Table of Contents

Introduction .................................................................................................................... 2 Basin Location and Tectonic Settling ............................................................................ 3 Basin Evolution and Stratigraphy .................................................................................. 5

The South Atlantic Ocean ...................................................................................... 5 Field Study 1: ............................................................................................................... 10

Field Location and Discovery ................................................................................. 10 Bounds or Extent of the Field .............................................................................. 10 Discovery of the Rabi-Kounga field .................................................................... 10 Rabi Discovery ..................................................................................................... 10 Kounga Discovery ................................................................................................ 10 Structural or stratigraphic characteristics ............................................................. 11

Characteristics of the Source Rocks ......................................................................... 15 The Melania Shales ............................................................................................. 15

Reservoir Characteristics .......................................................................................... 16 Dentale Reservoir Rock –fig7 and fig13 .............................................................. 16

Production History ....................................................................................................... 18 Pressure Regime ....................................................................................................... 18

Field Study 2 ................................................................................................................ 19 Field Location and Discovery ................................................................................. 19

Márcio Rocha Mello, Barry Jay Katz, AAPG Petroleum systems of South Atlantic margins ......................................................................................................................... 19

Discovery of the Gamba field. ............................................................................. 19 Source Rocks Characteristics ................................................................................... 20

Melania Shales ..................................................................................................... 20 Reservoir Characteristics, Trap and Seal Mechanisms ........................................ 20 Production History ............................................................................................... 20

Discussion .................................................................................................................... 21 Conclusion .................................................................................................................... 25 References ................................................................................................................... 26 Appendix ...................................................................................................................... 28

PART B: CASE STUDIES

Study Area: Gabon Basin

IntroductionA petroleum system is defined by a mature source rock supplying hydrocarbons into a

migration network and, ultimately into a hydrocarbon trap. (Magoon1988).

The Gabon basin (figure 1) is divided into the northern and southern sub-

basin, separated by the N’ Komi fault zone. This project will focus on the southern

sub-basin.

Gabon, the third largest crude oil producing nation in sub Saharan Africa

holds a great potential for hydrocarbon exploration and production in this present

decade, in spite of its recent decline in oil production.

However Gabon has experienced a steady reserves growth. Gabon’s proven

reserves increased from 1.3 billion barrels in 1996 to 2.5 billion barrels in 2002.

The two fields that will be the point of focus is one of Gabon’s largest field

which is the Rabi-Kounga field and the Gamba field which have continued to dwindle [1].

Gabon’s biggest potential lies within the pre-salt section. The pre-salt section

lies below the salt seal and the post-salt refers to sediments deposited above the salt

seal (see Fig 11). The onshore and shallow water areas of the South Gabon Basin have

been successfully exploited but not in the deep water areas. New 2D seismic data

from CGGVeritas is helping to reveal the pre-salt potential before the 10th Gabonese

License Round scheduled between June and December 2010.[2]

1 Petroconsultants2 GeoExpro.com

Basin Location and Tectonic SettlingThe Gabon Basin is located at the limit of two old cratons which have been stable

since ~2Ga. The South Gabon basin stretches from the continental shelf in the east to

the ocean-continent boundary in the west. The sedimentary basins of Gabon (North,

South and Interior sub-basin, figure 1) form part of the salt basin complex that

stretches from southern Cameroon to Angola. They are limited to the north by the

Guinea Ridge and to the south by the Walvis Ridge (Figure 2). Both of these ridges

are volcanic basement features. This whole basin complex and the complementary

basins found on the Brazilian coast of South America were formed during the early

development of the South Atlantic, as Africa first rifted and then separated from

South America. This will be explained in detail in the Basin evolution and history.

The present-day coastline, basin boundaries and structural elements generally parallel

to the Phanerozic alignments mapped on the adjacent cratonic basement.

Figure 1

Gamba Field

Figure 2

Basin Evolution and Stratigraphy

The South Atlantic OceanThe Mesozoic tectonic evolution of the African plate has been characterized by three

main crustal rifting stages. Break-up of Gondwana led first to the Karoo rifting and to

the opening of the Central Atlantic Ocean by separating North West Africa and North

America (180 Ma), and more recently (20Ma) to the opening of the Red Sea-Gulf of

Aden and to the rifting in the East African Rift System. In between, during Late

Jurassic time (~150-140Ma), continental rifting began between South America and

Africa and led to the opening of the South Atlantic Ocean (Fig. 3) and to the

formation of the West and Central African Rift System (Binks and Fairhead, 1992;

Davison, 1999; Kampunzu and Popoff, 1991; Nurnberg and Muller, 1991; Teisserenc

and Villemin,1990).

