Diagenetic evolution of the Ansian–Pliensbachian Adigrat Sandstone, Blue Nile Basin, Ethiopia

Journal of African Earth Sciences 56 (2010) 29–42

Contents lists available at ScienceDirect

Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci

Diagenetic evolution of the Ansian–Pliensbachian Adigrat Sandstone, Blue Nile Basin, Ethiopia A. Wolela Department of Petroleum Operations, Ministry of Mine and Energy, Kotebe Branch Office, P.O. Box 486, Addis Ababa, Ethiopia

a r t i c l e

i n f o

Article history: Received 1 September 2008 Received in revised form 12 May 2009 Accepted 20 May 2009 Available online 7 June 2009 Keywords: Adigrat Sandstone Blue Nile Basin Diagenesis Ethiopia Triassic-Jurassic

a b s t r a c t The Blue Nile Basin is located in the Central Plateau of Ethiopia. The basin consists of Precambrian basement, Palaeozoic and Mesozoic sedimentary rocks and Tertiary volcanic rocks. The sedimentary successions and Adigrat Sandstone reach a maximum thickness of 3000 and 800 m, respectively. The formation is composed of mudstone, finely laminated siltstone, very fine-grained cross-bedded sandstone, coarse to medium-grained sandstone, massive to crudely cross-bedded gravely sandstone and massive to crudelybedded conglomerate. In the Blue Nile Basin, the Adigrat Sandstone was deposited in alluvial fan, fluviatile and lacustrine depositional environments. The formation has a complex diagenetic history and cemented by silica, carbonate, kaolinite and hematite with minor amounts of dolomite, illite, chlorite and feldspar overgrowths. Depositional environment, burial history and diagenetic processes are the major factors, which control the porosity and permeability of the Adigrat Sandstone. Primary porosity is preserved due to framework grain stability. Dissolution of carbonate cements created a certain amounts of secondary porosity in medium to coarse-grained sandstones, siltstones and mudstones facies. The porosity and permeability reach up to 20.4% and 710 mD, respectively. The medium-coarse-grained sandstones are porous and potential for oil and gas reservoir, whilst low-permeability siltstones and mudstones are possible gas reservoir. Ó 2009 Published by Elsevier Ltd.

1. Introduction Porosity, permeability and other reservoir properties of sandstones are the results of both primary depositional environment and diagenetic alteration (Bjørlykke et al., 1989). Primary porosity can be reduced by chemical cementation, mechanical compaction and pressure solution. Secondary porosity can be created through dissolution of unstable framework grains and cements as well as carbonate and framework leaching by meteoric waters during shallow burial (Hayes, 1979; Schmidt and McDonald, 1979a; Bjørlykke, 1983, 1993; Worden and Burley, 2003). Reconstruction of the diagenetic history of sandstones helps to understand the evolution of porosity. The Adigrat Sandstone lies either on the Precambrian basement or the Palaeozoic sedimentary rocks (Mohr, 1962; Kazmin, 1975; Getaneh, 1991; Wolela, 1997). The Adigrat Sandstone reaches a maximum thickness of 800 m (Amuru-Jarty area), and pinches out between the Precambrian basement rocks and the Tertiary volcanics in south-western part of the basin (Assefa and Wolela, 1986; Tamrat and Tibebe, 1997). Petroleum exploration in the Blue Nile Basin targeted the Adigrat Sandstone, the Debre Libanose Sandstone and the Antalo

E-mail address: [email protected] 1464-343X/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.jafrearsci.2009.05.005

Limestone as a reservoir objective. Hence, it is important to evaluate thoroughly the sedimentation, and diagenetic evolution of the Adigrat Sandstone, which in turn help to identify the most favourable areas for petroleum accumulation. Only scanty information is available regarding the diagenetic evolution of the Adigrat Sandstone. The diagenetic history is established from a combination of transmitted light microscopy, and electron microscopy studies. This paper discusses the diagenetic evolution of the Adigrat Sandstone. 1.1. Background geology Basement rocks in the Blue Nile Basin consists of Precambrian acidic to basic rocks including quartzites, granites, granodiorite gneisses, hornblende-biotite gneisses, diorite, metasediments and metavolcanics (Kazmin, 1975; Wolela, 1997, 2002). These are overlain unconformably by a Permo-Tiassic ‘‘Karroo” succession around 450 m thick (Fincha area), interpreted as alluvial fan and fluviatile deposits (Wolela, 1997). Contrasting with the Karroo succession in the Ogaden Basin (Worku, 1988; Worku and Astin, 1992), up to 200 m of Karroo rocks may have been erosively removed in the Blue Nile Basin (Wolela, 1997). The Karroo succession is unconformably overlain by the up to 800 m thick (Amuru-Jarty area), fluviatile-dominated Adigrat Sandstone (Fig. 3). This is composed of conglomerates, sandstones,

30

A. Wolela / Journal of African Earth Sciences 56 (2010) 29–42

siltstones and mudstones. In the Blue Nile Basin, the Adigrat Sandstone is 450 m thick at Dejen, 800 m thick at Amuru-Jarty, 750 m thick at Fincha area, 200 m thick in the Arjo area and 150 m thick in Ejere area (Assefa and Wolela, 1986; Serawit and Tamrat, 1996; Wolela, 1997; 2006). The upper part of the Adigrat Sandstone is composed of alternating carbonaceous mudstones, carbonaceous siltstones and sapropelic coals (Assefa and Wolela, 1986; Wolela, 1991). Coal-bearing sediments are interpreted to have been deposited in lacustrine depositional environments. Palynomorphs include Corollina spp., Caamospora tender, Dictyophyllidites mortonii and Exesipollenites tumulus. Overlying the Adigrat Sandstone is a 50 m thick Transitional facies (Gohatsion area) with alternating shales, limestones, sandstones, dolostones and evaporites (Fig. 3). This is overlain by the transgressive Gohatsion Formation and Antalo Limestone which reach a maximum thickness of 1140 m in the Blue Nile Basin. The Gohatsion Formation and Antalo Limestone thicken towards the north and northeast. The Bathonian-Oxfordian (420 m thick) Gohatsion Formation in the Dejen area is composed of dolostones, gypsum, mudstones, marls and shales, with common algal stromatolites, green algae, foraminifera, gastropods and bivalves. In the Blue Nile Basin, during the early Callovian – early Oxfordian, a major transgression covered the whole of East Africa and led to the deposition of up to 720 m thick Antalo Limestone in Weleka River section (cf. Bosellini, 1989; Bosellini et al., 1997; Russo et al., 1994; Wolela, 1997). This formation is composed of limestones (skeletal packstone-wackestones, oolitic-skeletal packstones) alternating with black mudstones and black shales. Bioclasts include brachiopods, corals, algae, gastropods and echinoids. Overlying the Antalo Limestone is the Lower Cretaceous Mugher Mudstone (320 m thick), and the Barremian-Cenomanian Debre Libanose Sandstone (280 m thick) (Getaneh, 1991; Wolela, 1997) (Fig. 3). Some 50 m of erosion is assumed to have occurred before the onset of Tertiary volcanic (basaltic) flows. The basalts that overlies the sedimentary sequences in the Dejen and Gohatsion areas revealed 26.9–29.4 Ma years, approximately equivalent to the Oligocene (Hofmann et al., 1997; Kieffer et al., 2004), and coeval to the radiometric age of basalts in South-western Plateau of Ethiopia (cf. Davidson and Rex, 1981). (Fig. 4)

At the end of the Cretaceous, rifting began in the Gulf of Aden area and led to the formation of the Red Sea and the Main Ethiopian Rift (McConnel, 1972; Kent, 1974; Shackelton, 1978; Bunter et al., 1998; Korme et al., 2004). The Karroo rift system is thus dissected by the Main Ethiopian Rift which separates the Blue Nile Basin from the Ogaden Basin. The northeast–southwest and north– northeast to south–southwest trending fault systems of the Main Ethiopian Rift are exposed in the western rift escarpment in the eastern part of the Blue Nile Basin (Wolela, 2006). 1.3. Sedimentation The Adigrat Sandstone is widely distributed in the Blue Nile Basin, Ogaden Basin and the Mekele Outlier (Beyth, 1972; Getaneh, 1991; Russo et al., 1994; Bosellini et al., 1997; Hunegnaw et al., 1998). It covers an extensive area in the Blue Nile Basin and forms vertical cliff exposures in Dejen, Gohatsion, Amuru-Jarty, Fincha, Gendebret-Jeldu and Ejere (near Jema River Bridge) (Figs. 1 and 2). In the Blue Nile Basin, the Adigrat Sandstone can be divided into six major facies based on sedimentary structures, grain size and lithofacies association: (1) mudstone, (2) finely laminated siltstone, (3) very fine-grained cross-bedded sandstone (4) coarse to medium-grained sandstone, (5) massive to crudely cross-bedded gravely sandstone and (6) massive to crudely-bedded conglomerate. The Adigrat Sandstone resulted from denudation and peneplanation of the Precambrian basement rocks and pre-existing Palaeozoic sediments. The north-western Ethiopian highlands were the possible provenance area for the sedimentation. Considering the overall characteristics, the Adigrat Sandstone is dominated by a mixture of alluvial fan, meandering river and lacustrine deposits

