Petroleum Geochemical Assessment of The Lower Congo Basin: G. A. Cole A. G. Requejo D. Ormerod Z. Yu A. Clifford
Petroleum Geochemical Assessment of The Lower Congo Basin: G. A. Cole A. G. Requejo D. Ormerod Z. Yu A. Clifford
Petroleum Geochemical Assessment of The Lower Congo Basin: G. A. Cole A. G. Requejo D. Ormerod Z. Yu A. Clifford
Chapter 23
G. A. Cole Z. Yu
A. G. Requejo* A. Clifford
D. Ormerod
BHP Petroleum
Houston, Texas, U.S.A.
*Consultant to BHP
Present address: Geochemical Solutions International
The Woodlands, Texas, U.S.A.
Abstract
Petroleum geochemical data and burial history, thermal, and fluid flow models show that the southern
Lower Congo Basin contains dual petroleum systems. The older petroleum system includes a presalt type
I lacustrine source (Barremian Bucomazi Formation time equivalent) that is present but not well under-
stood in the deeper offshore Lower Congo Basin. The lacustrine synrift grabens are generally obscured by
salt on seismic profiles, and there have been few well penetrations except on the shelf. However, based on
models of burial history, thermal evolution, and fluid flow, we have found that the lacustrine sequence
matures offshore and that migration is focused updip toward the shelf where the reservoired oils correlate
to this source. A younger petroleum system consists of type II marine clastic shales of the
Cenomanian–Turonian (Upper Cretaceous) Iabe Formation that have charged the overlying Tertiary sand-
stones in deep-water areas. These turbidite reservoir sandstones, although charged primarily by the
Cretaceous Iabe marine clastic source, show some evidence of mixing with early oils from the Tertiary
Landana Formation. Mixing of oils between the two petroleum systems is limited to those areas where the
Loeme Salt has evacuated sufficiently to allow migration through an assumed permeable salt weld.
325
326 Cole et al.
PETROLEUM GEOLOGY
Rift phase strata deposited during the Early
Cretaceous extensional opening of the South Atlantic
have economic importance as both reservoir facies and
highly prolific hydrocarbon source rocks. Tectonic
regimes (Figure 1) associated with continental separation
can be divided into four stages (Brice et al., 1982), from
oldest to youngest:
• prerift,
• synrift,
• synrift II, and
• postrift (with regional subsidence).
Prerift sediments were deposited on faulted metamor-
phic basement prior to major continental rifting. This
section consists of a sandy fluvial–lacustrine sequence
that is up to 1000 m in thickness (Brice et al., 1982). In the
synrift I stage, a range of graben and half-graben troughs
along the early rifted margins of western Africa and
Brazil formed the depocenters for organic-rich lacustrine
shales. The lake systems that developed within the
grabens were progressively filled by lacustrine turbidites
that graded laterally and upward into organic-rich shales.
In the coastal basins of Angola (Lower Congo and
Kwanza), this rifting stage produced the bituminous
shales of the Bucomazi Formation and its time equiva-
lents, the primary synrift source rocks for major oil accu-
mulations along the west African margin (Burwood et al.,
1995; Burwood, 1999). These organic-rich shales can
attain a maximum thickness of 1800 m (McHargue, 1990). Figure 1—Schematic stratigraphic section for the Lower
Beyond Angola, time-equivalent organic-rich lacustrine Congo Basin showing the primary and secondary source
shale deposits along the South Atlantic margin include rock and reservoir units. After Burwood et al. (1990).
the Marnes Noires and Melania Formations of Congo and
Gabon, respectively, as well as the Lagoa Feia, Guaratiba,
and Mariricu Formations of Brazil.