Sedimentary basins associated with the opening of the South Atlantic and

located north of the Walvis Ridge-Rio Grande Rise complex include, Ivory Coast,

Barreirinhas, Benue, Potiguar, Rio de Peixe, Arripe to name a few.

North & South Gabon

Fig.3. The three major tectono-sedimentary South Atlantic domains with the location of the intracontinental and marginal basins in relation to Gondwana mega-discontinuities during the opening of the South Atlantic Ocean, modified after Popoff (1988)

The West African rifted margin is divided into two main domains by the

oceanic Walvis Ridge (see Fig. 3 and 4). South of the ridge there is a volcanic margin

extends from South Africa to Namibia), and is characterized by seaward dipping

reflectors (Gladczenko et al., 1997).

Along the Gabon Margin, onset of rifting started in Neocomian–Berriasian

times (144 Ma-million years ago) based on dating of the oldest rift sediments

corresponding most exclusively to fluviatile basal sandstones. In Cabinda (Fig. 5), a

volcanic layer found on top of the pre-rift sequence has been dated at 140Ma75 Myr,

corresponding to the Jurassic–Cretaceous boundary (Brice et al., 1982). The end of

rifting has been dated and associated with the magnetic anomaly M0 at ~118. Based

on these ages, rifting duration can be approximated to 25 Myr. The Gabon Margin is

segmented by major normal faults parallel to the present-day coastline and by NE–

SW trending strike-slip faults defining zones with partly different tectonic and

stratigraphic histories. The N’Komi fracture zone divides the Atlantic Basin into the

North Sub-Basin and the South Sub-Basin (Fig. 5).

Fig.4 Present-day configuration of the South Atlantic Ocean with sea floor topography derived form satellite data (Smith and Sandwell, 1997).Box shows study area

Fig 5 shows a simplified geological map of the studied area along the South

Western African coast focus on the South Gabon Basin.

From Fig 5 Locations of seismic lines and wells (eg G1 and G2), industry data

provided by Norsk Hydro are reported.

Offshore shallow reflection seismic lines stretch from the coast to the oceanic

crust over 80, 49 and 47 km for the lines 1, 2 and 3 respectively. Wells are mainly

located on the continental shelf.

Some of the southern major fault zones have been identified along the Gabon

Basin by onshore outcrops, reflection seismic, well data and gravity anomalies.

The oldest pre-rift sediments along the Gabon Margin are found in the north,

in the Interior Basin, which is separated from the offshore Atlantic Basin by the

Lambarene Horst as shown in Fig 5.

They consist of Late Carboniferous to Triassic-Jurassic fluviatile and

lacustrine. The oldest sediments are of the Precambrian age. Continental rifting began

in Berriasian time ~144ma and led to the formation of extensional faults and therefore

to series of garbens and horst.

The sedimentary section of Gabon is found in three major sub-basins as previously

showed in figure1.

The Gabon sedimentary section is deposited over a series of horst and garbens that

developed during the rifting of the metamorphic basement complex (Figure 2). These

metamorphics are highly variable ranging from granites and schists to metasediments.

This variability is reflected in the range of magnetic susceptibility seen in these

basement rocks. This magnetic variability leads to uncertainties in the interpretation

of aeromagnetic data especially in the deeper parts of the basin where supporting data

(seismic and well data) is poor or absent.

Fig 6

Fig 6

The identification of structural features on aeromagnetic data is further

complicated by the presence of extrusive volcanics in northern offshore Gabon and by

the boundary between the oceanic and continental crusts. (Figure 6)

As seen in the above figure the Rabi-Kounga oil and gas reservoir belong to

the pre-salt Gamba and Dentale sequence of the Dianongo Basin which form part of

the Lower Cretaceous African-South American rift valley system. Other gas and oil

field like the Gamba-Ivinga can be seen also.

