Petroleum geology of Africa: new themes and developing technologies, Volume 1

296 Pages·2003·34.56 MB·English

by  Geological Society of London

Petroleum Geology of Africa: Ne w Themes
and Developing Technologie s
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It is recommended that reference to all or part of this book should be made in one of the following ways:
ARTHUR, T.J., MACGREGOR, D.S. & CAMERON, N. (eds) 2003. Petroleum Geology of Africa: New Themes
and Developing Technologies.. Geological Society, London, Special Publications, 207 .
TARI, G., MOLNAR, J. & ASHTON, P. 2003. Examples of salt tectonics from West Africa: a comparative
approach. In: ARTHUR, T.J., MACGREGOR, D.S. & CAMERON, N. (eds) 2003. Petroleum Geology of Africa:
New Themes an d Developing Technologies. Geological Society, London, Special Publications, 207 ,
85-104.
GEOLOGICAL SOCIETY SPECIAL PUBLICATION NO. 207
Petroleum Geology of Africa: New Themes
and Developing Technologie s
EDITED BY
T. ARTHUR
Burnham, Slough, Berkshire, UK
D. S. MACGREGOR
Sasol Petroleum International, UK
and
N. R. CAMERON
Global Exploration Services, UK
2003
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Contents
Acknowledgements v i
PURDY, E. G. & MACGREGOR, D. S. Ma p compilations and synthesis of Africa's petroleum
basins and systems 1
HEMSTED, T. Second and third millennium reserves development in African basins 9
BURKE, K., MACGREGOR, D. S. & CAMERON, N. R. Africa's petroleum systems: four
tectonic 'Aces' in the past 600 million years 2 1
COWARD, M. P. & RIES, A.C. Tectonic development of North African basins 6 1
TARI, G., MOLNAR, J., & ASHTON, P. Examples of salt tectonics from West Africa: a
comparative approach 8 5
KARNER, G. D., DRISCOLL, N. W. & BARKER, D. H. N. Syn-rift regional subsidence across
the West African continental margin: the role of lower plate ductile extension 10 5
MACGREGOR, D. S., ROBINSON, J. & SPEAR, G. Play fairways of the Gulf of Guinea
transform margin 13 1
MORGAN, R. Prospectivity in ultradeep water: the case for petroleum generation and
migration within the outer parts of the Niger Delta apron 15 1
LUNING, S., ADAMSON, K. & CRAIG, J. Frasnian organic-rich shales in North Africa:
regional distribution and depositional model 16 5
VAN DER SPUY, D. Aptian source rocks in some South African Cretaceous basins 18 5
BURWOOD, R., REDFERN, J. & COPE, M. J. Geochemical evaluation of East Sirte Basin
(Libya) petroleum systems and oil provenance 20 3
DICKSON, W. G., DANFORTH, A. & ODEGARD, M. Gravity signatures of sediment systems:
predicting reservoir distribution in Angolan and Brazilian basins 24 1
DRUMMOND, J. M., KASMI, R., SAKANI, A., BUDD, A. J. L. & RYAN, J. W. Optimizing 3-D
seismic technologies to accelerate field development in the Berkine Basin, Algeria 25 7
PARNELL, J. & CHEN, H. Application of fluid inclusion studies to understanding oil charge,
Pre-Salt succession, offshore Angola 27 5
Index 28 5
Acknowledgements
This book is the outcome of the diligence and enthusiasm of the authors, and the generous sponsorship
provided by industry. Shell International Exploration and Production made a substantial donation which
enabled colour illustrations to be included for the academic papers. WesternGeco Ltd Multiclient funded
the production of the CD. The generosity of both these companies is greatly appreciated.
Companies that funded colour figures for papers with which they were associated are Anadarko Algeria
Company, PGS Geophysical, Vanco Energy Company and Veritas DGC. Their support, both in financial
terms and in permitting their staff to publish, is gratefully acknowledged.
Our gratitude must also be extended to the staff of the Geological Societ y in London and to the
Geological Society Publishing House in Bath. The referees, listed below, gave freely of their time and
energy to review the papers and provide help and guidance to the authors.
