Geology And Coal Potential Of Somaliland

4          Potential coal areas

Coal deposits have been reported in many sites in Somaliland. The following occurrences have been visited on two separate field trips in the country (July 2006 and July 2007). Locations of the study area are shown in Figure 2.

4.1 Jurassic deposits

Location: Gal Hamud, north of Bihendula (45°09.885′ E; 10°09.713′ N)

In the area of Gal Hamud, eight kilometers north of Bihendula, coal stringers showing branches and trunks are deposited in the Lower Jurassic sandstones and conglomeratic beds of the Adigrat Formation which underlie the Bihen Limestone (Figure 4). The coal is contained in 5–8 centimeters thick lenses of sandstone (Figure 5). Analyses of the coal samples reported by Grundstofftechnik (1983) suggest that the coal is a sub-bituminous type with low ash contents (8.1%), volatile matter content of 27.6% and relatively high calorific values (6,251 kcal kg^–1).

Figure 4 Basal clastics of Adigrat Sandstone showing conglomerate consisting of detrital quartz, Bihendula area, south of Berbera (see online version for colors)

Figure 5 A thin seam in Adigrat Sandstone, Gal Hamud, south of Berbera (see online version for colors)

4.2 Cretaceous deposits

Location: Hed-Hed, south of Onkhor (46°14.484′ E; 10°33.802′ N)

Hed-Hed is located about 25 kilometers south of the coastal village of Onkhor. The coal deposits occur approximately 4 kilometers upstream in the Hed-Hed gorge from its junction with the Hodmo ravine. The main outcrop occurs on the bottom of the eastern side of the gorge. The coal occurrences at Hed-Hed were the first to be recognized in the country and have since been the most extensively studied. Hed-Hed has the greatest thickness and highest quality of coal so far exposed in the country. The coal-bearing sedimentary strata dip 5° to the south and occur in the upper part of the Cretaceous within the Yesomma Sandstone, which is overlain by massive Eocene Auradu limestone (Figures 6 and 7).

Farquharson (1924) has documented the coal-bearing that outcrop in the area and recent fieldwork has also confirmed the occurrence of seams that are exposed at the base of the Hed-Hed gorge. Two further seams were discovered during the drilling campaign carried out by Grundstofftechnik (1983), making a total of four coal seams to have been recognized in the area within a 20 meters

Upper Cretaceous sequence and a horizontal extent of at least 500 meters in a north-south direction. The thickness of the coal seams varies considerably over the distance with some of the seams extending much further laterally. The stratigraphy of the coal-bearing sections drilled in the Hed-Hed area is shown in Figure 8. The seams consist of multiple layers of carbonaceous material interbedded with clay, shale, marlstone and sandstone. In general, the coal is black, hard and compact with a conchoidal fracture. The seams are designated from top to bottom as shown in Table 1.

Table 1            Description of the coal seams in Hed-Hed

Figure 6 Hed-Hed gorge, south of Onkhor, showing gently dipping seams occurring in the upper part of the Cretaceous within Yesomma (Nubian) Sandstone (see online version for colors)

Figure 7 (a, b) Hed-Hed gorge showing gently dipping seam (see online version for colors)

Figure 8 Stratigraphy of the seams in Hed-Hed gorge (see online version for colors)

The coal was sampled and analysed by Farquharson (1924), Aden and Peeters (1981) and Grundstofftechnik (1983). The results of the analyses are presented in Table 2. It is not known whether the variations observed in the results reflect the petrological differences in the coal seams, differences in sample collection procedure, chemical and physical changes which occurred after the samples were taken, or maybe due to differing analytical procedures.

For example, the moisture ranges from 8% when analyzed by Grundstofftechnik (1983), to 13.24% and 17.4% when analyzed by Farquharson (1924) and Aden and Peeters (1981) respectively. The calorific values show values ranging between 4,712 kcal kg^–1 and 5,550 kcal kg^–1, with high ash content from 8.2% to 11.78%, and high volatile matter content at 33.9% to 38.1%. As a result, the coal is classified as being between sub-bituminous and lignite (Unsworth et al., 1991).

Samples from the drilled wells at Hed-Hed were examined geochemically and by means of coal petrography and palynology. Mineral matter from all seams shows a predominance of quartz and kaolinite (Ganz, 1990). This corresponds to the largely terrestrially derived palynomorph assemblages. Furthermore, the intercalations of clastics (quartz sandstones and siltstones) between the seams indicate stages of higher energy potential, probably related to fluvial inundations (Hidalgo et al., 2002).

The maceral and total organic carbon (TOC) analyses of samples are highly variable (Table 3). The analyses indicate that Liptinite (18%–90%) and Huminite (10%–73%) are the dominant macerals assembles (Ganz, 1990). Inertinite contents are very low (< 10%) indicating that the seams were possibly deposited in a large shallow basin which was rarely subaerially exposed, thus the conditions favorable to the formation of inertitine macerals, like oxidation and forest fires, were uncommon during peat accumulation (Taylor et al., 1998). In addition, the palynological analyses of core samples from shallow boreholes in the Hed-Hed area yielded predominately terrestrial microfloras (Schrank, 1990). This suggests that the coals have developed under humid conditions in tropical lowland with rivers, fresh lakes, and swamps.

Moreover, low sulphur (0.65%) content of the Hed-Hed coal indicates depositional conditions without marine influence possibly deposits of upper delta plain. This is because high sulphur content in coal is generally explained by the proximity of the peat to sea waters that can provide an abundant source of sulphur during deposition (Cohen et al., 1984; Given and Miller, 1985).

