HYDROCARBON POTENTIAL OF
THE VOLTAIAN (VOLTA)
GNPC – TEMA; 7TH FEBRUARY, 2007.
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• ABOUT THE VOLTAIAN SEDIMENTARY BASIN
• REGIONAL GEOLOGY AND STRATIGRAPHY
• GEOPHYSICAL AND GEOLOGICAL DATABASE
• HISTORICAL REVIEW OF OIL & GAS EXPLORATION IN THE VOLTAIAN
• EFFORTS SO FAR AT GNPC
• REGULATORY FRAMEWORK
• HYDROCARBON POTENTIAL
• DEDUCTIONS AND CONCLUSION.
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GHANA’S SEDIMENTARY BASINS
ONSHORE − 107,100 KM²
OFFSHORE – 49,900 KM²
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ABOUT THE VOLTAIAN SEDIMENTARY BASIN
• The Voltaian (or Volta) Basin is the single
largest sedimentary basin in Ghana. It
occupies about 40% of the total landmass of
Ghana, covering about 103,600km2.
• Basin stretches from Ghana into Togo, Benin,
Burkina Faso and Niger.
• Basin is believed to have a petroleum system
in place, with favourable conditions for
generation and accumulation of commercial
quantities of hydrocarbon.
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COMPARATIVE SIZE OF VOLTA BASIN WITH
REST OF GHANA’S SEDIMENTARY COVER
AREA SIZE (km²) UNDER
103,600 0 0 103,600
Coastal Onshore 3,500 0 0 3,500
23,000 4,869 21.2 18,131
(0 – 200m)
26,900 22,127 82.3 4,773
(200 – 3000m)
Area Total 157,000 26,996 17.2 130,004
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• Offshore sedimentary basins - formed as a
result of the separation of the African and South
• The Voltaian Basin is an epicontinental interior
sag basin associated with the Pan-African
Orogeny of ~ 600 million years ago and was
formed as a result of the collision of the West
African Shield (of the Birrimian System) with the
Benin-Nigerian Shield (of the Dahomeyan
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• The basin is an asymmetrical depression filled with thick
sediments of Late Precambrian (Proterozoic/Infra-
Cambrian) to Early Paleozoic era. Age differentiation so
far difficult due to rarity of diagnostic fossils.
• Oldest Voltaian sediments overlie unconformably on
Lower (“Birrimian”) - Middle (“Tarkwaian”) Proterozoic
• Sediment thickness is estimated to exceed 6,000m in the
deepest portions in the east and south-east.
• Some sediments are considered as young as Devonian-
Carboniferous in age.
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• Depositional environment is shallow-marine to
• Sedimentation is mainly clastic, with some carbonates.
The lithology comprises sandstones, siltstones, shales,
mudstones and limestones. The sandstones are
massive and conglomeratic in places.
• Sediments, especially lower (older) members, are gently
folded and with normal block-faulting. Dips recorded in
Premuase #1 wildcat measured 1 - 4 deg.
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GEOPHYSICAL & GEOLOGICAL DATABASE
• Aeromagnetic data over the entire the basin, shot
between 1960 and 2006 (currently as part of EU project):
• 1960 and 1961 surveys by Hunting Geology & Geophysics Ltd for
• 1969 surveys by Prakla-Seismos Ag for Uranerzbergbau Gmbh;
• 1971 survey by CGG for Shell International Petroleum.
• 2006 (ongoing EU-funded airborne surveys being coordinated by
• Ground Gravity data covering parts of the basin.
• Limited seismic data (208km of 2-D data).
• Shallow Premuase-1 well (1,167m or 3,830 feet) data.
• Limited data on ten (10) hydrodynamic wells drilled to
~700m in the 1960s by the Soviets.
• Diverse documentations on various aspects of the
basin’s geology and geophysics, particularly the ff:
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GEOPHYSICAL & GEOLOGICAL DATABASE (CONTD.)
• Data currently available at GNPC:
• 11 individual reports (including on Premuase – 1 well)
• 16 maps and sections (incl 3 of 4 seismic sections)
• 7 suites of well logs on sole wildcat, Premuase – 1
• Field/Borehole samples including:
– Washed & dried drill cuttings from 2 locations: Buipe - SGST #7 and
– Core chips from 4 locations: Nasia - SGST #5, Tamale – SGST #2,
Tibagona – SGST #1 and Yendi – SGST #6.
