Delineation-of-sub-basalt-sedimentary-basins-in-hydrocarbon-exploration-in-North-Ethiopia_2010

Delineation of sub-basalt sedimentary basins in hydrocarbon exploration in North Ethiopia

Tilahun Mammo

Department of Earth Sciences,Addis Ababa University,P.O.Box 1176,Addis Ababa,Ethiopia

a r t i c l e i n f o

Article history:

Received 18December 2008Received in revised form 11December 2009

Accepted 20December 2009Available online 13January 2010Keywords:

Plateau basalts Oil seepage Wereilu basin

Hydrocarbon potential Gravity survey Ethiopia

a b s t r a c t

In Northern Ethiopia oil seepage could be traced ?owing through fractured basalts at the Mechela river bed near Wereilu town.These rocks make up part of the huge volume of Ethiopia’s Oligocene-Miocene Plateau basalts and associated rhyolites that cover most of the central and northern part of the country.They overlie the marine sedimentary formations of Triassic–Cretaceous age and constitute one of the largest visible ?ood basalts on the face of the earth.

2-D and 3-D analyses of the gravity ?eld have been performed to determine the structural pattern and subsurface density distributions beneath the thick volcanic sequences.The resulting images offer signi?cant new insights into the structural pattern and geophysical characterization of the study area.A NW–SE elongated basin of signi?cant dimension has been localized directly beneath the oil seep at Wereilu.The basin is a graben formed within and by the NW–SE trending structures of the Karroo rift system.A younger generation of faults in the NE–SW direction has affected the basin exerting signi?cant control on the geometry and perhaps on the sedimentation pattern that might have played a major role in hydrocarbon accumulation and localization.

The nature and thickness of the sub-volcanic sedimentary succession,attaining a signi?cant thickness of more than 5km,coupled with the overlying thick volcanic sequences providing the necessary thermal gradient for the maturation of the organic material create a favorable condition for the generation and accumulation of hydrocarbon deposit.

1.Introduction

As petroleum exploration activities expand in Ethiopia all known sedimentary basins have become focus of intensive studies.Increasing demands 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 succes-sions which contain both potential source and reservoir rocks (Fig.1).This study concentrates on a particular locality called Wereilu in the Blue Nile Basin in northern Ethiopia where oil seepage is observed that can be traced through fractures in the volcanic rocks.

Hydrocarbon explorations in sub-basalt sedimentary forma-tions have been ignored for quite a long time due to miscon-ceptions by both petroleum geologists and oil companies (Summer and Verosub,1989).The misconceptions mainly arise from the conclusion that volcanic activities would generally over-

cook and destroy the oil-and gas-bearing strata.These miscon-ceptions are now gradually being eroded away as understandings widened on the positive role the volcanic activities actually play in the thermal maturation of the sedimentary basins.It is now believed that sedimentary basins overlain by volcanic rocks may contain signi?cant reserves of hydrocarbons and consistent relationship seems to exist between hydrocarbon production and volcanic activities (Summer and Verosub,1989,1987).The known hydrocarbon accumulations under the plateau basalts of the Algerian Sahara in North Africa (Benelmouloud and Zhuravlev,1989),the promising hydrocarbon potential beneath the thick lava ?ows of the volcanic provinces of Parana in South America (Milani et al.,1990),of Columbia in North America (Morrison et al.,1996),of Siberia in Russia (Benelmouloud and Zhuravlev,1989;Odintsova and Drobot,1983)are of great interest for future explorations in the sedimentary basins that are associated with Plateau basalts.

Geophysical explorations in volcanic covered regions have suffered serious setbacks.The conventional multichannel seismic re?ection techniques which are the standard investigation tools in hydrocarbon exploration fail to give reliable seismic images.The

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sub-basalt sedimentary sequences as well as the intra-basalt features have rendered the seismic method useless.The main problems being the low signal-to-noise ratios,reverberations,and statics problem and common-midpoint smearing (Jarchow et al.,1991a ).Measures taken to tackle these problems –using large explosive sources to raise the signal level and to avoid the need to

stack thereby eliminating the effects of static shifts and common midpoint smearing and acquiring wide-aperture recordings to minimize the effect of reverberation –seem to give better results to certain degrees (Jarchow et al.,1990,1991a,b ).

