3. Geological Background
1. Ethiopia is located in eastern Africa in the southern Red Sea region.
2. The topography of Ethiopia consists of a central high plateau bisected by the
Ethiopian segment of the Great Rift Valley into northern and southern highlands
and surrounded by lowlands, more extensive on the east.
3. Plateau height: 1500-3000m above sea level.
4. The layers of rocks and big pieces of stone and gravel can be seen frequently,
indicating that the surface soil is very thin with an average thickness between 0.5 to
1 meter, overlying possible bedrock or large boulders.
6. • Blocks belongs in the semi desert area with three geographical terrains that includes
mountain, hills and plain, which causes the field operation inconvenient.
7. Challenges in exploration
seismic survey projects
a) Access to our blocks/areas limited due to rough topographical surface and this has
affected seismic survey lines planning;
b) Difficulties in measuring and identifying the best routes of survey lines which
reflects the poor data collection of subsurface; and
c) Health, Safety and Environmental (HSE) matters.
• Seismic projects typically cover tens to hundreds of source and receiver points
within each square mile. This require extensive planning, scheduling, and
communication resulting in a constant need for a variety of surface maps and
models.
• Hence, GIS can be used as a tool to provide multiple benefits for such operation,
especially in areas of rough terrain.
8. Regional Geological Mapping using
GIS
This process will evaluate the targeted areas
by comparing and re-evaluate potential
targets. During the evaluating of geological
structures of the areas, integrated geological
maps were generated .Hardcopy maps were
geo-referenced to verifying coordinate
systems for Ethiopia and overlay with other
datasets in single coordinate systems.
9.
10. Seismic Survey Planning
• Careful planning which can result in more cost-effective acquisition and processing
can be achieved using GIS
• Before the first shot is fired geophysicists must determine the best route to reveal
the subsurface target.
• Factors that must be taken into account:
1. seismic line must be as straight as possible for better data acquisition –how flat is
the topography.
2. Subsurface structures:
a. Inline seismic basically must follow the dipping topographical structures (Fig.6 ).
b. Crossline seismic must follow the strike of the topographical structures for
accurate seismic reflection data (Figure 6)
11. Those factors reflect the survey lines determination. By integrating the
datasets and visualizing using GIS the exploration geophysicists can shift
the lines to a new location. This will reduce time of accessing and
dropping seismic sources while in operations.
12.
13. Integrated Geological Maps
• GIS can tie these data such as satellite imagery, digital elevation model
(DEM), seismic surveys, surface geology studies together to the location in
question and allow us to overlay, view and manipulate the data in the form of a map
to thoroughly analyze the potential of seismic survey lines locations.
• Through the integrated datasets using GIS, raster data, such as aerial photos or
satellite imagery, can be incorporated with vector data, and surface culture, such as
contours, topographic landmarks, digital elevation model or points of interest can
be presented
14.
15. Benefits of GIS/Conclusions (A)
1. Time saved in designing seismic survey by:
• Less exposure to HSE risks by planning the best routes of seismic direction
• issues of coordinate’s discrepancy can be resolved.
2. Costs saving
3. Sharing of integration of and access to centralized databases via computer networks
or the internet through web-based GIS.
4. Linkage of multiple software applications and formats.
5. The project data will be archived into centralized GIS database
17. Why is it a new trend?
• Increasing demands for oil and gas for new targets have necessitated expanding the
exploration activities to the volcanic covered provinces.
• One such target is the huge volume of the Plateau basalts, known also as Trap
basalts, beneath which lie the Mesozoic sedimentary successions which contain
both potential source and reservoir rocks (Fig. 1).
• It is generally believed that volcanic activities would generally overcook and
destroy the oil- and gas-bearing strata.
• Hence, hydrocarbon explorations in sub-basalt sedimentary formations have been
ignored for quite a long time.
18. The conventional multichannel seismic
reflection techniques which are the standard
investigation tools in hydrocarbon exploration
fail to give reliable seismic images. The sub-
basalt sedimentary sequences as well as the
intra-basalt features have rendered the seismic
method useless.
Hence, we will suggest a sequential data
acquisition and processing schedule towards
developing effective and realistic sub-basalt
exploration strategy.
Plateau basalt distribution in Ethiopia
and neighboring countries
19. Geological and tectonic setting
• The easternmost part of the study area is in Afar depression and is characterized by
flat and low lying topography averaging about 600 m above sea level.
• The geology is characterized by varied rock types and geologic structures (Fig. 2).
The basement rocks in the study area are Precambrian rocks composed of
metamorphosed volcanic, sedimentary and intrusive rocks that were accreted during
the Pan-African orogeny.
20.
21. • The Middle Triassic–Lower Jurassic sandstone which is equivalently known as the
Lower Sandstone or the Adigrat Sandstone generally overlies the Karroo
successions.
• Most prominent in the study area are the thousands of meters thick massive
volcanic sequences that overlie the Mesozoic successions.
• The intense Oligocene volcanism that produced these sequences of flood basalts
and rhyolites are indicative of hot upper mantle beneath the Plateau.
