Sedimentology and diagenesis of Neogene sediments in the central Kenya Rift Valley
Ego, John Kimaiyo
Sedimentation in the central Kenya Rift between latitudes 0°30'N and 0°45'N is controlled primarily by tectonics and volcanism. Rift-floor subsidence led to the formation of many basins in which fluviatile, lacustrine and colluvial deposits have accumulated from the Miocene epoch to the present. Major lava flows in the region capped most of the Neogene rift sediments after sedimentation in the individual basins, but episodic tectonic uplift and subsidence within the main half-graben have exposed the sediments in some areas, notably west of Lake Baringo. Clastic sediments along the rift shoulder in these basins are derived from metamorphic basement rocks, volcanic rocks or a combination of both sources. This thesis describes the detailed sedimentology and diagenesis of selected Neogene siliciclastic sediments in the central Kenya Rift basins and assesses the potential hydrocarbon reservoir and source rocks in the region. Petrographic and mineralogical analyses of siliciclastic rocks from throughout the region reveal that, in general, sands derived from basement rocks are better potential hydrocarbon reservoirs than those derived from volcanic sources. Sands of volcanic derivation are commonly susceptible to early diagenetic formation of authigenic clay minerals, zeolites, iron oxides and calcite cements, which reduce the porosity and permeability of the rocks before hydrocarbons can be come entrapped. X-ray diffraction and electron microprobe analyses of authigenic analcimes in the Miocene Tambach Formation show that they formed by the reaction of clays with saline, alkaline pore fluids, whereas the analcimes in the Kapkiamo and Poi lacustrine sub-basins of the Ngorora Formation likely formed from zeolite precursors, that themselves originated from reaction of volcanic glass with alkaline water. Authigenic analcimes in the sediments of the Ngorora Formation occur as pore-lining and pore-filling cements and predate authigenic smectites. The paleolakes of the Poi and Kapkiamo basins of the Ngorora Formation (Member C) were saline and alkaline (commonly, pH > 9.5) at certain stages and periodically underwent desiccation. Early diagenesis is strongly controlled by groundwater levels and their fluctuations, while the effect of temperature likely becomes more significant with burial. The results of a diagenetic study of selected Miocene siliciclastic sediments in the central Kenya Rift show that the greatest hydrocarbon reservoir potential exists in fluviatile arkosic or quartzose sands that have not experienced much early diagenesis and retain much primary porosity.