On the other hand,the resistivity contrast between basalts and sediments make the electrical methods (primarily magneto-tellurics)alternative techniques in sub-basalt exploration (Hautot et al.,2006;Manglik and Verma,1998;Morrison et al.,1996;Stanley,1984).Rao and Reddy (2005)suggest a sequential data acquisition and processing schedule towards developing effective and realistic sub-basalt exploration strategy.They suggest that prior to any seismic studies,gravity surveys followed by magne-totelluric and deep resistivity sounding should be undertaken to obtain good results.They af?rm that such procedure yielded fruitful results in the volcanic covered region of India.

In this study,gravity data were collected over the broad Ethio-pian plateau and processed to map possible sub-basalt basins and understand the structural set up of the region.The gravity signals obtained are analyzed to de?ne depths to the various gravity sources,map the important trends and structures that are identi-?able in the anomaly ?eld,and delineate the geometry and lateral extent of basin that may exist beneath the Trap basalts.

2.Geological and tectonic setting

The study area,located within the Blue Nile basin (or Abay basin)in North Ethiopia,is roughly bounded by coordinates 9 –12 N and 37 –41 E.The easternmost part of the area is in Afar Depression and is characterized by ?at and low lying topography averaging about 600m above sea level.More than half of the study area,on the other hand,is marked by very rough topography with heights reaching as high as 3600m above sea level.Many rivers with deep gorges including the Blue Nile river and its tributaries cover quite a good portion of the area.

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,sedi-mentary and intrusive rocks that were accreted during the Pan-African orogeny (Abdelsalam and Stern,1996).Overlying these

o o

Fig.2.Geologic map of the region under study (modi?ed after Tefera et al.,1996).

Fig.1.Plateau basalt distribution in Ethiopia and neighboring countries (modi?ed after Wilson and Guiraud,1992).

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rocks are the Upper Permian–Lower Triassic continental Karroo sediments,predominantly sandstone and shale,generally believed to have been deposited in alluvial fan and ?uviatile settings (Wolela,1997).The existence of Karroo sediments in the Blue Nile basin is reported by Mohr (1963),Jepsen and Athearn (1964),Assefa (1991),Russo et al.(1994)and Wolela (1997,2004,submitted for publication),and Tadesse (2007)though information about the thickness of these sediments is scarce.However,Beauchamp (1977)reported that about 650m.thick sediments are exposed near the bridge across the Blue Nile canyon.In another exposure near the canyon these sediments are reported to have thickness of about 450m.(Jepsen and Athearn,1964;Assefa,1991;Wolela,1997,2007).

The Middle Triassic–Lower Jurassic sandstone which is equiva-lently known as the Lower Sandstone or the Adigrat Sandstone generally overlies the Karroo successions.The unit is reported to have a thickness of 850m (Wolela,2007)and can be traced continuously to the Sudan border.It grades into about 400m thick shale and gypsum units collectively known as the Gohatsion Formation.This is overlain by Callovian–Kimmeridgian limestone consisting of fossiliferous carbonates interbedded with thin marl layers and becomes clay-rich with interbedded shale units in its uppermost part.Both the limestone and shale units have a total thickness of more than 750m and are referred to as the Antalo Group.This Group is unconformably overlain by the Kimmer-idgian–Cenomanian Upper Sandstone Formation consisting of two units namely,the Mugher Mudstone and the Debre Libanos Sand-stone (Assefa,1991;Wolela,1997),with the latter containing conglomerates and claystone as well.

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 ?ood basalts and rhyolites are indicative of hot upper mantle beneath the Plateau (Pik et al.,1998,1999;Ayalew et al.,2002;Wolfenden et al.,2004).The volcanic rocks make up one of the world’s major continental Large Igneous Provinces associated with rifting and/or domal uplift on the African plate.A simpli?ed stratigraphy of the Blue Nile basin is shown in Fig.3.

The Blue Nile basin is considered to be an NW–SE trending failed arm of the Karroo system in Ethiopia that was formed by the breakup of Gondwanaland during the Paleozoic–Jurassic times (Bosellini,1989;Russo et al.,1994).As such the idea that this basin is actually a continuation of the Ogaden basin in SE Ethiopia is gaining acceptance among geoscientists (Bosellini,1989;Russo et al.,1994;Atnafu,2003;Korme et al.,2004;Wolela,2007)(Fig.4).Studies also indicate that both the Blue Nile and the Ogaden basins are largely fault-controlled.The Ogaden basin is ?lled with the Upper Paleozoic continental Karroo sediments (Raaben et al.,1979;Worku,1988;Worku and Astin,1992;Haile,1998;Hunegnaw et al.,1998)that have variable thicknesses but reaching a maximum thickness of more than 4000m.in El-Kuran trough (Raaben et al.,1979;Bosellini,1989;Haile,1998).

