3. PETROLEUM
Petroleum has been known to occur at Dagaha Shabell at the east end of Bihendula Range since 1912, when samples of “a highly bituminous nature” were brought into Berbera by Somalis. In 1914, as the result of a report on three samples by the Imperial Institute, and of a further report on samples collected in 1913 by Mr. H. A. Byatt,[13] who himself visited the locality, it was decided by the Colonial Office to have an expert examination made of the occurrence. Since this decision there have been three authoritative reports issued on the Dagaha Shabell Field, viz.:
- By Mr. H. T. Burls, in 1914, for the Colonial Office. The conclusion come to in this report was that “ the existence of a thickness of at least 20 feet of shale saturated with rock oil at the base of a series of shales and sandstones, 500 feet in thickness, some of which are bituminous, render(s) it in my opinion most advisable to sink test wells to prove the extent and probable yield of a deposit that may develop into a valuable oilfield.”
- By Mr. Beeby-Thompson and Dr. Ball in 1918. This report was the outcome of a request made by His Majesty’s Government to the High Commissioner of Egypt that he would arrange for a careful investigation to be made of certain oil indications in Somaliland, with a view to determining whether the indications were those of an oilfield capable of commercial development. The report was published in Cairo in 1918.
After careful geological examination and detailed mapping of the vicinity of the occurrence, Mr. Beeby-Thompson and Dr. Ball came to the following conclusions:
“We have no hesitation in advising drilling operations at Daga Shabell in order to test the commercial worth of the deposits. Although the geological structure of the district does not conform with conventional anticlinal ideas, the essential conditions for the formation, retention and concentration of petroleum exist so obviously that they could not be ignored by the most pessimistic observer. So far, oil indications have only been located over an area of about fifty acres, but there is no reason to doubt that these outcropping oil-beds continue for a considerable distance to the south and south-west; and although they will be struck at increasing depths, the inclination of the beds is so gentle that they can be reached by the drill for at least two miles from the outcrop.”
After reviewing the situation and summarizing the favorable features, Mr. Beeby-Thompson concludes:
“The uncertainties already mentioned can only be removed by drilling. That oil will be found in some quantity is quite certain, and that the quality will be good is equally sure, but the drill alone will demonstrate the thickness of the series and the area of saturation.”
- By Drs. Wyllie and Smellie, of the Anglo-Persian Oil Company, in 1920.
In 1920, the Anglo-Persian Oil Company, becoming interested in the possibility of the occurrence of an oilfield on a large commercial scale in Somaliland, sent two of their geologists, Drs. Wyllie and Smellie, to make an investigation of the prospects of the occurrence of petroleum in the Protectorate. The area examined by them during the course of seven months was the central coastal district enclosed (on the map) by lines drawn from Zeyla to Meragelleh, from Meragelleh through Hargeisa and the base of the main scarp to Las Dureh, from Las Dureh to Ankor, and from Ankor through Berbera to Zeyla. The country to the east of Ankor and from the top of the scarp southwards to Abyssinia was not examined. Particular attention was devoted to Dagaha Shabell, and the stratigraphical geology of the whole area outlined was studied in order that the geology of Dagaha Shabell itself could be examined in proper perspective. As a result, the conclusions arrived at, both as to the geology of the occurrence and as to its possibilities, were in some respects different from those of Mr. Beeby-Thompson. In order to clarify the present position with regard to the field, it will be as well to set out briefly the main statements in each of the two reports.
- According to Mr. Beeby-Thompson, the rocks exposed in and around the oilfield can be classified into the following four well-marked groups:
(i) Alluvial deposits (recent sand and gravel).
(ii) A Limestone-Conglomerate series, consisting of beds of coarse limestone conglomerates alternating with sandstones (Miocene?).
(iii) Brown sandstones without clays (Jurassic).
(iv) The Shabell Series, consisting of alternations of clays and sandstones (Jurassic), some of the lower beds of which contain oil.
(i) The Alluvial Deposits comprise older limestone-gravels which cap the hills, and younger sands and gravels which cover a large proportion of the lower country.
(ii) The Limestone Conglomerate Series comprises thick strata of conglomerates consisting mainly of hard limestone pebbles and boulders somewhat loosely set in a sandy or calcareous matrix, alternating with whitish and yellowish sandstones of a rather friable nature. In some places the limestone pebbles and boulders are mixed with others composed of igneous and metamorphic rocks. The deposition of the series dates back at least to Younger Tertiary times and a Miocene age is provisionally suggested.
At some points along lines of fault, the limestone-conglomerate beds have been transformed into hard breccias by combined pressure and the action of mineral solutions circulating in the fault plane. The rocks called Daga Shabell, from which the oilfield has been named, are conspicuous upstanding masses of hardened fault-breccia formed in this way; the hardening appears to have been principally due to deposition of siliceous and calcareous matter between the fragments.