22.
23. Gravity data
• The gravity experiments consisted of collecting about 2000 data points which were
then merged with the pre-existing data of over 2270 points in order to better
analyze the density variations and relate these changes to the geologic structures.
• The gravity data were normally obtained at 2 km intervals.
• Three gravity meters – two Lacoste-Romberg and one Scintrex CG3 M – were used
to collect the gravity data.
• Localized gravity lows that could be associated to structural depressions are
observed throughout the central part.
• These depressions that are filled with low density materials could outline the
morphology and attitude of the sedimentary basins that might exist buried beneath
thick sequences of volcanic rocks.
24. Of all these
depressions, the one
with roughly NW
elongated shape
within the coordinates
39E– 39.8E and 9.7N–
11N deserves
particular attention.
Fig. 5B. Bouguer anomaly map of the region with
gravity station distributions
25. Regional geological and tectonic
Sedimentary basins
structures
• Various gravity analyses tools have been utilized to obtain a better image of the
subsurface and to understand the extent of the role the geologic structures played to
control the geometry of the basins.
Structural Features:
• The NW–SE structural trends could very well be associated with the Karroo rift and
• therefore constitute the oldest structures which are primarily responsible for the
formation of the Blue Nile basin.
• The basin since then has continuously been affected by the NNW–SSE as well as
by the younger NE-SW trending structures.
• The interaction among these three structures consequently results in the formation
of various sub-basins within the Blue Nile basin. As such accurate mapping of these
structures will be vital in hydrocarbon exploration in the region.
27. Wereilu Basin
• The interest for detailed investigation was kindled by the presence of oil seeps
which give evidence for the presence of hydrocarbons in the basin. For this we use
following techniques:
Horizontal gradient maxima
• As a prelude to modeling and to better visualize the geologic structures the
horizontal gradient magnitudes are calculated and the maxima in the horizontal
gradient were searched and located by passing a 5 5 data window over the
horizontal gradient magnitude grid.
Power spectral analysis
• The power spectra method is used to determine statistical mean depths to the
various interfaces of density contrasts.
29. Fig. 11. Depth estimations to the various startigraphic interfaces
in the Wereilu basin. using (a) Power spectral analysis and (b) 3D
Euler deconvolution
30. Density data
• Knowledge of the density values of the layers is essential to constrain the initial
model. Bulk density measurements were made on representative samples
collected from the horizons.
Initial model
• In this study, depths to the various interfaces are constrained by the known
geology as well as by the spectral analysis of the gravity field and by Euler
deconvolution. It is the preparation of an initial model that is constrained by the
available geological and geophysical information.
31. 2D analysis
• Two gravity profiles in the E–W and N–S directions (profiles AA and
BB, respectively in Fig. 10c) were developed and modeled. Both models show
more or less consistent results.
• The uppermost volcanic layer has an average thickness of 800 m and reaches a
maximum thickness of about 1200 m beneath highly elevated part of the plateau.
• The total thickness of the sedimentary sequence reaches about 5 km in the central
part of the basin and decreases to 2 km at the periphery in all directions.
• Modeling result also shows the presence of very thick Adigrat and Pre-Adigrat
Karroo sediments in the basin. A total thickness of about 3 km is modeled.
32.
33. 3-D gravity inversion
• All the information derived from the gravity data as well as the knowledge of the
local geology were put together to create a reasonable 3-D model of the subsurface
that is a crucial input to the inversion process.
• The model was created by dividing the volume of ground directly beneath the
survey area into a set of 3-D prismatic cells whose orientation, size and density are
appropriately adjusted keeping the density constant within each cell.
• A forward accurate 3-D simulation program is then run to obtain a synthetic gravity
data to compare with the real data
34.
35. Results and discussion
Tectonic implications
• The results obtained from different approaches are integrated to define the subsurface
structures for the Wereilu basin. The various tansformation techniques applied to the gravity
data revealed detailed regional and local structures.
• Although the Early Precambrian activity laid out the basic structural framework in the region
the tectonic features and the structural development of the basins as well as their history of
sedimentation may have been controlled and influenced by the Paleozoic extensional faulting
associated with Gondwana breakup which later on underwent episodic restructuring that
continued into the Mesozoic.
• The Gondwana breakup induced intracontinental rifting (Karroo rifts) around and within the
East African continental margin and deposition of very thick succession of continental Karroo
sediments in a series of separate fault-controlled aulacogen- like basins.
36. • The original Wereilu basin geometry, due primarily to Karroo rifting, is very
difficult to determine as it is continuously modified by the later reactivation process
as well as by the younger NS/NNE and NE trending structures.
• However, information retrieved from the 2-D and 3-D models show the basin to be
elongated in the SE direction. The length of this basin is about 80 km and the width
at the centre is roughly 25 km. The aspect ratio (length to width ratio) seems to be
roughly 3.2:1 on average.
• The available pieces of information put together suggest that the Wereilu basin has
similar architecture to the well-documented modern and ancient pull-apart basins.