– Other field geological/outcrop samples.
• Other data not yet in GNPC domain:
• Some ~ 31 individual published and unpublished reports.
• Data believed to be in domain of Prof N.A.
Bozhko/Zarubezhgeologia of Russia:
• Unspecified reports, sections, maps and other data.
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(AFTER N.A. BOZHKO, 1964, AND J.E. CUDJOE, 1971 – GSD)
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GEOLOLGICAL CROSS SECTIONS
(AFTER U. SULUTIU, 1968, AND J.E. CUDJOE, 1971 – GSD)
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• Phase I: Up to 1961- Geological Survey Department (GSD) efforts,
including P. Davis’ Gravity Survey of 1957 – 58 and early Soviet
Geological Team’s exploration efforts to determine hydrodynamics
of the basin.
• Phase II: 1962 - 70 – Soviet Team efforts (till 1965), and later
Romanian Team’s oil prospecting efforts. Continuation of GSD
geophysical (mainly gravity) data acquisition.
• Phase III: 1971 - 77 - Efforts by Shell International in magnetic, then
seismic data acquisitions. Eventual wildcat drilling of Premuase #1
• Fallow period : 1978 - ~1996 – No significant activity (for ~ 18 yrs.)
• Phase IV (Current): 1996 – Date - Exploration promotion and re-
evaluation of Basin’s potential by GNPC in cooperation with
prospective partners, notably Ghana Geological Survey and the
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HISTORICAL REVIEW (CONTD.)
• Reconnaissance geological mapping by Geological Survey Dept
(GSD) with detailed mapping in isolated areas.
• 10 hydrogeological wells drilled in the 1960s by the Soviets (SGST
wells) to average depth of 700m, with some of them encountering
traces of bitumen, heavy oil and gas indications.
• Ground Gravity data acquisition by Geological Survey Dept in the
• Wide grid aero magnetic survey by CGG in the 1970s, totaling
18,000 line-kilometers made up of groups of 4 flight lines at 4km
spacing and 11 tie lines at 45km spacing.
• Only 208 line kilometers of 2D seismic survey by Shell.
• 1 wildcat – Premuase-1 well by Shell in 1977 with results: dry,
plugged and abandoned.
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HISTORICAL REVIEW (CONTD.)
• Indications of heavy oil and gas in Soviet (SGST) wells
of the 1960s. Saline waters (of Cl – Na – Ca type
mineralization), usually associated with petroliferous
sediments, were also found in the wells.
• Bitumen was also found impregnated in limestone,
shale and sandstone sequences, of the Middle Voltaian
• Early attempts at evaluating hydrocarbon potential have
not been successful. Further attempts between 1996
and early 2000s were also not sustained.
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EFFORTS SO FAR AT GNPC
• In May 2005, GNPC established the Voltaian Project
Unit (VPU) and charged it with the major objective of
accelerating and coordinating programmes to promote
the Voltaian Basin to a marketable status. In
particular, the VPU was charged to:
– Develop strategies of opening up the Voltaian Basin to
– Develop strategies for raising funds for reconnaissance
– Collaborate with the Ghana Geological Survey to obtain
applicable set of data from the EU Airborne Surveys;
– Develop and implement complementary reconnaissance
exploration programmes to bring the Basin to a
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EFFORTS SO FAR (CONTD)
VPU WORK PROGRAMME
• The VPU of GNPC proposed a Work Program and Budget for
the Basin’s Geoscientific Study and Evaluation. It outlines
GNPC’s plan to undertake a systematic geologic and
geophysical study of the Basin within a 4½-year, three-
phase Study Program.
• Essentially, the Study involves Geological-Geophysical
Reconnaissance in the initial phase, to be followed by a
High-grade Evaluation in the second. It will culminate in a
Drilling/Evaluation programme in the final phase.
• This Study is expected to generate a catalogue of leads,
prospects, maps, reports and other important technical
documentation within a broad spectrum of a well-defined
Petroleum System. Its results are targeted at the
international petroleum industry and are expected to raise
the Basin’s status to an attractive and marketable level.
