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University of Africa Journal of Sciences
Tectonostratigraphic Development of Lacustrine Deposits of Mesozoic-Cenozoic Basins, Muglad
Basin, Sudan
0TRashid A. M. Hussein٭
Abstract
0TMuglad Basin exposed to tectonic and structural regime which
can be manifested clearly to characterize the tectonic history of
the basin. Accordingly, models for time span, mechanism,
deformation, and evolution of the Muglad were proposed.
0TMuglad Basin, is a rift basin developed in Mesozoic-Cenozoic
time and was initiated as an extensional basin or graben (half-
graben) to the direct south of the Central African Shear Zone
(CASZ). The basin opening occurred on a series of half-
grabens trending NW-SE in Muglad Basin. The structural style
is most convenient to be studied as assemblages of the
correlative structures formed under a certain geological
conditions, rather than to be studied as structural patterns such
as faulting and folding. Structural analysis based on seismic
data from Muglad Basin was set. The structural style of
continental or nonmarine in Muglad basin, can be categorized
into two major styles: thick skinned (basement-involved) and
thin skinned (faults restricted to sediments only) categories.
٭
Sudan University of Science and Technology- College of Petroleum Engineering & Technology
University of Africa Journal of Sciences (U.A.J.S., Vol.2, 14-44)
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0TThe tectonstratigraphic framework of the rifting of Muglad
Basin is a model of three phases was documented for Muglad
Basin: pre-rifting phase, syn-rifting phase of three diverse
episodes of rifting, and post-rifting sag phase.
Introduction 0TLacustrine, nonmarine, and continental basins are terms
interchangeably used by authors for those basins developed on
continental crust and dominated by continental sedimentary
infill. Most of the continental basins were formed in the
Mesozoic particularly in the Cretaceous; however, the oldest
continental basins were formed in Mid-Late Carboniferous and
Permian. Hydrocarbon exploration in lacustrine rift basins has
received inadequate conduct until the end of the 1970s. Some
major aspects highlighted the significance of rift basins such as
the recognition of lacustrine shales as good source rocks and
the notable successes of petroleum exploration in rift basins
(Begawan and Lambiase, 1995) and sand reservoir including
turbidites, deltaic, and fluvial types. Many publications
emphasized the economic and scientific interest of lacustrines
(e.g., Zhu Xiaomin and Xin Quanlin, 1987a and 1987b; Ponte
and Asmus, 1978; Ojeda, 1982; Estrella et. al., 1984 and
Hussein, R., 2008).
0TMuglad Basin is located in the south-central Sudan forms part
of a regionally linked intracontinental rift system (Fig. 1) that
crossed Central Africa.
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0TThe Muglad Basin is the largest of the NW-SE trending rift
basins in Sudan which is considered as a main component of
West and Central African rift system (WCARS). It extends
across at least 120,000 Km2, up to 200 km wide and over 800
km long (Bosworth, 1992) and locally contains up to 13 km of
Cretaceous-Tertiary sediments and appears to be terminated
against the Central African shear zone (CASZ) which extends
for at least 2000 km across Africa from the Atlantic coast in the
Gulf of Guinea through Cameroon, Chad, the Central African
Republic, and into Sudan (Schull, 1988 and Fairhead, 1988).
0TThis study is to establish and summarize the main features and
categories of structures and tectonic development of lacustrine
sequences during the geologic evolution of Muglad Basin, and
to propose a commonly acceptable opinion on the forming
mechanism of the basin.
0TApparent role of tectonic on the lacustrine basin and
reconstruction of structural style are characterized on the basis
of data acquired during oil exploration from Muglad basin
since Chevron time up to the recent discoveries.
0T Regional geology
0TMuglad Basin is a rift basin developed in Mesozoic-Cenozoic
time span. It was initiated as an extensional basin or half-
graben in the Late Jurassic/ Early Cretaceous to the direct south
of the Central African Shear Zone (CASZ) (Fig 2.). The deep
Cretaceous-Tertiary basins of south-central Sudan form part of
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a regionally linked intracontinental rift system that crossed
Central Africa (Fig. 2).
