PUT RIVER SANDSTONE – LOW COST RESERVOIR APPRAISAL IN A MATURE FIELD

DEAN, James¹, PIETRAS, Jeffrey T.², SELS, Roger¹, YANCEY, Alex¹, WHEELER, Tami¹, INCE, Don³, and THREADGILL, Greg⁴, (1) Greater Prudhoe Bay, BP Exploration, Alaska, 900 E. Benson Blvd, Anchorage, AK 99508-4254, [email protected], (2) BP Sakhalin, Inc, 501 Westlake Park Blvd, Houston, TX 77079, (3) Conoco Phillips, Alaska, 700 G Street, Anchorage, AK 99501, (4) Production, Exxon Mobil, 3301 C Street, Suite 400, Anchorage, AK 99503

The appraisal of the Put River Sandstone is an excellent example of low-cost reservoir appraisal of bypassed pay in a mature oil field. Field development is currently envisioned to include one to five production wells with one injection well.

The Put River Sandstone is located on Alaska's North Slope within the prolific Prudhoe Bay Unit. The Put River Sandstone, along with the Kalubik Formation, is part of the Cretaceous Put River interval that lies stratigraphically above the Lower Cretaceous Unconformity (LCU) and below the Highly Radioactive Zone (HRZ).

The sediments deposited in the Put River interval were locally derived from an exposed highland and represent an overall marine transgression following uplift and erosion associated with the LCU. Four laterally extensive sandstone lobes have been interpreted within the Put River interval. The Northern lobe comprises the entire Put River interval. The remaining three lobes (Central, Western, and Southern) are stratigraphically equivalent to the Northern lobe and are interbedded with the Kalubik Formation.

The Southern lobe contains the main development potential. In core, the sandstone is medium-grained, moderately-to well sorted with localized pebble layers and significant glauconite content. Average porosity and permeability for the southern lobe are estimated to be 19% and 173 mD, respectively.

Initial appraisal occurred concurrently with the early development of the Prudhoe Bay Field and included core data and RFT pressures collected in the Northern and Central lobes. In the early 1990's, RFT data was collected in the Western Lobe.

In 2004-2005, the Put River Sandstone was further evaluated by isolating the existing Ivishak completion and perforating the Put River in five wells. Production tests, fluid samples, and pressure data were collected in each lobe. This low-cost reservoir data was integrated with the earlier core, RFT, and existing log data to complete the reservoir characterization.

The appraisal data indicated the four lobes are isolated reservoirs with different fluids and pressures. The Southern Lobe contains an under-saturated black oil accumulation with 12.6 – 19.2 MMBO in place. Early screening of analog reservoirs indicates potential waterflood recovery to be approximately 20% - 35% of the oil in place.