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Wolela A.,Petroleum Licensing and Administration Directorate
Journal of African Earth Sciences | Year: 2014

Major element, K/Ar, sedimentation and palynology data are reported on Tertiary volcanic and sedimentary rocks from the Yayu and Delbi-Moye Basins, SW Ethiopia. In the Yayu and Delbi-Moye Basins, pre-rift volcanic (basalts) and post-rift volcanic (basalts) are separated by coal and oil shale-bearing sedimentary rocks. The basalts are tholeiitic in composition. The K/Ar data on the volcanic rocks range from 10.98. ±. 0.55. Ma (Lower Miocene i.e. Tortonian) to 111.19. ±. 2.83. Ma (Early Cretaceous i.e. Aptian). Inter-Trappean coal and oil shale-bearing sedimentary rocks are widely distributed in the Yayu and Delbi-Moye Basins. The NNW-SSE fault system developed grabens and half-grabens for the deposition of coal and oil shale-bearing sedimentary rocks. The sedimentary successions are dominated by fine to medium-grained sandstones, siltstones, mudstones, carbonaceous shales, carbonaceous claystones, coal and oil shale seams, and are characterized by meandering river and lacustrine depositional environments. Fresh water algae Botryococcus brauni, Pediastrum sp., Polypodii sporites favus and Polypodii sporites afavus and Pachydermites diederixi, are common palynomorphs in the studied coal and oil shales, and are indicators of lacustrine environment. The presence of Peregrinipollis nigericus is consistent with Oligocene to Miocene. The presence of Euphorbiaceae, Papilionacae, Melastomtaceae sp., Dodonaea, Martretia quadricornis, Rubiaceae sp. (pollen derived from tropical shrubs), Sapotaceae spp. and Meliaceae spp. are indicative of humid, tropical rainforest conditions. Extensive bio-assemblages confirm that the age of coal and oil shale-bearing sedimentary rocks to be Oligocene to Miocene. © 2014 Elsevier Ltd. Source


Wolela A.,Petroleum Licensing and Administration Directorate
Journal of African Earth Sciences | Year: 2014

Diagenetic studied in hydrocarbon-prospective Mesozoic rift basins were carried out in the Blue Nile Basin (Ethiopia), Ulster Basin (United Kingdom) and Hartford Basin (United States of America). Alluvial fan, single and amalgamated multistorey meandering and braided river, deep and shallow perennial lake, shallow ephemeral lake, aeolian and playa mud-flat are the prominent depositional environments. The studied sandstones exhibit red bed diagenesis. Source area geology, depositional environments, pore-water chemistry and circulation, tectonic setting and burial history controlled the diagenetic evolution. The diagenetic minerals include: facies-related minerals (calcrete and dolocrete), grain-coating clay minerals and/or hematite, quartz and feldspar overgrowths, carbonate cements, hematite, kaolinite, illite-smectite, smectite, illite, chlorite, actinolite, laumontite, pyrite and apatite. Diversity of diagenetic minerals and sequence of diagenetic alteration can be directly related to depositional environment and burial history of the basins. Variation in infiltrated clays, carbonate cements and clay minerals observed in the studied sandstones. The alluvial fan and fluviatile sandstones are dominated by kaolinite, illite calcite and ferroan calcite, whereas the playa and lacustrine sandstones are dominated by illite-smectite, smectite-chlorite, smectite, chlorite, dolomite ferroan dolomite and ankerite. Albite, pyrite and apatite are predominantly precipitated in lacustrine sandstones. Basaltic eruption in the basins modified mechanically infiltrated clays to authigenic clays. In all the studied sandstones, secondary porosity predominates over primary porosity. The oil emplacement inhabited clay authigenesis and generation of secondary porosity, whereas authigenesis of quartz, pyrite and apatite continued after oil emplacement. © 2014 Elsevier Ltd. All rights reserved. Source

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