The synrift I stage was followed by a period of regional to that observed in the present-day Walvis Bay area, or (2)
subsidence (synrift II stage), resulting in marine incur- oceanic anoxic events (OAE) resulting from an expanded
sions. This transitional sequence consists of lacustrine oxygen minimum layer.
carbonates and sandstones and alluvial clastics and is The Cenomanian–early Eocene section contains
marked by a transition from nonmarine lacustrine condi- several groups and constituent formations deposited
tions to marine conditions. In the Lower Congo Basin of under conditions conducive to petroleum source rock
Angola, saline waters initially entered from the south. By formation. These include the Upper Cretaceous Iabe and
the end of this stage, active marine transgression culmi- the Paleogene Landana Formations of the Lower Congo
nated in deposition of carbonates and a desiccation event Basin. The Iabe Formation encompasses the time period
that led to deposition of the Loeme Salt, a regional to during which OAE source rocks would have been
subregional seal between the synrift lacustrine petroleum deposited, although the main Iabe source rock develop-
system and the overlying marine petroleum system. ment occurred in Turonian–early Tertiary time. Salt
Following evaporite deposition, permanent marine movement during this time resulted in growth faults and
conditions and carbonate-dominated sedimentation the formation of salt domes and ridges. At the end of the
accompanied the postrift phase and passive margin Paleogene, the seaward edge of the continent foundered,
opening of the South Atlantic Ocean. The Albian–earliest producing a strong westward tilt and regional subsidence
Cenomanian Pinda and Cabo Ledo (Moita Seca) (postrift stage). During the Oligocene–Miocene, a thick
Formations are the first postsalt units documented regressive sequence consisting of turbiditic sands and
through well control. These units fully span continental silty clays was unconformably deposited across the older
clastics to marine shaly marls (700–800 m) and are shelf sequence. This clastic sequence built the continental
thought to contain secondary hydrocarbon potential. shelf to its present position. A generalized stratigraphic
Postsalt source rock deposition could have been influ- section highlighting reservoir and potential source units
enced by either of two mechanisms: (1) upwelling similar is shown in Figure 1.
Chapter 23—Petroleum Geochemical Assessment of the Lower Congo Basin 327
During Oligocene–Miocene deposition, the resultant to 10% and hydrogen indices (HI) from 650 to 800 mg
gravity sliding, or salt raft tectonics , produced detached HC/g TOC, suggesting type I kerogens. Kerogen kinetics
sections referred to as “Pinda bumps,” which are Upper of the richest intervals show the predominance of a single
Cretaceous shelf strata that are displaced seaward. These activation energy of about 56–58 kcal/mol, characteristic
bumps may represent source rock “mini-kitchens” that of type I kerogens.
could charge local structures depending on overburden Zone C consists of more calcareous, microlaminated
and heat flow considerations (sufficient to mature the carbonaceous strata containing predominantly amor-
source rock pods). Another important feature is the exten- phous organic matter deposited in shallower water,
sive faulting throughout much of the Tertiary section; anoxic, alkaline lake settings. TOC ranges from 2 to 5%
these faults likely acted as conduits for hydrocarbon and HI from 550 and 800 mg HC/g TOC, corresponding
migration. to more typical type II kerogens. Kerogen kinetics also
Significant salt diapirs and ridges are also evident in show a narrow distribution at a lower activation energy
the Lower Congo Basin and are most likely the result of (54 kcal/mol) than zone D, suggesting an organofacies
periodic halokinesis associated with tectonic activity change from that zone.
along numerous transfer fault zones. These diapirs Strata of zone B were also deposited under lacustrine
increase in size and frequency westward into the ultra- conditions but in shallower, brackish waters under less
deep-water areas of the Lower Congo Basin (Blocks 31 pervasive anoxia, marking the onset of “sheet drape”
and 32). The salt, which has been active since the Albian, sedimentation (synrift II) of Bouroullec et al. (1991).
acts to concentrate and encapsulate potential reservoir Organic-rich claystones have TOC contents of 7–13%;
sandstones in their vicinity. The Albian section may also otherwise TOC averages about 1%. Organofacies of the
contain porous carbonates that could act as reservoirs. richer intervals contain type II kerogens with intermedi-
ate HI values of 400 mg HC/g TOC. In contrast to the
underlying zones, kerogen kinetics show variable activa-
PETROLEUM GEOCHEMISTRY tion energies of broad distribution averaging 52
kcal/mol, suggesting mixed assemblages of organic
The geochemical aspects of a petroleum system are matter types.