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EFFORTS SO FAR (CONTD)
Notable efforts currently at GNPC are devoted
to accomplishing Phase I of the Program.
However, they also include:
• Technical contacts, data reviews, etc
between GNPC and interested investor
partners and service companies;
• Compilation and study/review of the basin’s
data inventory, useful data exchanges with
GSD, our reliable and dependable partner.
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EFFORTS SO FAR (CONTD)
• Technical liaison with GSD and by extension, with GEUS (of
Denmark) through its Technical Advisor (TA) to foster
familiarization and participation in the Voltaian Study/EDF Project,
and to identify further channels of technical cooperation.
Projections have also been made for a joint GNPC - GSD geological
fieldwork over the basin as a collaborative effort.
• GNPC/GSD Geotraverse Project in 2Q-2006 over the Buem and
Voltaian geological provinces of Ghana
– Conducted in collaboration with Prof. Pascal Affaton of CEREGE
– Objective: to collect fresh rock samples for dating to know more
about the geology of the Voltaian Basin as part of efforts toward
• GNPC participation in Geosciences workshops, as on the
Stratigraphy & Water Quality of the Voltaian Basin, organized jointly
by GSD, HAP (Hydrogeological Assessment Project), GEUS and
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EDF AIRBORNE SURVEYS
• The EDF (European Development Fund) through its “8 ACP GH 027”
project has provided financial and technical support to fund a 36-month
€40m project for Ghana’s Mining Sector Support Programme. GNPC is
keenly interested in the results of these surveys.
• Under the Terms of Reference for Airborne Geophysical Surveys, the total
area to be surveyed includes the Volta River Basin (93,000 km²) and the
Keta Basin (5,000 km²) totaling 98,000 km².
• The works are being conducted in 5 phases, with Phases 1 and 2 together
taking ~12 months:
• Phase 1: Geological outline through Radar and Optical Satellite
• Phase 2: Airborne Geophysical Survey for Magnetics, Gamma-Ray
Spectrometry (and Gravity?);
• Phase 3: Airborne Electromagnetic and Magnetic Geophysical
Survey of specific areas, following the completion and
interpretation of phase 2, using fixed wing Time Domain technology
– 12 months;
• Phase 4: Interpretation of the combined Geology and Geophysics –
• Phase 5: Production of factual and interpretation maps – 3 months.
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EDF AIRBORNE SURVEY LOCATIONS
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• The following Laws govern petroleum E & P operations in
– GNPC Law (PNDCL 64);
– Exploration & Production Law (PNDCL 84);
– Petroleum Income Tax Law (PNDCL 188).
• The above have been conveniently packaged into a Model
Petroleum Exploration and Production Agreement.
• The elements of the existing fiscal regime are contained in
PNDC Laws 84 and 188. The fiscal elements are:
– Carried Interest,
– Additional Interest (Pay Interest),
– Petroleum Income Tax, and
– Additional Oil Entitlement.
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REGULATORY FRAMEWORK (CONTD)
Ghana National Petroleum Corporation recommends a
downward revision of some fiscal elements and an increase
in exploration period for Voltaian Basin and Deep Water
operations, and the fixing of same in the Laws. These
proposals are currently receiving Cabinet attention.
Shallow Water Deep Water Voltaian Basin
Royalty 7.5% (from 10%) 5% (Unchanged) 6% (from 10%)
Carried Interest 10% (Unchanged) 7.5% (from 10%) 7.5% (from 10%)
Petroleum Income Tax 35% (Unchanged) 35% (Unchanged) 35% (Unchanged)
7 Years 10 Years (from 7
Exploration Period 10 Years (from 7 years)
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KEY PLAY ELEMENTS
Primary Objective: Middle Voltaian: Upp
Grnsh Gry Sandstone
• Secondary Obj: Lower Voltaian Oti beds
• Porosity: Fair, about 10-15% av;
Maximum ~ 20%, if
• Permeability: Fair.
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KEY PLAY ELEMENTS (CONTD)
• Source: Middle Voltaian - Variegated
Shale series. Type II, possibly
Type III kerogen.
• Seal: Variegated Shale/Argillaceous
• Traps: Structural: Horsts and fault
blocks, possibly anticlines.
