Well logs and ditch-cuttings information was used to study the subsurface of the Yowi oilfields, shallow offshore, in the eastern Niger Delta sedimentary basin. The essence of the study was to determine the gross depositional environments of selected and-bodies. Electrofacies trends from gamma ray log and mineral composition of ditch-cutting samples were used to reconstruct the depositional environments in the study area. A total of eleven (8) relatively thick sandstones reservoirs penetrated by five representative wells (A9, A12, A6, A4 and A2) occurred within the paralic Agbada sequence in the field. Seven log facies associations deduced from log trends revealed three major environments resulting from fluvial, marine and marginal marine processes. A coarsening upward log motif is interpreted to be deposits resulting from progradation, whereas fining upward trends are transgressive deposits. The presence of glauconite in all the wells in the field from about 1800ft down hole within the Agbada Formation shows that the environments were influenced by marine processes than continental. A predominantly marine, deltaic sequence strongly influenced by clastic output from the delta is inferred for the Yowi Field. Water depths fluctuated considerably and deposition occurred within a variety of littoral and neritic environments ranging from near shore barrier sand complexes to fully marine, outer shelf mudstones. An overall shallowing trend is observed on well log towards the base of the Qua Iboe Shale Member. The upper part of the well between the intervals of 2000 and 2450ft appears to have been deposited in a marginal marine setting. Below this depth down to about 5000ft may indicate relatively shallow marine conditions with normal salinities, well oxygenated bottom waters and periodic access to open marine conditions. Interval above 1800ft is the overlying continental Benin Formation. The associated environments based on log shapes and the presence of glauconite within the studied interval include; offshore/regressive bars, barrier foot, beach and shore face deposits.

Geochemical studies of sandstones from the three lithostratigraphic successions in the southern Benue Trough of Nigeria were undertaken for a geochemical characterization of the sandstones, and to assess their stratigraphic and source evolution.Major and trace elements data were obtained from outcrop sandstone samples. The SiO 2 /Al 2 O 3 , Fe 2 O 3 /K 2 O ratios and CaO contents have been used to characterize the Cretaceous sandstones into Al-rich and high and low Fe 2 O 3 /K 2 O ratio sandstones. Results indicate that there are geochemical features that display stratigraphic trends across the succession from the Asu River Group, Eze-Aku Group to the proto-Niger Delta succession which may imply a discontinuous evolution from different source terrains of Precambrian and Mesozoic ages that supplied the sediments. The Asu River Group sandstones have lower SiO 2 , Al 2 O 3 , Fe 2 O 3 and higher MgO; the Eze-Aku sandstones have higher TiO 2 , CaO, alkalis and lower MgO while the proto-Niger Delta sandstones have higher SiO 2 , Al 2 O 3 , Fe 2 O 3 and lower alkalis and CaO. These discontinuities signify the influence of tectonic impulses that affected the southern Benue Trough during the Cretaceous time. Changes in ratios of TiO 2 /Al 2 O 3 , Fe 2 O 3 , Cr and Zr suggest an increasingly mafic contribution to the depositional basin with time. The chemical index of alteration increases with time, possibly suggesting that a more intense weathering regime in the hinterland developed with time.

Mineralogical and chemical studies have been made of shale cuttings from the Akiri‐1 well in Oligocene — Miocene sediments of the Niger Delta. The < 2μm fraction from the 2,183 to 3,720m stratigraphic interval was analyzed by X‐ray diffraction. Kaolinite and chlorite are more abundant than other clay minerals. The illiteh mectite composition changes from a phase with 32% illite layers, to one with 82% illite layers over the 3,250 to 3,720m stratigraphic interval. Kaolinite decreased in shallow shales from 22 to 16% and increases to 29% in deep shales. Quartz increases in abundance with depth, while chlorite and plagioclase feldspar show no systematic trend with depth. Calcite is present in small amounts in all the samples. Variations in the bulk chemical composition of true shale with depth only show significant changes in the K2O and Fe2O3 content: other major chemical constituents show no systematic variations. The potassium appears to be derived from the pore water. The loss of smectite and abundance of Fe coincided with hydrocarbon occurrence. On the basis of the apparent systematic transformation of smectites to illites through mixed layers, the illite/smectite reference interval in the subsurface has been established: this interval coincided with hydrocarbon occurrence. Mineralogical and chemical changes observed are dependent on the pore system chemistry, the temperature, and possibly compaction effects.

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This paper deals with the regional and structural framework of the Cretaceous rocks in the Afikpo Basin located in the southeastern part of the Lower Benue Trough. Results from regional tectonics are presented together with those of the microtectonic analysis of microfaults in the Owutu-Afikpo-Adadama area in the basin.The Owutu-Afikpo-Adadama ridge at the north-central part of the basin marks the boundary between the Late Cenomanian-Turonian-Conianian sediments and the Campanian-Maastrichtian sandstones. This ridge trends N45ºE on average and is faulted in three main directions, namely: (1) N-S normal faults; (2) NE-SW strike-slip faults; and (3) NW-SE strike-slip faults. The faulted rocks along these brittle discontinuities are mainly cataclastics with internal fracture cleavage and sigmoidal quartz mosaics that are reminiscent of extensional deformation. The cataclasites often bear slickenside striations.The NE-SW and NW-SE strike-slip faults are the results of the youngest brittle events on the Owutu-Afikpo-Adadama ridge. These faults were reactivated after deposition of the Campanian-Maastrichtain sediments (post-depositional faults) and, therefore, are post-Maastrichtain. Microfault analysis of these fault trends suggests an extensional regime that prevailed in the Lower Benue Trough. These results are of significance because this event is least poorly understood and less documented in contrast to the Cenomanian and Santonian events that affected the older Cretaceous strata in this region.