identification of the primary source rocks responsible for Zone A marks the conclusion of Bucomazi sedimenta-
any commercial accumulation of oil and gas within a tion. These strata represent deposition in a shallow, low-
basin and the correlation of reservoired oil and gas to energy lacustrine environment, possibly indicating a
specific source units. Major source rock intervals within progressive contraction and drying out of the lake. A
the Lower Congo Basin are developed within both the predominantly oxic water column with dysoxic periods is
pre- and postsalt sections. Primary effective source units indicated by relatively low TOC contents (about 1%),
are likely associated with Late Cretaceous flooding events except at the basal interval where values exceed 5.0%. HI
in the deep-water areas and with the synrift lacustrine values up to 500 mg HC/g TOC are recorded in the
section both nearshore and onshore. organic-rich interval but are generally less than 400 mg
HC/g TOC where TOC is lower. Kerogen kinetic data
Synrift Presalt Lacustrine Source Rocks show a broad activation energy distribution (maximum 52
kcal/mol) typical of type II organic matter assemblages.
Previous geochemical studies in Angola have focused The focus of this chapter is the deep-water (>500 m) to
largely on the presalt succession in the coastal Lower ultra-deep-water (>1500 m) portion of the southern
Congo Basin. Burwood et al. (1990, 1992, 1995), Mycke Lower Congo Basin, where the presalt section has not
and Burwood (1995), and Burwood (1999) describe the been penetrated (the nearest penetrations are on the shelf
source rock potential and molecular characteristics of the to the east). Figure 2 shows a crossplot of TOC versus the
Bucomazi and Chela Formations. The wells studied S2 yield from Rock-Eval pyrolysis for potential source
(CABGOC 86-1 and 123-4) are in the Cabinda enclave rocks in the southern Lower Congo Basin. On the basis of
north of our study area. To summarize these papers, the this data and the limited presalt penetrations, we believe
presalt Bucomazi and Chela sequence is comprised of late development of good, thick organic-rich shales is ques-
Barremian–Aptian sedimentary rocks that were initially tionable within the lacustrine section. However, organic-
deposited in a freshwater environment which evolved rich potential source rocks exceeding 4–5% TOC are
into a brackish water to saline depositional setting. The evident within some wells. Burwood (in press) has
Bucomazi section has been subdivided into four zones, recently summarized source rock data for this area,
representing geochemically discrete organofacies and including both pre- and postsalt candidates.
depositional settings. Of greater importance is the kinetic breakdown of the
The oldest zone, zone D, is the basal Bucomazi interval lacustrine organic-rich shales. To address generation and
which consists of fissile, dark brown argillaceous silt- expulsion, a 1-D section was modeled using burial
stones containing predominantly amorphous organic history and thermal modeling software developed by
matter. These were deposited in a deep, anoxic, freshwa- Institut Français du Pétrole (IFP) and Beicip GENEX.
ter lake setting with periodic slumping of shoreline sedi- Results are shown in Figure 3 for the lacustrine section as
ments. Total organic carbon (TOC) contents range from 5 determined in a pseudowell constructed from a regional
328 Cole et al.
Figure 2—Crossplot
between TOC (in wt. %)
and the S2 yield from
Rock-Eval pyrolysis
showing that the two
primary source rocks are
the postsalt marine Iabe
Formation and presalt
lacustrine units
(Bucomazi Formation
time equivalents).
seismic dip line across the southern Lower Congo Basin localized deltaic systems that built into local accommo-
that was calibrated to the nearest well data. The presalt dation zones within an overall transgressive regime. The
section was not imaged at this site but was assumed to be overlying major regressive cycle is associated with
present; it has a thickness of 20 m, an average TOC of 6%, Eocene–Holocene deltaic clastics.