Fig 7

Rabi-Kounga

Gamba-Ivinga

Field Study 1:

Field Location and Discovery

Bounds or Extent of the FieldThe Rabi-Kounga oil and gas reservoir reaches a maximum thickness of 6000 to

7000m and ranges in age from Early Cretaceous to Recent. The Rabi-Kounga Field is

covered by a Production License of 129 km.

Discovery of the Rabi-Kounga fieldThe Rabi-Kounga field discovered by Shell Gabon in August 1985 lies two

degrees south of the equator in a remote, sparsely inhabited part of the primary rain

forest of equatorial Africa.

The Rabi-Kounga Field was discovered within a relatively well known Lower

Cretaceous rift valley province.

Rabi DiscoverySeismic was acquired in 1985 and another closure, Rabi, which was structurally

higher than Echira could be mapped on the Mandji-M'Bari high trend. M'Bari-1(Fig.

8-appendix) is located 7 km to the north, had been drilled some 200 m down-flank

from the Rabi culmination. The well called Rabi-2 discovered the Rabi Field in

August 1985. Taking into account the low velocities found in the post-salt sequence

of Rabi-2, a reinterpretation of the seismic data in late 1985, led to the definition of

another closed structure to the north of Rabi, the Kounga structure (Fig. 8-

appendices).

Kounga DiscoveryThe exploration well Kounga-1 was drilled in 1986 on a closure 1.7 km (1

mile) west of M'Bari-1 and encountered the Base Salt objectives hydrocarbon bearing

over a gross interval of 68 m. A total of 22 m of gas and 46 m of oil were logged (Fig.

9-appendices). Most important of all, Kounga-1 established the continuity with the

Rabi accumulation. Fluid contacts, reservoir pressures, and crude characteristics

similar to those of the Rabi wells were observed. Further appraisal drilling which took

place during 1986 and 1987 (wells Rabi-4 to 13) confirmed that the two areas were

linked and formed only one structure.

In summary, the Rabi-Kounga discoveries derived from marked improvements

in the quality of the seismic data acquired since 1981 and the higher resolution of the

objective horizons obtained from better static corrections and velocity determinations.

Structural or stratigraphic characteristicsBased on seismic interpretation and well data, the Rabi-Kounga Field now

appears at Top Gamba level as a North-South trending anticlinal feature, almost 14

km long and 4 km wide. The eastern flank is dipping into a North-South trending fault

bounding the Rabi-Kounga structure to the East. The western flank is much flatter,

and in this direction, the edge of the field locally remains to be ascertained. The

anticlinal axis is affected by cresta1 faults which are related to the bending of the

Rabi- Kounga structure over an old Dentale core in post-Gamba times. Deep seated

transcurrent movements are locally reflected at Gamba level. The change in direction

of the structural axis in the northern part of the field may be taken as evidence of such

deep-seated transverse faults.

Below the Base Gamba unconformity, the Dentale anticlinal core is

subdivided by NNE-SSW faults into several tilted blocks which rise toward the NW.

The above shows source rocks that are abundant in the pre-salt sequence. It shows the

Kissenda and Melania shales on the Plateau, together with organic shale intercalations

of the Dentale formation in the trough

Source:-GEOEXPRO-Hunting the Pre-SaltThe stratigraphic cross-section shows Pre-salt traps comprise tilted fault blocks sealed by shales or salt and structural closures

associated with salt movement. Drape structures are also expected over the fault blocks (Gamba reservoir). Post-salt traps comprise drapes over salt domes sealed by shales or combination structural/stratigraphic traps of sand-rich channels within turbidite systems.

Unconformities

Melania, Source rk of Rabi-Kounga field and Gamba Field

Reservoir Rk of Rabi-Kounga

Ezanga Salt

Figure 11

Figure 123

Chronostratigraphic chart of the South Gabon Basin based on high-resolution stratigraphy

The Eustatic curve shows the eustatic change (as opposed to local change) as a result

in an alteration to the global sea levels, such as changes in the volume of water in the

world oceans or changes in the volume of an ocean basin in this case the Atlantic

Ocean.

