ACTIVE SEEPS, INVESTMENT CLIMATE DRAW INTEREST TO UGANDA

May 1, 1995
D. Keith Patton University of Colorado, Denver Reuben Kashambuzi, Ernest N.T. Rubondo Petroleum Exploration & Production Department Entebbe, Uganda Production has not been established in Uganda, but the hydrocarbon generating capacity of its rift basin is clearly evident. Reports of oil seeps date to 1925 to observations by Wayland,1 who relates 52 hydrocarbon occurrences in and around Lake Albert. Although not all remain active, nine oil seeps in the rift basin are confirmed active at

D. Keith Patton
University of Colorado,
Denver

Reuben Kashambuzi, Ernest N.T. Rubondo
Petroleum Exploration & Production Department
Entebbe, Uganda

Production has not been established in Uganda, but the hydrocarbon generating capacity of its rift basin is clearly evident. Reports of oil seeps date to 1925 to observations by Wayland,1 who relates 52 hydrocarbon occurrences in and around Lake Albert. Although not all remain active, nine oil seeps in the rift basin are confirmed active at present.2

The Albertine graben, a part of the East African rift system, dominates the geology of Uganda (Fig. 1). Other, larger basins of similar evolution in Africa include the Sirte basin and the Gulf of Suez basin. These prolific basins account for one third of Africa's total reserves.

The Albertine graben is underexplored. Several stratigraphic tests have been drilled, but the penetrations are shallow. The deepest well in Uganda was drilled in 1938 to total depth of only 1,232 m, although sedimentary thickness in the basin exceeds 4,000 m.2

A favorable geologic framework, a paucity of drilling, live oil shows, an interest by the government in promoting exploration, and the newly established favorable investment climate combine to make Uganda an attractive option for oil and gas exploration.

GEOLOGIC SETTING

The western arm of the East African rift system trends roughly north-south with the segment in Uganda striking northeast through the country (Fig. 1). The feature is discontinuous and includes at least 32 basins, one of which, the Albertine graben, falls within Uganda. The basins are bounded by dip-slip and oblique-slip fault systems that are typically 100 km long.3 Depth to pre-rift basement varies by several kilometers from basin to basin.

Structural style is characterized by rifting due to tension. High-angle normal-fault segments with throws of 1,000-6,000 m border the rifts. The normal faults are en echelon and linked by oblique-slip transfer faults.3 Basin development dates to the Miocene, although rifting was influenced by earlier structural trends which affected the basement.2

In the Albertine graben, throws of the boundary faults vary from 600 m to more than 1,500 m and may exceed 3,000 m; fault planes dip a relatively steep 60-80.1 Volcanic activity is noted but is limited to the southern part of the Albertine graben and is associated with the early stages of basin evolution.

Recent aeromagnetic and follow-up ground gravity and magnetic surveys indicate that the Albertine graben is divided into smaller sub-basins (Fig. 2). It includes several depocenters: Lake Albert (Lake Mobutu), Semliki, Lake Edward, and Lake George. Depth to basement of the depocenters typically exceeds 3,000 m, and reprocessed aeromagnetic data have yielded depths to basement in excess of 5,000 m, (Fig. 3). Recent gravity and ground magnetic surveys carried out by the Petroleum Exploration and Production Department (PEPD) indicate that depths to basement of the order of 6,000 m are possible.

The geological framework of western Uganda can be reviewed by considering the two tectonic phases which make up its history: the prerift phase and the syn-rift phase.

Rocks of the pre-rift sequence begin with Precambrian basement which is composed of predominantly gneisses and varying amounts of quartzites and granites.2 Unmetamorphosed pre-rift clastics which may range in age from Permian to Jurassic are preserved in some areas. Included are tillites, variegated mudstones, sandstones and conglomerates.

Syn-rift sediments are composed of Miocene to Quaternary lacustrine and fluviatile sediments on the basin flanks (Figs. 4 and 5). The Kisegi formation of Lower Miocene to Pliocene age consists of conglomerates and coarse sandstones which fine upward into sands and shales (Fig. 5). Lateral changes are quite variable resulting in discontinuous units. The basal Kaiso formation is dated as early Pleistocene from fossil mammals and fresh-water molluscs (Fig. 5). The interval is followed by late Pleistocene to Holocene terrace deposits of clays, silts, sands and gravels.

RESERVOIR

Good reservoirs are developed in sandstones of the Tertiary syn-rift section with porosities measured from cores exceeding 30%. The reservoirs are confirmed by flow rates measured at 1,200 b/d from water wells. In addition, older intervals of the pre-rift stage which crop out in Zaire are clastic-rich and contain sandstones adequate for reservoir development. Stacked sequences of good reservoir quality sand.stones are exposed on the edge of the Semliki basin in Uganda (Fig. 6).

SOURCE

The presence of adequate source rocks may be inferred from surface oil seeps which have been known for many years. An outcrop of sandstone at Kibiro (Fig. 7) is saturated with tarry oil of 14.7 gravity having a sulfur content of 0.31%.1 2 Two nearby boreholes encountered heavy asphaltic oil of 15.9 gravity as well. Kikubu outcrops of conglomeratic sandstones are saturated with oil of estimated 25-35 gravity API.2 The only deep test, Waki-B1 drilled in 1938, encountered shows in sands and shales of the Tertiary. The well encountered a total of 95 m of oil shales with TOC exceeding 6%.

