FACIES ARCHITECTURE AND CHARACTERIZATION OF FLUVIAL DOMINATED SANDSTONES OF NORTHERN BIDA BASIN, NORTH-CENTRAL NIGERIA

Abstract

This research characterized the reservoir geometry and architecture of fluvial dominated sandstones of the northern part of Bida Basin, North-central Nigeria. Detailed regional field studies were carried out and good snap shots of the vertical profiles of outcrop sections were taken at various locations visited. A careful study at an outcrop and reservoir scales as well as on photomosaics led to recognition of various architectural elements such as channel elements (CH), downstream accreting macroforms (DA), sandy bed forms (SB), lateral accretion deposit (LA) and overbank fines (OF), which record varied deposition and infilling mechanisms within the channel confines and adjacent sub-environments. The sandstone geometry and dimensional analyses revealed e average channel width-to-depth (W/D) ratios of 36.45, 19.37, 32.93 and 18.53 from channelized sand bodies in Manigi, Bida and Rabba areas respectively, and these values indicate that channel elements in these areas were generally mobile channels were filled by a process of channel migration or switching within a single major channel. The architectural element analyses show that reservoir architecture is scale dependent. Thus, on an inter-well to field scale, three reservoir architectures are recognized in the study area, namely: reservoir architecture 1 (mainly layer cake), reservoir architecture 2 (mainly jigsaw puzzle) and reservoir architecture 3 (labyrinth) style. Layer cake reservoir architecture is exemplified by Rabba outcrop, jigsaw puzzle reservoir system is illustrated by potential reservoirs in Kawo area, and labyrinth architectural styles characterizes Manigi’s outcrop. This range of reservoir systems suggests that the potential reservoirs for hydrocarbons in the northern Bida Basin could be complex and far from simple, and that a variety of reservoir properties is not impossible. It could be concluded from the field evidences that the porosity-permeability (poroperm) values, vertical sweep efficiency, production performance and ultimately hydrocarbon recovery factors may be drastically reduced as one moves hydrocarbon production from layer cake through jigsaw puzzle to labyrinth reservoir system (e.g. from Rabba via Kawo to Manigi type reservoirs). The study recommends that an adequate knowledge and recognition of the reservoir architectural styles are critical to designing the initial field development plan (FDP) at the development phase in exploring for and production of hydrocarbon in the northern part of Bida Basin.