1.2. Structural history NE Africa has been affected by several phases of rifting. The first ‘‘Karroo” phase (Late Carboniferous to Triassic), accompanying initial break-up of Gondwanaland, led to the formation of north–south, northwest–southwest and northeast–southwest oriented rift basins including the Ogaden and Blue Nile Basins (Raaben et al., 1979; Worku and Astin, 1992; Gebre Yohanes, 1989; Salman and Abdula, 1995; Wolela, 1997; Hunegnaw et al., 1998). The basinal deposits are known as the Karroo Group. The Blue Nile Basin is interpreted as a northwest–southeast trending failed arm of the Karroo rift system (Russo et al., 1994; Wolela, 1997; Ketela, 2007). A second phase of rifting in the Early to Middle Jurassic marked the beginning of the disintegration of Gondwanaland into separate blocks. At the same time, a marine transgression resulted in the deposition of marine sediments in the Ogaden Basin, Blue Nile Basin, Southern Red Sea areas, in which carbonates, evaporites and clastic sediments were deposited (Salman and Abdula, 1995; Wolela, 1997; Hunegnaw et al., 1998; Bunter et al., 1998). During Miocene, normal fault blocks developed, possibly reactivated along northwest–southeast trending Karroo Rift trends. These are exposed in the Bichena, Fincha, Dejen areas and Abay River (Wolela, 2008).

Fig. 1. Location map of the studied area in the Blue Nile Basin.

A. Wolela / Journal of African Earth Sciences 56 (2010) 29–42

31

Fig. 2. Geological map of the Blue Nile Basin (as fault omitted) (modified after Kazmin, 1972).

(Wolela, 2008). The conglomerate and gravely sandstone facies are interpreted as alluvial fan sediments. The basal conglomerate passes up vertically into gravely sandstone, possibly indicating proximal to distal alluvial fan sedimentation. The Karroo Rift system favoured the accumulation of the alluvial fan. Coarse to medium-grained sandstone siltstones and mudstones are interpreted collectively as meandering river deposits (channels, point bars and flood-plain fines). The presence of several upwardfining sequences, flood-plain sediments, cyclic sedimentation, thickness/width ratio, lateral accretion surfaces, suspension-load/ bed-load ratio and calcrete are indicators of meandering river sedimentation (Wolela, 2008). The presence of carbonaceous sediments (shales, claystones, mudstones and coal seams) suggests reducing environment, possibly a lacustrine depositional environment. In humid areas, floodplains contain swamps and/or lakes that are usually covered with dense vegetation, which may develop into coal seams (Wolela, 1991). Detail of the sedimentation of the Adigrat Sandstone is given in Wolela, 2008. 2. Methodology Sections were logged and samples were collected from the Adigrat Sandstone. Samples were cleaned in an ultrasonic water bath.

All sandstone samples were impregnated with blue resin (bluedyed araldite) for petrographic study to highlight porosity. Resinimpregnated thin sections were examined using a transmitted light microscope, and their modal composition and porosity evaluated by point counting 500 points per thin section. Gold-coated chip samples were examined under a JEOL 6400 scanning electron microscope equipped with energy dispersive X-ray analysis (EDX) system with accelerating voltage 10–15 kV, to study the morphology, mineral composition, distribution and paragenesis of the authigenic minerals and pore-throat geometry. Cathodoluminescence studies were conducted on carbon-coated polished thin sections. A CL detector attached to a JEOL 6400 with an accelerating voltage of 20 kV to differentiate the quartz grains from their overgrowths. Quantitative analyses were carried out using a JEOL 733 superprobe with an accelerating voltage of 15 kV, probe current of 1  10 8 A and spot size 1 lm to identify mineral composition, mineral transformation and zoning in cementing minerals. A ZAF software programme was used to calculate weight percentage of the oxides in each analysis. Vertically and horizontally cut samples were examined for permeability under a Stimlab System Limited EPS FPP 300 Field Probepermeameter. The probe is pressed against the rock surface with a controlled force. At a constant pressure, nitrogen is passed through

32

A. Wolela / Journal of African Earth Sciences 56 (2010) 29–42

Fig. 3. (a) Chrono and lithostratigraphic section of the Blue Nile Basin (data from Getaneh, 1991; Wolela, 1997; 2002; 2006). (b) Texture and sedimentary structures and depositional environments of the Adigrat Sandstone, Dejen section.

a small opening into the probe and flow rate is recorded when a steady state is reached. Leakage between the rock and the probe is prevented by an elastic closed rubber seal. The inlet opening of the probe radius is 1 mm. The measured flow rate is converted into permeability using calibration curves.

3. Framework composition The average mode of 63 sandstone samples from the Adigrat Sandstone is Q92.1 F7.5 L0.9. Monocrystalline in type (80–90%) predominate polycrystalline quartz. The monocrystalline quartz re-

33

A. Wolela / Journal of African Earth Sciences 56 (2010) 29–42

vealed straight to undulose extinction. The polycrystalline quartz grains exhibited curved to sutured boundaries. The presence of monocrystalline and polycrystalline quartz grains indicates possible derivation from composite sources (volcanic, metamorphic and pre-existing sediments). The replacement quartz grains and overgrowths margin by carbonate cements contributed to the angularity of the quartz grains. Rounded clay-coated grains indicate the original high-sphericity of quartz grains. XRF results of the quartz arenite samples revealed up to 91.6%-SiO2. Quartz arenite in terms of chemical composition are generally distinctive in containing >80% SiO2 (Condie et al., 1992) (see Table 1). Feldspar grains are also one of the important components, predominantly of K-feldspar (orthoclase and microcline) and minor amounts of plagioclase. Some feldspar grains revealed partial or complete alteration. Extensive alteration and dissolution of feld-

spars and replacement by carbonate and clay minerals indicated that feldspars were formerly more abundant. Moldic porosity and honeycombed grains are results of complete and partial dissolution. Rock fragments are uncommon in the Adigrat Sandstone. Detrital mica and heavy minerals (zircon, rutile, ilmenite and titanium oxide) exist as accessory minerals. The Adigrat Sandstone is a coarse-to-fine-grained, sub-angular to rounded, modertly sorted quartz arenite, sub-arkose and arkose (cf. Folk, 1968) (see Fig. 4). 4. Diagenetic sequences The Adigrat Sandstone has a complex diagenetic history. The authigenic minerals reflect changes in the pore water chemistry. The relative sequence of the diagenetic events is shown in Fig. 5. The following paragenetic sequence was established from the stud-

Table 1 Texture, framework grains, cementing minerals and porosity of the Adigrat Sandstone. Sample no

Locality

Location on map 2

WA-10 Wa-11 WA-12 WA-13 WA-14 BN-01 BN-02 BN-03 BN-04 BN-05 BN-06 BN-07 BN-08 ABSS-1 ABSS-2 ABSS-3 ABSS-4

Dejen

A

BN-09 BN-10 BN-11 BN-12 WA-25 Wa-26 WA-27 WA-9 ABBSS-1 ABBSS-2 ABBSS-3

Gohatsion

FISS-1 FISS-2 FISS-3 ABFISS-4 ABFISS-5 ABFISS-6 ABFISS-7 ABFISS-8

Fincha

ABESS-1

Ejere (Jema River Bridge)

