University of Khartoum

Sedimentology of the Late Jurassic / Cretaceous - Tertiary Strata of the NW Muglad and the Nile Rift Basins, Sudan

Sedimentology of the Late Jurassic / Cretaceous - Tertiary Strata of the NW Muglad and the Nile Rift Basins, Sudan

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Title: Sedimentology of the Late Jurassic / Cretaceous - Tertiary Strata of the NW Muglad and the Nile Rift Basins, Sudan
Author: Osman Mahmoud, Abdullatif
Abstract: This study investigates the sedimentological, mineralogical, geochemical and thermal maturity aspects of the Late Jurassic / Cretaceous to Tertiary strata of the North West Muglad and the Nile Block rift basins in Sudan. The study is based on data obtained from eight exploration wells. Different techniques have been used such as XRD, XRF and discriminant analyses. For thermal maturity evaluation, the clay diagenesis as well as observed and predicted maturity indices have been used. The clay mineralogical analyses of the strata along the well profiles reveal three distinct zones: upper, middle and lower, which can be correlated laterally across the rift basins. The upper zone consists of kaolinite, smectite with minor illite and/or chlorite. The middle zone consists of kaolinite, smectite, mixed- layer smectite/illite, illite, vermiculite and/or smectite/vermiculite. The lower zone consists of kaolinite, illite, chlorite and mixed-layer smectite/illite. The above zones suggest the influences of environmental, tectonic and burial diagenetic processes. The kaolinite and smectite (both are mostly pedogenica11y-derived) indicate a warm humid climate interrupted by dry seasons. Moreover, the predominance of kaolinite throughout the sedimentary succession suggests, in part, a detrital origin, from reworking of lateritic crusts on rift basin flanks and adjacent land-masses. The comparatively lower kaolinite abundances in the Nile basin may perhaps indicate less humidity or more aridity. This suggestion is supported further by the higher abundances of illite, chlorite and vermiculite which all indicate physical weathering under drier conditions and/or active relief rejuvenation. Consequently, this may also suggest that the Nile basin might have slightly different tectonic and/or climatic evolution. The clay geochemistry further shows preferential enrichment and depletion of' certain chemical elements in the rift basin environments. For instance, the more mobile elements (Mg, Ca, K, Rb) are enriched distally in the downstream floodplain, lacustrine and marine environments. In contrast, the less mobile elements (Ti, Al, Cr, Ca) remained proximally in the upstream channels and alluvial fans environments. Additionally, kaolinite and smectite show parallel distribution in the upstream and downstream environments respectively. This geochemical and mineralogical pattern indicates the influence of hydrolytic processes as controlled by the intensity of leaching as well as climatic and environmental conditions. Deviations from the above mentioned patterns may be attributed partly to post-depositional digenetic changes. Moreover, marine environment, show slightly higher abundances of the more mobile elements than those found in the floodplain and lacustrine environments. In contrast, the discriminant analysis of the marine and terrestrial environments (using clay geochemistry) suggests indecisive discrimination between these environments, which could partly be attributed to hydrolytic and diagenetic changes that might have modified the original clay composition in these environments. The Late Jurassic and Tertiary marine strata in the rift basins suggest a connection to tectonic and paleogeographic developments in east Africa. The smectite transformation shows the progressive decrease of smectite layers in the mixed-layer smectite/illite with the increase in subsurface temperature; this transformation indicates the influence of burial diagenesis. Three levels of smectite transformation have been recognized with a range of 90 to 20% smectite layers in the mixed layer, smectite/illite. This transformation occurs between 54° and 137° C. Although temperature seems to be the principal controlling factor in the smectite transformation, it appears that other factors might have influenced the reaction. Finally, the thermal maturity evaluation shows that the percentage of smectite in the mixed-layer smectite/illite decreases with the increase in both observed and predicted maturities, revealing the close relation between smectite transformation and temperature related changes in organic matter. Significant smectite transformation occurs in the Late Jurassic / Middle Cretaceous early rifting phase strata. These strata are indicated by both observed and predicted maturity indices as marginally to moderately mature. The largely immature Late Cretaceous/ Tertiary strata, however, show insignificant smectite transformation. Some discrepancies between the observed and predicted maturities are attributed to variation in burial and/or geothermal history of the rift basins. Moreover, the time-temperature modeling agrees with the observed maturity and indicates that oil generation is confined to the early rifting phase strata. The modeling also suggests an earlier timing of oil generation in the Companion (70-76 Ma) in both rift basins, and a later timing in the Oligocene (27-34 Ma) and in the Middle/ Upper Miocene ( 9-13 Ma) reported in the NW Muglad and the Nile block basins respectively .
URI: http://khartoumspace.uofk.edu/handle/123456789/13867
Date: 2015-06-22


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