Origin of the Petroleum.—The occurrence of the oil in the clays and sandstones of the Daban Series suggests two possible modes of origin:
(1) That it has been derived from carbonaceous material in the Daban beds themselves.
(2) That it has been derived from a series below the Daban beds at Dagaha Shabell, and what now appears is escaping upwards along a fault-plane.
- The Daban Series undoubtedly contains abundant carbonaceous material at the horizon outcropping at Khal Der, where both lignite seams and chocolate-colored carbonaceous clays occur. Moreover, to judge from the average dip of the beds of the series and their extension along the direction of the dip, the thickness of the series must be over 10,000 feet, and very possibly other carbonaceous strata occur at lower levels than those found. So far as has been observed, however, these carbonaceous beds show no bituminous or kerogenous characters, and the structure of the series shows no favorable indication of the occurrence of oil in it.
- That the Bihendula Jurassic Series of limestone and black carbonaceous shales does extend eastwards under Dagaha Shabell is, as pointed out by Wyllie and Smellie, supported by the occurrence of the small Jurassic outlier at Ida Kabeita, and other remains of Jurassic beds occur farther east at Ambal and Bihen Gaha, in both cases with black carbonaceous shales underlying limestone. In the opinion of the writer, therefore, the more feasible explanation of the presence of the oil is that under the Daban Series at Dagaha Shabell occurs the Jurassic Series containing, at least originally, carbonaceous or kerogenous shales. Distillation of these shales gave rise to petroleum which has been kept in the Dubar Sandstone overlying the Jurassic. Subsequent faulting brought about a means of escape for the oil along the fault-plane, but the Daban clays above have more or less sealed this fault-plane, and the oil which is now escaping is the small portion which has percolated through the clays. The distillation of the shale has possibly been brought about by the intrusion into them of dolerite dykes or sills. Wyllie and Smellie have mentioned the presence at the base of the Jurassic in Bihendula of a bed of basaltic lava. The present writer, however, during an examination of the Jurassic outcrops at Daba Dulla in Western Somaliland, found the black carbonaceous shales that underlay the limestone intruded by a dolerite dyke which had violently dislocated the strata, had more or less baked and bleached them and had produced in them traces of secondary minerals. South-east of Hemal other black shales under Jurassic limestone were found to have been similarly affected. The basalt at the base of the Jurassic in Bihendula may be, as Wyllie and Smellie consider, a lava, or it may be a sill, especially as the evidence of successive flows in the outcrop was by no means clear. If it is a lava, it is, with one exception, the only occurrence[14] of pre-Jurassic or early Jurassic basalt noted in the Protectorate and it can have had no effect on the production of the oil. If it is a sill, then it is probably of the same age as the basaltic dykes at Daba Dulla and Hemal.
Faulting has produced the essentials of an anticlinal structure in the oilfield, and there is no doubt some thickness of Dubar Sandstone overlying the Jurassic which has acted as a reservoir for the distilled fluid, otherwise the latter would have all escaped before the Daban beds were laid down.
As regards the free circulation of the oil along the fault-plane and the statement of Wyllie and Smellie that the high degree of mineralization was probably caused by the escape of oil from below, the writer is unable to agree with the amount of importance attached to them. The degree of mineralization is by no means greater than is observable, say, in the sandstones north of Khal Der, where there is no suspicion or suggestion of the occurrence of oil, and one would normally expect that, as a faultplane is always to some extent a water-channel, the degree of mineralization could quite feasibly be produced by ordinary circulating ferruginous water. The mineralization may have been caused by the escape of oil, but there is no more reason for accepting this view than that it has been caused by these circulating surface solutions. Moreover, if it were due to the escape of oil, it is fairly certain that some traces of oil—some solid residuum— would have been left along the channel equally with the ferruginous impregnation, yet not a trace—seepage or residuum—could be found anywhere along the line of the north fault through the sandstones. It is probable, that, owing to the sealing of the fault by the Daban Clays, it is only at a certain spot that any oil can seep through, and that the present shafts mark this spot.
With reference to the statement of Wyllie and Smellie that the seepage has had an erratic and a declining career, and giving the information obtained by them as to the character of the wells, the writer found the facts in 1923 somewhat different from those found by the former in 1920. Owing to the fact that the holder of the oil concession has not manned his concession, and that, consequently, no work or observations have been carried out at the occurrence since 1920, the actual history of the wells or shafts since that date is unknown. In 1923, however, oil was certainly found in two of the shafts:
(1) In the shaft (22 feet deep) put down by Wyllie and Smellie, oil now occurs for a depth of two feet and is being periodically carried away by camelmen. This was the shaft about which they stated that, five months after digging, it was found destitute of oil with the exception of slight sweatings on the walls.