0TThe instigation of intracontinental rifting within West and
Central Africa was concomitant with gradual breakup of
Gondwana, particularly with separation of South America from
Africa, involving their inherent connection.
0TThe interior of West and Central Africa exposed to strong and
extensive rifting (Guirad and Maurin, 1992; Binks and
Fairhead, 1992), which synchronized with the Pacific Plate
subduction in the Mid/Late Jurassic-Early Cretaceous (Fig. 3).
0TFig. 1: Location Map of Muglad Basin.
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0TThe first phase of subsidence was quite active and extensive,
characterized by early-stage of rifting and late stage of thermal
contracted sagging which continued up to the Santonian where a
number of basins (particularly those are trending E-W) later uplifted
and exposed to erosion.
0TFig. 2 : Regional Geology and Structure of Muglad Basin Manifesting Early Cretaceous Extension and Strike-slip-dominated Rift Basins in West and Central Africa-Noting two groups of basins with distinct orientations, NE- SW- trending and NE-SW- trending, respectively. CASZ- Central African. (Modified from Genik 1993)
0TThe basin inversion was attributed to the far-field effect of
initial collision of the African and Eurasian plates along the
Alpine orogenic belt (Fig. 4). The NW-SE trending Tenere and
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Muglad rift Basins escaped the inversion, because these basins
axes are sub-parallel to compressional stress direction caused
by the collision (HcHargue et al 1992).
0TFig. 3: Regional Tectonic Setting Illustrating Onset of Opening of Atlantic Ocean on the Western Side and the Indian Ocean in the Eastern Side of Africa during the Late Jurassic to Early Cretaceous. Note: Muglad Rift Basin Initiated as a Result of the Transform Fault which extended into African Continent and leading to Strike-Slip fault of Central African Shear Zone (CASZ) (Modified from Fairhead & Green, 1989).
0TThe second phase of subsidence of the basin initiated as a
result of the compressional stress caused by the collision. Then,
the direction of movement of the African plate changed relative
to Eurasian plate that escorted to crustal-scale horizontal
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extension in NE-SW direction in association with wrench-
related basin inversion along the CASZ.
0TFig. 4: Regional Tectonic Setting Illustrating Collision of Eurasian and African Plates along the Alpine Orogen during Late Cretaceous Creating NW-SE Oriented Compressional Stress and Inducing NE-SW Trending Tensile Force Leading to Subsidence in Muglad and Tenere Basins (Modified from Fairhead & Green, 1989).
0TThe second phase continued until the Middle Eocene, when the
most intense collision occurred along the Alpine orogenic belt,
resulting in closure of the Tethyan ocean and exerting a very
strong influence on basin development in interior of Africa.
Most of Mesozoic rift basin came to an end during the Middle
Eocene, Except for the NW-SE- trending Tenere and
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Sudanese rifts (HcHargue et al 1992).
0TThe third phase of subsidence began in the Late Eocene and
occurred in parts of Muglad Rift Basin as transtensional
tectonic regime. Rejuvenation of the Central African shear
Zone appears to have occurred several times with dextral
movement in Late Cretaceous-Early Tertiary times producing
narrow subsiding rift basin infilled with Cretaceous- Tertiary
sediments (Browne and Fairhead, 1983; Browne et al. 1985).
The existence of Lower Cretaceous sediments within the basins
supports the idea that these basins were structurally developing
during early Cretaceous times, possibly as response to dextral
shear. In western Sudan, the Central African shear zone
transforms most of its dextral displacement into the southern
Sudan Rift which is a major extensional basin that extends into
a southeasterly direction through southern Sudan into Anza rift
in the northern Kenya (Bosworth, 1992 and Reeves et al.
1986).