and an HI of 600 mg HC/g TOC. In this and other burial The uplift in onshore Angola, in combination with
history and thermal models presented (both 1-D and 2- major lowstands of sea level, generated large submarine
D), we used default kinetic parameters as described in the fan systems within the offshore Lower Congo Basin.
literature (Tissot and Espitalié, 1975; Ungerer et al., 1986; Deeply incised canyon systems on the shelf to slope
Ungerer and Pelet, 1987; Sweeney and Burnham, 1990). region can be seen within the shelfal area. These acted
This simulation showed that the lacustrine source rock, if both as bypass mechanisms for the submarine fan sedi-
present at this location, would have expelled its hydro- ments and as a means of focusing the sediment load. The
carbons during the early Miocene. These results indicate sands were shed from the major delta system that
that this source rock could have contributed significant emptied into the north–south trending mini-basins
hydrocarbons into any presalt traps (assuming the pres- formed by salt and raft tectonics. The sands filled from
ence of a Loeme Salt seal) or could have contributed the north to the south, with the paths between the mini-
hydrocarbons into the postsalt section (Tertiary sand- basins varying with salt halokinesis. As particular
stones or Pinda carbonate reservoirs) through salt with- diapirs, salt walls, and turtleback structures switched the
drawal windows (welded zones). rate of accommodation within a mini-basin, the likely
overall path of the turbidite flows varied. It is this combi-
Postrift Marine Source Rocks nation of postsalt source rock “pod” development and
reservoir deposition/trap development that is important
The earliest postrift section is the extensive Loeme Salt, in the deep-water and ultra-deep-water petroleum
which formed during the early phase of separation of the systems of the southern Lower Congo Basin. Of even
African and South American continents. The postrift more critical importance is where these source rocks
phase comprises the regressive Albian–Cenomanian matured and how they charged the Tertiary turbidite
carbonates that encompass shallow-water to platform sandstones.
facies with minor shelfal sandstones. A major source rock The most dominant postsalt source rock is the marine
sequence, the marine clastic Iabe Formation (Ceno- clastic Cenomanian–Turonian Iabe Formation, which
manian–Turonian), occurs in the Upper Cretaceous post- includes intervals having in excess of 10% TOC with
salt section. Other contributing source rocks may be the excellent oil-prone quality (Figure 2) (Burwood, 1999).
uppermost Albian–Cenomanian Moita Seca and the Secondary source rocks are the Albian–lower Ceno-
lower Tertiary Landana. Overlying these are a series of manian Moita Seca marine shales and marls, which have
Chapter 23—Petroleum Geochemical Assessment of the Lower Congo Basin 329
Figure 3—Calculated
curves for oil and gas
expulsion, expulsion effi-
ciency, temperature, trans-
formation ratio, and vitri-
nite reflectance for a
presalt Bucomazi time
equivalent (lacustrine)
source unit in a pseudo-
well in the deep-water part
of the southern Lower
Congo Basin. (Units used
for the temperature plot are
in °C/100 in order to
display at this scale.)
Figure 4—Calculated
curves for oil and gas
expulsion, expulsion effi-
ciency, temperature, trans-
formation ratio, and vitri-
nite reflectance for the
postsalt marine clastic Iabe
source unit in a pseudo-
well in the deep-water part
of the southern Lower
Congo Basin. (Units used
for the temperature plot are
in °C/100 in order to
display at this scale.)