Field structural studies in the Blue Nile basin reveal NW/NNW,NE/NNE,NS,and E-W trending fault systems.The interaction between these fault systems must have in?uenced and controlled the subsurface stratigraphy and could have a major control in the attitude and behavior of the layers related to hydrocarbon gener-ation and migration.These faults must have also controlled the shape of the sedimentary basins that might occur beneath the thick trap basalts.

More information on the geology and structures of the area can be obtained from Assefa (1991),Atnafu (2003),Bosellini (1989),Russo et al.(1994)and Wolela (2004).3.Gravity data

The gravity experiments consisted of collecting about 2000data points which were then merged with the pre-existing data of over 2270points in order to better analyze the density variations and relate these changes to the geologic structures.The gravity data were normally obtained at 2km intervals.In Wereilu area where oil seepage was observed along the Mechela river the gravity interval was reduced to 1km for detailed investigations.

Three gravity meters –two Lacoste-Romberg and one Scintrex CG3M –were used to collect the gravity data.One of

the

Formation Thickness (m)Lithology

Period

Era

Cenozoic

Tertiary

Cretaceous Jurassic

Triassic

Upper Paleozoic

Mesozoic

Paleozoic Precambrian

Volcanics

Upper sandstone Antalo Group Goha Tsion Adigrat sandstone Pre Adigrat

Basement

1100

560750850400

650

Formation Karroo sediments

Quaternary

Sediments 130Fig.3.Simpli?ed stratigraphic column of the Blue Nile basin.

Fig.4.Karroo rift system in Ethiopia (modi?ed after Bosellini,1989;Russo et al.,1994;Wolela,2007).

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gravimeters was consistently used to establish gravity base stations which were uniformly distributed over the area.The gravity network was tied to the 1967gravity bases.The station elevations are determined using Trimble 4000si differential GPS unit.All the GPS stations are in turn referenced to known triangulation stations of the national elevation network that not only eliminates the random errors associated with the use of different GPS networks but also help reduce the data to the national datum that facilitates data integration.

The standard gravity reductions are applied on the observed gravity readings.There is signi?cant topographic variation in the area.On the plateau the topography varies between 2600and 3600m above sea level.In the east in Afar the elevation decreases down to about 600m.Terrain corrections are performed to elimi-nate the effect of the topographic variations.Density measure-ments on representative samples show that a density value of 2.72gm/cc is more appropriate for Bouguer and terrain corrections.The terrain corrections are calculated using Digital Elevation Model.The complete Bouguer anomaly map and the gravity point distri-butions are shown in Fig.5.Statistical analyses show that full terrain corrections range from 0.0to 8.45mGal with a mean value of 3.09mGal and a standard deviation of 2.29mGal.The terrain corrections fall below 2mGal for 45%of the data,range from 2to 5mGal for 32%;5to 7mGal for 15%and 7to 8.45mGal for 11%of the data.Two distinct zones are observed in the map.The entire central part (the plateau)is characterized by very low gravity anomalies ranging from à280to à200mGal.The eastern and south-eastern margins (Afar and the Main Ethiopian Rift,respec-tively)show much higher Bouguer anomalies.Localized gravity lows that could be associated to structural depressions are

observed throughout the central part.These depressions that are ?lled with low density materials could outline the morphology and attitude of the sedimentary basins that might exist buried beneath thick sequences of volcanic rocks.Of all these depressions,the one with roughly NW elongated shape within the coordinates 39 E–39.8 E and 9.7 N–11 N deserves particular attention.It has an anomaly contrast of more than 50mgal from the surrounding region.It is in this basin that both Wereilu town and the oil seepage at Mechala river are located.

4.Regional geological and tectonic structures 4.1.Sedimentary basins

The complexity of the region makes it dif?cult to clearly outline the geometry of the possible basins and to see detailed tectonic features in the un?ltered gravity maps.Many anomalies of various size,shape and orientations might remain hidden in raw maps.Various gravity analyses tools have been utilized to obtain a better image of the subsurface and to separate shallower features from deeper ones and to understand the extent and the role the geologic structures played to control the geometry of the basins.As the deeper features are of no interest in this study they are removed from the Bouguer anomaly.