3 www.sciencedirect.com

Sources of Interest

Rifting along the Gabon margin

The diagram above is a schematic section running across the Dianongo Basin from

the edge of the crystalline basement outcrops in the East, to the Atlantic coast in the

West. (illustrates the geological setting)

There is only one accumulation where the Dentale Formation is productive, and that is

Rabi-Kounga, see above. The trapping is insured by the salt (explained later) and

Dentale reservoirs are oil bearing because they are included inside a close high.4

The Rabi-Kounga oil and gas reservoirs belong to the pre-salt Gamba and

Dentale sequence of the Dianongo Basin which form part of the Lower Cretaceous

African-South American rift valley system.

The sedimentary prism reaches a maximum thickness of 6000 to 7000m and

ranges in age from Early Cretaceous to Recent. The geological setting is illustrated in

fig 5. The pre-salt source rock sequence is composed of fluvio-lacustrine sand and

shale deposits ranging in age from Neocomian to Aptian, (refer to chronostratigraphic

chart of the South Gabon Basin, fig 12), which have been deformed into tilted blocks

by basement tectonics and syn-sedimentary faulting. The sequence which includes the

Dentale sand reservoirs of the Rabi-Kounga Field and the organic rich Melania shale

source rocks represents the fill of a continental rift valley.

The evaporitic deposits and their thin underlying clastics of Aptian age

(Gamba Formation) which uncomfortably overlie the rift valley sediments reflect the

start of a marine transgression. Their deposition corresponds to a transition phase

when rifting ceased and continents started to separate.

Characteristics of the Source Rocks

The Melania Shales

• Deposited in quiet euxinic rift lakes formed during the late rift phase of the

Cretaceous, these organic rich lacustrine sources shales contain oil prone Type

1 kerogen shales.

• The Melania Formation contains source rocks of very high quality with an

average of 6.1% TOC by weight.

• While the maximum thickness is just about 800 meters

• In the east when approaching the basement outcrop Type I kerogen, passes

into Type III. As illustrated in the Figure 5/1 appendix, it has either been

eroded, or is immature or over-mature in many areas so that it is of interest as

a source of oil only in the Gamba-Ivinga high area.

4 Gabon and Douala Basin, PETROCONSULTANTS

• In short, although it is of very high quality, the potential of this source rock is

in fact limited in this study of the South Gabon Sub-basin

• The organic rich intervals have an organic carbon content that averages 8-10%

but may reach 20%.

Reservoir Characteristics

Dentale Reservoir Rock –fig7 and fig13

The Barremian age dentale sandstone provides the main reservoir for the

hydrocarbons contained in the Rabi-Kounga Field. In the Rabi-kounga field the

sandstone bodies are interpreted as being laid down in stacked channels. Individually,

the reservoirs are discontinuous, but there are many of them. They are in some cases

in communication with each other, giving a thickness of about 30m (Boeuf et al.,

1991). The sandstone is generally poorly consolidated at the relatively shallow depth

of 1100m at which they occur in this field giving excellent reservoir characteristics

with:

1. Average porosity close to 30%;

2. Permeability up to 1D Darcies

But in the southern offshore area, reservoir quality deteriorates, and thickness

decreases. The Dentale Formation (Fig.7 and 13 appendix) consists of sands and

shales and is interpreted as a fluviatile to lacustrine-fluvio-deltaic sedimentary

complex. The sands, mainly channel sands, are discontinuous but abundant in the

upper part of the sequence which becomes shalier at depth – lower Dentale and

Cardita formation. The source of hydrocarbon is in the intra-Dentale brownish shales

which are rich in lignitic and coal debris.

Gamba Reservoir Rock

The gamba formation is a good reservoir objective only if its thickness exceeds about

20m. At shallow depths as in Rabi-Kounga the porosities range from 20to 30% and

permeabilities may vary from 100 to over 1000 mD

The Gamba sands are generally massive, well sorted and homogeneous and are

interpreted as deposited in a lacustrine/coastal environment. Over the field, their

reservoir characteristics are generally very good. Some tightly cemented dolomitic

layers occur at the top of the Gamba sands

Migration Pathways, Faults and Traps and Seals Mechanisms

There is a complex pathway documented in the West Africa Salt Basin. This

involves the migration of pre-salt sourced oil to post-salt reservoirs. Such migration

pathways are indicated by comparison of the fingerprints of such oils and source

shales which appear to match. The Gamba sands act as a migration route. Thus to

reach the post-salt section the upper Gamba shales could be thin or the Gamba must

be fault juxtaposed with the younger section.