TOC values of up to 3% with oil-prone quality, and the amounts of oil and gas expelled at calculated (and cali-
lower Tertiary Landana, with 4% TOC and oil-prone brated) temperatures, and the vitrinite reflectance values.
quality (Figure 2). The Tertiary Landana remains only a The estimates are based on a 75-m-thick Iabe source rock,
potential source rock across most of the deep-water with a TOC of 4% and an HI of 550 mg HC/g TOC, which
region because it may not be sufficiently mature to expel was observed in one of the salt-rafted source pods in the
hydrocarbons. In some localized areas, however, it has deep-water part of the Lower Congo Basin. As illustrated
apparently contributed to accumulations, as indicated by in Figure 4, this unit can attain sufficient maturity to expel
oil–source correlations (see below). significant amounts of oil and associated gas within the
As before, the kinetic breakdown of the postsalt deep-water parts of the basin. Expulsion has occurred
marine type II Iabe organic-rich shales is significant. To quite recently, mainly within the last 15 m.y. Since trap
address generation and expulsion for the Cretaceous Iabe formation occurred during Oligocene–Miocene time,
source unit, the same 1-D section discussed above was expulsion and migration timing is very favorable for the
used to determine the efficiency of expulsion, the trapping of oil and gas.
330 Cole et al.
Figure 5—Crossplot of
the carbon isotopic
compositions of satu-
rate and aromatic
hydrocarbons isolated
from soils and source
rock bitumens.
tive source units. The C24/C21 tricyclic terpane ratio is ciently mature) could be the Tertiary Landana Formation
generally lower for the postsalt marine oils and source (formerly the Tertiary Iabe). Figure 9 shows a plot of the
rocks in comparison to presalt oils and lacustrine source biomarker ratio oleanane/C30 hopane versus the C26/C25
rocks. tricyclic terpane ratio. Oleanane is an indicator of organic
As summarized above, the two primary source rocks contribution from angiosperm land plants, and its occur-
for the deep-water Lower Congo Basin oils are the lacus- rence is commonly observed in post-Cretaceous source
trine presalt section and the postsalt marine clastic Iabe rocks (Ekweozor et al., 1979; Ekweozor and Telnaes,
Formation. A possible secondary source (where suffi- 1990). Based on this plot, some oils and shows in the
332 Cole et al.
Figure 8—Crossplot of
the C26/C25 tricyclic
terpane ratio and the
C24/C21 tricyclic terpane
ratio for oils and source
rock bitumens.
southern Lower Congo Basin contain elevated oleanane Basin. Figure 10 shows the regional distribution of these
levels that strongly suggest some contributions from a families. The shelfal oils (Pinda carbonate plays) were
Tertiary source rock. The only source unit with adequate derived mostly from the lacustrine presalt section,
source richness and potential that also contains olenane is whereas the deeper water areas are dominated by Iabe-
the Tertiary Landana Formation (Figure 9). derived oils or mixtures. This distribution is controlled by
On the basis of these data, we recognize two broad two factors: (1) the stratigraphic section, primarily where
families or types of oils in the southern Lower Congo the Loeme Salt acts as a seal and isolates the two petro-
Chapter 23—Petroleum Geochemical Assessment of the Lower Congo Basin 333
Figure 11—Schematic drawing from a regional seismic line across the southern Lower Congo Basin showing the general-
ized stratigraphic section, wells, and source rock intervals. This line is the basis for the 2-D PetroMod model. The only major
interval not shown is the thin (3–10 m) Loeme Salt “sheet,” which may act as a seal between the presalt and postsalt petro-
leum systems.
flow continued updip toward the shelf and some flow migration. The salt rafted Iabe source pods formed in
migrated through the salt welds. The oil and gas was such a way that no migration occurred toward the shelf.