Jerry Anthony Davi d Ellis
Ray Bate Ia n Jarvis
David Boote Gar y Karner
John Brewster Stev e Lawrence
Nick Cameron Dunca n Macgregor
Andy Carr Justi n Morrison
Danny Clarke-Lowes Mik e Norton
Chris Cornford E d Purdy
Neil Craigie Bil l St John
Mark Davies Gabo r Tari
Ian Davison Hanna h Walford
John Dewey Chri s Walker
Peter Dolan Joh n Zumberge
Map compilations and synthesis of Africa's
petroleum basins and systems
E. G. PURDY1 & D. S. MACGREGOR2
1Foxbourne, Hamm Court, Weybridge, Surrey, KT13 8YA, UK (e-mail: [email protected])
2Sasol Petroleum International, 93 Wigmore Street, London, W1U 1HJ, UK (e-mail:
duncan. mac g re g or @ sasol. com)
Abstract: The purpose of this short contribution is to provide an overview of our current state
of knowledge of Africa's petroleum systems as an introduction to the detailed volume accounts
that follow. Toward that end we introduce a set of maps on a supplementary CD compiled by
Purdy as part of a confidential report provided to industry subscribers some ten years ago. The
maps include subsurface as well as surface structural information but, because of their vintage,
have not taken account of more recent information. Nonetheless, the regional geological frame-
work apparent on the maps has not changed and, in that sense, the maps are as relevant today
as when they were first compiled. Moreover the maps serve as a useful building block on which
more recently acquired exploration data can be readily added by others using facilitating com-
puter programs.
Map compilations basement and below the first age-diagnostic fos-
sils.
The map's basin classification follows the ter-
Two map compilations are provided on the sup-
minology of Kingston et al. (1983) with two major
plementary CD (Purdy 2003). The first is entitled
modifications. The first is the designation of delta
Exploration Fabric of Africa an d includes not only
sag (DS) for large basins that are dominated by
conventional outcro p geolog y bu t also pertinen t
delta fill . Th e secon d i s th e additio n o f a W
subsurface structural trends, including the depths
subscript for those basins in which the structural
and bottom-hole formation s of shallo w onshor e
style has been markedly affected b y subsequent
boreholes that have exploration significance (e.g.
wrench-fault movements. Some of these may turn
in Botswana, Sudan and Nigeria). Bathymetry is
out to be expressions of a single divergent wrench
also shown along with the location, bottom-hol e
couple rather than separate phases of interior frac-
formations and thickness of section penetrated by
turing and wrenching. Also indicated is the gen-
the Joint Oceanographic Institutions for Deep Earth
eralized stratigraphi c age of each stage of basin
Sampling (JOIDES) and Deep Sea Drilling Project
evolution. For example the Niger Grabens desig-
(DSDP) holes prior to 1989.
nation of IF/IS (K -Tp/T ) translates as an Early
The second compilation ma p is entitled Sedi- L N
Cretaceous t o Paleogene Interio r Fractur e Basin
mentary Basins of Africa an d illustrates the areal
succeeded by an Interior Sag Basin during the Neo-
extent and distribution of basins within the general
gene. This classification is important in providing
evolutionary framewor k o f Afric a describe d b y
an indication of the type of structures generally
Burke et al. (2003) The basins are classified by
present and, in some instances, provides some indi-
name an d ag e o f principal hydrocarbon-bearin g cation of the size of structures that may be present
reservoir play, illustrated against an outcrop back- by comparison to structural sizes known to be gen-
ground of Precambrian basement, infra-Cambrian erally associated with specific structural styles. It
sediment, volcanic s (mainl y Mesozoi c an d should be noted tha t the seaward extent o f the
Tertiary) an d importan t subsurfac e trends . Th e mapped basin s wa s arbitraril y limite d t o th e
delineation of infra-Cambrian sediments is viewed 1000m water-dept h contour . I n man y Wes t
as particularly important because there are several African margina l sa g basins , however , recen t
basins in Africa that are underlain by significan t exploration in deeper waters has demonstrated the
thicknesses o f unmetamorphosed, unfossiliferou s occurrence of Tertiary reservoir intervals that are
sediments lyin g abov e Precambria n crystallin e demonstrably more important than the older Cre-
From: ARTHUR, T. J., MACGREGOR, D. S. & CAMERON, N. R. (eds) Petroleum Geology of Africa: New Themes and
Developing Technologies. Geological Society, London, Special Publications, 207, 1-8. 0305-8719/037 $ 15
© The Geological Society of London 2003.
2 E . G. PURDY & D. S. MACGREGOR
taceous reservoi r play s identifie d o n th e map . uplift and erosion of the infra-Cambrian interval.
These major required revisions are detailed in the With these difficulties in mind, the prospectivity of
following discussion. the infra-Cambrian section is considered to be less
than encouraging.