Juxtaposition by faulting is also common in both the pre-salt and post salt.

Erosional juxtaposition at regional unconformities also occurs primarily in the presalt.

The Gamba sands may rest uncomfortably in Melania shales, which may in turn rest

uncomfortably on Lucina shales. The Gabon Margin is segmented by major normal

faults parallel to the present-day coastline and by NE–SW trending strike-slip faults

defining zones with partly different tectonic and stratigraphic histories.

From the above it is obvious that although such migrations are documented, very specific local circumstances must prevail for them to occur.

Salt (Ezanga Salt) provides the seal for the Dentale and Gamba reservoir.

Traps related to the Dentale Formation are characteristically arcuate, narrow

enlogated and broken by faults into separate tilted compartments. They may also

suffer from unsatisfactory intra-Dentale sealing conditions. Seals are not sufficiently

well developed in the more prospective upper part of the succession to hold large oil

Fig 15 -

accumulations against fault-traps except perhaps where an absence of the overlying

Gamba sandstone allows pressure sealing to develop below the salt.

Potential hydrocarbon traps are manifested as horst, tilted fault blocks and

stratigraphic traps (pinchouts, truncations and aggradational carbonate build-ups).

These traps are potentially sealed by syn-rift shale (Vebo Shale) and Aptian age

transitional-phase evapourites (Ezanga Salt)

Production HistoryRabi-Kounga Field Production Characteristics

The above shows the field Production characteristics

Production testing yielded maximum rates of 448 m3/d on a 5/8 inch choke from the

oil zone, and 280000 m3/d on a 1/2-inch choke from the gas interval.

Pressure Regime

The technical challenges of obtaining a high oil recovery in the Rabi-Kounga

reservoir environment, therefore, centre on making the best possible use of the very

large fluid expansion potential available by avoiding excessive free gas production

which may result in resaturation losses.

In Rabi, the potential for such losses is compounded by the presence of a continuous

overlying sand unit (Gamba Sand Member) which enables fluid communication to

occur between compartments. Through the conduit of the Gamba Sand Member, any

pressure depletion can possibly be communicated via the gas cap without direct oil

drainage.

Field Study 2

Field Location and Discovery

The second field study is the Gamba Field.

The field was discovered in 1967 and had since produced over 200 million

barrels of oil. Little information is present about this field because very little

documentation was done on this field5. It produced oil from sandstones of the Gamba

formation. The Rabi-Kounga field produce from this formation but its source rock is

the same as the Gamba field which is the Melenia source rocks.

The oil is trapped in an aerially large, low relief anticlinal complex that

drapes the basement high of the Gamba Horst adjacent to Hinge Zone 1. (See figure

GAB 1-appendices and fig 2) The anticlinal complex is broken into three fields by

east-west faults that offset the Gamba Horst. Vertical seal to the Gamba sands is

provided by both Gamba shales and the overlying Ezanga salts. The Rabi-Kounga

field has seals which is also Ezanga salts (evporites)

Discovery of the Gamba field.The discovery of the Gamba field involves geological field work, aerial

photography, air magnetometry, gravity and a small amount of seismic reflection

(440Km) and refraction surveys (131Km). On the basis of the results of

aeromagnetics and refraction seismic two highs were identified. The first well,

Kissenda-1, was dry. No reservoirs or significant shows of hydrocarbons were

encountered. However, the second well drilled close to the coast, in the later part of

1963 discovered the Gamba Field (see fig Gab 1). The oil was found at a much

shallower level than expected, in a transgressive sand at the base of a Lower

Cretaceous salt sequence (Gamba Sst., see Figs 7 and 9 appendix).

5 Márcio Rocha Mello, Barry Jay Katz, AAPG Petroleum systems of South Atlantic margins

Source Rocks Characteristics

Melania Shales

The oil is sourced primarily from the organic rich lacustrine Melania shales

(see figure 16) that subcrop the base Gamba unconformity over large areas of the

Gamba Horst (refer to Fig 11).The organic rich but also thin Gamba shales may have

contribute a minor part of the oil.

Refer to the Rabi-Kounga field source rock for the oil window and kerogen type as it

has the same source rock as the Gamba field

Gamba Shales

These are organic rich shales of the upper members of the Gamba formation.