The various numerical methods used in separating the deeper features from the shallower (Gupta and Ramani,1980;Hearst and Morris,2001;Nettleton,1976)have been evaluated and two approaches have given better results and provided roughly similar gravity anomaly features.In the ?rst approach the gravity ?eld is upward continued to various heights to extract gravity signatures at the corresponding depths following the theory of Jacobsen (1987)which stated that upward continuation to a height of 2Z is used to extract the ?eld associated to sources at and below depth Z .Hence to extract the gravity ?eld associated to depths of 2.5,5,10and 15km the Bouguer gravity map of Fig.5is upward continued to a height of 5,10,20and 30km,respectively (Fig.6).As the continuation height increases more of the smaller features and shorter wavelengths disappear.Hence,to obtain a residual map of the region with Wereilu basin as focus of attention it is found appropriate to upward continue the gravity ?eld to a height of 30km to make sure that any trace of the basin is completely eliminated.Subsequent subtraction of this from the Bouguer gravity gives a residual anomaly map for the basin.The polynomial ?tting method used as a cross check seemed to give comparable results for the Wereilu situation.When the second-order polynomial surface of the regional anomaly is removed from the observed gravity ?eld a residual anomaly is obtained that agrees to certain degree with the result of the ?rst approach and give similar locations of residuals.Of the two,however,the upward continuation technique gives a more detailed anomaly features and is used in this study.

The resulting residual gravity ?eld is given in Fig.7.All gravity responses that originated from sources deeper than 15km are eliminated from the residual anomaly.A prominent roughly pear-shaped Wereilu basin becomes more evident as the effect of the masking deeper gravitational effect is removed.This basin seems to be separated from another basin in the north by approximately E–W trending structural feature.Both these basins ?ank the Plateau-Afar margin that is marked by well developed border faults.Some of the gravity lows in the western half of the area do not show up in the residual map indicating deeper origins.4.2.Structural features

To have a better understanding and a clearer picture of the subsurface structures and to study the major tectonic features

that

Fig.5.Bouguer anomaly map of the region with gravity station distributions.

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have shaped the relief of the concealed Precambrian basement and consequently the morphology of the basin and sub-basins,?ltering and edge enhancement techniques are applied to the gravity ?elds.

To delineate contacts and faults the magnitudes of the hori-zontal gradient (Cordell and Grauch,1985;Roest and Pilkington,1993),the analytic signal (Blakely,1995;Nabighian,1972,1974;Roest et al.,1992)and the local wave number (Smith et al.,1998;Thurston and Smith,1997)of the gravity ?elds were computed.The maxima in each of these were searched and located by passing a 5?5data window over the grid following the automated procedure outlined in Blakely and Simpson (1986).The results from the three methods are all similar and the superimposed plots (Fig.8a)show several dominant structural trends that have well de?ned alignments in NW/NNW and NE/NNE directions.The NE/NNE structures are seen cutting through the NW/NNW structures indicating younger origin.On the other hand,a reactivation of the NW/NNW trending structures took place at a later time as evi-denced by the cross-cutting and offsetting of the NNE trending structures along the entire eastern margin.The outline of NW trending structural lineaments,most probably delineating about 200km wide mega basin amid noisy background can be traced from the plots.

Fig.8b shows the structural interpretation of the superimposed maxima.The structural lineaments are interpreted as faults from knowledge of the local geology.

All the methods consistently offer signi?cant new insights into the structural framework of the region.The resulting interpreta-tions not only con?rm the structures previously recognized in the area from geology but also yield important results in the charac-terization of structural framework not recognized before.Three major structural lineaments that roughly follow NS,NW/NNW,and NE/NNE trends and which could be correlated to regional tectonic elements have been identi?ed.These structures could have played a prominent role of controlling the morphology of the Precambrian basement and consequently the orientation and shape of the entire Blue Nile Basin and the number of sub-basins.The NW–SE struc-tural 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 explora-tion in the region.