Main African reservoir plays
There follows a summary of each of the main res- Palaeozoic within North African interior sag
ervoir plays identified on the map. These are sum- basins
marized with respect to age and basin type and are
The North African Palaeozoic play is presented in
based o n innumerabl e reference s an d persona l
this volume by Coward & Ries (2003) and in an
communications with industry workers, with only
earlier publication by Boote et al. (1998). Reser-
the key sources listed as references. For each of
voirs ranging in age from Cambrian to Early Car-
the identified reservoir plays, a brief assessment is
boniferous in the Ghadames (Grand Erg), Illizi,
made both of the proven petroleum provinces dis-
Hamra, Murzuk and Ahnet basins contain around
cussed by Hemsted (2003) and of frontier basins
18.5 billion barrels oil equivalent (BBOE), consti-
that may be analogous to these. A view is also
tuting 8% of Africa's petroleu m resource. Over
taken of the critical exploration factors and techno-
half of these reserves are oil in the Cambrian Hassi
logies applicable to each category. Finally, obser-
Messaoud Field . Hydrocarbon s wer e generate d
vations made on key reservoir and petroleum sys-
from Silurian and Devonian source rocks predomi-
tems are simplified into a number of themes that
nantly durin g Mesozoic reburial , pre-Hercynia n
characterize curren t hydrocarbo n exploration o n
generation havin g bee n largel y ineffective ,
the continent . Fo r furthe r detail s o f th e cite d
although the Ahnet Basin gas reserves could, as
reserves, please refer to the tables and references
Coward & Ries (2003) point out, be exceptions to
in the following paper by Burke et al. (2003)
this genera l rule . Th e impact o f the secondar y
Devonian source rocks has been significant for the
Infra-Cambrian reservoirs within interior most recent discoveries and these are described in
sag and fracture basins this volume by Liming et al. (2003) Technologies
that are contributing and will continue to contribute
Widespread areas of interior central and southern
to the discovery of more petroleum in these Palaeo-
Africa are characterized by the occurrence of thick
zoic systems include improved seismic imaging in
sequences of unfossiliferous sediments overlying
areas of unfavourable surface conditions, such as
Precambrian basement and conformably underly-
shifting sand dunes (Drummond et al. 2003)), and
ing the earliest fossiliferous beds, which are typi-
shallow carbonate occurrences. Improve d petro-
cally of Silurian age. The largest of these include
physical and engineering practices in some of the
the Zaire Interior Basin, the Barotse Basin of Ang-
tighter reservoirs, particularly the Ordovician flu-
ola, the Etosha Basin of Angola and Namibia, the
vioglacial sands , will undoubtedly contribute to
Kalahari Basin of Botswana and the Volta Basin
increased recovery of in situ hydrocarbons.
of Ghana.
The Silurian source rock is widespread across
These beds are typically referred to as 'infra -
most of the frontier basins in this class (Macgregor
Cambrian' and are assigned a Vendian-Ordovician
1996; Boote et al 1998) , parts of the Kufra Basin
age. Such strata are productive on the Arabian plate
being an exception. Consequently, the critical fac-
of southern Oman, where the petroleum syste m
tors for frontier areas in this basin category are bur-
benefits fro m a thick Mesozoic overburden , but
ial history and preservation. The absence of a sig-
there have been no infra-Cambrian discoveries in
nificant Mesozoi c buria l phas e i n mos t o f th e
Africa. This might reflect a lack of drilling as there
frontier basin s seem s t o b e a ke y differenc e
are no more than a handful o f true exploration
between these and the productive basins.
wells i n the basins concerne d and , withi n this
sequence, a number of good source rocks hav e
been mentioned in the literature. However, these Karroo megasequence within interior
are frequently assessed as being overmature. More-
fracture and foreland basins
over, burial history relative to phases of trap forma-
tion, uplift and potential flushin g i s difficul t t o The 'Karroo ' i s a broad lithostratigraphi c term
assess. These problems are manifested in the best- applied to sediments extending from the Late Car-
documented basi n o f Zaire (Daly e t al. 1992) . boniferous 'Dwkya ' glacia l sectio n u p t o th e
Here, Lawrence & Makazu (1988) have struggled Triassic-Liassic 'Stormberg' volcanics. These infill
to find source levels in outcrop which are not over- interior fracture and foreland basins developed dur-
mature and additionally coul d have undergone a ing or following Permo-Carboniferous tectonism.