saturating the Cretaceous section and some gas had All migration was vertically above the pods and/or
reached the Tertiary sandstones at the 20–50 km section of enhanced by the associated faulting, and this occurred
the line. only in the deep-water areas, not the shelf. Thus, the shelf
At 8.2 Ma (Figure 14), the Cretaceous postsalt Iabe would not be expected to contain postsalt derived oils, as
source had attained early maturity and was generating the Iabe source was too immature to effectively charge
and expelling some oil and gas to saturate the surround- any Tertiary reservoirs, which is confirmed by the oil
ing rock. The presalt lacustrine source units continued family distributions. At this time, oil was still migrating
expelling volatile oil and associated gas. At that time, toward the shelf from the presalt lacustrine source and
flow continued updip toward the shelf with some charge also through the salt windows at the 20–50 km part of the
migrating through the salt welds. This oil and gas was line. If the lacustrine presalt oil was actively migrating
saturating the Cretaceous section, and some gas had through the windows and into the Cretaceous and
reached the Tertiary sandstones at the 20–50 km part of Tertiary reservoirs, some mixed oil types would be
the line. expected. However, geochemical characteristics of oil
At 4 Ma (Figure 15), the Cretaceous Iabe source was at shows and accumulations in the deep-water southern
peak expulsion and expelling sufficient oil and associated Lower Congo Basin suggest generation from either the
gas to oversaturate the surrounding shales and siltstones. Iabe postsalt source or a mixture with the Tertiary
The hydrocarbons began to actively migrate from the Landana, but no discernible contribution from any
source pods formed by the salt rafts. Migration was presalt sources. This suggests that salt windows are most
predominantly vertical at this time. Of particular interest likely rare or nonexistent, limiting migration through the
is the morphology of the pods and how these controlled Loeme Salt sheet. The salt welds probably consist of resid-
Chapter 23—Petroleum Geochemical Assessment of the Lower Congo Basin 335
Figure 12—
Transformation ratio for
pre- and postsalt source
rocks along the dip line in
the southern Lower
Congo Basin. The presalt
lacustrine source is over-
mature in the deep-water
area, expulsion mature
along the shelf–slope,
and immature on the
shelf. The postsalt Iabe
source is postpeak expul-
sion mature to overma-
ture in the deep-water
areas and immature to
early mature for expul-
sion along the
shelf–slope to shelfal
areas.
Figure 13—Petroleum
saturation and flow direc-
tions (green arrows) at 12
Ma, Lower Congo Basin.
ual salt and anhydrites that still form a sufficient seal to justify a Tertiary play in the deep-water area of the south-
preclude any presalt lacustrine oil migration. ern Lower Congo Basin.
Present-day total petroleum saturation (Figure 16) and
petroleum saturation derived exclusively from either the
Cretaceous Iabe (Figure 17) or the presalt lacustrine CONCLUSIONS
source rocks (Figure 18) indicate that sufficient hydrocar-
bons have been expelled from the postsalt Iabe source to Two primary source rocks can be identified within the
charge traps above the mature source pods. Presalt lacus- southern Lower Congo Basin. The first is a presalt lacus-
trine contributions would therefore not be required to trine sequence time equivalent to the Bucomazi
336 Cole et al.
Figure 14—Petroleum
saturation and flow direc-
tions (green arrows) at
8.2 Ma, Lower Congo
Basin.
Figure 15—Petroleum
saturation and flow direc-
tions (green arrows) at 4
Ma, Lower Congo Basin.
Formation, which consists of clastics with intervals of gen with initial HI values of 350–550 mg HC/g TOC.
high TOC and HI that contain mostly oil-prone type I This source rock is syn- to postdepositionally thickened
kerogen assemblages. Source rock data for this sequence due to salt tectonics (rafting). Also, salt rafting has
are sparse in the southern Lower Congo Basin, but some removed this source from some areas of the southern
thin intervals with high measured TOCs (>5%) and good Lower Congo Basin, increasing the source risk in local-
residual HI values are evident. The second source rock is ized areas.
the postsalt (Cenomanian–Turonian) marine clastic Iabe Secondary source rocks include the clastics and marls
Formation. TOC values in this unit exceed 5% in some of the Albian–Cenomanian Moita Seca, with TOCs reach-
wells, and pyrolytic yields suggest a marine type II kero- ing 2–3% and mixed oil/gas proneness, and the Tertiary
Chapter 23—Petroleum Geochemical Assessment of the Lower Congo Basin 337
Figure 16—Present-day
total petroleum satura-
tion. Flow directions are
not shown, but are similar
to those in Figure 15.
Figure 17—Present-day
petroleum saturation
derived from the postsalt
Cretaceous Iabe source
rocks.