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OVERVIEW OF ANALOGOUS BASINS IN ALGERIA
The following clips are based on a
presentation, circa 2005, “Contribution
of the Exploration Activity in
Renewing the Reserves – the Case of
Algeria” by Rabih Lounissi of
Sonatrach, Algeria’s major indigenous
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ALGERIA - SEDIMENTARY BASINS
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PLAY TYPES BY COMPARISON
• Algeria (Proved case histories) • Voltaian (Projected)
– Objective zones (Reservoirs): – Objective zones (Reservoirs):
Є (Pz) thro’ Eoc-Mio (Cz). Infra-Є (Vt1) Oti beds thro’ Mid-
Voltaian (Vt2) Upp Grnsh Gry Sdst.
– Reservoir lithologies: Mainly – Reservoir lithologies: Sdst, Congl
Sdst and Lmst. Sdst and Carbonates.
– Discoveries: – Discoveries: Nil (Not yet
• Oil: Tr, D, O, C, Є, S and K. applicable).
• Oil & Gas: Tr, D, O and C.
– Source rocks: Pz Mid-Voltaian
• Gas: D, O, Tr, C, S and Є. Variegated Shales.
– Source rocks: O, S and D – Seals: Pz Mid-Voltaian Variegated
(Frasnian). Argillaceous Carbonates.
– Seals: Tr shales. – Traps:
• Structural – Horst blocks,
Fault traps, possibly
– Traps: Mainly Structural – Anticlines.
Horst blocks, Fault traps, • Stratigraphic: Possibly
Anticlines; and Stratigraphic. pinchouts.
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ALGERIA – EXPLORATION SUMMARY
• Earliest exploration activities in the Cheliff Basin in the 1890s.
• First commercial discovery in Oued Gueterini Basin in 1948.
• Discovery of Hassi Messaoud and Hassi R’Mel giant fields in 1956.
• Between 1953 and 2004 in Algeria, the following advances were
made in petroleum E & P:
• ~ 530,000 km of 2D seismic acquisition – i.e. 0.4 km per km²
• ~ 6,000 km² of 3D seismic acquisition; and
• ~ 1,400 wildcats and appraisal wells - i.e. 9 wells per 10,000
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• On the basis of investigations conducted so far on the
Voltaian Basin, and based on experience from
analogous basins that are petroliferous in North
Africa, South America, and elsewhere, what is the
outlook for hydrocarbon prospectivity of the Volta
• Clearly, there is a bright and challenging outlook for
the hydrocarbon prospectivity and future exploration
potential of the Volta Basin. These stem from fact
There really exists a thick sedimentary cover in the
basin of at least 6km thick in the deepest sections.
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Structural as well as stratigraphic traps are likely to
occur in various subsurface locations.
The occurrence of sandstones and limestones in the
succession may serve as suitable reservoir rocks.
On account of the indications of heavy oil and gas,
bituminous and other hydrocarbon indications that
were encountered in the 10 SGST wells, this Basin
most probably harbours favourable conditions for
the generation and accumulation of commercial
quantities of hydrocarbon.
Known oil and gas discoveries already exist in
analogous basins in North Africa, Brazil and
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•The Voltaian Sedimentary Basin has a very bright outlook for its hydrocarbon
exploration. There really exists a thick sedimentary cover of at least 6km in its deepest
• There must certainly be oil + gas in the Voltaian Basin waiting to be tapped. On
account of the thickness of its sedimentary cover and the bituminous and other
hydrocarbon indications encountered in the Soviet wells of the 1960s, this Basin is
deemed to possibly harbour favourable conditions for generation and accumulation of
commercial quantities of hydrocarbon.
• There are possible structural and stratigraphic leads that could provide favourable
architecture for generation and accumulation of commercial quantities of hydrocarbon.
• The occurrence of sandstones and limestones in the succession, may serve as
suitable reservoir rocks.
• Known oil and gas discoveries and production already exist in analogous basins in
North Africa and elsewhere in the world.
• A very large domestic market for gas and oil in Ghana waits impatiently to be fed from
discoveries and production from the Volta Basin.
• Political, fiscal and legal environments in Ghana provide a lot of incentives for
attracting and sustaining investments in hydrocarbon exploration in the Volta Basin.
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