Mesozoic reburia l phas e tha t post-date d majo r Despite fairly significan t exploration in sediment
COMPILATION OF AFRICA'S PETROLEUM BASINS 3
of this age in Madagascar and South Africa, sig- Jurassic-Early Cretaceous in interior
nificant hydrocarbon occurrences are limited to the
fracture/marginal sag basins
near-outcropping Bemolanga tar sands of Madaga-
scar, with a reported resource of 22 billion barrels Reservoir plays of this age are developed alon g
in-place. This oil occurrence is tied to thick Per- oceanic margins that rifted durin g the Jurassic,
mian shallow marine or lacustrine shales that have namely the northwestern African margin, the east-
not as yet been recorded outside of Madagascar ern Mediterranean margi n and the East Africa n
(Raveloson et al. 1991) and which seem to be lim- margin. With the exception of a number of Egyp-
ited to narrow rifts developed at this time. The tian rifts, such as the Shushan Basin (Keeley et al.
thick Permian coals that occur elsewhere seem to 1998), some small petroleum systems in Morocco
be predominantl y gas-pron e (thoug h occasiona l and the small Songo-Songo discovery in Tanzania,
these reservoirs have been surprisingly unproduc-
torbanites ar e reported) an d high maturities are
tive i n exploratio n t o dat e an d hav e certainl y
reported fro m man y basins (e.g. Kreiser e t al .
underperformed relative to other basins of this age
1998).
elsewhere in the world. While it is clearly difficult
The critical factors for Karroo plays thus seem to
to generalize over such wide areas, it can be said
be presence of an oil-prone source rock, maturation
that the plays most typically comprise either Jur-
timing relative to structuring , and the potential
assic carbonates (of variable reservoir quality) or
destructive effects of the numerous phases of uplift
Early Cretaceous elastics (e.g. Slind et al. 1988;
that have occurred. The main unexplored areas are
Jabour e t al. 2000) . Unlik e th e younger plays
the narrow rifts of Zambia, Zimbabwe and Mozam-
described below in younger marginal sag basins,
bique. A typical exploration case study in one of
there seems to be no clear regional source level.
these is provided by Barnes et al. (1995).
Lean Late Jurassic source rocks and Early Cre-
taceous reservoirs are a combination that appears to
work in some of the Egyptian basins and in Songo-
Triassic and Early Jurassic in interior sag Songo, but extensive geochemical analyses along
many of the margins indicate only sporadic source
and fracture basins
quality. Many of the East African basins concerned
have also undergone upper stage uplift in the Ter-
Triassic sands in Algeria, Tunisia and Libya are tiary, whic h may hav e caused remigratio n an d
one of Africa's most significant plays, with an esti- flushing. Possibilities may exist along these mar-
mated 31 BBOE of reserves, over half of which is gins for Triassic-Liassic lacustrine source rock s
gas in the giant Hassi'R Mel Field. These consti- within the rift sequences (e.g. Jabour et al. 2000),
tute abou t 14 % of Africa' s petroleu m reserve s as has been locally observed in the Newark rifts of
(Boote et al. 1998). The majority of reserves lie the US Atlantic margin. In many basins, however,
in fluvia l sand s belo w Triassi c sal t an d ar e these are very deep and may lie, or have previously
Palaeozoic-sourced. There are also major reserves lain, in the gas window.
at thi s leve l i n th e Sir t Basin , source d strati -
graphically downwar d fro m Cretaceou s sourc e Early-Late Cretaceous in interior
rocks (Burwood et al. 2003). Imaging of undrilled
fracture/marginal sag basins
traps below shifting sand dunes and salt is a criti-
cal-technology technique that has led to consider- The continenta l margin s develope d durin g th e
able recent success (Drummond et al. 2003). Early Cretaceous are considerably more productive
Triassic reservoirs were also developed in wide- and also can be shown to contain a series of prom-
spread Tethyan rifts and plays are developing in ising new plays. The margins concerned exten d
areas such as Morocco, where a recent discovery from Senegal to South Africa, with discoveries to
is speculated to be sourced from the Silurian. A date concentrated in the Aptian Salt Basin between
separate play is developed in Triassic-Early Jur- Nigeria and Angola (Coward et al. 1999). Most
assic fluvial sands in syn-rift sequences in Somalia traps are anticlinal, related to salt tectonics (Tari et
al. 2003). A series of petroleum systems in these
and Ethiopia (Tietz 1991), which, at least in the
areas is now well documented (Tiesserenc et al.
case o f Somalia , i s a n extensio n o f a pla y
1989; Katz 2000; Schoellkopf & Patterson 2000),
developed at this level in rifts in Yemen. Deep bur-
with a total resource base in Cretaceous reservoirs
ial and complex burial/uplift histories characterize
of around 13 BBOE, or 6% of Africa's petroleum.
much of this play, and the best possibilities may lie
Several petroleum system s can be simplisticall y
in areas such as the Darror Rift of eastern Somalia,
subdivided into pre-salt sources charging pre-salt
distant from th e areas of uplift aroun d the Afa r
and post-salt reservoirs, and post-salt source rocks
Plume and Gulf of Aden.
charging post-salt reservoirs. Syn-rif t Early Cre-