University of Khartoum

Facies, Depositional Environments and Reservoir Quality of the Turonian-late Senonian Zarga Formation-Darfur Group in Heglig and Unity Fields, Muglad Rift Basin, Sudan..

Facies, Depositional Environments and Reservoir Quality of the Turonian-late Senonian Zarga Formation-Darfur Group in Heglig and Unity Fields, Muglad Rift Basin, Sudan..

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Title: Facies, Depositional Environments and Reservoir Quality of the Turonian-late Senonian Zarga Formation-Darfur Group in Heglig and Unity Fields, Muglad Rift Basin, Sudan..
Author: Saida Osman Mohamed, Mustafa
Abstract: This study investigates the subsurface sedimentary facies, depositional environments and reservoir quality of the Turonian-Late Senonian Zarga formation of Darfur group in Unity and Heglig fields in the Muglad rift basin, Sudan. The principal objective of this study is to characterize the facies, paleoenvironments and reconstruct a facies depositional model, to characterize the sandstone petrography and provenances and to investigate the reservoir geology and quality. The methods of study included lithofacies analysis based on conventional cores and wireline logs, and sandstone petrographic analysis, heavy and clay minerals analysis. The core lithofacies analysis shows the cross trough-bedded sandstone, ripple cross-laminated sandstone, low-angle planner crosssbedded sandstone, laminated sandstone, siltstone and mudstone, laminated to massive siltstone, mudstone and massive to laminated shale. Based on the lithofacies analysis it reveals that Zarga formation compnses, from top to bottom, of three majors units, fluvial-dominated, fluvial-lacustrine and lacustrine-dominated unit. These units generally correspond to the upper, middle and lower parts of Zarga formation respectively. The fluvial unit is composed of stacked fining upward sequences of fine to medium grained channel sandstone and overbank floodplain claystone and siltstone facies. The fluvial/lacustrine units consist of mixed, interbedded fine to medium sandstone and claystone facies. The lower lacustrine-dominated unit consists mainly of claystones and siltstones unit interbedded lacustrine and meandering stream facies. These units reflect the fluvial and lacustrine system response to lower base level change as controlled mainly by tectonics and climate. The channel-fill geometry of sandstones may be characterized by narrow isolated channel in a vertical or isolating stacking manner formed by vertical aggradation within a relatively stable channel under condition of rapid subsidence. Single-story sandstone bodies in Zarga formation wide average range (45-430m) and average thickness range (O.7-4.3m). The he tero lithic units include abandoned channel and overbank/floodplain deposits average wide range (49-897m) and average thickness range (1.1-1 Om). The width/depth ratio> 15 reflected sheet sandstone bodies. The heavy mineral analysis reveals three zones consisting first of ultrastable heavy minerals (zircon, tourmaline, rutile), second of metastable heavy minerals (staurolite and kyanite) and the third zone of the unstable heavy minerals (hornblende and garnet?) .The heavy minerals zone indicates that the main source area rock are metamorphic, igneous and recycled sedimentary rocks. The heavy mineral distribution in Zarga formation appears to be controlled by environments, source area, tectonics, weathering and transportation processes and intrastratal solutions. The clay minerals recognized in Zarga formation include smectite, kaolinite, smectite/illite, illite and chlorite. The clay minerals distribution reflects the control of climate, tectonics, source rock, environment and diagenetic process. The Zarga reservoir sandstone are mostly arkosic arenite, which are mainly fine to medium grained, poorly sorted, subangular to angular and cemented by argillaceous and calcite cement. Secondary porosity is present and introduced as a result of partial or complete dissolution of carbonate cement, feldspar and clay cement. Reservoir quality of Zarga formation seems to be controlled by macro-scale of facies, sub-environment, geometry, depth, and diagenetic alteration such as compaction, cementation and diagenetic alteration and dissolution are of prime importance. Zarga formation sandstone represents fair to good quality oil reservoir in Unity and poor in Heglig fields.
URI: http://khartoumspace.uofk.edu/handle/123456789/14132
Date: 2015-06-22


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