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Evolution and Evaluation of a Shallow Marine LRLC Reservoir With Advanced Well-log Measurements: A Case Study from Offshore Nigeria

Abstract

Low resistivity low contrast (LRLC) reservoirs are often difficult to evaluate since important pay zones could be overlooked or completely bypassed if accurate and comprehensive evaluation program is not considered. It's imperative to understand the geological evolution of such reservoirs to understand their distribution and associated petrophysical properties and their variation in the field. The Tertiary shallow marine clastics LRLC reservoirs in the study area are interpreted to be deposited in deltaic environment offshore Nigeria in predominantly distal environment. Borehole image interpretation suggests that the low resistivity contrast is attributed to dispersed shale, probably due to the coating of clay around the sand grains in the toe part of a delta front in major coarsening up and feeble fining up sequences; sedimentation being controlled by a developing delta. Petrophysical data recorded at high resolution correlates indicating main causes of LRLC pay zone are often the clay content and distribution and small grain sizes within the reservoir rock resulting in low resistivity values and more irreducible water associated to the increased surface area. Advanced nuclear magnetic resonance (NMR) measurements are integrated with spectroscopy data to evaluate shaliness, porosity and permeability level-by-level, better defining any possible minerals contribution and nullifying the clay effect. High resolution NMR detailed rock description and magnetic resonance (T1 and T2) signal distributions portrays heterogeneous grain sizes distribution with increased amount of finer grains and small clay particles coating the sand grains or filling the pore throats between framework grains, hence altering the reservoir properties. NMR diffusion-based logging and resulting fluid maps confirms the presence of oil. The analysis of porosity, water volumes and saturation components computed from the petrophysical data revealed such reservoir, despite its complexity and lowered overall rock quality, could be produced free of water breakthrough. This case study highlights the interesting relation between the depositional facies in LRLC environment and its relation with the development and evaluation of petrophysical environment. The workflow adopted in this study helps to characterize and exploit similar reservoirs offshore Nigeria with better evaluation results; and producibility prediction.