Deciphering the Impact of Subsurface Geology on Reservoir Quality Indicators: Petrophysical Evaluation from Niger Delta
Authors: Ossai Okechukwu , Cavalleri Chiara , Shrivastva Chandramani , Claverie Michel 
Published: May 2016

Abstract
Petrophysical properties of subsurface are controlled by the geological evolution of the formation; consisting of structural framework, depositional history and diagenetic changes. Similar well log response can be observed on conventional logs in different settings, often misguiding the petrophysical evaluation compared to the reality, realized due to production discrepancies and challenges in history matching.
The Niger Delta marginal marine clastics reservoirs have traditionally been treated as well-characterized for formation evaluation over the years; however, reservoir distribution and quality could be quite variable due to the heterogeneities that went unnoticed while taking production from massive sands. Moving away from the best developed sands, different units were analyzed in the study area with advanced wireline measurements of tri-axial induction resistivity, nuclear magnetic resonance, and dielectric dispersion in the backdrop of geological understanding developed with the borehole images in nearby wells. An attempt has been made to understand the reservoir rock quality and producibility in response to the geological evolution of the subsurface.
Thinly bedded reservoirs are discovered and added to the reserves estimate, hitherto overlooked in the study area. Different sand units are studied, trying to understand the geological factors that could impact their producibility behavior. Multiple innovative reservoir quality indicators are listed and further analyzed honoring the geology from different combinations of advanced logging measurements. These include: invasion profile, high resolution volumetric analysis using sand and shale definition from image logs, irreducible water analysis and movable fractions from dielectric dispersion measurement, and magnetic resonance T1, T2 and diffusion constant measurements.
Different sand units studied with respect to the developed indicators capture the heterogeneity and resolve for more precise reservoir summation. The well-test results in the study area validate this innovative approach of integrating the geological information with high resolution advanced well-log measurements for developing reservoir quality indicators.

Introduction
Niger delta is a prolific hydrocarbon province (Figure-1) and has been producing over many decades through different fields scattered in Tertiary clastics reservoirs. From the Eocene to the present, the delta has prograded southwestward, forming depobelts that represent the most active portion of the delta at each stage of its development (Tuttle, 1999). Distribution of reservoir sands is quite complex when one moves out of the easily recognizable massive sands. Rock quality and reservoir distribution can be quite variable due to heterogeneities and differently developed sand-shale packages. Besides, some of the reservoirs are stacked sand bodies with variable salinity among the different hydraulic units.
It is imperative to understand the geological setting; structural and depositional to comprehend the petrophysical response in the sub-surface.