Geology: East Africa study highlights regional framework using geoscience syntheses

Physiographic index map showing the geographic extent of the East Africa study area.

The search for economic accumulations of hydrocarbons along the East Africa continental margin has long been overshadowed by industry's historically heavy operational focus in North Africa and West Africa. This situation is changing, especially as acreage opportunities lessen and acreage entry costs continue to generally escalate in West Africa.

East Africa beckons, and national oil companies and agencies are more eager than ever for foreign investment in the exploration sector. Excellent regional-quality geological and geophysical data sets from East Africa, analyzed using modern GIS and visualization tools, yield fresh insights on the area's prospectivity.

Properly corrected and enhanced gravity and magnetic anomaly data have widespread utility in acreage evaluation activities, such as those in new venture areas. Applications include the mapping of sedimentary depositional trends and basin architectural elements (basin and sub-basin trends, structural axes, hinge zones, strike-slip deformational zones, block-faulted basement framework).

These data can be combined or fused with other data (heat flow, sediment and crustal isopach data, seismicity data, crustal type, crustal age) to gain additional insight on basin heat flow, hydrocarbon generation and maturation, and the most likely hydrocarbon migration paths from delineated oil generative fetch areas. A limited number of examples are presented here to convey a sense of the wide-ranging utility and application of such data and associated derived products.

Onshore trends

The Western Branch (including the Albertine Rift, Tanganyika Basin, Rukwa Basin, and Malawi Basin) and the Eastern Branch (Gregory/Kenya Rift) of the Cenozoic East African Rift System are coincident with regional gravity anomaly maxima. It should be noted that gravity anomaly minima associated with lower density rift sedimentary fill are observable if the broader regional gravity effects discussed here are removed in a gravity anomaly residualization process.

Interpretation of these regional gravity anomaly maxima trends, in the context of rifting processes, implies that they mark the location of thinned crust over areas of mantle-doming and crustal extension. The gravity maxima arise from positive density contrasts created by denser material associated with mantle doming and igneous intrusive activity within the stretched crust.

Older rift basin systems are also evident. The northeast-trending Selous Basin, a failed rift of Permo-Triassic age, is demarcated by a positive "saddle" in the regional gravity anomaly field. The regional gravity anomaly maxima trends, together with surface geomorphological data, are helpful in predicting the locations of rift valleys having the thickest and most prospective continental-type sedimentary deposits.

In the Albertine Graben of Uganda, numerous documented surface oil seeps confirm the existence of hydrocarbon source beds and the implied generation, expulsion, and migration of hydrocarbons.

Crustal foundation

The eastern side of the island of Madagascar is marked by a regional gravity anomaly maximum, as is the Madagascar Plateau (submarine feature) south of the island. The gravity maximum that trends north along eastern Madagascar arises in part from basement, which is exposed in outcrop along the eastern half of the island. An additional likely source of the gravity maximum is the dense igneous underplating and intrusions formed in Upper Cretaceous time, when the Madagascar Island/Plateau region was associated with a large igneous province (LIP) event known as the Madagascar Flood Basalt Episode.

The mantle plume responsible for this magmatic episode presumably helped cause the later rifting and northeast drift (Gondwana breakup) of India away from Madagascar in the Late Cretaceous.

An argument exists as to whether the Madagascar Plateau is floored by continental-type or oceanic-type crust. This question can be partially addressed by considering the interpreted crustal thickness results from this study. Crust of thickness intermediate between that typical of continental-type and oceanic-type crust is interpreted to underlie the Madagascar Plateau. The submerged plateau region may represent heavily modified continental crust - possibly attenuated and reworked by thermal plume impingement during the LIP event.

The prospective Majunga and Morondava Basins offshore western Madagascar correspond to well-defined gravity anomaly minima. The Majunga salt basin has attracted recent industry interest, whereas the Morondava Basin has been the focus of considerable previous exploration activity - being the site of the Bemolanga tar sand and Tsimiroro heavy oil discoveries.

Leakage, seal integrity

Earthquake epicenters (circular blue and red-colored symbols) displayed against residual gravity anomaly data in the Ruvuma Delta/Basin area. Oil seeps and the Mnazi Bay gas discovery are located in the Ruvuma Delta. The white dotted lines depict interpreted strike-slip elements or fracture zones associated with the Davie Fracture Zone system.

The Ruvuma Delta straddles the border (defined by the Ruvuma River) between Mozambique and Tanzania. The Davie Fracture Zone, an impressive NNW-trending right-lateral strike-slip zone, occurs just seaward of the Ruvuma Delta. The delta is roughly coincident with a basin by the same name.

