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The " Turkana Grits " : Potential Hydrocarbon Reservoirs of the Northern and Central Kenya Rifts

Abstract : Over two thirds of the world’s giant oilfields are found in two principle tectonic regimes; continental passive margins and continental rifts. The preferential formation of hydrocarbons in rifts is attributed to the proximal juxtaposition of high grade, lacustrine source rock units with medium to high grade reservoir rocks - a consequence of both faulting and sedimentation in the resulting accommodation space, which in many cases may locally modify the prevailing climatic conditions. In one of such basins, the Lokichar Basin in the Kenyan Rift, over 600 million barrels of recoverable oil have been discovered. The principle reservoir unit in this basin is the Lokone Sandstone that belongs to a larger family of sandstones called the ‘Turkana Grits’, arkosic sandstones that are sandwiched between metamorphic basement and mid-Miocene volcanics. The hydrocarbon proclivity of the Lokone Sandstones as reservoir units motivated further study of the ‘Turkana Grits’, as potential hydrocarbon reservoirs. In this work, three sedimentary formations, i.e. Kimwarer Formation, Kamego Formation and Loriu Sandstones, which have not been previously fully characterized from chronostratigraphic and sedimentological point of views were studied through detailed logging. Over 170 samples were collected to determine, detrital and authigenic components, the main cementation zones in the different outcrops, and, from lithofacies analysis, the depositional environments. Volcanic and intrusive samples were also characterized and used for 39Ar-40Ar dating. Three superposed depositional environments were determined for the Kimwarer Formation, a distal fluvial channel, an alluvial fan and a floodplain depositional environment. The diagenetic study shows cements change from dominant hematite at the base to calcite within the middle zones and back to hematite towards the top of the Formation. These cementation episodes occur during early and relatively late diagenesis in low temperature conditions (<80 °C), under significant mechanical compaction. A minimum deposition age at ca. 18 Ma (Early Miocene – Burdigalian) has also been set for the Kimwarer Formation. The Kamego Formation evolves from fluvial to floodplain depositional environments and is dominantly cemented by hematite. Calcite cement is only noted in the lowermost 5m. A thin lava flow interbedded with the topmost sediments of the Kamego Formation gave a minimum deposition age of ca. 20 Ma for most of the sediments. The Loriu Sandstone is composed predominantly of fluvial channel deposits. The main cements are calcite, hematite and kaolinite clays. A cross-cutting dyke suggests a minimum deposition age of ca. 18.5Ma. A final reservoir analysis of the Turkana Grits shows that while compaction and cementation are dominant agents of porosity reduction, the Turkana Grits are generally poor to moderately good reservoir units. The Lokone Sanstone has been proven to have sub-surface porosities ranging between 10 - 20% and permeabilities as high as 3 darcies (Africa Oil Corporation, 2011). For petrographic analyses, the Kimwarer Formation has been ranked as having the second best reservoir potential with porosities as high as 20% in some sections of its studied stratigraphy. The Kamego Formation also has good potential but is not as highly ranked owing to the huge component of volcanic material that have a greater propensity to diagenetic alteration. No good porosities were noted for the Loriu Sandstone and hence this formation has been ranked 5th amongst the Turkana Grits
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George Muia. The " Turkana Grits " : Potential Hydrocarbon Reservoirs of the Northern and Central Kenya Rifts . Applied geology. Université de Rennes 1, 2015. English. ⟨tel-01379989⟩

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