(2) In the shaft next to it to the north (34 feet deep) oil now occurs (with a little water due to rain) for a depth of two feet. Owing to the lack of any record since 1920, no estimate of the rate of seepage is possible.
Without doubt, as the wells (or shafts) are in the tug valley itself, and as the tug is occasionally a fast-flowing stream, the oil has been from time to time floated out, and the wells have been partly filled with alluvium, so that, unless they are cleaned out again, a considerable time may elapse before any oil is again visible in them. Wyllie and Smellie state that so far as the seepage can be taken as an index of the amount of petroleum present in the sands which come to, or near, the surface, that amount can only be very small. The vital questions, however, are how far this seepage can be taken as an index and whether the bulk of the oil does not exist in sands which do not come to, or near, the surface. The quantity of seepage— on the hypothesis (with which Wyllie and Smellie are in accord) that the oil has originated from the kerogenous shales of the Jurassic Series— depends chiefly on the amount of oil present in the reservoir sands and on the degree to which the clays of the Daban Series have sealed the fault-plane, and as it is probable that this sealing has been effective, the value of this seepage as an indication of the amount of oil obtainable by boring may be very small.
It is believed, on reasonable grounds, that the beds underlying the petroliferous clays and sandstones are Daban and Dubar Sandstones, which act as a reservoir for the oil, and the Jurassic Series from which the oil was distilled. What, however, is the extent of these series and consequently what is the size of the oilfield can only be surmised.
In view (1) of the lack of knowledge of the beds that have given rise to the oil, the beds that at present actually contain it, their extent and thickness,
(2) Of the continual seepage of appreciable quantities of oil and the fact that the clays of the Daban Series can act as a seal to the fault-plane,
(3) Of the fact that boring to prove whether oil exists in considerable quantities would require at first to be carried out only to a depth of between 600-800 feet in sandstones, shales and clays,
(4) Of the statement even of Wyllie and Smellie that boring to this extent on the spot indicated would explore the strata in which the greatest results may be expected, and would in any event decide whether the field is worthy of attention; in view of these considerations, the writer is strongly of opinion that this boring should be carried out. Where so much doubt exists, where such positive evidence of the existence of oil is present, and where the cost of boring will be, comparatively, so small, it is of little value to devote attention to possible explanations and to hypotheses which, under present circumstances, cannot be tested by evidence. What are necessary are further facts and these can only be obtained by boring. If the oil has originated from Jurassic shales, then, to judge from the size of the outcrops of these beds at present in existence and in consideration of the probability that only a branch of the Jurassic sea extended into the central part of the “Guban,” the extent of the oilfield would appear to be limited. On the other hand, as Jurassic shales have been found as far east as Ambal and Bihen Gaha, as Jurassic, limestones occur within eight miles of the coast near Dubriat, and as evidence exists that the Jurassic rocks between Lower Sheikh and Berbera were formerly, before erosion, spread over a fairly wide area, it is possible that those unexposed and covered up by Daban beds are also of considerable extent, seeing that they have been largely preserved from erosion.
The present concession for boring for oil in the Dagaha Shabell neighborhood, should, in default of work being promptly done on the field, be terminated on the expiry of the present period of extension. The conditions attaching to the grant of a concession particularly for oil exploration or development include, or should include, that work should be regularly carried out on the property. If this work is not carried out regularly, the grant should be withdrawn so that any other person or party desirous of obtaining the concession and willing and able to observe the conditions may be permitted to do so. As matters at present stand with reference to the Dagaha Shabell occurrence, no development work of any consequence whatever has been done by the present holder of the concession— despite several periods of extension of the grant—and the fact that the concession is held, effectually prevents anyone else from doing any work on the property. All necessary preliminary geological work has been carried out, suitable sites for the preliminary bores have been fixed, even estimates of the cost of machinery and of the actual boring have been prepared, and all that is now needed is the placing of the machinery on the ground and the services of a qualified superintendent of the boring machinery and operations. Experience elsewhere shows that, where a genuine desire exists to proceed with the work, there is no difficulty in obtaining such a man, and even if there were, it is possible in these days to have the whole of the work carried out by boring contractors, who provide their own machinery and their own engineering staff. Undoubtedly, after the receipt of a concession, some little time is usually required before operations can be begun, but when a genuine desire to proceed with the work and the necessary capital exist, the period of the concession has not elapsed before tangible progress has been made.
Petroleum Indications Elsewhere.