H

ABGESS-1 BN-13 BN-14 BN-15 BN-16

Gendebert-Jeldu Arjo

8 1

Ambo-1 Ambo-2 Ambo-3 Ambo-4

Ambo

B

C

D

E

F

G

ABESS-2

J

Texture

Framework grain Roundness

Sorting

Qtz

m-c m f-m f f f f-c f f-m m-c m m m f-m m-c f-m c

r s.r s.a r r s.a s.a s.r s.r s.a sa s.r s.r s.r s.r s.r s.a-s.r

w m m w w w p w m m w m m w p w p

76.0 68.0 65.2 73.0 66 61.5 58.5 70.5 79.5 58 61.5 66.4 78.5 35 65 30 36

2.8 3.2 5.8 2 9 8.5 6.5 8.4 12.5 16 5.8 43 2 59 10

m-c c f-m f-m m m-c m f f m-c f

s.r s.r s.r s.a s.r s.r s.r s.r s.a s.r s.r

m m m p m m m m p m m

82.6 87.2 71.2 90.0 91.6 73.4 64.4 56.3 24.2 64.5 64

8.2 9.8 1 5.6 0.4 7.8 6.2 3.2 30.1 15.2 5

0.2 0.6 1.2 1.1 10

m-c f-m f-m f m-c f f-m m-c

s.r-s.a s.a s.a s.r s.a-s.r s.a s.r s.a

p p m m p m m p

47.4 40 60.3 40 47 45 37 30.4

2.1 4 2.4 5 10 10 20 18.2

2.5 10 5.3 5 5 10 2.6 2

f-m

s.r

m

35

17

f-m

s.r

m

35.6

5.2

m-c m-c f-c f-m f

s.r s.r s.r s.r a-s.a

w m m m m

85 67.8 63.5 78 29

5 5.2 5.5 12 30

f m m m

a-s.a a-s.a s.a-s.r s.r-s.a

m m m w

25 25.6 25 70

45 46.2 40 5

Qtz = quartz, Fsp = feldspar, Rfg = rock fragment. Opq = opaque minerals, Cal = calcite, Dol = dolomite, Hem = hematite.

Fsp

Mica

Grain size

Opa

Clay

Rfg

0.8 0.4 0.2

2.5 2.5 1.5 1.5 3 1 0.4 1.8 10 3 2 4

1 0.6 2.8 1.2 0.8

1.2 2 1.2

5.4

0.6

1 1 2 3.2 1

Cementing minerals Cal.

Dol.

9.8 6.8 8 26.8 26 22 14.5

0.2

13.4 4 1 9.6

7.6 15 14.6 5

4

4 8 5 11 3.6 4 5 8.4 10 15 8 10

0.5 20 16

6 3

9 4.5 1

1

15 1 5 0.8 0.4

5

30 4

2.2 13.2 1.4

5

1 0.6 2 6.4

0.6

1

1.2

1

1

7.1

1.5

5.2 2 4

0.6

17.5 27.1

2.9

4.4 0.4 14.6 3 3 6.8

5.8

34.5 9 10

10 8 11

0.2

30 40 20 40 30 30 20 7

29.2

15 5 3 5 7 5 20.4 13

1

27

4

10

5

26

20

21.4 20 2.5 1

10 5 10 5.5 20

3 1 9 4 1

1.1 0.6 1 0.5 2 2 3

4 7.6

10

Porosity

Hem.

1

19

1

6 4 5 4

5 5

21 17 20 12

34

A. Wolela / Journal of African Earth Sciences 56 (2010) 29–42

Fig. 4. Ternary plots for Adigrat Sandstone in the Blue Nile Basin based upon quartz, feldspar and rock fragments (after Folk, 1968), and based upon tectonic setting (after Dickinson and Suczek, 1979). Quartz, feldspar and rock fragments data are changed to 100%.

ied sandstones (1) early concretionary calcite, (2) grain dissolution, (3) illite and/or hematite grain-coating, (4) quartz overgrowths, (5) feldspar overgrowths, (6) calcite cementation, (7) dolomite cementation, (8) mechanical compaction, (9) cement and unstable framework grains dissolution, (10) kaolinite precipitation, (11) euhedral quartz crystals, (12) hematite precipitation, (13) illite precipitation and (14) chlorite precipitation. Minor amounts of barite cement were identified by microprobe analysis. The paragenetic context of barite is not clear. Barite is possibly derived from either alteration of plagioclase and K-feldspar or from the overlying gypsum and limestone-rich Gohatsion Formation. Each component and its significance in the diagenetic sequence are addressed separately below with descriptions, photos and geochemical data (Figs. 5–10).

4.1. Concretionary calcite cement Concretionary calcite are found sporadically throughout the entire section. SEM studies and EDX identification revealed the presence of small scattered concretions 30–100 lm in diameter. Concretionary calcite is considered to be the earliest environment-related mineral. It occurs as patchy distribution around the quartz and feldspar grains (Fig. 7a). 4.2. Grain dissolution Dissolution of unstable framework grains took place at early diagenesis when unsaturated gravity-driven meteoric water was introduced into the diagenetic environment. The dissolution of

Fig. 5. Burial history and major diagenetic sequences of the Adigrat Sandstone, Blue Nile Basin.

A. Wolela / Journal of African Earth Sciences 56 (2010) 29–42

35

Fig. 6. (a) Thin section photomicrograph showing grain-coating illite–hematite (I/H), quartz overgrowths (Qo), porosity (P) and late-stage hematite cementation (H). Note extensive syntaxial quartz overgrowths. Triassic-Jurassic Adigrat Sandstone, WA-11, outcrop sample, Dejen section, Blue Nile Basin (field width 1.1 mm, plane light). (b) Thin section photomicrograph showing poikilotopic calcite cementation, (C) and quartz grains (Q). Triassic-Jurassic Adigrat Sandstone, WA-13, outcrop sample, Dejen section, Blue Nile Basin (field width 1.1 mm, cross-nicols). (c) Thin section photomicrograph showing poikilotopic calcite cementation (C), quartz grains (Q) and quartz overgrowths (Qo). Note quartz grains and quartz overgrowths replaced by calcite cement. Triassic-Jurassic Adigrat Sandstone, WA-14, outcrop sample, Dejen section, Blue Nile Basin (field width 1.1 mm, cross-nicols). (d) Thin section photomicrograph showing poikilotopic calcite (C), quartz grain (Q) and quartz overgrowths (Qo). Note replacement of calcite from the periphery towards the centre of the crystals. Triassic-Jurassic Adigrat Sandstone, WA-27, Gohatsion section, Blue Nile Basin (field width 1.1 mm, cross-nicols). (e) Thin section photomicrograph showing generation of secondary porosity (P) due to dissolution of pore-filling and replacive carbonate cements, quartz grains (Q). Note the corroded grain margins. Triassic-Jurassic Adigrat Sandstone, WA-11, outcrop sample, Dejen section, Blue Nile Basin (field width 1.1 mm, plane light). (f) Thin section photomicrograph showing late-stage hematite cementation in secondary pore spaces. Q = quartz, Qo = quartz overgrowths and H = hematite. Note extensive syntaxial quartz overgrowths. Triassic-Jurassic Adigrat Sandstone, WA-10, outcrop sample, Dejen section, Blue Nile Basin (field width, 1.1 mm, plane light).

unstable framework grains liberated ions of potassium, sodium, calcium, magnesium, aluminium, silicon and iron for the precipitation different minerals. 4.3. Early illite and/or hematite grain-coating Illite and/or hematite are considered to be the earliest authigenic minerals. They are well-developed around detrital grains as partial or complete coatings and show high birefringence around the grains (Fig. 6a). SEM studies and EDX identification also confirmed the presence of authigenic illite and/or hematite cementation. Illite and/or hematite grain-coating reduced primary porosity. 4.4. Quartz overgrowths Volumetrically, silica cement is one of the most abundant cementing minerals in the studied sandstones (Figs. 6a and 7a–c

and f). Quartz overgrowths account up to 8% of the total volume. As the pore water was supersaturated with Si4+ ions authigenesis of quartz overgrowths took place. Generally, quartz overgrowths tend to grow in optical continuity around the detrital grains. In places, these projections merged to produce large euhedral crystals (Figs. 6a and 7b). Cathodoluminescence microscopy helps to differentiate the overgrowths from the detrital grains (Fig. 7c). Quartz overgrowths exist in two forms as syntaxial overgrowths (Fig. 6a) and euhedral crystals (Figs. 7b and 8c) throughout the entire formation ranging from 5 to 70 lm in diameter and 10 to 100 lm in length. Fully developed euhedral quartz crystals indicate that the availability of pore spaces, and silicon ion supersaturated pore water. Some of the quartz overgrowths are partly corroded by calcite cement to develop irregular morphology (Fig. 6c and d). Quartz overgrowths are more common in the coarse to medium-grained sandstone than in the fine-grained sandstones and siltstones. Quartz overgrowths reduced porosity.