The more or less similar aspect ratio of 3:1 is found.
37. Hydrocarbon potential
• The geology of the Wereilu area possesses the requirements necessary for the
generation and accumulation of hydrocarbons.
• The over 5 km thick sedimentary sequence deposited in a basin of adequate size,
the presence of source rocks, reservoir rocks, cap rocks and a favorable thermal
regime as well as appropriate structural traps are the necessary elements present to
make Wereilu a potentially promising basin.
• The well developed fault systems could also act as important passages of petroleum
migration.
38. Surface indications
• Within the basin direct evidence of the presence of hydrocarbonsexists in the form
of oil seepage.
• Fresh oil seeps through fractures in the plateau basalts and is deposited as black
tars.
• The entire exposed basalt layer along the Mechala river shows the seeps spread out
from the many fractures exposed at the river floor.
• The seeps indicate that source rocks have generated and expelled oil.
• Evidently the source has been subjected to temperature high enough to generate oil
but not high enough to destroy it.
39. Source rocks and maturity
• Possible source rocks are the black shales of the Gohatsion formation, the Antalo
limestone and the shales of Pre-Adigrat Karroo sediments.
• The black shale of the Antalo limestone with its total organic content value (TOC)
of up to 7% and mean vitrinite reflectance (Ro) of upto 0.9% constitutes an
excellent source rock.
• TOC greater than 2% and Ro between 0.5 and 1 are considered to be good source
rocks.
• Recent studies conducted on the oil seep indicated that the source rock for the oil
was a Mesozoic shallow marine.
40. Reservoir rocks
• The potential reservoir rocks are the Adigrat and pre-Adigrat Karroo sediments
and the Upper Sandstone.
• The porosity and permeability measurements performed on these rocks
(Wolela, 2004, 2007, submitted for publication) indicate that the Adigrat and
upper sandstones have porosities.
• These values make these rocks fair to good reservoir rocks.
41. Traps and seals
• The plateau basalts which lie directly above the reservoir rock (Upper Sandstone)
obviously make an ideal cap rock.
• The faulted basement block with a graben structural style setting could very well
serve as structural trap.
• The possibility of having stratigraphic traps could also be expected since facies
changes could not be ruled out in the environment of sediment deposition typical of
similar rift types.
42. Conclusion
o From the analysis of gravity data an improved geologic model for the Wereilu basin was obtained.
o The geological layers were identified and the outline of the basin were determined.
o A 3-D inversion of the gravity field clearly shows that theWereilu basin is a graben formed within
and by the NW–SE trending normal faults which later on was affected by the younger NE–SW
trending structures.
o It is clear that these structures exerted significant control on the geometry and perhaps on the
sedimentation pattern of the Wereilu basin. Structural factors might have played a major role in
hydrocarbon accumulation and localization.
o The nature and thickness of the sub-volcanic sedimentary succession, reaching a maximum thickness
of about 5 km, coupled with the overlying thick volcanic sequence providing the necessary thermal
gradient for the maturation of the organic material create a favorable condition for the generation and
accumulation of hydrocarbon deposit.
43. o The most suitable structural traps could be associated with the faulted grabens in
the central part of the basin right beneath the seep.
o Another trap could be at the extreme south eastern part of the basin adjacent to the
NS trending border faults.
o Noting that the oil seepage observed along Mechala river bed is within theWereilu
basin and considering the implication obtained from the extensive analyses carried
out in this work that the Wereilu basin extends down to a considerable depth, it
seems evident that this basin qualifies to be appropriate for further exploration
work to determine the economic presence and extent of the hydrocarbon
accumulation as evidenced by the seepage.
44. REFERENCES
• Delineation of sub-basalt sedimentary basins in hydrocarbon exploration in
North Ethiopia.
• A First Course In Geophysical Exploration and Interpretation, Robert E.
Sherif, University of Houston, 1978
• GIS: The Exploration and Exploitation Tool, Kirk A.
Barrel, Geodynamics, Inc. Houston Texas, USA (AAPG).
Breg(k) = exp(-2 k z0)Bslab(k) = exp(-2 k z1) – exp(-2 k z2)
Clear NW and NNE/NE trending structures are observed on the horizontal gradient map (Fig. 10c). It is seen thatthe basin is completely described and formed by these structures. A clear NE trending fault is observed in the middle of the mapcrossing the entire length of the basin. It is found out that oil seepage occurred at the north eastern part of the basin along andfollowing these NE trending structures.
(Fig. 11a) shows a discrete series of linear segments with slopes proportional to the average depth of the density interface. Basement is estimated at a depth of about 6.35 km. The depth to the upper surface of the Mesozoic sediments is about 0.9 km. Clear NW and NNE/NE trending structures are observed on the horizontal gradient map (Fig. 10c). It is seen that the basin is completely described and formed by these structures. A clear NE trending fault is observed in the middle of the map crossing the entire length of the basin. It is found out that oil seepage occurred at the north eastern part of the basin along and following these NE trending structures.
