and Kenya rifts (Fairhead, 1986; Schull, 19881.

During Phase 1, up to 126 Ma the boundaries between the blocks corresponded to major frac- tures representing reactivated Pan African crustal discontinuities; only minor actual rifting occurred

(Fig. 5a) during this time. The period from 126 to 98 Ma is the early rift stage (equal to Popoff, 1988b, syn-rift Phase I). The rifts became fully developed across middle Africa at about 108 Ma

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-1 3143 Biotite gneiss, pegmatite Basement 434-489 (R) b. Termit Gosso Lorom outcrop Basalts, dolerites, tuffs Extrusives < 1 to 10 (RI c. Termit Iaguil-1 2486 Hornfels, schist Basement 266 + and < 116 (R) **

c. Termit Iaguil-1 1250 Alkalic diabase Sill 8.6 + 0.5 (RI d. Termit Dilia Longrin-1 1987 Granite Basement 190*7 (R) e. Termit Sedigi-1 2095 Rhyolite, basalt Dike 5 85 (Sl f. Termit Sedigi-2 2103 Rhyolite Dike 5 85 (Sl g. Termit Kumia- 1 4100 Dolerite Sill I 95 (Sl h. N’Dgel Edgi N’Dgel Edgi-1 2776 Metamorphics quartzite,

micaschist, phyllite Basement Pan African (?I i. Bongor Naramay-1 1256 Dolerite Sill 52-56 (RI j. Doba Kassi-1 2766 Granodiorite Basement 568 k 26 (R) k. Doba Beboni-1 3496 Gneiss Basement 474 * 17 (RI 1. Doseo Tega- 1 1875 Basalt Sill 97 + 1.2 (RI m. Doseo Keita-1 2395 Altered andesitic basalt Sill Ill0 (S) n. Doseo Kikwey- 1 1100 Altered basalt Sill 101.1 * 1.1 (R)

(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.

RIFT BASINS IN NIGER. CHAD AND THE CENTRAL AFRICAN REPUBLIC 173

(Figs. 5a and b and 6) and were filled with en- tirely continental sediments, This age of rifting is supported by paleontological data from wells drilled in the Tenere, Bongor and N’Dgel Edji basins in Niger (J.P. Cohn, pers. commun., 1990 and Y.Y. Chen, pers. commun., 1991), from the Muglad basin in Sudan (Schull, 1988) and from the Anza basin, Kenya (National Oil Corporation of Kenya, 1990), as well as from outcrops flanking the Benue basin (Popoff, 1988a,b) and eastern Niger rifts (Faure, 1966; Greigert and Pougnet,

1966). This rift period clearly correlates with the

early rift phase in West Africa and Brazil (Popoff,

1988a). During the late stage of Phase 4, at about 98

to 75 Ma (Fig. 5~) a late rift to sag stage followed [equal to Popoffs (1988a,b) syn-rift Phase II] and was accompanied by a marine transgression which reached the study region from the Tethys to the north via Mafi and Algeria and from the South Atlantic to the south through the Benue trough, as indicated from the respective microfauna

CETACEOUS

PERMO~TRIASSIC

PALEOZOIC

PRECAMEMIAN

CENOZOIC VOLGANiB

SEDIMENT THICKNESS

I ta12 KM

, o,L + DRY

A-A’ - STRUCTURAL CROSS SECTIONS

1.2.3 - SEISMIC

ilu la_b..c...NAM@ WELLS TABLE

Fig. 3. Simplified isopach of the WAS with surrounding generalized surface geology. Named well locations and outcrop are: a = Seguedine #I; 6 = Goss.0 Lorom outcrop; c = Iaguil-1; d = Dilia Longrin-I; e = Sedigi-I; f= Sedigi-2; g = Kumia-1; h = N’dgel

Edgi-1.

174 G.J. GENIK

(Furon, 1963; Faure, 1966; Murat, 1972; Petters, 1978, 1979; Allix et al., 1981; Petters and Ekweo-

zor, 1982; Benkhelil and Robineau, 1983; Du-

faure et al., 1984; Reyment and Dingle, 1987). This transgression is coincident with the world- wide Late Cretaceous eustatic sea level highstand (Haq et al., 1987). Micropalaeontological data from exploration wells tends to corroborate this bi-directional marine flooding. The epeiric seas attained their maximum eastern extent at about 80 Ma when they reached the western most end of the Doba basin (Fig. 5~). Well data shows that several thousand metres of Late Cretaceous marine sediments filled the WAS, whereas the CAS received similar thicknesses of purely continental sediments.