36

A. Wolela / Journal of African Earth Sciences 56 (2010) 29–42

Fig. 7. (a) SEM photomicrograph showing facies related concretionary calcite (C) and quartz overgrowths (Qo). Note the diagenetic sequence that the quartz crystal precipitated on concretionary calcite. Triassic-Jurassic Adigrat Sandstone, WA-12, outcrop sample, Dejen section, Blue Nile Basin (scale bar 100 lm). (b) SEM photomicrograph showing extensive quartz cementation (Qo) with well-developed euhedral hexagonal crystals. Triassic-Jurassic Adigrat Sandstone, WA-11, outcrop sample, Dejen section, Blue Nile Basin (scale bar 10 lm). (c) SEM-CL photomicrograph showing extensive quartz overgrowths (Qo) and quartz grains (Q). Triassic-Jurassic Adigrat Sandstone, WA-11, outcrop sample, Dejen section, Blue Nile Basin (scale bar 100 lm). (d) SEM photomicrograph showing pore-filling feldspar overgrowths and crystals (Fo) and illite (I). Triassic-Jurassic Adigrat Sandstone, WA-13, outcrop sample, Dejen section Blue Nile Basin (scale bar 10 lm). (e) Back-scattered electron photomicrograph showing replacive calcite cement (C), corroded quartz grains (Q) and hematite (H). Triassic-Jurassic Adigrat Sandstone, WA-27, outcrop sample, Gohatsion section, Blue Nile Basin (scale bar 100 lm). (f) SEM photomicrograph showing diagenetic sequence of authigenic minerals. Late-stage kaolinite (K) precipitated on early stage quartz crystals (Qo). Triassic-Jurassic Adigrat Sandstone, WA-10, outcrop sample, Dejen section, Blue Nile Basin (scale bar 10 lm).

4.5. Feldspar overgrowths K-feldspar overgrowth is the second abundant type of overgrowths in the Adigrat Sandstone. Feldspar overgrowths are rare compared to the quartz overgrowths. As the activities of K+, Al2+ and Si4+ increased, precipitation of feldspar took place. Feldspar overgrowths observed both in thin section and SEM occluding pore spaces. The authigenic K-feldspar occurs as overgrowths around the detrital feldspar grains (Figs. 7d and 8f) range from 5 to 45 lm in diameter and 10 to 100 lm in length.

part of the Adigrat Sandstone. Thin section and SEM studies revealed that calcite cementation occur as poikilotopic cement (Fig. 6b–d). Replacement of quartz and feldspar and their overgrowths by calcite cement generally took place either inwards from the grain boundaries or outwards from grain centre towards the margin of the grain (Fig. 6c and d). The replacement of quartz and feldspar overgrowths by calcite cement indicates late-stage authigenesis. The replacement of grains by carbonate cement resulted in grain corrosion. Calcite cementation reduced primary porosity.

4.6. Calcite cementation

4.7. Dolomite cementation

Volumetrically, calcite is the most important pore-filling carbonate cement in the studied sandstones, and accounts up to 5% of the total volume. It is well distributed in the lower and upper

Minor amounts of dolomite occur as pore-filling cement. It exists in the form of euhedral crystals ranging from 10 lm to 30 lm.

A. Wolela / Journal of African Earth Sciences 56 (2010) 29–42

37

Fig. 8. (a) SEM photomicrograph showing pore-filling booklet of kaolinite (K). Note the co-precipitation of booklet and pseudohexagonal kaolinite. Triassic-Jurassic Adigrat Sandstone, WA-10, outcrop sample, Dejen section, Blue Nile Basin. (scale bar 10 lm). (b) SEM photomicrograph showing pseudohexagonal kaolinite (K) filling secondary pore spaces. Note the diagenetic sequence that late-stage hematite (H) precipitated on kaolinite crystals (K). Triassic-Jurassic Adigrat Sandstone, WA-11, outcrop sample, Dejen section, Blue Nile Basin (scale bar 1 lm). (c) SEM photomicrograph showing intergranular pore-filling hematite (H) and late-stage authigenic quartz crystals (Qo). Note (1) corroded quartz grains (Q) and (2) late-stage quartz crystals precipitated on early stage quartz overgrowths (Qo). Triassic-Jurassic Adigrat Sandstone, WA-27, outcrop sample, Gohatsion section, Blue Nile Basin (scale bar 1 lm). (d) SEM photomicrograph showing booklets of kaolinite (K), incipient feldspar overgrowths (Fo) and incipient quartz overgrowths (Qo). Note (1) illitization (I) of kaolinite and (2) co-precipitation of booklets and pseudohexagonal kaolinite. Triassic-Jurassic Adigrat Sandstone, WA-10, outcrop sample, Dejen section, Blue Nile Basin (scale bar 10 lm). (e) SEM photomicrograph showing pore-filling illite (I) and chlorite (Ch). Triassic-Jurassic Adigrat Sandstone, WA-11, outcrop sample, Dejen section, Blue Nile Basin (scale bar 10 lm). (f) SEM photomicrograph showing pore-filling authigenic feldspar (Fo) and chlorite (Ch) and porosity (P). Triassic-Jurassic Adigrat Sandstone, WA-11, outcrop sample, Dejen section, Blue Nile Basin (scale bar 10 lm).

4.8. Mechanical compaction

4.10. Kaolinite precipitation

Mechanical compaction took place at a relatively late-stage diagenesis, after precipitation of major cementing minerals. It has a limited significance in the reduction of primary porosity. The original roundness of some of the grains was protected from destruction by earlier cementation.

Volumetrically, kaolinite is the most abundant type of authigenic clay mineral in the studied sandstones, and account up to 5%. It occurs as pore-filling cement forming dark brown patches in thin section. Kaolinite precipitation took place in the primary and secondary pore spaces as the pore water supersaturated with silicon and aluminium ions. Two morphotypes of kaolinite were identified. Kaolinite forms clusters of booklets (Fig. 8a) and well-developed euhedral, blocky crystals (Fig. 8b). The pseudohexagonal kaolinite plates range from 1 lm to 5 lm across and up to 8 lm in length. The authigenic kaolinite crystals were best developed in areas where authigenic feldspar is poorly developed.

4.9. Cement and grain dissolution Partial and complete dissolution of unstable grains and authigenic cements took place in the studied samples. The introduction of aggressive unsaturated acidic pore water resulted in the dissolution of carbonate cements and unstable framework grains to create secondary porosity (Fig. 6e and f).

38

A. Wolela / Journal of African Earth Sciences 56 (2010) 29–42

4.11. Euhedral quartz crystals SEM revealed the presence of small crystals of quartz with euhedral faces ranging from 1 lm to 10 lm in diameter (Fig. 8c). The faces of the secondary quartz crystals are fresh, not replaced by carbonate cements. 4.12. Hematite cementation Thin section and SEM studies revealed the presence of hematite cement as pore-filling cement (Figs. 6a and f and 8c). It is closely associated with platy kaolinite (Fig. 8b). Hematite distribution appears to be controlled by the enrichment of the pore water chemistry with respect to ions of iron and distribution of the pore spaces. Hematite cementation accounts for up to 2%. The late-stage hematite precipitation also occluded porosity. 4.13. Illite precipitation Minor amounts of illite occur as poor to well-crystallised lathlike blades and fibre-like morphology (Fig. 8e). In some samples, illite fibres are associated with kaolinite (Fig. 8d). It is typical feature of the coarse to medium-grained sandstones. 4.14. Chlorite precipitation SEM and XRD studies confirmed the presence of minor amounts of chlorite (Fig. 8e and f). Chlorite is the least abundant type of clay mineral in the Adigrat Sandstone. Chlorite occurs in rosette morphology. 5. Discussion and interpretation The major portion of the Adigrat Sandstone consists of light grey to reddish brown quartz arenite, arkosic and sub-arkosic arenite. The light to light grey colour in the lower part of the Adigrat Sandstone indicate reduction of ferric oxides into ferrous oxides, whereas the reddish brown to red colour indicates the presence of diagenetic minerals (hematite and goethite). The grain size of the Adigrat Sandstone is variable, ranging from fine to coarse-grained, but medium to coarse-grained sandstone predominate. The grains are usually sub-rounded to rounded with a moderately sorted nature indicating moderate transportation. Quartz invariably predominates greatly over feldspar framework grains. The abundance of monocrystalline quartz reflects an igneous provenance, whilst the polycrystalline quartz indicates a metamorphic or igneous origin (cf. Ingersoll and Suczek, 1979; Morad et al., 1991). The presence of abundant kaolinite minerals is probable clue to the presence of more feldspar in the past. The dissolution, alteration and replacement by calcite and clay minerals are the major factors to reduce the amount of feldspar grains. The presence of heavy minerals is an indicator of igneous and metamorphic provenance. Zircon indicates an acidic igneous provenance, whereas rutile indicates igneous and metamorphic provenances (Whitten and Brooks, 1972). Minerals known to be stable under condition of high pore fluid temperature as found deep burial are apatite, the TiO2 polymorphs (i.e. rutile, anatase and brookite), zircon and tourmaline (Morton, 1984). These minerals are also stable in acidic ground water except apatite (Morton and Hallsworth, 1994), which is good indicator of provenance for sediments. 6. Pore water chemistry It is possible to suggest a model of evolving pore water chemistry to explain the following sequences from early to late diagenesis