0TStructural setting 0TThe most considerable recent debate is the nature of
extensional fault system in the upper crust whether they are
dominantly listric (e.g. Gibbs, 1987, 1989) or they are
essentially planner (e.g., Jackson, 1989). Gibbs, 1984 used a
half-graben model to describe rift-basin geometry. To
investigate the nature of extensional fault system it is more
convenient and reflective to correlate the structural assemblage
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than to investigate the patterns of individual type of structure
such as folding or faulting. Harding and Lowell 1979 studied
the structural style as assemblages of the correlative structures
formed under a certain geological conditions which could be
studied collectively in order to attain and predict hydrocarbon
traps. The structural style of Muglad basin -which is one of the
continental or nonmarine basins- can be categorized into two
major styles: thick skinned (basement-involved) and thin
skinned (faults restricted to sediments only) categories. The
thick skinned (basement-involved) includes: (1) strike-slip or
wrench fault assemblage, (2) compressive blocks and thrusts,
(3) extensional blocks and (4) warping arches, domes and sags.
The thin skinned includes: (1) detached thrust- fold
assemblage, (2) detached normal fault assemblage, (3) salt
structures, and (4) shale structures. Variety of structural styles
prefers allocation in certain plate-tectonic environments and
associated with the basin tectonic settings, mechanism of basin
formation, and sedimentary sequences. The various geometries
of the Sudanese rift-basins are variations on a half-graben idea
(D. Craig Mann, 1989).
0TIn this study the structural style of nonmarine basin in Muglad
Basin is characterized by extensional and/ or transtensional
structural styles resulted from faulting system activities during
Early Cretaceous stage and Early Tertiary. Therefore, Muglad
Basin is categorized as thick skinned (?) and thin skinned fault
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assemblage.
0TThick skinned (basement-involved) category 0TExtensional basement-faulted blocks assemblage: In Muglad
Basin the extensional sediment-basement-faulted blocks are
thick skinned generally asymmetric in configuration and
present large scale tilting blocks (shearing) (Fig.5) seismic
lines SD77-59 and SD83-28 with steep fault dips that can be
classified into two categories: antithetic and synthetic normal
faults, and half-grabens which are the basic configurations of
several complex rift-faulted basins (Fig. 5).
0TFig. 5: Seismic section (Line 3-Muglad block) showing several complex rift-basins and their mimic subbasins indicating extensional basement-faulted blocks assemblage. Note: overall basin is composed of at least 6 mimic subbasins. 0TThey may also present some step fault blocks and horsts and
grabens combination (Fig. 6).
0TThe authentic shape of thick-skin fault is still controversial;
however, in Muglad Basin they developed in response to planar
or listric (or dogleg) shaped half- graben (Fig.7) originating
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within the deep crust or mantle (thick-skinned) creating pull
apart.
0TThick-skin faults connect to the underlying crustal detachment
either on a “flat” or on a “ramp” on the basal detachment are
common in Muglad Basin. Fault displacement is transferred
between synthetic thick-skin faults by “transfer faults”.
Transfer faults have been modeled as near vertical intersecting
the main detachment faults at right angle (Lister et al. 1986 and
Ebinger et al. 1987).
0TFig. 6: Presents Seismic Cross Section Showing Some Step Fault Blocks (Horsts and Grabens Combination).
0TNevertheless, in Muglad Basin transfer faults with this
geometry (right angle) are rare.
0TDeformation in extensional basement-faulted blocks
assemblage might be contemporaneous and/ or deuterogenic.
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The subsiding mechanisms of the basins are ascribed to the
block-faulting of the basement despite the type of the basin was
post-orogenic, composite block-faults, or intracratonic rift
basins.
0TThus the majority of the faulting system in Muglad Basins are
contemporaneous, i.e. growth faults. However; comparing with
the pre-basin faults, they are deuterogenic faults. The rate of
continental sedimentation plays a significant role on the
erosion of fault scarp depending on the pace of the fault
displacement. When the rate of sedimentation is low with rapid
fault displacement, fault scarps eroded back.
0TAlso, erosion can occur on the opposite updip side of a basin,
when rotation of the block by thick skin faulting creates a new
base level for sedimentation in the basin low; the updip edges
commonly erodes to fill the newly formed depression. The
compensation between basin extensional and subsiding rates
controls the dip of contemporaneous faults.