The analyses also indicate that the coal in Hed-Hed gorge falls into the rank of sub-bituminous with vitrinite reflectance (%R_o) ranging from 0.38 to 0.44. The vitrinite reflectance values also generally increase with depth (Table 3). However, tectonic events during the formation of the coal seams that caused horsts and grabens (Bosellini, 1992) and rapid differential compaction of peat and sand-dominated sedimentary sequences on the delta plain may have differentiated the area into various smaller basins separated by local highs (Grundstofftechnik, 1983).

Location: Guveneh Range (46°01.020′ E; 10°27.010′ N)

Guveneh hills are located about 32 kilometers north of Las Dureh and 30 kilometers southwest of Hed-Hed. Gellatly (1960) visited the area and recognized a seam that is exposed within the Yesomma sandstone near Haro gorge on the western margin of the Guveneh hills. The seam is 30 centimeters thick and consists of laminated coal associated with carbonaceous shale. Grundstofftechnik (1983) revisited the area and reported three gently dipping coal-bearing horizons that are exposed in the area. They are located in the uppermost part of the Cretaceous succession, with a similar stratigraphic setting as the coal seams of Hed-Hed.

In July 2007, the author visited the Guveneh area and recognized that seam 1 consists mainly of 10 centimeters thick beds of coal fragments within a sandy matrix. Seams 2 and 3 are situated about 50 meters below seam 1 and consist of black carbonaceous shale. The lowermost bed of horizon 2 is up to 17 centimeters thick and consists of a soft clayey coal. The three seams can be followed to at least 3 kilometers to the south and show almost unaltered composition. Samples collected and analyzed by Grundstofftechnik, (1983) suggest that the coal is of generally low quality with high ash contents (49.7%) and low calorific values (2,521 kcal kg^–1) (Table 2).

The plant fossils in the coal-bearing horizons reported by Grundstofftechnik (1983) indicate that peat was deposited in swamps where the vegetation was not dense enough to develop clean coal beds. Similar depositional environments are suggested for other Upper Cretaceous coal layers such as the Hed-Hed seams.

Other coal seams of varying thicknesses have been reported at various locations in the country (Aden and Peeters, 1981; Grundstofftechnik, 1983) including several Cretaceous deposits along the escarpment approximately north and east of Erigavo (e.g., Har Araale and Ham-Ham). In addition, Fantozzi and Ali-Kassim (2002) have reported two lignite horizons within the continental arenaceous formation of Yesomma Sandstone in Dhurbo area, northeast of Somalia. All of these deposits have been formed in Upper Cretaceous strata but none can be regarded as being sufficiently enough to warrant commercial development.

4.3 Oligocene-Miocene deposits

Location: Daban basin, southeast Berbera (45°12.825′ E 10°20.858′ N)

Oligocene-Miocene lignite deposits have been found on the southern slopes of Al Wein Range of Biyo Gure gorge about 25 kilometers southeast of Berbera (Daban Basin). The seams have been reported by Farquharson (1924) and Hunt (1960). Abbate et al. (1987) and Abbate et al. (1993a) have also recognized thin lignite horizons within the basin. On a recent visit to the area, the author observed two seams at the bottom of the gorge in two separate locations about three kilometers apart. The maximum thickness of each seam is about 30 centimeters and the horizontal outcrop distances are about 30 meters, although the seams probably extend over a wider area within the Daban Basin. However, in weathered surfaces correlation between coal seams becomes impossible.

The seams crop out within the lower Daban Group, which consists of shales, clays and sandstones. The Daban succession shows a variation from shallow marine at the base to continental sediments at the top (MacFadyen, 1933; Sagri et al., 1989; Abbate et al., 1993a). The lower Daban Group has a very high content of shallow marine fossils, such as nautiloids, nummulites, ostracods, and gastropods (MacFadyen, 1933; Sagri et al., 1989; Abbate, et al., 1993a). In contrast, the upper part of the sequence contains few fossils such as Cichlids (fresh-water fishes) (Abbate et al., 1993a) and silicified trees (MacFadyen, 1933). The coal-bearing sections are overlain by a gypsum-dominated unit. The coal deposits consist mainly of lignite (dull-bright coal) with numerous cleats, breaks and conchoidal fractures.

Figure 9 A seam in the lower Daban Series which consists of shales, clays and sandstones, Daban Basin, southeast of Berbera (see online version for colors)

The coal was sampled and analyzed by Farquharson (1924), the results of which are presented in Table 2. The analysis suggests that the deposits are of high quality with low ash (9.32%) content and relatively high calorific values (5,733 kcal kg^–1), corresponding to a rank of hard lignite to sub-bituminous coal. However, analysis of samples taken by Grundstofftechnik (1983) suggests that the coal is of relatively poor quality with high percentages of ash (43.80%) and sulphur (17.94%) and low calorific value  (3,915 kcal kg^–1). The vitrinite reflectance (R_o) for the coal samples ranges between 0.48 and 0.54 (Grundstofftechnik, 1983), which is considered high for lignite. This deposit has the potential to be used as an energy source for the cement factory in Berbera, which is located less than 8 kilometers from the deposit, and as feedstock for a power station.

Preservation of fossils (e.g., gastropod shells) in the coal seams suggests that water acidity was probably very low (Pickford, 1986). In addition, gypsum-dominated unit overlying the coal seams indicates that the sulphate content in waters was probably raised as a result of evaporation under dry conditions. Both low acidity and high sulphate content in water heavily enhances sulphur content. These observations suggest that the coal seams were deposited in an estuarine environment that was subjected to marine transgressions, which increased the sulphur content of the peat. In addition, high ash contents indicate inadequate isolation of peat-forming environments from clastic influx (Koukouzas and Koukouzas, 1995). This interpretation is consistent with sedimentological studies of Abbate et al. (1993a) which suggested that the lower Daban Group is deposited on a restricted lagoon to deltaic environment.

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