(Fig. 9): (i) an influx of meteoric water facilitated leaching of unstable grains to librate ions of potassium, sodium, calcium, magnesium, aluminium, silicon and iron for the authigenesis of different minerals. (ii) As the pH increased, precipitation of graincoating illite followed by quartz overgrowths, feldspar overgrowths, calcite and dolomite cementation took place under alkaline conditions. (iii) The introduction of acidic pore water once more in the diagenetic environment activated further leaching of unstable grains and carbonate cements. (iv) Reaction between acidic pore water and carbonate cements increased the pH value and precipitation of kaolinite, late-stage quartz crystals, illite and chlorite took place. 6.1. Authigenesis of concretionary calcite (calcrete) The presence of calcrete is consistent with a semi-arid to arid climate conditions and flood-plain accretion. Calcrete developed where evaporation is greater than precipitation, and represent a near-surface accumulation of calcium carbonate (Orhan, 1992). Calcrete formed in the alluvial sediments in poorly drained areas under a semi-arid climate (e.g. Suttner and Dutta, 1986; Wright and Tucker, 1991; De Ros et al., 1994; Ramos, 1995; Tucker, 2001). Calcretes formed on Magnesium-poor alluvial sediments, whilst dolocretes developed on magnesium-rich lake sediments (Alonso-Zarza et al., 1992). 6.2. Authigenesis of grain-coating illite and/or hematite The ions released from the dissolution of unstable minerals control the type of minerals to be precipitated in the diagenetic environment. It is noted by some workers that carbonates and unstable framework grains leached by meteoric water at shallow burial depth (e.g. Bjørlykke, 1983, 1993; Giles and Marshall, 1986; Worden and Burley, 2003). In semi-arid and arid areas under an oxidising environment iron is present as ferric oxides and enters into the

Fig. 9. Logarithmic activity diagram for potassium oxide–silicon oxide–aluminium oxide–water system, showing progressive evolution of fluid chemistry from (1) brackish surface conditions to successively precipitate, (2) illite, (3) illite and Kfeldspar, (4) quartz, (5) kaolinite and (6) illite and quartz. Phase boundaries for 50 C, 150 bars (from Bjørkum and Gjelsvik, 1988).

A. Wolela / Journal of African Earth Sciences 56 (2010) 29–42

39

Fig. 10. (a) SEM photomicrograph showing dolomite (D) dissolution and generation of secondary porosity. Note (1) the diagenetic sequence that kaolinite (K) precipitation post-dated dolomite dissolution and (2) dolomite dissolution from the centre towards the crystal margin. Triassic-Jurassic Adigrat Sandstone, WA-10, outcrop sample, Dejen section, Blue Nile Basin (scale bar 10 lm). (b) SEM photomicrograph showing dissolution of calcite crystals (C). Triassic-Jurassic Adigrat Sandstone, WA-12, outcrop sample, Dejen section, Blue Nile Basin (scale bar 10 lm). (c) Back-scattered electron photomicrograph showing the generation of secondary porosity (P) by dissolution of pore-filling and replacive calcite. Note intragranular pores, elongated pores, oversized pores, hematite cement (brightest mineral in the pore spaces), quartz grains (Q), quartz overgrowths (Qo) and (6) remnant calcite cement in the pore space. Triassic-Jurassic Adigrat Sandstone, WA-11, outcrop sample, Dejen section, Blue Nile Basin (scale bar 1000 lm). (d) Back-scattered electron photomicrograph showing generation of secondary porosity (P) due to the dissolution of calcite (C). Note oversized pore, intragranular pores, elongated pores, titanium oxide (Ti), hematite (H) and quartz grain (Q). Triassic-Jurassic Adigrat Sandstone, WA-9, outcrop sample, Gohatsion section, Blue Nile Basin (scale bar 100 lm). (e) Back-scattered electron photomicrograph showing extensive carbonate cement dissolution. Note porosity (P), elongated pores, oversized pores, intragranular pores, mica (M), quartz overgrowths (Qo) and corroded quartz grains (Q). Triassic-Jurassic Adigrat Sandstone, WA-11, outcrop sample, Dejen section, Blue Nile Basin (scale bar 1000 lm). (f) Back-scattered electron photomicrograph showing generation of secondary porosity (P), dissolution of feldspar (Fs), Dolomite (D), hematite (H) and corroded quartz grains (Q). Triassic-Jurassic Adigrat Sandstone, ABFISS-4, outcrop sample, Fincha section, Blue Nile Basin (scale bar 1000 lm).

crystal lattice of the clay or coatings around the framework grains (Besly and Turner, 1983; Tucker, 2001). Grain-coating illite was the first phase to be precipitated when the activity of K+ and H4SiO4 was sufficiently high (Ali and Turner, 1982). 6.3. Authigenesis of quartz and feldspar overgrowths Quartz and feldspar overgrowths post-dated grain-coating illite and/or hematite. The main sources of silicon ions include: (i) percolation of meteoric water with a high silica content, (ii) dissolution of unstable framework grains, (iii) pore water exchange from the interbedded mudstones and siltstones and (iv) mass and fluid exchange from the underlying Karroo sediments and overlying Gohatsion Formation.

There is no evidence of intergranular pressure solution to explain silicification. The grains show point-contacts and long-contacts indicating little or no pressure solution. Extensive quartz cementation in the Adigrat Sandstone reflects: (i) initially claypoor sediments with higher permeabilites allowed silica-rich solution circulation and (ii) the higher amount of detrital quartz provided the necessary sites for the nucleation of quartz overgrowths (cf. Franklin and Tieh, 1989; De Ros et al., 1994). The timing and source for quartz cementation are still points of debate. Giles et al. (1992) and Bjørlykke et al. (1992) stressed that sandstones normally do not become significantly quartz-cemented until the burial depth exceeds 2–2.5 km. After the authigenesis of quartz overgrowths, the pore water supersaturated with respect of potassium, aluminium and silicon

40

A. Wolela / Journal of African Earth Sciences 56 (2010) 29–42

ions for the authigenesis of feldspar overgrowths. The possible source for the authigenesis of K-feldspar was related to the dissolution of detrital K-feldspar.

by the replacement of iron-bearing grains of dolomite cements (Hubert et al., 1976).

