When we Dare to Look Deeper: Drilling Downflank on Alternative Fluid-Fill and Reservoir Architecture Concepts for a Carbonate Field in the Sultanate of Oman

Abstract

The development of thin oil rims in carbonate reservoirs requires good understanding of structural setting, reservoir architecture and transition zone saturations. Fields that have a tectonic and/or geochemical history after initial charge are likely to challenge standard assumptions of fluid distribution, contacts and saturation-depth relationships. This paper is a case study illustrating downflank field extension opportunities in an oil rim, related to post-charge tectonics affecting fluid distribution and contacts. The structure of this article encompasses four different aspects:begins by explaining the geological concepts that are being postulated for downflank hydrocarbon potential,proposes the alternative methodology of concept-driven analysis for log data interpretation,explains in detail the methodology applied to existing field data for reservoir architecture and fluid fill description,summarises the outcome of the appraisal well with respect to alternative concepts. The workflows that justified the placement of appraisal wells downflank followed the philosophy of concept-driven analysis where data is used to eliminate hypothesis rather than averaged into one a-priori assumption or average fitting equations. The placement of the pilot appraisal well (at a depth interval and location where previous models predicted water-fill) has been enabled by the identification of stratigraphic rock types, regional variability of fracture intensity and the prediction of tilted contacts. The results of an appraisal well drilled in 2017 confirm the alternative concepts proposed from concept-driven analysis of legacy log data:–Flank reservoir thickness improvement due to post-deposition crestal erosion of best facies.–Fracture density reduction towards the northern flank of the dome structure.–Tilted oil contacts deepening towards the flank and related to paleo-charge.–Relatively dry oil production from deeper depth intervals with low oil saturation due to transition zone water mobility. The drive mechanism and development options for the field should be investigated further.