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Journal of Petroleum Geology, Vol. 30(4), October 2007, pp 389-402 389 SOURCE ROCK POTENTIAL OF THE BLUE NILE (ABAY) BASIN, ETHIOPIA A. Wolela* The Blue Nile Basin, a Late Palaeozoic – Mesozoic NW-SE trending rift basin in central Ethiopia, is filled by up to 3000 m of marine deposits (carbonates, evaporites, black shales and mudstones) and continental siliciclastics. Within this fill, perhaps the most significant source rock potential is associated with the Oxfordian-Kimmeridgian Upper Hamanl
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  389  Journal of Petroleum Geology, Vol. 30(4), October 2007, pp 389-402 ©2007 The Authors. Journal compilation©2007 Scientific Press Ltd SOURCE ROCK POTENTIAL OF THEBLUE NILE (ABAY) BASIN, ETHIOPIA A. Wolela* The Blue Nile Basin, a Late Palaeozoic – Mesozoic NW-SE trending rift basin in central Ethiopia,is filled by up to 3000 m of marine deposits (carbonates, evaporites, black shales and mudstones)and continental siliciclastics. Within this fill, perhaps the most significant source rock potential isassociated with the Oxfordian-Kimmeridgian Upper Hamanlei (Antalo) Limestone Formation whichhas a TOC of up to 7%. Pyrolysis data indicate that black shales and mudstones in this formationhave HI and S 2 values up to 613 mgHC/gCorg and 37.4 gHC/kg, respectively. In the Dejen-Gohatsion area in the centre of the basin, these black shales and mudstones are immature for the generation of oil due to insufficient burial. However, in the Were Ilu area in the NE of the basin, theformation is locally buried to depths of more than 1,500 m beneath Cretaceous sedimentary rocks and Tertiary volcanics. Production index, T  max, hydrogen index and vitrinite reflectancemeasurements for shale and mudstone samples from this areas indicate that they are mature for oil generation. Burial history reconstruction and Lopatin modelling indicate that hydrocarbons havebeen generated in this area from 10Ma to the present day.The presence of an oil seepage at Were Ilu points to the presence of an active petroleumsystem. Seepage oil samples were analysed using gas chromatography and results indicate thatsource rock OM was dominated by marine material with some land-derived organic matter. ThePr/Ph ratio of the seepage oil is less than 1, suggesting a marine depositional environment. n-alkanes are absent but steranes and triterpanes are present; pentacyclic triterpanes are moreabundant than steranes. The black shales and mudstones of the Upper Hamanlei LimestoneFormation are inferred to be the source of the seepage oil.Of other formations whose source rock potential was investigated, a sample of the PermianKarroo Group shale was found to be overmature for oil generation; whereas algal-laminated  gypsum samples from the Middle Hamanlei Limestone Formation were organic lean and had littlesource potential  *Department of Petroleum Operations, Ministry of Minesand Energy, PO Box 486, Addis Ababa, Ethiopia.email: Wolela_am@yahoo.com INTRODUCTION The NW-SE trending Blue Nile (or Abay) Basin is aLate Palaeozoic - Mesozoic intracratonic rift basincovering an area of about 120, 000 sq. km in centraland NW Ethiopia (Fig. 1). The sedimentary successionin the basin reaches a maximum thickness of 3,000m. In previous studies, the Oxfordian-KimmeridgianUpper Hamanlei (Antalo) Limestone Formation has been investigated as a potential source rock (Wolela,1997; 2002; 2004). Oil seepages occur in the bed of the Mechela River near Were Ilu in the NE of the basin (Fig. 2), indicating the presence there of anactive petroleum system. However, little informationis available regarding potential source rocks or geochemical characteristics of the Were Ilu seepageoil.This paper reports on the hydrocarbon potentialof the Blue Nile Basin, focussing in particular on potential source rocks (black shales and mudstones)in the Upper Hamanlei Limestone Formation. Thegeochemical characteristics of the Were Ilu seepageoil are also briefly reviewed. Key Words: Blue Nile Basin, source rocks, Upper HamanleiLimestone Formation, Ethiopia, Were Ilu, oil seepage.  