(Oligocene and younger) Malembo, which contains up southern Lower Congo Basin, the Landana is mainly
to 2% TOC with gas/minor oil proneness. The Malembo immature for expulsion. However, in deep-water areas
is not mature over most of the basin, whereas the Moita where burial was maximized from the Tertiary fill, the
Seca could have contributed hydrocarbons into the Landana could be sufficiently mature to expel hydrocar-
system. The lower Tertiary (Paleocene–Eocene) Landana bons and has apparently contributed to accumulations,
Formation also contains moderate to excellent source as indicated by oil–source correlations
richness and potential. This unit is mostly marine in Key risks regarding source aspects of the petroleum
character, with some contribution from land-derived systems in the southern Lower Congo Basin are (1) source
organic mater (as evidenced by biomarkers). In the presence, that is, whether organic-rich lacustrine shales
338 Cole et al.
Figure 18—Present-day
petroleum saturation
derived from the presalt
lacustrine source rocks.
are present, and (2) the thickness and lateral extent of between the postsalt marine clastic sourced system
mature postsalt Iabe marine clastic shales. and the presalt lacustrine system. The salt acts as a
Stable carbon isotopes and biomarkers readily illus- barrier that prevents presalt charge from reaching
trate the relationships between Lower Congo Basin oils Tertiary or Cretaceous reservoir intervals in the
and potential source rocks (presalt marine Iabe, undiffer- deep water and focuses the charge laterally to
entiated Tertiary, and presalt lacustrine), as well as identi- updip, shelfal areas. Salt seal therefore controls
fying oil families. Available data indicate that the most fluid flow and migration of the presalt lacustrine-
diagnostic biomarker parameters include distributions of derived hydrocarbons. This is consistent with
tricyclic terpanes, most notably the C26 and C25 observed oil occurrences of presalt derived oils.
homologs, and the hopane/sterane ratio. Another key 4. Charge in the deep water is controlled by the salt-
geochemical property is the canonical variable derived rafted source pods whereby the source has been
from stable carbon isotopic compositions (Sofer, 1984). thickened by the salt movement. It is primarily
Using these relationships, oils can be correlated to either vertical above the slightly overpressured Tertiary
the postsalt Iabe marine clastic source or the presalt lacus- strata, then modified laterally by the turbidite
trine source. Limited mixing of these principal oil types, sandstone packages.
or between the Iabe and the Tertiary Landana sources, is
also indicated in some oils. On the basis of the data presented, we conclude that
A regional dip line was modeled (using 2-D PetroMod) the southern Lower Congo Basin contains dual petro-
to address expulsion timing and fluid flow (migration) leum systems. The deep-water Tertiary plays are
from the two primary source rock intervals. We drew charged primarily by the Cretaceous Iabe marine clastic
several principal conclusions from the basic model: type II source unit, whereas the shelfal areas are domi-
nated by oils derived from the presalt lacustrine
1. Both source rock systems are mature in the deep sequence (Bucomazi time equivalent). Mixing is limited
water parts of the basin, given the thermal regime to those areas where the Loeme Salt has evacuated suffi-
for this region. On the shelf, the postsalt system is ciently to allow migration through an assumed perme-
immature due to minimal burial, whereas the able salt weld.
presalt system is immature to oil mature.
2. Oil shows and discoveries of postsalt origin
should predominate in Tertiary deep-water sand- Acknowledgments—We thank Sonangol and BHP
stones, whereas the shelf contains primarily lacus- Petroleum for permission to use and publish these data. We also
trine-sourced presalt oil accumulations. This is acknowledge constructive comments by Ralph Burwood and
consistent with observed oil occurrences in this Barry Katz on an earlier version of this manuscript.
part of the southern Lower Congo Basin. Discussions with Bill Brumbaugh were helpful in developing
3. The Aptian Loeme Salt forms a regional seal some of the petroleum geology concepts.
Chapter 23—Petroleum Geochemical Assessment of the Lower Congo Basin 339