7. Porosity evolution and reservoir characteristics 6.4. Authigenesis of carbonate cements 7.1. Porosity evolution After the precipitation of quartz and feldspar overgrowths the pore water became enriched with respect to calcium, magnesium, carbonate and bicarbonate ions. The most likely source for the calcite cementation include: (i) dissolution of concretionary calcite, (ii) dissolution of ferromagnesian minerals, (iii) pore water exchange from the interbedded mudstone and siltstone and from the overlying muddy facies. Previous studies of sandstone diagenesis suggest that carbonate cements in sandstone often originated in mudstones) (e.g. Füchtbauer, 1967a; Schmidt and McDonald, 1979a), (iv) the percolation of pore water from the overlying carbonate and evaporate-rich Gohatsion Formation, which is composed of dolostone, gypsum, limestones and minor anhydrite and (v) percolation of pore water from the underlying Karroo sediments. 6.5. Authigenesis of late-stage clay minerals The pH value was increased by carbonate cements and acidic pore water interaction. Supersaturation silicon and aluminium ions facilitated precipitation of kaolinite (Curtis, 1983). The aluminium and silicon ions necessary for kaolinite precipitation were possibly derived from the dissolution of feldspar grains and their overgrowths. Kaolinite can crystallize during both shallow and deep burial (Weaver, 1989). The presence of abundant kaolinite in the Adigrat Sandstone indicated that quantitatively large amount feldspar before dissolution. Generally kaolinite is confined to the purest sandstones where its growth was not inhibited by detrital clays (Füchtbauer, 1967a). Booklets of kaolinite precipitated directly from the pore water, whilst the pseudohexagonal platy kaolinite is an alteration product from feldspar (cf. Irwin and Hurst, 1983). Kaolinization of feldspar produces silica corresponding to 40% of the altered feldspar, which may precipitates as quartz cement (Bjørlykke, 1983). The late-stage quartz crystals probably took place by taking advantage from kaolinization of K-feldspar. An average of 33 ppm silica is adequate to initiate precipitation of quartz in pore space (Blatt, 1979). Illitization of kaolinite indicates an increase in burial depth and crystallinity as the paleotemperature increased. The transformation from kaolinite to illite might have been favoured by potassium ions from the dissolution of K-feldspar and its overgrowths. Kaolinite has a very marked effect on the porosity, and the limited microporosity between kaolinite crystals is either not filled with oil or oil can not be produced from such pores (Bjørlykke, 1984). Illitization and chloritization indicate an increase in burial depth and crystallinity. Following deposition, the Adigrat Sandstone underwent continuous burial, and in Mid-Cretaceous the Adigrat Sandstone entered a depth of 2 km (see Fig. 5). At the present time, the Adigrat Sandstone is found at a depth of 4 km. The burial history of the basin supports the transformation kaolinite into illite and chlorite. 6.6. Authigenesis of hematite The presence of late-stage hematite is related to progressive loss of Fe–Ti oxides from the heavy mineral fraction in the sandstones with progressive burial (cf. Milliken et al., 1994). The possible sources of hematite cement are: (i) the dehydration of limonite, (ii) iron derived from the dissolution or alteration of iron silicates, (iii) the oxidation of ilmenite and magnetite and (iv) iron released

Depositional environment, diagenetic evolution and tectonic setting are the three major factors to control the porosity and permeability trend of the Adigrat Sandstone. Fluid exchange from the overlying and underlying strata had an effect on the diagenesis, porosity and permeability of the Adigrat Sandstone. Transmitted light microscopy and electron microscopy studies revealed the presence of mixed porosity (primary and secondary) (Figs. 6e and 10c–f). The mineralogical composition, grain size distribution, sorting and the type of clay minerals (kaolinite, illite and chlorite) are additional factors in controlling the porosity and permeability trends of the studied sandstone. Quartz cementation prevents framework collapse and preserved significant amounts of primary porosity. In a few localities, silicification has overshadowed the generation of secondary porosity (Figs. 6a and 7b). Extensive carbonate cements, clay minerals and hematite cement reduced primary porosity. Porosity reduced by carbonate cements can be recoverable through dissolution, whilst the porosity lost by clay minerals and mechanical compaction is unrecoverable. Thin section and SEM studies confirmed that the generation of secondary porosity in the Adigrat Sandstone. Secondary porosity is created by partial to complete dissolution of unstable grains and carbonate cements (Fig. 10a–f). The most extensive leaching is therefore expected in sandstones which are exposed to highly under saturated pore water over the longest period of time (Nedkvitne and Bjørlykke, 1992). The presence of corroded grains, inhomogeneity of packing, internal grain dissolution (honeycombed grains), remnant (floating) cements and grains in pore spaces, moldic pores, grain fracturing and oversized pores are good indicators of the generation of secondary porosity. Of the five genetic types of Schmidt and McDonald (1979b) three types were identified in the studied sandstone: (i) dissolution of framework grains and matrix, (ii) dissolution of authigenic pore-filling cements and (iii) dissolution of authigenic replacive cements. Elongate pores, moldic pores and oversized pores are the common types of secondary porosity in the studied sandstones. Booklets of kaolinite crystals stacked several microns in diameter reduce porosity but not permeability (Loucks et al., 1980; Loucks et al., 1984). Parnell (1987) pointed out that kaolinite clearly reduces porosity but still exhibits an effective microporosity because permeability between pores is not so restricted. 7.2. Reservoir potential The presence of carbonate cement in the Adigrat Sandstone is not perceived as indicating the elimination of the sandstone’s potential as a hydrocarbon reservoir. The introduction of acidic pore water in the diagenetic environment facilitated the dissolution of unstable framework grains and carbonate cements to create secondary porosity. There are porous potential reservoir horizons in the Adigrat Sandstone. The channel and point bar deposits are fair to good reservoir rocks in the studied sandstones. The coarse to mediumgrained sandstone is more porous and permeable than the fineclastic sediments (siltstones and mudstones). The potential reservoir sandstones are mainly found in the lower and middle part of the formation. The porosity and permeability in the Adigrat Sandstone reach up to 20.4% and 710 mD, respectively (Table 2).

A. Wolela / Journal of African Earth Sciences 56 (2010) 29–42 Table 2 Porosity and permeability of the Adigrat Sandstone formation. Sample no

Locality

Environments

Porosity in %

Permeability (mD)

WA-10 Wa-11 WA-12 WA-14 BN-01 BN-02 BN-03 BN-09 BN-10 BN-11 WA-25 Wa-26 WA-27 ABBSS-3 FISS-1 FISS-2 FISS-3 ABFISS-7 ABFISS-8 ABESS-1 ABESS-2 ABGESS-1 BN-13 BN-14 BN-15 Ambo-1 Ambo-2 Ambo-3 Ambo-4

Dejen

Fluviatile

7.6 15 14.6 5 4 8 5 4.4 0.4 14.6 3 5 6.8 11 15 5 3 20.4 13 10 20 10 5 10 5.5 21 17 20 12

400 710 500 200 160 200 100 50 10 560 200 250 300 200 150 50 20 400 430 200 350 500 100 300 120 650 600 400 450

Gohatsion

Fincha

Ejere (Jema River Bridge) Gendebert-Jeldu Arjo

Ambo

8. Summary and conclusions 1. Mudstone, finely laminated siltstone, very fine-grained crossbedded sandstone, coarse to medium-grained sandstone, massive to crudely cross-bedded gravely sandstone and massive to crudely-bedded conglomerate are the dominant facies types in the studied sandstone. Considering the depositional environments, the Adigrat Sandstone is dominated by a mixture of alluvial fan, meandering river and lacustrine deposits. 2. The continental red bed dominated Adigrat Sandstone is characterised by fine to coarse-grained, moderately to well sorted sub-mature to mature quartz arenite, arkosic and sub-arkosic arenite and predominantly cemented by silica, carbonate and phylosilicates. 3. The diagenetic evolution, porosity and permeability characteristics of the Adigrat Sandstone are controlled by the depositional environment, burial history and diagenetic processes (i.e. pore water chemistry and circulation). 4. The Adigrat Sandstone has a complex diagenetic history. The most common pore-filling minerals are early illite/hematite grain-coating, quartz and feldspar overgrowths, carbonates (calcite and dolomite), clay minerals (kaolinite, illite and chlorite) and hematite. 5. Reconstruction of the diagenetic history helps to understand the porosity evolution of the Adigrat Sandstone. Grain-coating illite and/or hematite, silica and carbonate and mechanical compaction reduced porosity. Early silica cementation favoured framework stability and preserves significant amounts primary porosity. 6. The porosity of the Adigrat Sandstone is characterised by hybrid porosity (primary and secondary). Of these, secondary porosity predominates. The dissolution of carbonate cements facilitated the generation of secondary porosity. The presence of corroded grains, inhomogenity of packing, internal grain dissolution (honeycombed) grains, floating grains and cements in the pore

41

spaces are good indicators for generation of secondary porosity. Elongate pores, moldic pores, oversized pores and primary pores are the common types of porosity in the studied samples. 7. The Adigrat Sandstones has porosity and permeability reach up to 20.4% and 710, mD, respectively. The coarse to mediumgrained sandstone facies is the most porous and permeable, and are potential reservoir for oil and gas deposit, whilst the fine-grained sandstones and finely laminated siltstones facies are possible gas reservoirs.