Azimuthal (strike)?ltering along the NW and NE directions have been performed to isolate and individually analyze these linea-ments to understand the role these structures played in the evolution of particularly the Wereilu basin.Fig.9a shows ?ltered output in which all NE trending features are removed.Fig.9b,on the other hand,shows the anomaly map with all the NW trending features eliminated.When these two ?gures are compared with the residual Bouger anomaly map of Fig.7valuable information is revealed for Wereilu basin.With all the NE trending features removed the pronounced and well-built Wereilu basin remains intact.The basin morphology,on the other hand,is highly altered when all the NW trending features are eliminated.A signi?cant NE striking structural feature appears in the south-east and a relict of the basin remains in the north-west near Wereilu town where seepage occurs.These imply that the single basin which

was

Fig.6.Upward continuation of the gravity ?eld to a height of 5,10,20,and 30km to image sources buried at the depth of 2.5,5,10,and 15km,respectively.

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initially formed within and by the NW trending structural features was later on affected by the NE trending faults.The seepage might as well be controlled by the interactions of the NW and NE trending structures.5.Wereilu basin

Construction of geological model and geological interpretations corresponding to the geophysical observations were done on the more promising Wereilu basin.The residual gravity anomaly of the basin after the removal of upward continued ?eld to a height of 30

km is shown in Fig.10a.The NW trending basin is clearly outlined.The residual gravity anomalies vary from à42.59mgal to 42.21mgal.The gravity pattern in the eastern margin of the basin shows a strong gradient shouldering the plateau-Afar margin.The SE end is further affected by NE structures resulting in the slight deepening of the basin.Moderate gradient is seen in the other directions.The interest for detailed investigation was kindled by the presence of oil seeps which give evidence for the presence of hydrocarbons in the basin (Fig.10b).5.1.Horizontal gradient maxima

Prior to constructing a 3-D geologic model of the basin,however,attempts were made to extract the necessary relevant information from the gravity data itself using various approaches.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?5data window over the horizontal gradient magnitude grid (Blakely and Simpson,1986)on the isolated Wereilu basin.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.5.2.Power spectral analysis

The power spectra method is used to determine statistical mean depths to the various interfaces of density contrasts.The azimuth-ally-averaged power spectrum method of Spector and Grant (1970)was applied to the data in order to determine the top of density contrasts causing gravity anomalies.The logarithm of the power spectrum plotted versus radians/km (Fig.11a)shows a discrete series of linear segments with slopes proportional to the average depth of the density interface (Banks et al.,1977;Karner and Watts,1983).Six horizons which roughly correspond to the known local stratigraphy are recognized.The lower surface of the

Precambrian

Fig.7.Residual gravity anomaly map of the area after sources deeper than 15km are

removed.

a Fig.8.(a)Superimposed horizontal gradient magnitude,analytic signal amplitude,and the local wave number computed from the gravity ?elds.(b)Interpretation of the structural lineaments and contacts.

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Basement is estimated at a depth of about 6.35km.The depth to the upper surface of the Mesozoic sediments is about 0.9km.5.3.3-D Euler deconvolution

Euler deconvolution method (Reid et al.,1990;Thompson,1982)is a powerful and one of the effective direct methods used to gain preliminary information on the position,depth and nature of the gravity anomalies.The 3-D Euler homogeneity equation is given by

ex àx 0Td A =d x tey ày 0Td A =d y tez àz 0Td A =d z ?N eB àA T

where (x 0,y 0,z 0)is the position of a gravity source whose ?eld A is detected at (x ,y ,z ).B is a constant background ?eld and N is the structural index which depends on the shape of the gravity source.The above over-determined equation is solved using the least-squares method within the moving window to get the source coordinate (x 0,y 0,z 0).

Appropriate structural index and moving window size settings are necessary to get good results from Euler deconvolution.These settings were obtained after a number of trials and the method was applied to the data to estimate depths to the causative bodies and also delineate geologic contacts (boundaries).The Euler solutions were calculated for different structural indices (SI ?0,0.25,0.5,0.75,1,1.25,1.5,2,2.25,2.5,3).The deconvolution method produces sprays of solutions which are then subjected to post-processing by applying Rodin algorithm (Mikhailov et al.,2003;Widiwijayanti et al.,2003)to remove the poorly-constrained and poorly-clustered ones keeping only those solutions with relatively high geometric concentrations.By statistical means the solutions are then split into groups of dense clusters identifying causative sources.Fig.11b shows the resulting Euler solutions for structural index of 2which gives the tightest clustering.The Euler solutions cluster around the perimeters of the basin and emphasize both the NW and NE trending linear clusters.These linear clusters closely match the NW and NE trending faults localized in the horizontal gradient map of Fig.10c.The 5.5–6.8km depth sets of solutions correspond to the basement depth and agree well with the result obtained from the power spectral method.