They are organic rich marine shale but are usually less than 50m thick so have limited

generating potential as stated before but may be of local significance.

Reservoir Characteristics, Trap and Seal MechanismsThe structural development at the Gamba level is probably due to both post-

depositional compactions across the horst, together with minor reactivation of the

horst, probably in the Albian.

The migration of oil into the structure probably began in the latest Cretaceous

and has continued through to the present day with the main oil generating phase

occurring in the Paleogene.

As stated before the vertical seal of the Gamba sands is provided by both

Gamba shales and the overlying Ezanga salts. These are the same types of seal found

in the Rabi-Kounga field of the Gamba and Dentale formations. Gamba formation has

porosity from 20-30% and permeability from 100mD to 5D.

Production HistoryThe field, which came on stream in February 1967, has now produced some 42

million stm3 and still accounts for a production of some 1500 stm3/d.

DiscussionTo start the discussion of the findings the comparison of the fields in the

Gabon Basin would be compared to another field in the same basin and a different

basin.

The Rabi-Kounga and Gamba field has many similarities. The both fields have

the same source rock which is organic rich lacustrine Melania shales. Also noteworthy

is that the two fields has the same reservoir rock which is the Gamba sands

Gamba Reservoir inProperties Rabi-Kounga Field Gamba FieldPorosity 20-30% 20-30%

Permeability 100 -1000mD 100mD to 5D

Seals Salt Salt, vertical

The above compares the Gamba reservoir sand in 2 different fields

Current production in Rabi-Kounga field= 280,000 m3/d, Gamba field=1,500stm3/d

The field that also has similar characteristics to the Rabi-Kounga is the La

Luna field in the Maracaibo Basin in North- Western Venezuela. The trends that are

similar include the depositional history, lithology, sedimentary bodies. The time

period of the La Luna formation is the Cretaceous period in which the Melania Shales

in Gabon basin also originated from. To compare the depositional environment of the

La Rosa field sands to that of the Dentale reservoir is that the La Rosa field was

deposited in a fluvio-deltaic environment which consisted of continental deposits

overlaying an unconformity which cut Eocene units. The Dentale reservoir sands

depositional environment is related, in that it is a fluviatile to lacustrine-fluvio-deltaic

sedimentary complex which also cut the Eocene unconformity. The thickness of the

dentale reservoir is about 30m thick unlike that of the La Rosa sands range form 5-60

feet.

The comparison of porosity and permeability in the Gabon sub-basin and the

Maracaibo basin is summarise as follows

Basin South Gabon-Sub basin Maracaibo basinProperties Melania source rk La Luna source rk

Age Cretaceous Cretaceous

Mode ofDeposition Quiet euxinic rift lakes

Shelf to slope environment under anoxic conditions

Kerogen type

type I to III Type II

TOC 6.1% 11.2%

The Source Rocks

Basin Gabon South Sub-basin Maracaibo basin

Properties Dentale formation

Gamba formation (Rabi-kounga)

La Rosa Barua

Porosity 30% 20-30% 25-30% 8-20%

Permeability up to 1000mD 100 -1000mD 100-1126mD 50-200mD

lithology sandstone sandstone sandstone sandstoneDepositional Environment

fluviatile to lacustrine-fluvio-deltaic

lacustrine/coastal fluvio-deltaic formed during the mountain building process

Te

rtia

ry

Pa

leog

ene

Ne

og

ene

Mio

cen

eP

aleo

cene

Eoc

ene

PERIOD

Te

rtia

ry

Cre

tace

ous

LA

TE

EPOCH LITHOSTRATIGRAPHY

Olig

ocen

e

Pliocene

Pleistocene

CE

NO

ZO

ICM

ES

OZ

OIC

ER

A

Major eustatic sea level fall

East Africa Basin, Albertine graben sandstone

East Africa Basin, Albertine graben, sandstone

Gabon Basin, Melania shales

Miocene to recent time, several eustatic sea level changes occurred

Basin, Source Name, Type

Maracaibo basin, La Luna Shales

Michigan Basin limestone with interbedded thin shales

Gabon basin unconformites

EOCENE unconformity in Maracaibo basinAnd Gabon basin

Cre

tace

ous

ME

SO

ZO

IC

Jurassic

Triassic

PA

LE

OZ

OIC

PRECAMBRIAN

Permian

Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Cambrian

Car

boni

fero

us

EARLY

PERIOD EPOCH

ER

A

Lithostratigraphy

East Africa Basin con’t

Eastern Venezuela Basin, Querecual formation, Mudstone

South Gabon Sub basin, Gamba and Dentale sand/shale sequence of the Dianongo Basin Form part of African-South American rift valley system.