Acknowledgments Earlier drafts of this manuscript were read by J. Parnell, and his comments are gratefully acknowledged. Thanks also extended to E. Geirlowski-Kordesch and A. Ruffel for constructive and valuable reviews of this paper. The manuscript benefited substantially from critical commentary by Dr. Tilahun, M., and Dr. Kumpulainen, R. referees of the journal. Most of the work was carried out at the Geofluid Laboratory, Geochemical Laboratory and Electron Microscopy Unit, School of Geosiences, the Queen’s University of Belfast. The staffs of these institutions are acknowledged for their support and help. This research work made possible by the Ministry of Mines and Energy, Ethiopia and the School of Geosciences, the Queen’s University of Belfast, UK. References Ali, A.D., Turner, P., 1982. Authigenic K-feldspar in the Bromsgrove Sandstone Formation (Triassic) of central England. Journal of Sedimentary Petrology 52, 187–198. Alonso-Zarza, A.M.A., Wright, V.P., Calvo, J.P., Garcia-Delcyra, M.A., 1992. Soil land scape and climatic relationships in the middle of the Madrid Basin. Sedimentology 39, 17–35. Assefa, A., Wolela A., 1986. The Geology and Coal Occurrences of Arjo, Wellega. Unpublished Report, Ministry of Mines and Energy, Addis Ababa. Besly, B.M., Turner, P., 1983. Origin of red beds in a moist tropical climate (Etruria Formation, Upper Carboniferous, UK). In: Wilson, R.C.L. (Ed.), Residual Deposits: Surface Related Weathering Processes and Materials, vol. 11. Geological Society of London, pp. 131–147 (special publication). Beyth, M., 1972. Palaeozoic sedimentary basin of the Mekele Outlier, northern Ethiopia. American Association of Petroleum Geologists Bulletin 56, 2426–2439. Bjørkum, P.A., Gjelsvik, N., 1988. An isochemical model for formation of authigenic kaolinite, K-feldspar and illite in sediments. Journal of Sedimentary Petrology 58, 506–511. Bjørlykke, K., 1983. Diagenetic reaction in sandstones. In: Parker, A., Sellwood, B.W. (Eds.), Sediment Diagenesis, vol. 115. D. Reidel Pub. Co., Dordrecht, Holland, pp. 169–213. Bjørlykke, K., 1984. Formation of secondary porosity: how important it is? In: McDonald, D.A., Surdam, R.C. (Eds.), Clastic Diagenesis, vol. 37. American Association of Petroleum Geologists Memorandum, pp. 277–286. Bjørlykke, K., Ramm, M., Saigal, G., 1989. Sandstone diagenesis and porosity modification during basin evolution. Geologische Rundschau 78/1, 243–268. Bjørlykke, K., Nedkvitne, T., Ramm, M., Saigal, G., 1992. Diagenetic processes in the Berent Group (Middle Jurassic) reservoirs of the North Sea. An overview. In: Morton, A.C., Haszeldine, R.S., Giles, M.R., Brown, S. (Eds.), Geology of Brent Group, vol. 61. Geological Society of London, pp. 263–287 (special publication). Bjørlykke, K., 1993. Fluid flows in sedimentary basin. Sedimentary Geology 86, 137– 158. Blatt, H., 1979. Diagenetic processes in sandstone. In: Scholle, P.A., Schluger, P.R. (Eds.), Aspects of Diagenesis, vol. 26. Society of Economic Paleontologists and Mineralogists, pp. 141–157 (special publication). Bosellini, A., 1989. The continental margins of Somalia: their structural evolution and sequence stratigraphy. Memorie della Scienze Geologiche 41, 373–458. Bosellini, A., Russo, A., Fantozzi, P.L., Assefa, G., Tadesse, S., 1997. The mesozoic succession of the Mekele Outlier (Tigrai Province, Ethiopia). Memorie della Scienze Geologiche 49, 95–116. Bunter, M.G., Debretsion, T., Woldegiorgis, L., 1998. New development in the pre-rift prospectivity of the Eritrean Red Sea. Journal of Petroleum Geology 21, 374– 400. Condie, K.C., Noll, P.D., Conway, C.M., 1992. Geochemical and detrital mode evidence for two sources of early proterozoic sedimentary rocks from the Tonto Basin Supergroup, central Arizona. Sedimentary Geology 77, 57–76. Curtis, C.D., 1983. Geochemical studies on the development and destruction of secondary porosity. In: Brooks, J. (Ed.), Petroleum Geochemistry and Exploration of Europe, vol. 12. Geological Society of London, pp. 113–125 (special publication).

42

A. Wolela / Journal of African Earth Sciences 56 (2010) 29–42

Davidson, A., Rex, D.C., 1981. Age of volcanism and rifting in southwestern Ethiopia. Nature 283, 657–658. De Ros, L.F., Morad, S., Paim, P.S.G., 1994. The role of detrital composition and climate on the diagenetic evolution of the continental molasses: evidence from Cambro-Ordovician Guaritus sequences, south Brazil. Sedimentary Geology 92, 197–228. Dickinson, W.R., Suczek, C.A., 1979. Plate tectonics and sandstone composition. American Association of Petroleum Geologists Bulletin 63, 2164–2182. Folk, R.L., 1968. Petrology of Sedimentary Rocks. Hemphill Publishing Company, Austin, Texas. 182 p. Franklin, S.P., Tieh, T.J., 1989. Petrography, diagenesis, and reservoir properties of the Dakota Sandstone of west Lindrith Field, Rio Sarriba County, New Mexico. In: Colson, E.B. (Ed.), Petrogenesis and Petrophysics of Selected Sandstone Reservoirs in the Rocky Mountain Region. Rocky Mountain Association of Geologists, Denver, pp. 117–133. Füchtbauer, H., 1967a. Influence on different types of diagenesis on sandstone porosity. Proceedings of 7th World Petroleum Congress 2, 353–369. Gebre Yohanes, H., 1989. Facies, Depositional Environments, Diagenesis and Hydrocarbon Potential of the Jurassic Hamanlei Formation (Carbonateevaporite Rocks) in Ogaden Basin SE Ethiopia. M. Phil. Thesis, University of Reading, 183p. Getaneh, A., 1991. Lithostratigraphy and environment of deposition of Late Triassic to Early Cretaceous sequences of the central part of Northwestern Plateau, Ethiopia. Neues Jahrbuch für Geologie und Paläontologie: Abhandlungen 182 (3), 255–284. Giles, M.R., Marshall, J.D., 1986. Constraints on the development of secondary porosity in the subsurface: re-evaluation of processes. Marine and Petroleum Geology 3, 243–255. Giles, M.R., Stevenson, S., Martin, S.V., Cannon, S.J.C., 1992. The reservoir properties and diagenesis of the Berent Group: a regional perspective. In: Morton, A.C., Haszeldine, R.S., Giles, M.R., Brown, S. (Eds.), Geology of Brent Group, vol. 61. Geological Society of London, pp. 289–327 (special publication). Hayes, J.B., 1979. Sandstone diagenesis – the hole truth. In: Scholle, P.A., Schluger, P.R. (Eds.), Aspect of Diagenesis, vol. 26. Society of Economic Paleontologists and Mineralogists, pp. 127–139 (special publication). Hofmann, C., Courtillot, V., Feraud, G., Rochette, P., Yirgu, G., Ketefo, E., Pik, R., 1997. Timing of the Ethiopian flood basalt event and implication for plume birth and global change. Nature 389, 338–341. Hubert, J.F., Reed, A.A., Carey, P.J., 1976. Paleogeography of East Berlin Formation, Newark Group, Connecticut Valley. American Journal of Sciences 276, 1183– 1207. Hunegnaw, A., Sage, L., Gonnard, R., 1998. Hydrocarbon potential of the intracratonic Ogaden Basin, SE Ethiopia. Journal of Petroleum Geology 21, 401–425. Ingersoll, R.V., Suczek, C.A., 1979. Petrology and provenance of Neogene sand from Nicobar and Bengal fans, DSDP sites 211 and 218. Journal of Sedimentary Petrology 49, 1217–1228. Irwin, H., Hurst, A.R., 1983. Application of geochemistry to sandstone reservoir studies. In: Brooks, J. (Ed.), Petroleum Geochemistry and Exploration of Europe, vol. 12. Geological Society of London, pp. 127–146 (special publication). Kazmin, V., 1972. Geological Map of Ethiopia. Geological Survey, Ethiopia, Addis Ababa. Kazmin, V., 1975. Explanation of the geological map of Ethiopia. Ethiopian Geological Surveys Bulletin 1, 1–15. Kent, P.E., 1974. Continental margin at East Africa: a region of vertical movement. In: Bark, A., Drake, C.L. (Eds.), The Geology of Continental Margins. SpringerVerlag, NewYork, pp. 313–320. Ketela, T., 2007. Integrated Geophysical Data Processing and Interpretation of Crustal Structure in Ethiopia with Emphasis on the Ogaden Basin and Adjacent Areas. Ph. D. Dissertation, The University of Texas at El Paso, 238p. Kieffer, B., Ardt, N., Lapierre, H., Bastien, F., Bosch, D., Pecher, A., Yirgu, G., Ayalew, D., Weis, D., Jerram, D.A., Keller, F., Meugniot, C., 2004. Flood and shield basalts fro Ethiopia: magmas from the African superswell. Journal of Petrology 45, 783– 834. Korme, T., Acocella, V., Abebe, B., 2004. The role of pre-existing structures in origin, propagation and architecture of faults in the Main Ethiopian Rift. Gondwana Research 7, 467–479. Loucks, R.G., Douge, M.M., Galloway, W.E., 1980. Importance of Secondary Leaching Porosity in Lower Tertiary Sandstone Reservoir along the Gulf Coast, Bureau of Economic Geology. Geological Circular, University of Texas. pp. 80–102. Loucks, R.G., Douge, M.M., Galloway, W.E., 1984. Regional control on diagenesis and reservoir quality in Lower Tertiary Sandstones along the Texas Gulf Coast. In: McDonald, D.A., Surdam, R.C. (Eds.), Clastic Diagenesis, vol. 37. American Association of Petroleum Geologists Memorandum, pp. 15–46. McConnel, R.B., 1972. The geological development of the rift system of Eastern Africa. Geological Society of America Bulletin 83, 2549–2572. Milliken, K.L., Mack, L.E., Land, L.S., 1994. Elemental mobility in sandstone during burial: whole rock chemical and isotopic data, Frio Formation, South Texas. Journal of Sedimentary Research 64, 788–796.