(iii) The Brown Sandstones without Clays are a series of well-bedded rocks which are faulted down against the Shabell Series forming the line of hills bounding the oilfield to the north-east. They are well marked off in the field from the sandstones which occur in the Shabell Series by their color and by the absence of any intercalated clays; but it is quite likely that they are really the uppermost beds of a single great group comprising both them and the Shabell Series. The clays in the latter become noticeably less and less prominent the higher one ascends, and possibly there is a continuous conformable passage from the Shabell Series upwards into the Brown Sandstones without Clays. It seems most likely that the beds are Jurassic, corresponding with the sandstones overlying comformably the Jurassic limestones of the Bihendula hills further west.
(iv) The Shabell Series forms the lowest member of the geological column visible in the immediate neighborhood of the oilfield, and is a great series of alternations of red and purple clays with white, brown and greenish sands and sandstones. . . . By far the most strongly-marked feature of the lower beds is the abundance of red and purple clays. As one ascends in the series, the clays diminish in abundance, and the sandstones increase in thickness, possibly passing gradually into the “Brown Sandstones without Clays.” . . . Where the Shabell Series is faulted up against the limestone-conglomerates, the sandstone beds have generally been hardened to quartzite, frequently with an increase in the amount of iron oxide present, giving the rock a deep reddish-brown color. Oil-bearing sands and sandstones occur at intervals in the lowest half of the series, from the lowest point exposed up to a geological horizon some 850 feet higher.
As regards the age of the Shabell Series, the same remarks apply as in the case of the “Brown Sandstones without Clays.” The beds are most probably Jurassic.
The oilfield is sharply limited on the northeast and northwest by faults which have thrown up the oil-bearing Shabell Series against younger rocks. There are three main faults. One, which shows the greatest movement, crosses the gorge to the north of the Daga Shabell Rocks, throwing down the limestone-conglomerates to the north. Another extends from the former past Daga Shabell and follows a sinuous course south-westward along the flanks of the hills; this fault has let down the conglomerates to the north-west and has thrown up the Shabell Series to the south-east. The third follows the flanks of the hills in a south-easterly direction and has only been sufficient to throw down the “Brown Sandstones without Clays” alongside the Shabell Series.
The general effect of the earth-movements, which have resulted in the tilting and faulting of the locks, is to bring up the lowest beds of the Shabell Series to the surface at the most northerly point of the oilfield, where two of the main faults converge to meet the third one, and it was near this point that the seepage of oil from the beds occurred which led to the discovery of the field.
With reference to the question of the source of the oil, Mr. Beeby-Thompson considers three possible ways:
(a) The oil may be a product of migration from deeper unexposed strata.
(b) The contained oil may be the result of chemical action arising from local circumstances.
(c) The oil may be indigenous to the strata in which it now occurs, and be derived from organic matter contained in the beds.
In regard to (a), he states that the fact that oil is mainly concentrated where the three major faults converge naturally arouses the suspicion that the oil has migrated upwards in the highly fractured area of that region, but the absence of all indications of oil in the adjacent porous conglomerate and sandstone series speaks against this hypothesis; and further, were it true, it is exceedingly unlikely that no traces would have been found of its existence along the disturbed fault—zones that cut off the Shabell Series.
Dismissing (b) as, under the circumstances, improbable, he holds that (c) appears most satisfactorily to explain the origin of the oil, and states that one strong argument for considering the deeper unexposed beds of the Shabell Series to be the main oil-bearing series is found in the fact that impregnation of the upper sands is observable only in the vicinity of the minor fault already alluded to. He adds that these upper sands appear to have derived their oil-contents through the medium of this fault from a deeper source, which it is reasonable to conclude is the lower Shabell group, whose upper horizons are alone visible.
- Drs. Wyllie and Smellie, after an investigation of the stratigraphy not only of the Dagaha Shabell neighborhood, but of the country as far east as Ambal and as far west as Eilo Range and Meragelleh—i.e., of a much wider area than that examined by Mr. Beeby-Thompson—arrived at the following results:
They separated off as Dubar Sandstone a sandstone series lying mostly conformably on top of the Jurassic limestone, and, in places, below the Eocene limestone.
A point of great stratigraphical importance, they were the first to recognize the presence at Daban and on the southern slopes of Al Wein, near Khal Der well, of the series of alternating greenish and red sandstones and purplish, green and chocolate-colored clays called conveniently the Daban Series. Owing probably to the limited area examined by Mr. Beeby-Thompson, he failed to note not only the stratigraphical relations and unusual character of these beds, but even their existence. As already mentioned, this series contains thick beds of brown and red sandstone practically indistinguishable from Dubar Sandstone except by consideration of the associated beds.
