A Step Change in Lower Shuaiba Reservoir Facies Interpretation Triggers New Discoveries

Abstract

The Shuaiba formation is one of the key contributors to production across the Middle East region including the Sultanate of Oman. However, the play was thought to be largely “creamed” – where the best prospects have been exploited in the concession area of Petroleum Development Oman. The remaining potential in areas of low relief was thought to be limited, with concerns around small volumes and early water breakthrough. However, advances in seismic, drilling and logging technologies coupled with new geological and petrophysical interpretations have boosted the outlook of Shuaiba across North Oman. This has been strengthened by a realization of the role of facies to trap hydrocarbons. When low Hydrocarbon Saturations are observed, it is important to understand if a structure was not charged at all, or if low Hydrocarbon Saturation is an artifact of poor facies encountered. In the worst case, misinterpreting the role of facies may result in missing opportunities. This paper consists of several case studies of Lower Shuaiba reservoirs. Existing petrophysical and geological studies identified several facies in Lower Shuaiba, with rudist facies being the best in terms of reservoir properties (porosity, permeability, and hydrocarbon saturation). All facies have relatively constant porosity in the range of ~23-26 p.u., and almost 100% calcite composition. The comparison of observed Archie Saturation compared to saturation height function is commonly used for facies identification in discovered fields, where free water level and fluid properties are known. However, this approach doesn’t help during the exploration stage. Formation pressure data are usually scattered, but mobility profiling could be helpful. We found that the best facies are clearly visible from nuclear magnetic resonance (NMR) logs largest porosity bins. Sidewall core, formation pressure mobility profiling, and downhole sampling prove this observation. As a result, several fields were discovered despite showing low or missing pay in exploration pilot holes. Horizontal sidetracks geosteered through the best facies showed good commercial rates during well tests. This information supplements the general understanding of Lower Shuaiba in Northern Oman for fields where no core has been taken. This paper demonstrates how advanced technologies and integrated interpretation triggers new discoveries where basic wireline logs show ambiguous results. This success has brought the play back into focus with follow-up opportunities identified in different segments.