Mohr, P.A., 1962. The Geology of Ethiopia. Addis Ababa University Press, Addis Ababa. 268p. Morad, S., Ajet Bhattacharya, I.S., Al-Aasm, I.S., Ramsyer, K., 1991. Diagenesis of quartz in the Upper Proterozoic Kaimur Sandstones, Son Valley, central India. Sedimentary Geology 73, 209–225. Morton, A.C., 1984. Stability of detrital heavy minerals in Tertiary Sandstones from the North Sea Basin. Clay Minerals 19, 287–308. Morton, A.C., Hallsworth, C., 1994. Identifying provenance-specific features of detrital heavy mineral assemblages in sandstones. Sedimentary Geology 90, 241–250. Nedkvitne, T., Bjørlykke, K., 1992. Secondary porosity in the Berent Group (Middle Jurassic) Huldra Field, North Sea: implication for predicting lateral continuity of sandstones. Journal of Sedimentary Petrology 62, 23–34. Orhan, H., 1992. Importance of dust storms in the diagenesis of sandstone: a case study, Entrada Sandstone in the Ghost Ranch area, New Mexico, USA. Sedimentary Geology 77, 111–122. Parnell, J., 1987. Secondary porosity in hydrocarbon-bearing transgressive sandstones on unstable lower continental shelf, Welsh Borderland. In: Marshall, J.D. (Ed.), Diagenesis of Sedimentary Sequences, vol. 36. Geological Society of London, pp. 297–312 (special publication). Raaben, V.P., Komenyev, S.P., Lissin, V.N., Kitachew, W.T., 1979. Preliminary Report on the Evaluation of Petroleum Prospects of the Ogaden Basin, Ethiopia. Ethiopian Institute of Geological Surveys (Note No. 112). Ramos, A., 1995. Transition from alluvial to coastal deposits (Permian-Triassic) on the island of Mallorca, western Mediterranean. Geological Magazine 132, 435– 447. Russo, A., Getaneh, A., Balemwal, A., 1994. Sedimentary evolution of the Abay River (Blue Nile) Basin, Ethiopia. Neues Jahrbuch für Geologie und Paläontologie: Monatshefte 5, 291–308. Salman, G., Abdula, I., 1995. Development of the Mozambique and Ruvuma sedimentary basins, offshore Mozambique. Sedimentary Geology 96, 7–41. Schmidt, V., McDonald, D.A., 1979a. The role of secondary porosity in the course of sandstone diagenesis. In: Scholle, P.A., Schluger, P.R. (Eds.), Aspects of Diagenesis, vol. 26. Society of Economic Paleontologists and Mineralogists, pp. 175–207 (special publication). Schmidt, V., McDonald, D.A., 1979b. Textural recognition of secondary porosity in sandstones. In: Scholle, P.A., Schluger, P.R. (Eds.), Aspects of Diagenesis, vol. 26. Society of Economic Paleontologists and Mineralogists, pp. 209–225 (special publication). Serawit, A., Tamrat, M., 1996. The Geology of Gundo-Meskel and Ejere Area, North Shoa, Abay Basin. Ministry of Mines and Energy, Addis Ababa, 16p. Shackelton, R.M., 1978. Structural development of the East African Rift System. In: Bishop, W.W. (Ed.), Geological Background to Fossil Man: Recent Research in the Gregory Rift, East Africa. Scottish Academic Press, p. 55. Suttner, L.J., Dutta, P.K., 1986. Alluvial Sandstone composition and palaeoclimate, I. Framework mineralogy. Journal of Sedimentary Petrology 56, 329–345. Tamrat, M., Tibebe, G.S., 1997. The Geology of Ginde Mendebert-Jeldu and AmuruJarti Areas (East Wellega and Western Shoa), Abay Basin. Ministry of Mines and Energy, Addis Ababa. Tucker, M.E., 2001. Sedimentary Petrology: An Introduction to the Origin of Sedimentary Rocks, Third ed. Blackwell, Science Ltd. 260p. Weaver, C.E., 1989. Clays, Muds and Shales. Elsevier, Amsterdam. 820p. Whitten, D.G.A., Brooks, J.R.V., 1972. Dictionary of Geology. Penguin Books Ltd., London. 493p. Wolela, A., 1991. Coal and Oil Shale Occurrences and their Geological Setting in Ethiopia. Unpublished Work, Ministry of Mines and Energy, Addis Ababa. 205p. Wolela, A., 1997. Sedimentology, Diagenesis and Hydrocarbon Potential of Sandstones in Hydrocarbon Prospective Mesozoic Rift Basins (Ethiopia, UK, USA). Ph. D. Thesis, The Queen’s University of Belfast, 238p. Wolela, A., 2002. Hydrocarbon Potential of the Blue Nile Basin. Ministry of Mines and Energy, Addis Ababa. 150p. Wolela, A., 2006. Sedimentology and Petroleum Systems in the Blue Nile Basin. Ministry of Mines and Energy, Addis Ababa. 138p. Wolela, A., 2008. Sedimentation of the Triassic-Jurassic Adigrat Sandstone Formation, Blue Nile Basin, Ethiopia. Journal of African Earth Sciences 52, 30– 42. Worden, R.H., Burley, S.D., 2003. Sandstone diagenesis: the evolution of sand to stone. In: Burley, S.D., Worden, R.H. (Eds.), Sandstone Diagenesis: Recent to Ancient. International Association of Sedimentologist, pp. 3–44 (reprint series 4). Worku, T., 1988. Sedimentology, Diagenesis and Hydrocarbon Potential of the Karoo Sediments (Late Palaeozoic to Early Jurassic), Ogaden Basin, Ethiopia. M. Phil. Thesis, University of Reading, 222p. Worku, T., Astin, T.R., 1992. The Karroo sediments (Late Palaeozoic to Early Jurassic) of Ogaden Basin, Ethiopia. Sedimentary Geology 76, 7–21. Wright, V.P., Tucker, M.E., 1991. In: Wright, V.P., Tucker, M.E. (Eds.), Calcrete. International Association of Sedimentologists, pp. 1–22 (reprint series 2).