Optimization analysis of organic carbon concentration and sedimentation rate in the Anza Graben, Kenya.

Abstract

The Anza Graben is a major NW-SE structurally oriented sedimentary basin that lies in the northern part of Kenya roughly between the latitudes 0° 30'N and 4°N, longitudes 36° 30'E and 41°E. Three basins: North, Central and South Anza define the graben and the main sub-basins comprise Chalbi in the northwest, the Yamicha in central and southeast and the Intermediate Tilted Blocks in the middle. The evolution, stratigraphy and environments of deposition of the sedimentary sequences of the basin, are discussed together with the post Gondwanaland break-up marine incursions in the graben. Burial plots based on studies of five selected wells have been used to construct burial history curves for the named sub-basins and to calculatethe TTI (Time-Temperature Index) indices. The results redefine maturity profiles and the hydrocarbon generation history of the basin. The oil window is determined from TTI in this study to occur from 2500 m to Total Depth (TD) for Bellatrix-l and 1500 m to TD·for Sirius-l both of which are in the Chalbi sub-basin. In the Yamicha basin the window runs from 2500 m to TD (Ndovu-l) and 2550 m to TO for Anza-l wells. The Intermediate Tilted Blocks area, Duma-l , has the window between 2500-3100 m depth. The scope of heat flow in the basin is generally moderate. The average geothermal gradient for the graben is 24.9°Clkm. Chalbi sub-basin, owing to its vicinity to the main Kenyan rift records a slightly higher value of 26.6-27.6°C/km. In the Yamicha area 22.2-24.0°C/km is observed while 23.9°C/km is noted in the Intermediate Tilted Blocks area. Empirical relationships between the paleosedimentation rates and Total Organic Carbon values give optimum sedimentation rates of 30-90 m/M. Y for the Chalbi sub-basin with a corresponding T.O.C. value of 0.9 - 1.4% which is well above the 0.5% prospective threshold.