5.4.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.The volcanic rocks show wide range of density values re?ecting their varied composition.Fewer density measurements were made on the sedimentary layers due to poor exposure.The basement density value of 2.78gm/cc was obtained from the conversion of wide-angle seismic re?ection/refraction wave velocities (6.2km/s)(MacKenzie et al.,2005;Makris and Ginzburg,1987)using the density–velocity relationship of Nafe and Drake (1957).A summary of the density values is shown in Table 1.5.5.Initial model

A starting point in modeling is the preparation of an initial model that is constrained by the available geological and geophysical information.In this study,depths to the various interfaces are constrained by the known geology as well as by the spectral analysis of the gravity ?eld and by Euler deconvolution.Depth to the basement is also constrained by wide angle re?ec-tion/refraction data (MacKenzie et al.,2005;Makris and Ginz-burg,1987).Both geological and geophysical information together with measured density values of the various layers

are

Fig.9.(a)Azimuthal (strike)?ltering of the Bouguer anomaly map with all NE trending structures suppressed.(b)Same ?ltering with all NW trending structure

removed.

Table 1

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systematically put together to de?ne the initial model that could be a reasonable input to the modeling algorithm.

Accordingly the following density and thickness values are used as a starting model in both 2-D and 3-D modeling.

1.Uppermost layer (volcanic rocks)with density value of

2.72gm/cc and thickness of 900m.

2.The second layer (Upper Sandstone Formation)with density value of 2.34gm/cc and thickness of about 450m.

3.The third layer (Antalo Group)with density value of 2.50gm/cc and thickness of 750m.

4.The fourth layer (Gohatsion Formation)with density of 2.40gm/cc and thickness of 700m.

5.The ?fth layer (Adigrat Sandstone and the pre-Adigrat Karroo sediments)with density value of 2.38gm/cc and combined thickness of about 3km.

6.The six layer (Precambrian basement)has density value of 2.78gm/cc.

5.6.Density structures inferred from 2-and 3-D analysis

The information described above are used in the two-and three-dimensional analysis to obtain depth estimate to the various

horizons and outline the basement geometry.In the 2-D analysis the gravity model response calculation is based on the method of Talwani et al.(1959)as implemented by GM-SYS software (North-western Geophysical Associates,Inc.).The programs are interactive allowing visual evaluation and comparison of model derived and observed ?elds.

Two gravity pro?les in the E–W and N–S directions (pro?les AA and BB,respectively in Fig.10c)were developed and modeled.Both models show more or less consistent results (Fig.12).Six lithologic horizons,well de?ned basin and faults are clearly modeled.The uppermost volcanic layer which was modeled using a mass density 2.72gm/cc has an average thickness of 800m and reaches a maximum thickness of about 1200m beneath highly elevated part of the plateau.The total thickness of the sedimen-tary sequence reaches about 5km in the central part of the basin and decreases to 2km at the periphery in all https://www.360docs.net/doc/2914710531.html,pa-rable thickness for the sediment is observed from magnetotelluric study conducted in the adjacent area within the Blue Nile basin (Hautot et al.,2006).Modeling result also shows the presence of very thick Adigrat and Pre-Adigrat Karroo sediments in the basin.A total thickness of about 3km is modeled.It is informative to note the presence of more than 4km thick Karroo sediments in the SW Ogaden Basin as observed from a regional seismic line (Haile,1998

Fig.10.Residual Bouguer anomaly map of the Wereilu basin (a),Oil seepage within the Wereilu basin (b),and horizontal gradient maxima of the basin (c).

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The deepest,central part of the basin is a graben formed by normal faults which have played signi?cant role in forming and shaping the morphology of the Wereilu basin.It is through these faults that oil seepage takes place.

In the 3-D analysis the iterative algorithm of Cordell and Hen-derson (1968)is used to obtain the depth and geometry of the basement.With a given gravity anomaly grid and some restrictions with relations to the mass distribution and depth estimates it is possible to calculate a 3-D structural model automatically by successive approaches.The derived model is speci?ed in terms of a planar reference surface and a density contrast.The gravity ?eld of the model is calculated at each point of the grid,compared and adjusted to the observed ?eld to modify the ?rst and successive models until a good agreement is reached.The 3-D depth model thus obtained is shown in Fig.13a.This technique gives a complete view of the geometry of the basement surface where the depth ranges from about 2km to more than 6km in the central part of the basin.The basement morphology reveals that the Wereilu basin is elongated in the SE direction.