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

tectonic pulse at around 85 Ma when the regional

stress regime apparently changed abruptly. The tectonic pulse may have resulted from variations

in the spreading rate and direction between the equatorial and southern Atlantic plates (Fairhead and Binks, 1991) and/or by a N-S compression between the African and European plate (Guiraud, 1987, 1992, this volume). This tectonic pulse, which seems to have been chiefly transpressional in the region, lasted some 5 Ma as seen on seismic records. It caused folding and basin inversion in the Benue, Yola and Bornu basins (Popoff et al., 1983; Benkhelil, 1988; Avbovbo et al., 1986), as well as similar structuring in the southern Termit and Bongor basins. At the same time, the Agadez lineament (Figs. 2 and 3) (Guiraud et al., 1983) was reactivated, separating the Termit from the northern Niger rifts. Moreover, the Doba, Doseo and Salamat basins,

lb*

BONGOR BASIN

- ,i.

lw i2’

CHAD ,_,/--'-< SALAMAT BASIN .I ~1 _ ‘\

CAMERDD -k” 0 50 100 150 km

_ R <x ” OUADDA BASIN 8’

. . SCALE

LEGEND SEDMNT THICKNESS

cl 0 OUATERNARY-TERTIARY * OIL AN0 GAS

q 1-3 KM cl KU UPPER CRETACEOUS . OIL + DRY

q 3-5 KM cl KL LOWER CRETACEOUS F-F’ - STRUCTURAL CROSS SECTIONS

&j 5-6 KM cl p8 PRECAMBRIAN 1.3.5..--_- SEISMIC SECTIONS

2-6 KM / MAJOR FAULTS i., J. K. NAMED WELLS TABLE 3

Fig. 4. Simplified isopach of the CAS with surrounding surface geology. Named well locations are: i = Naramay-1; j = Kassi-1;

k = Beboni-1; l= Tega-1; m = Kikwey-1; n = Keita-1.

RIFT BASINS IN NIGER. CHAD AND THE CENTRAL AFRICAN REPUBLIC

which up to this time had effectively been a single emergent. Areas east of the Cameroon volcanic

basin, were separated into three discrete basins line (Fitton, 1980) in the Adamawa area were

by dextral movements along the Borogop fault. uplifted (Fig. 2) and eroded. Geochemical data

Additionally, anticlinal folds developed in parts indicates that up to 2500 m of sediments were

of the Muglad basin in Sudan (Schull, 1988) and eroded from the western part of Doba basin and

the Anza basin of Kenya. the Bongor basin. At the same time, the Goss0

Phase 4. (75-30 Mu). Following the preceeding Lorom volcanics developed in the northwest Ter-

tectonics a new rift cycle started in the WAS mit basin along the Agadez Lineament (Fig. 2).

Niger rifts and the CAS, Muglad and Anza rifts Elsewhere in this basin a 350-500 m thick blan-

with the accumulation of up to 4000 m of Maas- ket of continental sediments was deposited in the

trichtian-Paleogene continental elastics. Else- area centered on the present Lake Chad

where, the WCAS basins were essentially emer- (Mothersill, 1975).

gent (Fig. 5d). On the eastern side of Africa, rifting of the

Phase 5. (30-U Ma). During this the last major Red Sea area started about 40 Ma (Lowell and tectonic phase, the WCAS remained effectively Genik, 1972) and became fully developed during

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 F&P. relative to 126 Ma. (cl 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 system shown by dotted lines. Marine areas mapped in Nigeria, western most Niger, Mali and Western Algeria have been compiled

from literature (Reyment, 1972; Petters, 1981; Petters and Ekweozor, 1982; Reyment and Dingle, 1987).

176 G.J. GENIK

the Neogene as did the East African rift system pendent of the WCAS African rift system

(Rosendahl et al., 1986). The Red Sea-East (Fairhead and Green, 1989), over prints the for-

African rift system is genetically completely inde- mer and transects it at the Anza rift (Fig. 5d) in

W E 0 5KMS.

a.

b. 0 SKYS.

0 5 MI.

C C’ s BCNGORBASJN

N

C.

0 0 5KMS

0 5MI

D’ W

d.

0 5KMS +

E E’

E E

0 SKI45

F - F’ 0 SALAMATBASIN

5 MI

SE BcuuGOPFA"LTZMlE

NW 0

~:25c+l

&lo

7500

f. e.