They hold that the Shabell Series of Beeby-Thompson is, in fact, not of the same age as the Dubar Sandstone, and not of Jurassic, but of Kainozoic age. They state that the Dubar Sandstone was found and examined over a wide area from Dubar to Ambal, and nothing comparable with “ Shabell ” beds was found throughout its whole thickness, and, indeed, that with the exception of a 20 feet band of mudstone at Biyo Dader and some sandy shales at Ambal, no argillaceous sediments were found in this series. They consider that the “Shabell” beds represent a marginal or shoreward facies of the Daban beds, and that it follows that the earlier formations had already been faulted and eroded before the Daban-Shabell period started.
Of the “Brown Sandstones without Clays,” which in their geological map is marked off by faults as a hexagonal area of sandstone east of Dagaha Shabell, and which Beeby-Thompson and Ball regard as of Jurassic (Dubar?) age and as the formation into which the Shabell Series probably passes gradually upwards, they state that they are by no means satisfied that it does not really belong to the Dubar Sandstone. Along the fault which bounds it on the north side are numerous masses of what appears to be solid Eocene limestone and one small mass of gypsum. According to them the direct evidence is not sufficient to prove that these erratic masses are not remnants of an Eocene limestone and gypsum cap which lay on top of a horst of Dubar Sandstone. But, they assert, seeing that elsewhere along this line of fault, enormous boulders of Eocene limestone are definitely seen to form part of the conglomerates (Dagaha Shabell Rock itself), there is, perhaps, a balance of reason in favor of regarding the sandstone mass as belonging to the uppermost “Shabell“ beds.
They believe that the rock called Dagaha Shabell is, in the main, a solid mass of Eocene limestone and not a sheared conglomerate as interpreted by Messrs. Beeby-Thompson and Ball. Similar large masses of Eocene limestone can be seen along the Agagwein Ridge.
With regard to the origin of the oil, Drs. Wyllie and Smellie believe that the petroleum is derived from the shales and mudstones of the underlying Jurassic. The “Shabell” beds being an overlapping series laid down in an irregular basin bounded mainly by faults, it was found by them impossible to predict what strata would be found below the lowest exposed beds, which are the petroliferous sandstones and clays. But that the Bihendula beds— Jurassic limestone and carbonaceous shales—extend eastwards underneath Dagaha Shabell is supported by the presence of the small Jurassic outlier at Ida Kabeita, and, adopting the view that Jurassic beds are the source of the petroleum, it must be granted that those beds lie uneroded below the Dagaha Shabell seepage, and that probably some thickness of Dubar Sandstone covers the Jurassic. Moreover, if it can be assumed that the fault-plane has been adequately sealed by the plastic clays of the “Shabell” beds, then a concentration of oil might be expected either in the Shabell sandstones outcropping underground against the fault, or in the Dubar Sandstone which probably underlies them.
They profess to be unwilling to make this latter assumption, because they regard the degree of mineralization shown by the sandstones along the east-west fault from Dagaha Shabell westwards as evidence of the free circulation of mineralizing solutions along the fault-plane, and as indicating a high degree of probability, not only that a mobile oil could also find a passage, but that the degree of mineralization has been produced by solutions attending the escape of the oil.
They further state that the Dagaha Shabell seepage has had an erratic and a declining career. Of three shafts, A, B and C, put down by Messrs. Thompson and Ball, they assert that these geologists reported a yield of from two to three gallons per day, mostly from shaft B; that shaft B continued to yield oil for a time, until it was found full of water with only a thin scum of oil. At the time of their visit in 1920, they state, shaft B had ceased to yield, and shaft C never apparently yielded much. Shaft A was re-excavated under their supervision, but five months after digging, the pit was found destitute of oil, with the exception of slight sweatings.
Their final conclusions were:
(i) With regard to possibilities of accumulation of oil in beds which do not come near the surface, but terminate underground against the fault, large accumulations are highly improbable, though small quantities may still be found.
(ii) No area of promise besides Dagaha Shabell was discovered.
(iii) The amount of petroleum to be obtained by drilling is, in their estimation, so limited that the expenditure involved in transporting and running a standard drilling rig cannot be recommended (to the Company).
(iv) In view of the limited possibilities at Dagaha Shabell, the best policy would be —in the event of a being considered advisable—to test by means of a portable rig capable of reaching 600-800 feet. This would save expenses, would expose the strata in which the greatest results may be expected, and would in any event decide whether the field is worthy of further attention.