Analytical evaluation of the seeps has been carried out. Analysis of the seep at Kibiro by Simon Petroleum Technology5 indicates an early to middle-mature, organically rich source. A common source is indicated for oil from four samples in the Kibuku/Kisegi region by Fina Research.6 The crude from these samples is highly sulfurous, non-waxy and marine derived; a mixed environment, both marine and terrestrial, is inferred.

SEAL

No production has been established in Uganda and the effectiveness of seals remains a question. However, prolific production from fields in the Gulf of Suez and Sirte basins confirms the presence of adequate seals in rift basins. Mixed clastics are typical of rift-basin fill, and the presence of adequate source rocks suggests shales that can act as seals will be present as well.

STRUCTURE

Structural style of the Albertine graben is dominated by down-to-the-basin block faulting characteristic of rift basins2 (Fig. 8). Trend of the larger faults parallels the boundaries of the rift, although other fault trends are also recognized. Hydrocarbon traps associated with the faulting include dip reversals against the fault planes and rollover into the faults. Listric fault block rotation should produce anticlinal features.

EXPLORATION HISTORY

Exploration for oil and gas in Uganda dates to 1913, when the government granted an option to W. Brittlebank. Additional licenses to others followed in 1920-21, but little happened. The first serious assessment of the oil and gas potential of Uganda was made by government geologist E. J. Wayland, who documented hydrocarbon occurrences in the country.1 Activity by private companies increased during the period from the late 1920s to about 1940 with reconnaissance field surveys, acquisition of gravity data, the drilling of shallow stratigraphic tests, and the drilling of two deeper exploratory tests. The period from 194790 was dominated by work by the Uganda Geological Survey and included a World Bank funded Petroleum Exploration Promotion Project initiated in 1983. Since 1991 the Petroleum Exploration and Production Department of the Ministry of Natural Resources has been active in geological mapping and gravity and magnetic surveys. A part of this work has been accomplished in conjunction with the University of Columbia, the University of Leeds, and the University of Lubumbashi.

LICENSES

The Uganda government has defined three Exploration Areas (EAs) in the Albertine graben. Although not currently defined as EAs, other areas in the country may be applied for. The EAs are depicted on Fig. 7 and are based on a five-minute grid system. EA1 comprises some 8,910 sq km, EA2 is 6,885 sq km, and EA3 is 5,913 sq km. The government recognizes that industry may wish to apply for one or more of the EAs. The amount of acreage requested and awarded will depend on the work program proposed. An exploration license is awarded for a duration of 8 years. It is divided into three exploration periods:

Initial exploration period: 4 years
First extension period: 2 years

Second extension period: 2 years.

Mandatory relinquishments will be expected at the end of each exploration period. A production license is awarded for 25 years after approval of a development plan.

In granting licenses, the government will be seeking a certain minimum seismic commitment both onshore and offshore and an early well commitment to a depth adequate to evaluate a significant portion of the prospective section, both syn-rift and pre-rift. Work commitment will be tied to a minimum expenditure commitment.

FISCAL TERMS

An adequate fiscal program will achieve two objectives:

1. Encourage investment from petroleum companies for a wide range of potential field sizes under a variety of oil price and cost scenarios.

2. Ensure that the government achieves an equitable benefit from any oil reserves while satisfying the objectives above.

The fiscal regime is structured thus:

Government receives a royalty from the gross petroleum production within the Contract Area.

All exploration, development and production expenditures incurred by the company is to be recovered from all agreed percentage of oil and gas production after deduction of the royalty.

After recovery above a government/company split which will be negotiated will apply on the remaining production. Income tax rate is 35%. The government will consider alternative proposals.

ACKNOWLEDGMENT

This article is adapted from Petroleum Exploration Opportunities in Africa and Countries Beyond, by D. Keith Patton, published by PennWell Books, Tulsa, Okla.

REFERENCES

1. Wayland, E.J., 1925, cited in The hydrocarbon potential of the Albertine graben, Ministry of Natural Resources, Petroleum Exploration & Production Department, Republic of Uganda, Entebbe, 1993, 8 p.

2. The hydrocarbon potential of the Albertine graben, Ministry of Natural Resources, Petroleum Exploration & Production Department, Republic of Uganda, 1993, Entebbe, 1993, 8p.

3. Ebinger, C., Tectonic development of the western branch of the East African rift system, GSA Bull., Vol. 101, 1989, pp. 885-903.

4. Mukasa, R.K., Structure of the Albert and Edward rift basins (Uganda) from analyses of aeromagnetic data, post-graduate diploma in exploration geophysics dissertation, University of Leeds, 1993,

5. Bastow, M., 1989, Geochemical analysis of one oil seep sample, Uganda, Simon Petroleum Technology Report No. 6542/lc, 1989.

6. Myeke, B., Uganda seepage oil study, Fina Exploration and Production geochemistry report 92/11, 1992.

BIBLIOGRAPHY

Chatellier, J., and Slevin, A., Review of African petroleum and gas deposits, Journal of African Earth Sciences, Vol. 7, No. 3, 1987, pp. 561-578.

Clifford, A., African oil-past, present, and future, in Halbouty, M., ed., Future petroleum provinces of the world, AAPG Memoir 40, 1986, pp. 339-372.

Kashambuzi, R., Gravity and magnetic surveys of onshore Lake Albert basin, in Annual report of Petroleum Exploration and Production Department, Ministry of Natural Resources, Uganda, 1993,

Klemme, H.D., Petroleum basins-classifications and characteristics, Journal of Petroleum Geology, Vol. 3., No. 2, 1980, pp. 187-207.

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