ABSTRACT

The Turonian Amasiri Sandstone of the EzeAku Group was studied to review the characteristics of the sedimentary units, identify the sedimentary facies, and reconstruct the paleoenvironments and the paleogeography. Ten outcrop sections were measured and described in details.

SEDLOGTM was used to prepare the lithologic logs of the measured sections. The geologic map of the study area was also produced. Eleven lithofacies were identified. These are 1 the coquina limestone facies, 2 the bioturbated mudstone facies, 3 the muddominant heterolithic facies, 4 the massive finegrained sandstone facies,5 the bioturbated sandstone facies, 6 the horizontal laminated sandstone facies, 7 the ripple/wavy bedded sandstone facies, 8 the chaotic sandstone facies, 9 the bioturbated cross stratified sandstone facies, 10 the trough cross stratified sandstone facies and 11 the planar trough cross stratified sandstone facies. The litholog patterns show that the Amasiri Sandstone comprises predominantly coarsening upward successions with subsidiary fining upward and uniform graded units. On the basis of the lithofacies pattern and association, the Amasiri Sandstone is interpreted as deposited in a shallowing upward siliciclastic stormdominated shoreline. Paleocurrent analysis shows southeast, southwest bimodal paleoflow.

Petrographic analysis classifies the Amasiri Sandstone as texturally and mineralogically immature arkose. Diagenetic analysis and compaction data show that the contact index CI ranges from 1.3 3.8, and tightpacking index TPI ranges from 0.8 3.0. The depth of burial for the Amasiri Sandstone has been estimated to range from 1,113m 2,564m. An integration of the petrographic and paleocurrent data indicates a mixed provenance of medium high grade metamorphic and igneous source terrains as well as sedimentary terrain that lay northeast and east of the study area. The source terrains are mainly the Oban Massif, Obudu Plateau, and the folded and uplifted PreSantonian sedimentary units in the Mamfe embayment. On reservoir potential, the Amasiri Sandstone may not be a good hydrocarbon reservoir, though its angular unconformity with Nkporo Group could provide a good trapping element for the more porous and permeable neighbouring Afikpo Sandstone of Compano Maastritchian age which have been noted to be of good hydrocarbon reservoir quality.

1.1 Statement of the problem

The EzeAku Group is one of the Upper Cretaceous lithostratigraphic units in the Abakaliki Anticlinorium. The Santonian tectonic events had folded the sediments of the Abakaliki Anticlinorium into a series of northeasttrending folds constituting the Abakaliki Fold Belt Murat, 1972. The EzeAku Group flanks both sides of the Abakaliki Fold Belt while the older Asu River Group occupies the core Banerjee, 1981. At the western flank of the Abakaliki Fold Belt, the sediments of the EzeAku Group comprise calcareous shales with extensive limestone bands, while at the eastern flank, and in the Afikpo Synclinorium the study area, the EzeAku Group outcrops as northeast southwest trending sandstone interbeded with shales, refered to as the Amasiri Sandstone Simpson, 1954; Reyment, 1965; Whiteman, 1982.

The stratigraphic succession in the Abakaliki Fold Belt begins with the Early Cretaceous Asu River Group of Albian age. The Asu River Group is succeeded unconformably by the Eze Aku Group. There is an unconformity between the Albian and the Turonian sediments. This unconformity is indicated by the absence of the Cenomanian sediments in the study area. The lithostratigraphic succession within the study area comprises the Asu River Group and the EzeAku Group, represented by the Amasiri Sandstone Reyment, 1965, which forms extensive NE SW trending ridges. To interprete its depositional environment, Bernajee 1980 and Amajor 1987 studied the Amasiri Sandstone. While Bernajee 1980 suggested subtidal and tide dominated shallow marine depositional environment, Amajor 1987 disagreed and suggested deposition in storm dominated, shallow shelf environment. This controversy over the depositional environment of the Amasiri Sandstone still exists.

Furthermore, Agagu and Ekweozor 1982 studied the source rock characteristics of parts of the Lower Benue Trough and in their description of the geologic setting, stated that the Turonian EzeAku Group was overlain by the Amasiri Sandstone and Makurdi Sandstones, which Reyment 1965, Whiteman 1982, and Nwajide 1982 noted are lateral equivalents of the EzeAku Group.

To better understand the environment of deposition and paleogeographic history of the Amasiri Sandstone, a detailed sedimentological and facies analysis of the sediments is required. Process sedimentology is the detailed bed by bed description of sedimentary rocks for establishing the link between the deposit and the physics and hydrodynamics of the depositional process Shanmugam, 2006, 2007. It is the foundation for reconstructing ancient depositional environments and for understanding sandstone reservoir potential Middleton and Hampton, 1973; Shanmugam, 2006, 2007. Facies analysis on the other hand, is a rigorous, scientific approach to the interpretation of the characteristics of a sedimentary unit Middleton, 1973; Anderton, 1985; Reading and Levell, 1996; Walker, 1992, 2006; Nichols and Fisher, 2007; Nichols, 2009. These characteristics include depositional geometry, sedimentary structures, texture, color and biogenic content of the sedimentary units. These are imparted on the sediments by physical, biological and chemical processes of transport and deposition. By interpreting the sediments in terms of these physical, chemical and ecological conditions at the time of deposition, it is possible to ...Get Complete Material.