LEGEND ’

m RECENTMIOCENE

[PI PALEOGENE

m UPPERCRETACEOUS

m LOWERCRETACEOUS

k<pzl CRETACEOUSPALEOZOIC

m PRECAMBRIAN

Fig. 6. Seismically derived basin profiles, locations of a and b are on Fig. 3; c, d, e and f are on Fig. 4. (a) Tenere, Grein, Kafra

basins are well defined asymmetric half grabens. (b) Termit basin showing Late Cretaceous Phase 3 and Palaeogene Phase 4

normal faulting. (c) Bongor basin with normally faulted asymmetric flanks and Late Cretaceous structural inversion. (d) Doba basin

with Borogop boundary wrench fault and Late Cretaceous flexing. (e) Doseo basin with Early Cretaceous Phase 3 and Late

Cretaceous late Phase 3 ductile structuring. (f) Salamat basin with brittle transtensional Borogop fault zone and extensional fault

blocks along basin margins.


the border zone between Sudan, Ethopia and

Kenya.

Rity basin description and structure

WAS basins in Niger and Chad

The Grein, Kafra, Tenere and Tefidet are extensiona asymmetric rift basins which are located at the north end of the WAS (Fig. 2). These basins range in length from 250 to 500 km and in width from 40 to 70 km (Figs. 3 and 6a). They contain 3500-6000 m of Cretaceous non-marine and marine elastics and minor Palaeozoic to Jurassic pre-rift non-marine sediments (Fig. 7). The basins’ structura1 style is dominated by large tilted, normally faulted blocks (Fig. 8-l) which were active during the Early Cretaceous and Palaeogene. Phase 4, Palaeogene movements are

marked and tend to obscure the early rift Phase 3

faulting. The Termit basin, located further to the south,

is the largest of the WAS basins (Figs. 2 and 3). It is separated from the aforementioned basins by the Agadez Lineament (Guiraud et al., 19851, across which the regional Bouguer gravity patterns (Louis, 1970; Fairhead, 1988) as well as the structural styles change. The Termit basin is an extensional asymmetric rift, 575 km long and 150-200 km wide (Figs. 3 and 6b). Palinspastic fault reconstruction suggests extension of about 40 km (SNEA, pers. commun., 1990). Preliminary inte~retation of deep reflection seismic suggests a minimum Moho depth in the order of 26 km

(0.D. Baldwin and J.R. Gaither, pers. commun., 1991). Other geometric reconstructions using a similar Moho depth suggest basin extension of about twice this amount. The Termit basin con-

WEST AFRICAN RIFT SUBSYSTEM - NIGER, CHAD GREIN KAFRA TENERE TER~IT BON~OR AGE LIT~OLOGY

84 . SENON. 96- ALBIAN 108- APT~AN 3&\RfM

126 '.

i M

P

-

(U

MARINE

lzl SHALE

liza CARBONATES

NON-MARINE

$z&

lxl SILTSTONE

SILTEALY SANDSTONE SANzoNE

m VOLCANICS

l;,-r BASEMENT IGN-MTMPHC

* TESTED OIL/GAS

0 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 of

around 14,000 m (Fig. 7). These sediments are composed of 1000-3000 m of Early Cretaceous terrigenous elastics, 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, and dolerite sills were penetrated at Sedigi and Ku- mia, respectively (Fig. 3; Table 1). These volcanics have been dated stratigraphically to be no older than 85-95 Ma. Rhyolite outcrops to the

G.J. GENIK

south of Lake Chad are probably of Cenozoic age

(Wolff, 1964). In the northwest Termit basin the outcroping of Gosso Lorom basalts, dolerites and

tuffs (Fig. 3), range in age from Recent to 10 Ma (SNEA, internal commun., 1990; Wilson and Guiraud, 1992, this volume).

The two main structural styles over the major- ity of the Termit basin reflect predominantly Phase 3 extension (Fig. 6b) and late Phase 3, and Phase 4 sinistral wrench tectonics. Phase 3 NW- SW trending synthetic normal fault blocks (Figs. 8-2 and 9) are common on the Iaguil platform and in the central and western Termit area. Phase

(1)

W E I ‘“WI --m

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.


I \ \ I I 120 130 140

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.