To determine the validity of the result obtained,gravity ?eld is calculated from the derived depth model (Fig.13b)and compared with the observed gravity anomalies of Fig.10a.The residuals plotted in Fig.13c show errors of less than 1mgal between the observed and computed gravity values.5.7.3-D gravity inversion

All the information derived from the gravity data (from the various ?ltering results,spectral analysis,Euler deconvolution,2-D modeling,and the 3-D nonlinear modeling in the spatial domain)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 appro-priately adjusted keeping the density constant within each cell.A

forward accurate 3-D simulation program was then run to obtain a synthetic gravity data to compare with the real data.Adjustments in the width,location and density values were then made until a close match was obtained between them.

Using this as a starting model 3-D inversion was performed using the method developed by Li and Oldenburg (1998)aiming at recovering a 3-D density distributions that ?ts the observed data to within the accepted uncertainty levels.The problem can be rep-resented in a matrix form by

Gm ?d

where d and m are the observed gravity data and earth model vectors of dimensions N and M ,respectively.G is the forward operator describing the data as a function of the model.The inverse problem is solved using an optimized approach in which the best ?tting model is recovered by minimizing an objective function using an iterative process.The objective function includes terms that penalize the roughness in various spatial directions.It also contains a depth weighing function to resolve dif?culty arising from the non-uniqueness of the inverted models.

The 3-D recovered density model is shown in Fig.14a–c.The geometry of the sedimentary basin is well de?ned.The recovered density values for the sediments vary between 2.25and 2.55gm/cc.For the overlying trap basalt the density values are between 2.59and 2.88gm/cc and for the basement the densities are around 2.77gm/cc.In general the density values obtained from the 3-D inversion are similar to those obtained in the 2-D/3-D analysis and laboratory measurements.

6.Results and discussion 6.1.Tectonic implications

The results obtained from different approaches are integrated to de?ne the subsurface structures for the Wereilu basin.The

various

Fig.11.Depth estimations to the various startigraphic interfaces in the Wereilu https://www.360docs.net/doc/2914710531.html,ing (a)Power spectral analysis and (b)3D Euler deconvolution.

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transformation techniques applied to the gravity data revealed detailed regional and local structures.Even though much more information on the geologic structures than here-to-fore known was revealed by this study all the structures observed in the ?eld and identi?ed by geology correlate well with the lineaments interpreted from the gravity ?elds.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 in?uenced 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 aulac-ogen-like basins (Bosellini,1989).The NW trending structural features and pattern of sediment deposition seem to suggest the presence of these rifts in the study area.These features are seen as continuation of similarly trending structures in the Ogaden basin further southeast.

The Karroo stage of rifting and deposition was followed by the deposition of the Triassic-Early Jurassic basal clastic sediments (the Adigrat sandstone)which were primarily ?uviatile in origin.The Adigrat sandstones were later on covered by the Mesozoic sedimentary sequences of the Gohatsion formation and the Antalo Group.These Mesozoic sedimentary sequences are widely believed to be the consequence of a major transgressive-regressive cycle which affected the entire Horn of Africa.

A forced major sea regression most probably due to tectonic uplift in the Horn of Africa (Bosellini et al.,1997)caused the deposition of the Upper Jurassic sedimentary succession,the continental Upper Sandstone,which unconformably overlies the Antalo Group.

The voluminous Eocene–Oligocene plateau basalts and associ-ated rhyolites that overlie Upper Sandstone were the direct consequence and manifestation of the Afar mantle plume (Pik et al.,1998,1999;Ayalew et al.,2002).Closely associated with this activity is the rifting and continental break-up in the present-day Red Sea-Gulf of Aden-Main Ethiopian rift systems forming the Afar triple junction.The N–S/NNE–SSW and NE–SW trending structures in the study area are related to these rift systems.Being younger in age these structural features generally masked and obliterated the older NW trending structures.

The original Wereilu basin geometry,due primarily to Karroo rifting,is very dif?cult to determine as it is continuously modi?ed 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 80km and the width at the centre is roughly 25km.The aspect ratio (length to width ratio)seems to be roughly 3.2:1on average.