Illinois Basin, New Albany Shale.(deposition to migration)

Basin, Source Name, Type

North Sea Basin, Kimmeridgiam shales

Western Canada Basin, Evie shale

Maracaibo Basin con’t

Michigan Basin con’t

North Slope Alaska, Shublik, sandstoneMiddle to upper Triassic,

The above compares the basin’s source rock ages and type (shales or sandstone)

as they appear on the Geological timescale. Most of the source rocks formed in

the Cretaceous period.

Next the effects of sea level changes on the stratigraphy, sediment supply and deposition of various basins are to be compared to the South Gabon Sub-Basin.

Basin Effect of sea level changes on:Stratigraphy Sediment supply Deposition

Gabon

South Sub

Basin

Extensive

erosion and deep

channeling of

the older units

Lowstand events

introduced coarse

clastics into former

deep marine,

Submarine channels

sands, base-of slope

fans clastics and

deep-marine, and

basin floor fan

complex clastics

Coarse sediments

deposited on Eocene

unconformity

Michigan

basin

As time

progressed the

floor of each sea

became the

basin of its

successor

causing each sea

to be smaller

than its

predecessor

20-40 minor

regression and

transgressions

Different sea

types brought

different

sediments eg silt,

salt, organic

material, mud

Periods warm clear

seas, muddy seas,

desert conditions and

shallow and swampy

seas, these different

type of seas deposited

different type of

sediments

Illinois Basin

Withdrawal of

sea resulted in

valley system

with canyons

200 or more feet

deep and with

extensive karst

topography

Structurally open

basin filled with

thicker, finer

deep-water

sediments

Solution features

developed upon the

readily dissolved

Lower Ordovician

terrain

Maracaibo Basin

Lowstand

system tracts

and

Lowstand events.

Sediment supply

due to

Formation of deltas in

sea with drop in

eustatic sea level.

transgressive

system tracts.

transgression and

Fluvial erosion

Due to Regression

With respect to the Gabon Basin the, type of kerogen formed in the Melania source

rock in euxinic rift lakes are type I which suggested that the source is lacustrine,

hydrogen rich and generates oil accompanied by or followed by gas. In the east

(onshore) when approaching the basement outcrop Type I kerogen, passes into Type

III. This suggests that there is a change in the source of material being deposited. That

is from a lacustrine source (organic material deposited in an anoxic lake) to a woody

terrestrial source.

The types of faults present are normal faults parallel to the present-day coastline and

by NE-SW trending strike-slip faults defining zones with partly different tectonic and

stratigraphic histories.6

If a well was to be drilled it would be best to drill it where the reservoir is

compartmentalized by faults and effectively sealed at the top and the faultlines to

prevent migration to another low pressure reservoir.

ConclusionThe South Gabon Sub Basin holds great potential in its pre-salt in the Rabi-Kounga

oil and gas reservoirs which consist of the Gamba and Dentale sequence of the

Dianongo Basin which formed part of the Lower Cretaceous African-South American

rift valley system.

6 www.sciencedirect.com

References

Petroconsultant, 1989. Gabon Basin. The 8th Licensing Round, Gabon.

Dupre´, S., Bertotti, G., and Cloetingh, 2006. Tectonic history along the South Gabon Basin: Anomalous early post-rift subsidence. Marine and Petroleum Geology 24 (2007):151-172.

Discovery and Development of the Rabi-Kounga Field: A Giant Oil Field in a Rift

Basin Onshore Gabon. 1988. Gamba, Gabon:Shell Gabon Co.

Journal, CGGVeritas. 2009. Hunting the Pre-Salt. Geo ExPro 6

Appendix

Figure 16

1

3

2

Gamba/Invinga/Totou field

Rabi-Kounga

Petroconsultants

Figure 5/1