4 and 5 NNW-SSW trending principally antithetic, normal and transtensional fault blocks dominate the eastern and southeastern parts of the Termit basin in Niger and Chad (Figs. 6b, 8-3 and 9). Associated with this wrenching are NE-SW trending transpressional anticlines (Figs. 3 and 8-4). These Phase 4 and 5 structures are consistent with the contemporaneously aged left lateral wrench induced folds in the contiguous Bornu basin (Avbovbo et al., 1986) and in the Benue-Yola basins (Benkhelil, 1988).

In the Termit basin, oil (43”-46” API) and gas have been discovered in fault blocks and faulted anticlines in marine tidal Late Cretaceous reservoir sands, which were sourced and sealed by coeval marine to estuarine shales (Fig. 7). Oil of 20”-36” API has also been discovered in compa- rable structures in Eocene lacustrine sandstone reservoirs which are sealed by overlying Palaeo- gene shales and sourced from Late Cretaceous shallow marine and Eocene lacustrine shales.

N’Dgel Edgi graben is a small extensional WAS rift on the southwest flank of the Termit basin (Figs. 2 and 3). It is 150 km long, 40 km wide and contains approximately 3000 m of Early Creta- ceous chiefly continental coarse elastics. Horsts and tilted blocks have been mapped in this basin (Fig. 31.

The Bongor basin (Figs. 2, 4 and 6~) is inter-

preted to represent a partly exhumed, rotated, wrench modified, rift. This asymmetric basin is

300 km long, 75 km wide and contains an estimated maximum 7000 m of Early Cretaceous

continental siltstones, mudstones, shales and sandstones (Fig. 7). Minor volcanism is noted in the form of a dolerite sill radiometrically dated at

52-56 Ma in the Naramay well (Fig. 3; Table 1). The main structural styles in Bongor are extensional, rotated, large normal faults blocks of early Phase 3 age, and transpressional inverted anticlines of late Phase 3, probably Santonian, age (Fig. 6~).

CAS basins in Chad and CAR

The Doba basin is regarded as a rotated, wrench modified, rift (Figs. 4 and 6d). This asymmetric basin is some 300 km long, 150 km wide and contains up to an estimated 7500 m of Neo- comian to Recent, predominantly non marine sandstones, shales and mudstones (Fig. 10). The Early Cretaceous series is more than 3000 m thick. Similar but smaller types of rift basins have been mapped in Cameroon 50 km west of the Doba Basin (Maurin and Guiraud, 1990).

Structures in the Doba basin are primarily extensional rotated fault blocks (Figs. 6d and 11-l) of Early Cretaceous Phase 3 age. Trans- pressional anticlines (Fig. 11-2) of Santonian age are also present and form good hydrocarbon traps. On the west side of the basin, numerous easterly plunging half grabens, steep sided to the north (Fig. 11-l) are separated by an accommodation zone (Rosendahl et al., 1986; Fig. 4) from half grabens steep to the south on the east side of the basin. The individual fault blocks trend NW-SE subparallel to the basin margins (Fig. 12a), consistent with their early extensional origin. The younger anticlines are aligned in a northeasterly direction reflecting their origin from the Santo- nian right lateral wrench movements.

High gravity, average 34” API waxy oil is

trapped in anticlines, reservoired in Early Creta- ceous fluvial sandstones, and sourced and sealed by associated lacustrine shales. Low-medium gravity 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 also

generated from the Early Cretaceous lacustrine shales. It is reservoired in extremely porous and permeable (up to 10 Darcies) sandstones and is sealed by younger Cretaceous lacustrine and flood plain shales.

The Dmeo basin is a transtensional assymmet- ric complex rift in a “restraining bend” position (Christie-Blick and Biddle, 1985; Harding et al., 19851, which is located along the south side of the Borogop wrench fault zone (Figs. 3, 4 and 11-3). Dextral displacement in basement outcrops along the Borogop fault has been estimated at about 40-50 km (Cornacchia and Dars, 1983). Recent

G.J. GEi’ilK

in house work suggests that Early Cretaceous

dextral displacement of about 35 km (N-W. McAllister, pers. commun., 1991) along the Boro-

gop fault zone has caused transtensional subsidence of the Doseo basin. The Doseo rift is 480 km long, 90 km wide and contains up to 7700 m of mainly Cretaceous continental shales, sandstones and mudstones (Fig. 10). The Early Creta- ceous sediments make up to 5000 m of this inter- val, the Late Cretaceous elastics are about 2500 m thick and Tertiary sediments barely exceed 200 m. In this basin, the wells Keita, Kikwey and

Tega drilled 5-25 m thick sills consisting of altered basalt and andesitic basalt radiometrically dated at 97-101 Ma (Table 1; Fig. 4).

