Integrated static reservoir modeling of a Khuff reservoir, North Oman. 9th Middle East Geosciences Conference, GEO 2010.

Abstract

The Khuff Formation of the Arabian Peninsula comprises mixed carbonate/evaporitic sequences of Late Permian - Early Triassic age deposited on a widespread epeiric ramp attached to the Arabian Shield. The Upper Khuff oil and gas reservoir is characterized by lithological and reservoir quality heterogeneities as a result of both depositional history and diagenetic overprint. Further to its gas discovery and initial production, Petroleum Development Oman (PDO) has recently updated the Upper Khuff geomodel by integrating new data, such as reprocessed seismic volumes, appraisal well results, and core and fluid data. Depositional/stratigraphic modeling was carried out based on facies analysis of cores, log interpretation and outcrop analogues. This allowed a detailed description of the vertical evolution and lateral variations of the carbonate ramp, from open-marine shelf, to oolitic/skeletal shoals and mud-evaporitic tidal flats. A high-resolution sequence-stratigraphic framework was built for the reservoir zones combining subsurface data and North Oman outcrop analogues, enabling subdivision of the entire Upper Khuff into third-order sequences and characterization of the flow units in terms of depositional cycles. The resulting layering scheme has proved to be correlatable at field scale. Reservoir rock types were defined to describe the reservoir matrix behaviour combining sedimentological, diagenetic and petrophysical data. These were obtained by combining lithologies and depositional facies into one single classification scheme, thus allowing identification of rock volumes with similar reservoir quality. An appropriate Petrel grid was designed not only to capture the reservoir heterogeneity with sufficient detail, but also to prevent runtime excess. Petrophysical properties were populated in 3-D by conditioning to facies and rock types in order to capture the reservoir heterogeneities. By combining seismic fault pattern, curvature analysis and well interpretations, fracture models were generated and implemented in the dynamic simulation. The reservoir uncertainties were handled by defining their ranges and applying an experimental design process to evaluate their impact on stock tank oil initially in-place (STOIIP) and gas initially in-place (GIIP). Several static realizations were generated using combinations of these parameters. The final selection of subsurface scenarios was achieved by iteration with the dynamic reservoir simulation to capture the full range of reserves uncertainty.