The Ruvuma Basin is prospective - the Msimbati oil seeps and the Mnazi Bay gas discovery (Oligocene deltaic sandstone reservoirs) are situated near the coast in Tanzania. The Mandawa Basin, a salt basin, is located just northward of the Ruvuma Basin. A well-defined residual gravity anomaly minimum marks the location of the Mandawa Basin.

Interestingly, the Mandawa Basin gravity anomaly minimum is bifurcated by an interpreted north-south strike-slip element that is apparently related to the Davie Fracture Zone system. A right-lateral offset, not known if real or apparent, is clearly seen in the gravity minimum upon traversing eastward across the line of bifurcation.

A possible risk-related exploration concern in the Ruvuma Delta area stems from the close proximity of reservoir beds to this and other strike-slip elements associated with the Davie Fracture Zone, the latter of which appear to be seismically active. Earthquake epicenter information can be combined with residual gravity anomaly data (or other data/imagery) to aid the assignment of risk-weightings to possible "leak-prone" areas.

Source and maturation

Residualized gravity anomaly minima and maxima exhibit a strongly developed north-south fabric that "streams" northward away from the Southwest Indian Ridge - suggestive of an oceanic fracture zone association for the gravity anomaly lineations. A coast-hugging linear gravity anomaly associated with the Agulhas Fracture Zone makes a turn northwards along the Mozambique coast. Curiously, these interpreted fracture zone gravity anomaly lineations traverse the present Mozambique coastline and continue inland over the coastal plain.

The crust beneath the Mozambique Coastal Basin has been previously recognized as having an attenuated continental or quasi-oceanic type crustal nature. Recently, A. B. Watts (2000) concluded on the basis of flexural backstripping and gravity modeling that the crust underlying the coastal plain is likely of oceanic origin. The interpreted gravity anomaly fracture zone lineations noted here provide additional evidence of an oceanic-type crustal nature.

The ability to discriminate between continental-type crust and oceanic/transitional-type crust, as demonstrated here in the Mozambique Coastal Basin, has bearing on heat flow and hydrocarbon maturation issues - radiogenic heat production being less in oceanic-type crust (where convective heat transfer, or the lack of it, has a controlling influence). It is interesting then to examine the distribution of sediments in the coastal basin.

The tentative identification of oceanic-type crust beneath the coastal plain has hydrocarbon source implications. For instance, the Permo-Triassic lacustrine shales (Middle Sakamena Formation) considered to be the source for the Bemolanga tar sands and Tsimiroro heavy oils in Madagascar may be absent over parts of onshore Mozambique, which appear to be underlain by oceanic-type crust. This assumes that the sea floor is of Middle Jurassic age, the time when Madagascar-India-Antarctica separated and drifted southward relative to Africa along the Davie Fracture Zone).

Conclusion

North-sweeping gravity anomaly lineations related to oceanic fracture zones can be traced across the present coastline and into the onshore portion of the Mozambique Coastal Basin - an area previously suggested as being underlain by transitional-type or oceanic-type crust. The arrows denote two such positive, coast-crossing gravity anomaly lineations.

Widespread availability of powerful but inexpensive data management and visualization tools (GIS and 3D data rendering applications) places numerous multi-disciplinary geoscience data sets from the East Africa region squarely within grasp of today's explorationists.

These geological and geophysical data should not be ignored in the quest to better understand basin prospectivity. The value in routinely using such data to augment basin evaluation activities cannot be overstated.

The examples from East Africa demonstrate that even casual use of these data permits better understanding of the regional framework of an area and brings to the fore factors critical to decisions made in more block-specific or local settings. The outcome of combining and correlating such disparate earth science data is hard to predict until the exercise is actually done, though it nearly always leads to fresh insights and new exploration knowledge.

Study offers preliminary view

The recently completed study aids the high grading of hydrocarbon prospectivity in basin and block-specific settings and provides a regional geological context for such acreage evaluation. The project's geographic scope encompasses the onshore and offshore regions between the Gulf of Aden in the north and the tip of Africa in the south.

The study synthesizes a diverse assemblage of geoscience data that augment and support oil-finding activities. These include terrain, bathymetry, gravity, magnetics, seismicity, heat flow, seafloor age, crustal thickness, sedimentary thickness, and ancillary support data. Included are conventional hardcopy and digital data formats that can be integrated with other data, including wells, seafloor coring, heat flow sites, SAR seepage slick vectors, and others.