Depositional Environment Geochemistry and Diagenesis of the Aptian Albian Lacusterine Abu Gabra Formation Muglad Rift Basin Sudan

Abstract

The study investigates the depositional, geochemistry, and the diagenesis of the Aptain-Albian Abu Gabra Formation in the Muglad rift basin, which is one of the largest basins in the Sudan, in which the evolution and trend was controlled by tectonic activity. The Abu Gabra Formation (Aptian – Early Albian) represented the early phase of lacustrine development with thousands feet of organic rich lacustrine claystone and shale deposits, interbedded with fine -grained sandstone and siltstone. The facies analysis for the formation has made possible the establishment of a depositional model with three main subenvironments, fluvial, fluvio-lacustrine, and lacustrine sub-environments. Interpretation of sediments indicated that, the deposition resulted from a humid climate with shorter periods of dry climate, and lack of external drainage. The hot humid climate played a major role in the fluctuation of the lake levels, in addition to the active tectonics during the deposition of the Formation, thus leading to the cyclicity of the facies. Seismic sections showed that normal faults controlled deposition of Abu Gabra Formation with fining upwards sequences associated with low stand of the lake, and or floodplain and coarsening upwards sequences associated with the deltaic sub environment. From the seismic facies analysis it was found that the fluvial subenvironments were associated with the pre-rifting phase, the fluvio-lacustrine subenvironments were associated with the early rifting phase, and the deep lacustrine environments was associated with the high rate of subsidence. In the studied wells the proposed model of Abu Gabra Formation in Heglig field, suggested that, the coarse clastic sediments were generated at the border faults and then continued into deep water. Clay mineral analysis using XRD showed a vertical downward increase in diagentic minerals, mainly illite and chlorite, through an intermediate clay mineral Illite/Smectite. In the three investigated wells, illite is the second dominant clay mineral, and since the concentration of Smectite decreases with depth, it could be assumed that the smectite mineral has been changed to illite through the mixed layer. This change in composition is attributed to the dehydration of this mixed layer clay mineral. The above finding was supported by XRF, and SEM. SEM studies showed a vertical downward increase in diagentic clay minerals, mainly authigenic illite and chlorite, and authigenic kaolinite. XRF also supported the diagentic finding, major elements oxides K2O, Al2O3 Fe2O3 are slightly increased downward. The variation in major elements Ca, Mg, Na, Al, Ti, was insignificant due to the homogeneity of the Formation, which is dominated mainly by fine-grained organic rich sediments. From thin section studies the, the decomposition of K-feldspars provide K for the formation of illite diagentically, these analysis also indicated the presence of several diagentic cements and minerals, secondary silica cement, feldspar dissolution, late stage calcite, and authigenic chlorite filling pore spaces. Diagenesis in Abu Gabra formation played a major role in the accumulation and the transportation of the hydrocarbon to its reservoirs. In the three wells diagenesis associated with the disappearance of smectite mineral, started at depth 9800 feet (2956m) approximately. The three types of diagenesis were, diagenesis with burial depth, diagenesis due to fluid migration, and the geothermal diagenesis, all might have been active during the history of diagenesis in Abu Gabra formation