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0TFig. 7: Seismic section (A) form Muglad Basin and diagrammatic Section (B) showing listric or dogleg shaped half-graben originating within the deep crust or mantle (thick-skinned) conforming to the eroded fault scrap geometry. 0TWhen the extensional rate is higher, the dips are gentle but
when the subsidence rate is compensated by sedimentation, the
dips are steep. In other hand, the composite block-faulted and
postorogenic faulted basins have basement faults being
relatively steep in dips with planar fault surfaces. The basement
blocks of pull-apart basins caused by strike-slipping are
theoretically vertical in dips but practically they exhibit small
step-wise- basement-faulted blocks with gentle dipping angles.
(Fig. 8).
0TDetached normal faults assemblage: The detached normal
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faults assemblage thin skinned are not involved in the
basement. Thin-skin detachment faults are antithetic to thick-
skin detachment faults (Mann, D. Craig, 1989). They are
accommodated in the sedimentary facies on passive margins
and thick incompetent rocks inducing gravity slumping which
are rare in Muglad Basin due to the gravity differences of the
sediments and providing lubricant for detachment.
0TFig. 8: Seismic Section Exhibits Small Parallel Stepwise Basement-faulted Blocks with Gentle Dipping Angles. 0TThin skinned (basement not involved) category
0TIn Muglad Basin the detached normal faults are characterized
by listric plan within sedimentary sequences, steep near the
surface and flat at the deep depth. The detached normal faults
can be categorized in Muglad basin into the following
modes:(i) Common normal faults assemblage, (ii) Antithetic
normal faults assemblage, (iii) rollover anticlines or reverse
drag anticlines assemblage, and (iv) miscellaneous normal fault
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assemblage. (i) common normal faults assemblage are forming
as a result of occurrence of the basin under successive
extensional subsidence and sedimentation, thus, brittle sand
and clay beds might be fractured forming normal faults which
flattened out into plastic beds (e.g. salt or shale flowage) and
occurred mostly in the flanks of the filled rifts and associated
with changes of the thickness and sedimentary facies. (ii)
antithetic normal faults assemblage are often observed in
intracratonic rift basins. They habitually accommodated in the
central areas of the faulted depression, corresponding to the
depocenter. The listric faults firmed out into thick mudstones.
Sandford (1959) used experiment model which emphasized
that the normal faults propagate from outer to center when the
cabbage-like graben (Fig.9) is caused by detachment and
extension, or they propagate from the center outwards when the
graben is caused by upwelling of the basal rocks or structural
doming.
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0TFig. 9: Seismic section (SD82-43) from Muglad Basin Showing Antithetic Normal Faults propagating from the outer to center of the cabbage-like structure. The faults are Dominantly Listric within the Synrift Sequence. 0TThe antithetic normal faults can be established as a result of
local compensation of the extensional basal faults in interarc
rift basins and in several depressions of the postorogenic
basins. (iii) rollover anticlines or reverse drag anticlines
assemblage are major structural style of the progressive delta
sequences on passive continental margin (Bally et. al 1981,
Gibbs 1984); however, they can be seen in the rift infilling
sequence of the intracratoinc failed rift basins due to reverse
drag caused by the slipping and tilting of growth faults. They
are occur predominantly in the downthrown blocks of the
major boundary growth faults of both the northern and
southern depressions of the Kaikang Trough (Fig. 10 - a) with
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the axes parallel to the faults. Example is in the western side of
the dintersection SD 79-65 and 78-58 b. The rollover
anticlines are related to the gravity slumping along the growth
fault planes. So long as the sedimentary sequences in basins
were reversely dragged, accompanying the growth faults, the
induced anticlines can be categorized into this structural style
(10- b). The well developed plastic beds in the downthrown
block gave rise to growth fault flattened out into the plastic bed
and may form some rollover anticline. (iv) the other normal
faults assemblage are small normal faults which have no
relations to the basement faulted-blocks and couldn’t be
referred to the fractured system caused by basin extension.
