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Integrated Reservoir Characterisation in Support of a Challenging Oil-Rim Development - Northern Oman

Alsop, David *1; Angelatos, Matthew 1; Newn, Kenneth 1; Wong, See Heng 1
(1) PDO, Muscat, Oman.

Field scale reservoir characterisation of the Permo-Carboniferous, glacio-lacustrine Al Khlata Formation and the Permian shallow marine Lower Gharif Formation has been conducted in an attempt to identify mechanisms for unexpected production and drilling results including larger than predicted water-cuts. An interdisciplinary task-force was convened with the specific aim of integrating the depositional and structural framework with reservoir properties and production performance.

The field studied has a thin oil-rim within a largely unfaulted low relief structure at depths exceeding 3,000m TVDss. Reservoirs are typically tight with average porosities in the vicinity of 12%. Reservoir characterisation has allowed an understanding of fluid flow and production-related pressure response. At the larger scale, the reservoir can be subdivided into distinct hydraulic units separated by pervasive marine flooding shales. Within each hydraulic unit, fluid movement is controlled by permeability variations induced through a combination of grainsize variations and siliciclastic diagenesis. In this case, finer grained delta front sandstone facies exhibit significantly reduced permeability compared with the less abundant and coarser, cross-bedded channel sandstone facies. The observed permeability contrast yields a negative impact on producer performance with preferential movement of water and gas through very thin cross-bedded channels suppressing net oil production.

In support of the reservoir characterisation effort, targeted data acquisition has been utilised to confirm the mechanisms at play. This data acquisition exercise has also yielded further unexpected results regarding the relative permeability and fractional flow behaviour of these tight reservoirs.

This paper focuses on the reservoir characterisation effort undertaken to confirm mechanisms for unexpected production behaviour in a field for which there is no known analogue. In addition to core and log-based analyses, the supporting data acquisition program undertaken utilising an under-balanced rig will be outlined.

 

AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain