An Integrated Approach to Unlock Low Resistivity Carbonate Reservoir in North of Sultanate of Oman

Abstract

Predicting productivity and recovery in such setting as the Natih E Formation requires good understanding and representation of permeability distribution at all scales. An integrated approach is therefore crucial for reservoir description, simulation and forecast of reservoir performance to optimize reservoir management. The interpretation of hydrocarbon saturation in carbonate reservoirs is often challenging due to their complex and heterogeneous pore structures. Resistivity logs are the main pay-zone identifiers because of the resistivity contrast between oil and water zones. However, when pay intervals exhibit low resistivities reassessing these intervals can have significant implication in terms of reserves and field development. The Natih E reservoir is one of the major carbonate reservoirs in the Sultanate of Oman distinguishing itself by multi level heterogeneities that can give rise to complex geometries in terms of flow paths, porosity and permeability. The Natih E reservoir in the studied field is divided into three reservoir zones. The top zone is characterized by patchy leached fabric with local intense dissolution features, thin and thick nodules surrounded by background microporous matrix, with minor oil stain and minor oil saturation computed from logs. The middle zone is a 2 m moderately sorted rudstone with coarse packstone/Grainstone matrix with significant macroporosity, with strong oil staining and high log derived oil saturation. The top zone measured low resistivity and therefore it was not part of the early development, as production was focused on targeting the middle high saturation zone. However, a decision was taken to test the low resistivity reservoir zone. The test showed very good, dry oil production and hence additional data were acquired to better understand the complexity of the reservoir. The additional data acquisition included core, NMR and BHI logs, and pressure and downhole fluid identification measurements. The core data gave useful insights about dissolution features, pore structure and facies that impacted the resistivity log. NMR data resulted in a better understanding of the matrix pore size distribution and porosity validation. Downhole fluid samples proved the presence of hydrocarbon in the Low Resistivity Pay interval. This paper presents a case study in low resistivity pay in carbonate reservoirs, where the integration of all data sources in a multidisciplinary effort unlocked additional reserve and production of the field.

AAPG Datapages/Search and Discovery Article #90356 ©2019 AAPG Middle East Region Geoscience Technology Workshop, Low Resistivity Pay, Muscat, Oman, October 7-9, 2019