They concentrated in a certain interval, some of them are
related to the reservoir rocks and others concentrated in the
source rocks. They couldn’t be explained by deuterogenic
fractures. Two formation mechanisms could be suggested to
explain this kind of faults. One in the reservoir intervals, they
are caused by local gravity slumping of the differential
compaction due to the irregular distribution of many isolated
sandbodies in the deltaic area of a shallow lake basin and they
are irregular in both the directions and dip angles but in
Muglad basin can form by antithetic bedding plane faults not
slumping (Mann, D. Craig 1989, Bally et. al 1981, Gibbs
1984). The other one is that the faults in the source rocks are
resulted from minor fractures, which were caused by volume
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expansion in the process of oil and gas generation and were
displaced slightly under different overlying pressures (10- c).
0TFig. 10: (a) Rollover Anticline-: Example: West of Intersection of SD 79-65 7 78-58b. (b) Drag Fold Example: Amal -1 Structure. (c) Faulted anticline: Example: South of the Western Fault Step Zone between Seismic Lines SD 79-68 and SD 77-6. 0TStructural assemblage resulted from deformation of plastic
rocks: Structural assemblage of salts, shales and claystones are
formed due to the plastic flowage of rocks; however,
deformation adjoins some factors such as tectonic settings and
sedimentary conditions in order to provoke the formation of
this structural style.
0TBasement flexure-uplift assemblage: It occurs only in
intracratonic or stable platform. It is a resultant of vertical
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movement of the crust along faults due to the association of
faulted blocks in the basement on condition that completely
rigidized. This assemblage is not present in Muglad Basin.
0TBasement thrusts assemblage: It is characterized by thrust
faults and associated folds. The thrust faults cut both the
basement and the overlying sediments. It is one of the thin
skinned structural styles but is often misinterpreted, either as
thick-skin basement-involved faults with decreasing
displacement downwards, or as basin edge faults, which form
the other half of a misinterpreted full graben. On seismic
interpretations, reflectors below the detachment falsely appear
faulted, but with smaller apparent fault displacements than seen
above the detachment. This broken or faulted seismic character
below the detachment is artificially produced by complex ray
paths and lateral velocity variations across fault boundaries.
Another difficulty faces the recognition of detached thrust fold
assemblage in Mugald basin is the large variation in the size of
detachment. If the study area and seismic lines are too small to
see the whole fault system, large detachments can go
unrecognized.
0TDetached thrust folds assemblage: they normally occur in
sediments of the down-wrapping basins on cratons and on the
sides adjacent orogenies. This structural style deformation
happens where the original miogeosynclinal sediments on the
continental margin was upthrusted onto the craton due to
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terrain collision and the overlying sediments were compressed
and deformed, forming complex detached thrust folds which
extremely uplifted and induced gravity spreading and gravity
sliding while the craton basement reserved comparatively
stable due to its rigidity. The major parameters control the
grade and convolution of the detached thrust folds are the
magnitude of the colliding blocks, the thickness of the
sedimentary sequences and the intercalations of competent and
incompetent beds of the craton. Strike-slip or wrench fault
assemblage: strike-slip deformation has a middle stress. The
compressional or extensional field of stress can’t avoid
inducing some wrench deformations. An asymmetrical graben
structure with a tilted array of planar to listric faults was
produced (Fig 11). The faults within the prerift sequence in the
graben become sigmoidal as response to the internal
deformation in the graben. These faults become listric within
the synrift sequence (see fig. 11). The fault nucleation
sequence indicates both hanging-wall and footwall nucleation.
Many of the faults are nucleated relatively rapidly early in the
deformation history and are active throughout the deformation.
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0TFig.11: Seismic Section (SD78-14) Across Extension of Muglad Basin Showing Asymmetrical Development of Planar to Listric Growth Fault Arrays with a Complex Crestal-Collapse Graben System of Antithetic and Synthetic Faults. A = Pre-rift, B = Syn-rift, C = Post-rift. It illustrates Sharp Change of Thickness of Nayil and Tendi Formations Across Faults 0TMuglad Basin deformed in extensional-wrench. Extensional,
pull-apart basins are usually occurred (Crowell, 1973). In
addition, seismic line (GN98-020) from Muglad Basin (Fig.