390 Source rock potential of the Blue Nile Basin, Ethiopia Previous studies of the Blue Nile Basin includeGetaneh (1980, 1981, 1991) who described the claymineralogy and lithostratigrapy of the MiddleHamanlei (Gohatsion) Formation and the EarlyCretaceous Mugher Mudstone and overlying DebreLibanose Sandstone Formations. Assefa and Wolela(1986) reported on coal occurrences in the Getema(Arjo) area in the SW (Fig. 2). Mohr (1962) andSerawit and Tamrat (1995, 1996) investigated thegeology of the Jimma River and Gundo Meskel-Ejereareas, while Tamrat and Tibebe (1997) studied theGendeberet-Jeldu and Amuru-Jarty areas. Wolela(1997, 2002, 2004) investigated aspects of theevolution and hydrocarbon potential of the Blue NileBasin. GEOLOGICAL BACKGROUNDStratigraphy (Fig. 3) Basement rocks in the Blue Nile Basin consist of Precambrian basic to acidic rocks (Kazmin, 1975).These are overlain unconformably by a Permo-Triassic“Karroo” succession around 450 m thick, generallyinterpreted to have been deposited in alluvial fan andfluviatile settings (Wolela, 1997). In contrast to theKarroo succession in the Ogaden Basin (Tamrat andAstin, 1992), up to 200 m of Karroo rocks may have been erosively removed in the Blue Nile Basin(Wolela, 1997). The Karroo succession isunconformably overlain by the up to 850 m thick,fluviatile-dominated Triassic-Liassic AdigratSandstone Formation, composed of conglomerates,sandstones, siltstones and mudstones. The formationis 450 m thick at Dejen-Gohatsion, 850 m thick atAmuru Jarty, 750 m thick at Fincha River, 200 m thick in the Getema (Arjo) area, and 150 m thick in Ejeraarea (Assefa and Wolela, 1986; Serawit and Tamrat,1996; Tamrat and Tibebe, 1997). The upper part of the formation is composed of alternating carbonaceousmudstones, carbonaceous siltstones and sapropeliccoals (Assefa and Wolela, 1986; Wolela, 1991). Coal- bearing sediments are interpreted to have beendeposited in lacustrine depositional environments.Palynomorphs include Corollina spp., Caamosporatender  ,  Dictyophyllidites mortonii and  Exesipollenitestumulus . Overlying the Adigrat Sandstone is a 50 m 3638404244464816141210864 °°°°°°°°°° BlueNileBasin E T H I O P I A OgadenBasin SOMALIAKENYASUDANSUDANERITREAREDSEAGULF OF ADEN °°°° MainEthiopianRift Gambellabasin   Legend Miocene and later riftBlue Nile Basin SAUDI ARABIA WhiteNileRift YEMEN MekeleBasin Fig. 1. Location map of theBlue Nile Basin, central-NWEthiopia.  391  A. Wolela GendebertFinchaDejen Addis AbabaWeliso AmboNekemte Arjo    F   i  n  c   h  a   R   i  v  e  r 36°38°39°40°9°10°11°11°10°9°37° Abay River  Abay River Jimma River    GohatsionDebreMarkose36°37°38°39°40° 050100km TilliDebreBirhanWere IluLegendBichenaPrecambrianbasement rocksPalaeozoic-sedimentary rocks Mesozoic Volcanic rocksFaultOil seepageSampled areas N ContactWeleka River     C   E   N   O   Z   O   I   C    T  e  r   t   i  a  r  y Quaternary    P  a   l  a  e  o  g  e  n  e  Alluvium and volcanic Palaeocene-Miocene1100Volcanic    M   E   S   O   Z   O   I   C    C  r  e   t  a  c  e  o  u  s   L  o  w  e  r    U  p  Aptian-CenomanianDebre LibanoseSandstone240Portlandian-AptianMuger Mudstone320    J  u  r  a  s  s   i  c   L  o  w  e  r   M   i   d   d   l  e   U  p  p  e  r PortlandianTransitional Facies30 Oxfordian-Kimmeridgian Upper HamanleiLimestone720Bathonian-OxfordianMiddle Hamanlei(Gohatsion)Limestone350LiasTransitional Facies50    T  r   i  a  s  s   i  cU  p  p  e  r    M   i   d .   L  o  w  e  r Lower Triassic-Lias AdigratSandstone850Lower Triassic toUpper Paleozoic-Upper Permian450Basement rocksEraPeriodEpochFormation P R E C A M B R I A N    N  e  o  g  e  n  e Miocene-PlioceneVolcanicLith-ologyHydrocarbonsystemcomponentsSealLithology andenvironmentFluviatile sandstonelwith intercalation of siltstoneFluviatile mudstonewith intercalation of siltstone and sandstoneFluviatile and marinefaciesShelf marine limestonewith intercalationof shalesTidal flat and fluvatilegypsum,mudstonesiltstone, and limestoneFlvial and marinefacies Alluvial fan andfluviatile sandyconglomerate,sandstone andsiltstoneFluviatile sandstoneand shalesReservoir Reservoir SourceSealReservoir Reservoir/source (?) MaximumThicknessin (m) KarroosedimentsPalae-ozoicBasalt, trachyte,rhyolite withbeds of tuff  Fig. 2. Geological map of theBlue Nile Basin (simplifiedafter Kazmin, 1972).Fig. 3. Chrono- andlithostratigraphy of theBlue Nile Basin.  