In addition,the horizontal gradient,analytic signal and wave number maxima give the various structural features that affect the basin.Close observations at these maxima give indications of some component of strike-slip displacement in the faults.

The available pieces of information put together suggest that the Wereilu basin has similar architecture to the well-docu-mented modern and ancient pull-apart basins.The more or less similar aspect ratio of 3:1found by Aydin and Nur (1982)for the worldwide pull-apart basins seem to give additional weight to this

view.

Cenozoic Volcanics (d = 2.72)Upper Sandstone Formation (d = 2.34)Antalo Group (d = 2.50)Gohatsion Formation (d = 2.40)

Adigrat & Pre-Adigrat Karroo Sediments (d = 2.38)

Precambrian Basement (d = 2.78)

a b

Fig.12.Geologic models constructed for pro?le AA in the east–west direction (a)and pro?le BB in the north–south direction (b)(see location of the pro?les in Fig.10c).

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904

6.2.Hydrocarbon potential

The geology of the Wereilu area possesses the requirements necessary for the generation and accumulation of hydrocarbons.The over 5km 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.

6.2.1.Surface indications

Within the basin direct evidence of the presence of hydrocar-bons exists 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 ?oor.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.

6.2.2.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 re?ectance (Ro)of upto 0.9%(Wolela,2004,2007)constitutes an excellent source rock.TOC greater than 2%and Ro between 0.5and 1are considered to be good source rocks (Cercone,1984;Dow,1977;Peters,1986;Waples,1980).These values suggest that the source rocks have been subjected to nearly optimal temperature condition.Recent studies conducted on the oil seep indicated that the source rock

for

Fig.13.(a)Depth model for the Wereilu basin obtained by the 3-D iterative algorithm of Cordell and Henderson (1968).(b)Gravity ?eld computed from the obtained depth model.(c)Error between this computed gravity ?eld and the observed gravity ?eld of Fig.10a.

T.Mammo /Marine and Petroleum Geology 27(2010)895–908905

the oil was a Mesozoic shallow marine –lagoonal shale with mixed marine and terrestrial input and that the oil was expelled from an early mature source rock (IGI Ltd,2006)

6.2.3.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 as high as 20.4%and 22.2%and permeabilities as high as 710mD and 809.6mD,respectively.These values make these rocks fair to good reservoir rocks.

6.2.4.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.

7.Conclusion

From the analysis of gravity data an improved geologic model for the Wereilu basin was obtained.The geological layers were identi?ed and the outline of the basin were determined.A 3-D inversion of the gravity ?eld clearly shows that the Wereilu 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.It is clear that these structures exerted signi?cant 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.The basin is elongated in the SE direction and has a depth of about 6.5km at its central part.

The nature and thickness of the sub-volcanic sedimentary succession,reaching a maximum thickness of about 5km,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.The recent geochemical interpretation of the Wereilu seep bitumen indicated that the source rock for the oil was a Mesozoic shallow marine-lagoonal shale with mixed marine and terrestrial input and that the oil was expelled from an early mature source rock.The most suitable structural traps could be associated with the faulted grabens in the central part of the basin right beneath the seep.Another trap could be at the extreme south eastern part of the basin adjacent to the NS trending border faults.Noting that the oil seepage observed along Mechala river bed is within the Wereilu 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 quali?es to be appropriate for

further

Fig.14.Geologic model for the Wereilu basin obtained from 3-D inversion of the gravity ?eld with depth slices in the vertical and horizontal directions (a &b)and cutout depth section (c).

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906

exploration work to determine the economic presence and extent of the hydrocarbon accumulation as evidenced by the seepage.

As the results obtained from gravity data are generally non-unique attempts have been made in this study to reduce the non-uniqueness by incorporating as many analysis as possible and trying to accommodate information obtained from geology.Addi-tional information from other geophysical methods and future borehole data will de?nitely improve the geologic model given in this study.

Acknowledgements

The gravity data could not have been collected without the unreserved assistance and support of the following people:Abdella Kelil,Alemayehu Berhe,Feven Solomon,Mikias Wolde Selassie, Mualtu Tumoro,Olado Ollo,Biniyam Zimam,Zecharias Alemu, Abdulahi Sultan,Ababu Belachew,Eregetu Eyew,Berehane Meskel Mengesha,Araghe and Awol.I owe them many thanks.I am also thankful to Dr.William Bosworth and three anonymous reviewers for their constructive comments that have greatly improved the manuscript.

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