12) the Nayil and Tendi Formations are primarily deposited
and cover a thickness of 4 km. as a sedimentary drape strongly
controlled by faulting to indicate another intensive phase of
rifting.
0TThe resulted sub-basins were superposed upon the Darfur-stage
and bored faults were obviously formed by reactivating the
previous structures. In addition, many new intra-basin faults
were also created.
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0TFig. 12: Seismic line (GN98-020) from Muglad Basin showing negative flower structure as a cursor for the presence of wrench deformation. 0TThese newly formed faults, along with reactivated old ones,
often make up negative flower structures (see Fig.12)
indicating sinistral movement and which confirms the presence
of wrench deformation as one of the most typical criteria of
wrench deformation which is characterized by vertical strike-
slip faults without apparent vertical displacement. The dextral
en echelon faults in horizon and the alternation or combination
of the positive and negative flower structures in cross sections
verify the presence of wrench faults in the area. Figure 13
summerizes the major structure assemblages of Muglad Basin.
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0TFig. 13: Summarizing the Major Proposed Structure Assemblages of Muglad Basin.
Conclusions 0TGeometry and nature of the extensional detachment exert a
fundamental control on the evolution of extensional basin structures.
The horizontal or tilted detachments that generate extension above a
restricted area experiencing stretching induce extensional basin
geometries at the early stages of rifting. In these early stages rifts form
and are bounded by two steep bounding faults, and internal
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deformation within the asymmetrical rift is accommodated by an array
of planar faults.
- 0TOnce the planar faults penetrate the synrift sequence,
they become listric growth faults. Simple listric
extensional detachments are characterized by rollover
anticlines with gometrically necessary crestal-collapse
graben structures. Within the crestal-collapse grabens,
hanging-wall nucleation of new faults is dominant. At
high extension value, the synthetic crestal-collapse
graben faults develop into a fan of listric growth faults
in the synrift sediments. Superposition of successive
crestal-collapse grabens at high extension values
produces complex arrays of intersecting faults.
- 0TRamp/flat listric extensional fault systems are
characterized by a rollover anticline and crestal-collapse
graben system associated with each steepening-upward
segment of the detachment, and a ramp zone consisting
of a hanging-wall syncline and a complex deformation
zone. The hanging-wall syncline is characteristic of
ramp/flat detachment geometries. The ramp/flat
evolution of extensional basin structures developed
above major detachment surfaces can be generated by
horizontal or tilted detachments.
- 0TAssessment for nonmaring deposits of Muglad Basin
attained through integrating geophysical data, structural
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analysis, and petroleum geology.
- 0TIt has been proved that nonmarine petroleum basins have
very suitable geological conditions to form oil and gas
pools. A series of giant oil and gas fields has been found
besides lots of medium to small sized ones. This might
enhance understanding of petroleum geological
conditions of the Meso-Cenozoic nonmarine petroleum
basins, support and enriched the theory of the petroleum
formation and occurrence in nonmarine petroleum
basins.
- 0TIn Muglad Basin, the assessment of petroleum achieved
to zones, favorable reservoir–cap combinations and
favorable plays and prospects throughout the Muglad
Basin. some remarks are worthy to be stressed. 1. Oil
discoveries around the western flank of Unity oil field
confirmed the presence of the kitchen in which the
organic matters had been cooked and then generated oil
and/ or gas and latter by expulsion moved to the
adjacent areas. The paleostructure was mature enough to
trap the oil and later flow it from Tendi Formation to
Kaikang-1; 2. In the northern part of the basin near the
boarder of Fula subbasin, Abu Gabra Formation is well
developed, thus, it is the major Cretaceous source rock
in the area of the Unity oil field and; 3. The center of the
Muglad Basin was regarded less prospective because the
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Abu Gabra Formation is relatively thin due to the
gravity study and the structure in the area is young.
During the major lake development stage of a half-
graben, the block-faulting was strong, the lake was wide
and deep, thus, the source rock of large size and
reservoir bodies were developed.
0TReferences
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