Abstract Beach Energy has been the sole interest holder and operator of the 7,200 km 2 Lake Tanganyika South block since 2010. The block is located within the western arm of the East African Rift System. The prospectivity of the lake sequence was enhanced by large oil discoveries in the similar geological environment of Lake Albert in Uganda and in the eastern part of the rift in Kenya. The lack of wells drilled in the lake to date make predicting sedimentary sections difficult. In 2010 Beach Energy commissioned CGG to fly a FALCON Airborne Gravity Gradiometer (AGG) and a high-resolution airborne magnetic (HRAM) survey over the Lake Tanganyika South block in order to map the basin structural framework and the depth to magnetic basement. The FALCON AGG survey facilitated the imaging of the architecture of the rift zone and the interpreted sediment thickness provided an indication of prospective petroleum target areas. This information was used to plan a subsequent 2D marine seismic survey, which was shot in 2012. The preliminary results from the 2D marine seismic survey has confirmed a rifting structure similar to that encountered further north at Lake Albert in Uganda. A number of targets over tilted fault blocks, low-side rollovers and mounded features, have been identified for follow-up from the seismic sections. Natural oil seeps evident on the surface of Lake Tanganyika, which have been sampled and analyzed by Beach Energy, also indicate that a working petroleum system is present in the sedimentary section of the rift beneath the lake.

Abstract Driven by requests to provide carbonate analogs for subsurface hydrocarbon exploration in rift settings, we have identified and described select examples, summarized them from a carbonate perspective, and assembled them into a GIS database. The analogs show a spectrum of sizes, shapes and styles of deposition for lacustrine and marginal marine settings, wherein the types of carbonates inferred from seismic and cores (emphasis on microbialites, tufas, and travertines) can be illustrated.

Abstract Bottom sediments of the largest lake of the East African Rift system, Lake Tanganyika (length 650 km; maximum depth 1470 m; volume 18,800 km 3 ) (Fig. 1), were extensively studied between 1983 and 1986 by Project PROBE of Duke University (U.S.A.) and Project GEORIFT (1984-1985) of Elf Aquitaine (France), using a wide range of methods such as reflection seismology, piston coring, and dredging. Interpretation of multifold reflection seismic profiles collected by Project PROBE suggests up to 4 km of sediment has accumulated within local depocenters. In addition, seismic profiles exhibit several seismic discontinuities and associated sequences, interpreted to have resulted from large-scale, temporal changes in local tectonics and/or climate (Burgess and others, 1988; Scholz and Rosendahl, 1988). Our interpretation of Recent and Modern profundal sediments in Lake Tanganyika is based on high-resolution, 5-kHz seismic surveys, along with multiple Kullenberg cores from the north and south basins of the lake collected during the GEORIFT project, and our interpretation of littoral clastic and biogenic sedimentation is based on grab sampling, observations from SCUBA, and gravity cores collected by the University of Arizona (Cohen, 1990; Soreghan and Cohen, 1991). These previous studies were supplemented by gravity cores collected in the Burundian part of the northern basin during a 1992 joint field operation by the University of Arizona (U.S.A.), the INSU-CNRS (France), and the CASIMIR project (Belgium). In this paper, our goal is to illustrate fundamental differences in facies associations within Lake Tanganyika that are, to a large degree, controlled by the basin structure.

Distinctive compositional trends occur in sand suites collected from structurally diverse segments of Lake Tanganyika (East African Rift System). Climate (subhumid, tropical) and source lithology (mixed granitic and metasedimentary rocks) are generally invariant between the source areas studied. Therefore, differences in sand composition relate directly to differences in compositional modification during transport and deposition of the sands. Sand composition along contrasting segments of the lake basin can be related to the strong structural asymmetry of the basin. We point-counted medium sands from river mouth, deltaic, beach, and shallow-water environments of four structurally distinct, lake-margin settings: (1) escarpment margin (Pemba, Zaire site); (2) hinged margin (Nyanza Lac, Burundi, and Rumonge, Burundi sites); (3) accommodation-zone margin (Magara, Burundi site); and (4) axial margin (Ruzizi River site, Burundi). These four margin-types are typical of the structural morphology in strongly asymmetric half-graben rift basins. Using the Gazzi-Dickinson point counting methodology, margins two through four cluster tightly in QFL space, suggesting similar source lithology. Samples from the Pemba site (escarpment margin) are significantly depleted in feldspar. Because of the uncertainties in interpreting this difference, the escarpment margin sands were not included in subsequent analyses. Variations in sand composition occur between the remaining sites when using the traditional point-counting methodology. The suite of fluvial sands entering the lake basin along the accommodation-zone margin (Magara site) averages 38% rock fragments compared to an average of 15 to 20% and 18% rock fragments for the suite of fluvial sands entering the basin along the hinged or axial margins, respectively. This is a fundamental contrast that correlates directly to differences in drainage basin size. Reworking at the depositional site produces additional compositional differences across the structural margins. In comparing the sample suites for each site, it is the variability in the data that is more important than average composition in highlighting these differences. Minimal compositional modification occurs at the depositional site along both the accommodation-zone margin and the axial margin. In contrast, significant environmental overprinting occurs along the hinged margin sites. Limited compositional modification along the accommodation-zone margin results from the steep depositional gradient and the very narrow high-energy zone which limits reworking of the sediment. The limited reworking and the consequent minimal compositional modification along the axial margin are primarily a function of the high sedimentation rate. The compositional modification (enrichment in quartz) along the hinged margin is a function of low sediment accumulation rates coupled with low subsidence rates, leaving large regions of shallow-water substrate exposed to long periods of reworking. Chemical alteration of labile phases also occurs along the hinged margin through carbonate replacement of feldspar. Our results suggest that, with careful statistical analysis, it is possible to use sandstone composition to distinguish between tectonic margins typical of half-graben basins. Using this approach in ancient studies, it is necessary to collect from several coeval facies among different study sites and to constrain source lithology and climate. In addition, these results are most appropriate for humid-climate basins, where weathering processes are most extreme.