Depositional Environments, Facies Architecture and Reservoir Geology of Omdurman Formation (Upper Cretaceous) Around Khartoum, Sudan

Abstract

The study investigates the paleoenvironment, facies types and architecture and paleogeography of the upper Cretaceous sediments of Omdurman Formation around Khartoum The study also evaluates the facies heterogeneity and reservoir geology of Omdurman Formation as the surface analogue for age equivalent similar subsurface strata in the rift basins of interior Sudan. The investigations included surface and subsurface lithofacies analysis based on vertical and lateral profiles as well as water bore hole lithological logs. Laboratory investigations included grain size analysis, clay minerals analysis, heavy minerals analysis and petrographic analysis. Nine facies were recognized and these include matrix-supported massive pebble conglomerate facies (Gms), trough cross-stratified pebble conglomerate facies (Gt), planar cross-stratified pebble conglomerate facies (Gp), trough cross-stratified sandstone facies (St), planar cross-stratified sandstone facies (Sp), horizontally stratified sandstone facies (Sh), rippled cross-laminated sandstone facies (Sr), laminated siltstone facies (Fl), and massive mudstone facies (Fm). Six architectural elements were differentiated within the Omdurman Formation. The proximal and medial alluvial plain are characterized by gravelly and sandy channel elements (CHg and CHs) and sandy bedform elements (SB). Overbank deposits included proximal and distal flood plain elements (FFP and FFD), crevasse splay element (CS) and levee deposits (LV). Surface and subsurface facies analysis shows that Umm Badda Member is dominated by stratified siltstone facies (FI) and massive mudstone facies (Fm). This member is composed of complete and incomplete fining upward sequences, representing meandering streams with well developed flood plain fines and locally lake deposits. In contrast the Merkhiyat Member is composed of coarse-grained facies deposited by low sinuosity braided streams characterized by shallow channels, bars and uncohesive banks. This member shows fining upward sequences of vertically stacked channels and bars deposits. The depositional facies model of the Omdurman Formation can be represented by a coarsening upward cycle reflecting allogenic and autogenic controls on sedimentation. Base level change is controlled by gradual decrease of tectonic activity leading eventually to the dominance of an open or external drainage system during the deposition of the upper member.