In the light of the reports by Burls, Beeby-Thompson and Ball, and Wyllie and Smellie, and as the present writer was primarily engaged in an investigation of the mineral resources of the Protectorate with special reference to minerals other than petroleum, there was no necessity to make another full examination of and report on the oilfield; but in view of the considerable differences of opinion between the authorities advantage was taken of an opportunity to make a brief inspection of the field, and a short statement of the conclusions arrived at and the facts observed will be of some value. Having examined a much larger area of the Protectorate than even Drs. Wyllie and Smellie—an area stretching from Somadu in the west to the Italian frontier in the east—the writer has no hesitation in agreeing in the main with the interpretation of the geology of Dagaha Shabell as set out by Wyllie and Smellie. The Shabell Series consisting of alternating greenish and red sandstones and purplish, green and chocolate-colored clays exposed in the shafts and their vicinity are markedly distinct from either the Jurassic beds or the Dubar Sandstone, and this fact would have been realized by Beeby-Thompson and Ball had they, in their investigation of the oilfield, examined a wider tract of country than the Berbera-Bihendula track and the immediate neighborhood of Dagaha Shabell. The writer has examined the Dubar Sandstone (and, where existing, the Jurassic) beds from Kabri Bahr in the west to Las Gori in the east, and with the exceptions noted by Wyllie and Smellie and some sandy-clay beds on the track just west of Biyo Gulan (at the south end of Jirba Range), he has nowhere found any green, purplish or chocolate-colored clays in them, whereas in the Daban Series just north of Khal Der where the northern edges of the series outcrop on the slopes of Al Wein, the green and chocolate-colored clays are a pronounced and characteristic feature of the Daban Series. The latter, overlying the Eocene and Gypsum Series without any evidence of a thrust are undoubtedly younger than the Eocene, though their true age will not be known until the determination of the fossils collected has been completed.
With regard to Dagaha Shabell rock itself, the writer, having examined numerous Eocene limestone scarps in the Golis and elsewhere, is of opinion that it is part of a solid mass of Eocene limestone and not a sheared conglomerate. Undoubtedly on the weathered surfaces the rock exhibits a nodular or an indistinct conglomeratic structure, which is due to denser, yellower and more or less rounded portions of limestone being surrounded by indistinct shells of looser and greyer limestone. The appearance in the most advanced stage seen is somewhat similar to that produced by spheroidal weathering in basalts. If the Dagaha Shabell rock alone exhibited the structure, there would certainly be room for doubt whether it was not a limestone conglomerate. At many other places, however, notably on top of the Golis scarp south-west of Sheikh, and on the scarp at Geba Geba Pass, limestone showing the structure clearly forms the top of the limestone scarp, and as far as could be seen, the indistinctly nodular facies gradually merges into the massive Eocene limestone without any trace of an unconformity. Doubtless, examination of sections of the nodules and of the “shelly” portions will settle whether a true matrix is present or not, but even should a conglomerate be actually present on top of the Golis and Geba Geba limestone, it appears certain that the Dagaha Shabell rock is merely a mass broken off from a normal Eocene scarp and is not a sheared conglomerate in situ. It occurs in the ordinary conglomerate and may be an erratic.
The “Brown Sandstones without Clays” present a problem difficult of solution. Lithologically they are scarcely distinguishable from the Dubar Sandstone, and the sandstones exposed at Khal Der belonging clearly to the Daban Series are equally indistinguishable from it. They form an hexagonal mass bounded, most clearly on the north and west, by faults. Along the fault boundary on the north side are masses of Eocene limestone and one small mass of gypsum, and as the succession in many localities elsewhere is Eocene limestone surmounted by gypsum beds and underlain by Dubar Sandstone, it would appear that particularly near these two faults, the sandstone is really Dubar Sandstone. On the other hand, the limestone above the sandstone is a mass similar to but smaller than the Dagaha Shabell rock and may, therefore, be regarded as of the same origin. Moreover, exposed in the east cliff facing west and south-west, are whitish arkose sandstones, and farther east, chocolate-colored sandstones run underneath the cliff and under the arkose sandstones. On the west cliff, which belongs to the Daban Series, there are also, in ascending order, chocolate-colored and greenish sandstones, whitish arkose, red, slaty, yellow, and white sandstones, and finally limestone conglomerates containing masses (erratics probably) of Eocene limestone. There is, on the whole, a noticeable correspondence in the sequence in both cliffs, and on the east cliff the sandstone bands near the top are identical in all respects with the yellow sandstone beds exposed just north of Khal Der. The balance of evidence, therefore, is in favor of the “Brown Sandstones without Clays” being part of the Daban Series, and of Kainozoic and not of Jurassic age.
[su_button url=”https://saxafimedia.com/report-geology-and-mineral-resources-somaliland/10/” style=”soft” size=”12″ wide=”yes” center=”yes” text_shadow=”0px 0px 0px #FFFFFF” rel=”lightbox”]CONTINUE READING ON THE NEXT PAGE >[/su_button]