CENTRAL AFRICAN RIFT SUBSYSTEM - CHAD :M _I~ DOBA DOSE0 SALAMAT 1 I AGE I LITHOLOGY

MARINE

B SHALE

NON-MARINE m

SH-CLST.

i""-1 SILTSTONE

B SILTY/SHALY SANDSTONE

SANDSTONE

m VOLCANICS

BASEMENT IGN-MTMPHC

* TESTED OIL/GAS

0 TESTED OIL

Fig. 10. Generalized stratigraphic columns CAS, note thick development of Early Cretaceous shales in Doseo relative to Doba and

Salamat and near absence of Late Cretaceous sediments in Salamat.

RIFT BASINS IN NIGER, CHAD AND THE CENTRAL AFRICAN REI

111 I

DOEEO EMIN BOROQOP FAULT ZONE - -_-

G.J. GENIK

Fig. 11. Seismic section structural examples CAS, scales-horizontal as shown, vertical in seconds, locations on 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 Early Cretaceous fault blocks and Late Cretaceous roll-over on downside of listric blocks. (5) Doseo basin positive flower structure in ductile section, south (S) 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 basin

transtensional normal fault block with 2 second Early Cretaceous down to south (S) throw.

Structures in Doseo basin are greater in number, more varied, complex and more severely faulted than in Doba basin. This is due to the more pronounced wrench effects upon a more ductile stratigraphic sequence. Transtensional (Harding, 1985) and transpressional (Harding and Tuminas, 1988; Harding, 1990) structural styles dominate. Examples are: transtensional and extensional listric normal fault blocks (Fig. 11-41,

transpressional positive flower structures (Fig. 11-5) and anticlines (Fig. 11-61, transpressional basement-high fault blocks (Fig. 11-7) and transtensional 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 northeast aligned en echelon anticlines reflect the Santo- nian transpressional deformations discussed ear-

RIFT BASINS IN NIGER. CHAD AND THE CENTRAL AFRICAN REPUBLIC 183

lier (Fig. 4). This structural pattern characterizes

the dextral transtensional wrench formation of

this basin. The Early Cretaceous sediments contain

medium to high gravity 24”-39” API, paraffinic based oils and gas. The traps are anticlines and the reservoirs are fluvial, fluvio-deltaic and tur- biditic sandstones. The source and seals are thick lacustrine shales interbedded with the reservoirs.

The Salamat basin (Fig. 2, 4 and 6f) in eastern Chad and western C.A.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 the eastern end of the Borogop fault zone. Relative to the Doseo and Doba basins, the Salamat basin shows the greatest number of and the largest throws on major normal intrabasinal faults; this may reflect more brittle lithologic properties of the deformed sequence. The basin is 300 km long by 60 km wide; it contains an estimated 6000 m of largely Early Cretaceous terrigenous sandstones and minor shales (Fig. 10). Reservoir quality fluvial sandstones, lacustro-deltaic shales with source potential and flood plain-lacustrine mudstones capable of being seals have been drilled in the Salamat basin. The structure is characterized by very large, W-E trending, transtensional fault blocks (Fig. 11-S>, with throws up to 2 s of two way time (Fig. 6f).

Conclusions

The polyphased geologic evolution of the Cre-

taceous-Tertiary rift basins in Niger, Chad and the C.A.R. has resulted in the formation of nu-

merous traps for oil and gas that are typified by diverse extensional, transtensional and transpressional structural styles. The sediments in these basins contain continental to marine sandstone reservoirs, kerogenous lacustrine source shales and lacustrine to flood plain sealing shales. To date, 15 oil and gas discoveries have been made in the Termit, Doba and Doseo basins. The total prospective sediment volume exceeds 1 x lo6 km3. Geologic conditions favor finding additional significant hydrocarbon volumes in many of the ten studied WCAS rift basins.

I thank Exxon Company, International for the opportunity to prepare and publish this paper. I thank also Exxon’s present partners, Chevron Overseas (C.O.P.I.), She~i InternationaIe (S.I.P.M.) and Elf Aquitaine (SNEA(P)), as well as former partners, Texaco, Conoco and Amax Oil and Gas for their permission to use jointly acquired proprietary data.

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

I I

a. 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, Dose0 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.

184 G-I. GENIK

valued technical interpretations and I particularly

want 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 Norton and Yow Yuh Chen.

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Documents

Regional framework, structural and petroleum aspects of