The Athel: A Challenging Formation in South Oman
Authors: O&apos;Dell P.M. 
Published: December 2007

The Athel silicilyte is a deep, tight formation containing light oil and dissolved sour gas. Since the potential volume is large, there is interest in early development. However, since individual wells are very expensive, every opportunity to gather information must be utilized. Well testing (production tests, PVT sampling, production logging runs, and pressure transient tests) has been used extensively to characterize the reservoir, to guide appraisal activities, and to shape the ultimate development. Key issues to be resolved prior to development are initial and sustained productivity and project costs.

Production tests have demonstrated both challenges and opportunities in producing from this unique formation. Pressure transient tests have indicated that effective reservoir permeability is one tenth of cleaned core plug permeability. The difference is likely caused by the presence of excess salt in the formation. After injecting fresh water, salt saturated water is produced at a somewhat higher gross rate, leaving the formation stimulated. Production logging has been used to measure the fraction of pay contributing to production. Reservoir simulation models, based on well test results, have been used to predict initial rates and ultimate recoveries for various well types (vertical, multiple drainhole, and multiple hydraulic fractures). Project costs (number of wells required) are based on these reservoir simulation results.

Introduction

The Al Noor and Al Shomou Fields are located in the South Oman Salt Basin some 50 km west of Nimr Field. See the location map, Figure 1. Light, volatile oil (480 API), associated hydrocarbon gas containing H2S and CO2, water, and excess salt are present in the Athel Formation, a silicilyte reservoir at depths of around 4000 m. The infra-Cambrian Athel silicilyte consists mainly of micro-crystalline silica with low matrix permeabilities (0.02 md and less). For its depth, porosity is well preserved (porosity ranges from 0.15 to 0.25). The reservoirs are stratigraphically encased within salt and shale, and are geopressured (initial pressures are 67,000 to 80,000 kPa, or 19.8 kPa/m).

To date, four wells have been drilled on the Al Noor structure and two wells have been drilled on the Al Shomou structure. The first two Al Noor wells encountered 360 m of pay with permeability-thickness product of 8 to 9 md-m. Reserves of 15.0 × 10 m3 oil were established from expectation oil in place of 210 × 106 m3. The first Al Shomou well encountered 305 m of pay with permeability thickness product of 5 md-m. Reserves have not yet been booked, but there are plans to do so. Two down flank appraisal wells in Al Noor and one in Al Shomou have indicated substantial reservoir quality degradation towards the west and north west. Proper characterization of these reservoirs is required for further development.

Geology

The Athel silicilyte can best be described as an organically rich, finely laminated, micro-crystalline quartz (80–90%) rock with a uniform crystal size (2–3 m). The depositional model suggests a restricted shallow marine basin, most probably a rift setting, bounded by carbonate platforms along the rift margins. Silicilyte deposition is thought to have been restricted to topographical lows, controlled by early rifting. The diagenetic history suggests that limited porosity destruction occurred during the early burial phase of the silicilyte. Subsequent uplift forced water out of the slab by overpressures, resulting in halite precipitation. The silicilyte slabs are stratigraphically encased within salt and shale. They are characterized by a flat crest and relatively steep dipping flanks (between 5 and 20 degrees). Numerous faults cut through the slabs. The current fracture model suggest that all fractures are closed and filled with quartz, halite, or argillaceous material.

Al Noor Appraisal

Al Noor 1 was drilled in 1989 and Al Noor 2 in 1992. They encountered 360 m of pay with permeability-thickness product of 8 md-m, porosity of 0.23 and oil saturation of 0.83. The full Athel Formation was cored in Al Noor 2. P. 575^