The work done confirms the view of Wyllie and Smellie that with the exception of Dagaha Shabell, no place in the Protectorate so far examined offers any prospect of the existence of an oilfield. An anticlinal structure was found in three places:
(1) In Abassa Dadera tug, north of Borama.
(2) In Ambal Hill, south of Karam.
(3) At Biyu Dader.
- In Abassa Dadera tug the Jurassic limestone forming the bank of the tug is folded for a very short distance, the dip at one spot being 25° in a direction S. 30° W. and at another a few yards farther on, 23° in a direction N. 15° E. This folding, however, appears to be merely local, and though the tug has cut through the rock at right angles to the axis for a considerable depth, no trace of oil was to be seen.
- Ambal Hill at first sight appears to be a dome, but close examination shows that the west side is marked by a considerable thickness of sand, and as, farther north, two other similar Jurassic hills were clearly faulted on the west side, it is most probable that in Ambal Hill also the west side is faulted, but the fault is concealed by the sand.
- At Biyu Dader, to quote Wyllie and Smellie, “the evidence of true folding is very strong, a definite anticlinal arch being traceable for about four miles in an east-south-east direction with dips of 10° to the north-east and 10°-12° to the south-west.” Though the anticline, however, is largely broken down by faults, no vestige of oil or of oil residuum was to be found in the vicinity.
In Western Somaliland, where the greatest development of the Jurassic rocks is present, in spite of the fact that they are underlain by kerogenous shales, the sediments have no suitable cover, they rest in a floor of granite and hornblende gneiss either directly or with an intervening friable sandstone of no great thickness; they occur in faulted blocks having a steep scarp generally facing north-east, and a dip 25°-30° to the south-west. No indications of oil could be found in any scarp examined, and, though every well on the whole trek through the region was carefully examined, no oil films were discovered. Even at Meragelleh, where a large thickness of kerogenous shales underlies the limestone, and the beds are in places flat or nearly so, there is no cover to the rocks suitable for containing oil, and there is evidence that the gneissic floor is not far below the surface. The large escarpments of Bur Ad, Libaheli, Aga Sur, Eilo, Karimo, etc., all rest on a gneissic floor which is visible at many spots. Similar remarks apply to the exposures in the Central District, though in this district the overlying Dubar Sandstone is still preserved in several localities. At Ambal Hill, not only is there a lack of suitable cover to the Jurassic rocks, but there are no indications in the tug, in the wells, or elsewhere of any oil occurrence. At Bihen Gaha, the Jurassic Series occurs in a similar way to that in Western Somaliland, though both kerogenous shales and black foetid limestones form the base. In Eastern Somaliland, the Jurassic is not present, the sedimentary rocks are either old, probably Palaeozoic, slates with upturned edges, or Eocene and later limestones with neither suitable structure for the occurrence of oil nor any indications of its presence. The east-south-eastern, south-eastern and southern portions of the Protectorate were not examined. There are occasional rumors of oil indications having been found in the Baran District near the Italian frontier, but the hills and ridges in the south-east appear to be formed, at least in part, of Eocene limestone with overlying gypsum beds. Whether any limestone of older age, or any structures favorable for the occurrence of oil exist, there is at present no evidence to show.
Of the country south-east of Borama and south of Hargeisa, nothing at all is known. The presence of Jurassic limestone ridges a mile or two south-east, south-west, and south of Borama, indicate that this limestone extends for a considerable distance farther south than was apparently known to Wyllie and Smellie. According to the latter. Dr. Barnum Brown, of the Anglo-American Oil Company, was understood to believe that the Jurassic beds offered possibilities of oil in the down-faulted area north of Jig-Jiga where they are suitably covered. Whether the Jurassic in the vicinity of Borama is connected to the south and south-west with the Jig-Jiga limestone, and whether, in that event, there are possibilities of oil in the intervening country are questions outside the scope of the present work.
The oil from Dagaha Shabell has been analyzed by the Imperial Institute, by Sir Bovington-Redwood and at the Egyptian Government Analytical Laboratory, and some results are given in Mr. Beeby-Thompson’s report. It is to be noted, however, that all the samples of oil analyzed were obtained from near the surface and, having been exposed to the atmosphere for an unknown length of time, were necessarily of higher density or contained a smaller proportion of the more volatile constituents than may be expected from oil that has yet to be obtained from the sands at greater depths.
[su_button url=”https://saxafimedia.com/report-geology-and-mineral-resources-somaliland/11/” style=”soft” size=”12″ wide=”yes” center=”yes” text_shadow=”0px 0px 0px #FFFFFF” rel=”lightbox”]CONTINUE READING ON THE NEXT PAGE >[/su_button]