392 Source rock potential of the Blue Nile Basin, Ethiopia thick Transitional Zone with shales, limestones,sandstones, dolostones and evaporites (Fig. 3).This is overlain by the transgressive Middle andUpper Hamanlei Limestone Formations which reach amaximum thickness of 1,140 m, thickening towardsthe north and NE. The Bathonian-Oxfordian MiddleHamanlei Limestone Formation (420 m thick) iscomposed of dolostones, gypsum, mudstones, marlsand shales, with common algal stromatolites, greenalgae, foraminifera, gastropods and bivalves. TheUpper Hamanlei Limestone Formation (Oxfordian – Kimmeridgian), up to 720 m thick in the Blue NileBasin, was deposited during a major regionaltransgression which covered the whole of East Africa(cf. Bosellini, 1989; Russo et al  ., 1994). The formationis composed of limestones (skeletal packstone-wackestones, oolitic-skeletal packstones) alternatingwith black mudstones and black shales. Bioclastsinclude brachiopods, corals, algae, gastropods andechinoids. Intervals at least 30-50 m thick, composedof alternating beds of black mudstones, black shalesand limestones are exposed at Dejen and Jimma River,respectively (locations in Fig. 2). The black mudstones,shales and limestones are equivalent to the Agula Shaleof the Mekele Outlier (basin) in northern Ethiopia (Fig.1: Beyth, 1972), and to the Urandab Formation in theOgaden Basin (Raaben et al  ., 1979; Hunegnaw et al  .,1998).Overlying the Upper Hamanlei Limestone is theLower Cretaceous Muger Mudstone Formation (320m thick), and the Aptian-Cenomanian Debre LibanoseSandstone Formation (420 m thick) (Getaneh, 1991)(Fig. 3). Some 50 m of section is assumed to have beenerosively removed before the onset of Tertiary volcanicflows and trap volcanism dated at 49 Ma (Grasty et al  ., 1963). Structural history  NE Africa has undergone several phases of riftingduring the Phanerozoic. A “Karroo” phase (LateCarboniferous to Triassic) led to the formation of north-south, NW-SE and NE-SW oriented rift basinsincluding the Ogaden and Blue Nile Basins (Raaben,1979; Tamrat and Astin, 1992; Gebre Yohanse, 1989;Hunegnaw et al  ., 1998). Associated basinal depositsare known as the Karroo Group. The Blue Nile Basinis interpreted as a NW-SE trending failed arm of theKarroo rift system (Russo et al  ., 1994; Korme et al  .,2004) (Fig. 1). A second phase of rifting occurred inthe Early to Middle Jurassic. A coeval marinetransgression resulted in the deposition of marinesediments in the Ogaden, Blue Nile, Southern Red Seaand Mandwa areas including carbonates, evaporitesand siliciclastics (Wolela, 1997; Hunegnaw et al  ., 1998;Bunter  et al  ., 1998). During the Miocene, normal fault blocks developed, possibly reactivated along NW-SEtrending Karroo Rift trends. These are exposed inthe Bichena, Fincha, Dejen-Gohatsion and AbayRiver areas (Fig. 2).Beginning at the end of the Cretaceous, rifting began in the Gulf of Aden area and ultimately resultedin the formation of the Red Sea and the MainEthiopian Rift (McConnel, 1972; Kent, 1974; Bunter  et al  ., 1998; Korme et al  ., 2004) (Fig. 1). The Karroorift system is thus dissected by the Main EthiopianRift which separates the Blue Nile Basin from theOgaden Basin. The NE-SW and north-south to NNE-SSW trending fault systems of the Main EthiopianRift are exposed in the western rift escarpment inthe eastern part of the Blue Nile Basin (Fig. 2). MATERIALS AND METHODS Thirty one samples of potential source rocks werecollected from the Karroo Group, Adigrat SandstoneFormation, Middle Hamanlei Limestone Formationand Upper Hamanlei Limestone Formation. Thesamples came from the Dejen, Gohatsion, GendeBeret, Fincha, Weleka, Jimma and Arjo areas (Table1) and comprised (i) dark grey Karroo shale (onesample), (ii) lacustrine sapropelic coals from theupper part of the Adigrat Sandstone Formation (twosamples); (iii) algal-rich gypsum from the MiddleHamanlei Limestone Formation (three samples); and(iv) limestones and black shales and mudstones fromthe upper part of the Upper Hamanlei LimestoneFormation (25 samples). All samples were collectedfrom outcrop exposures as there are no wells in the basin.The samples were cleaned in an ultrasonic water  bath. Crushed black shale and mudstone sampleswere placed in a crucible and pyrolyzed at 300°C for 3 min, followed by programmed pyrolysis at 25°C/minute to an optimum temperature of 600°C in ahelium atmosphere. Standard S 1 , S 2 , S 3 and T max measurements were recorded with a  Rock-Eval II  instrument, as were the hydrogen index (HI) andoxygen index (OI). The production index (PI),defined as the ratio S 1 /(S 1 + S 2 ) (Peters, 1986), wasalso determined.Gold-coated black mudstones from the Upper Hamanlei Limestone Formation were examinedunder a  JEOL 6400 scanning electron microscopeequipped with an energy dispersive X-ray analysis(EDX) system with accelerating voltage 10 to 15 kV,to study the mineral composition and distribution of authigenic minerals. Other samples were mountedon aluminium pin stubs and polished to 0.5 μ m for visual analysis. A  Nicrphot-Fxa reflectancemicroscope attached to a  Fiber  optic light source,light collecting Pi, monochromatic filter, oilimmersion lenses (10x, 20x, 40x, 60x), and oil
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