Geology
Conservation Order No. 458A defines the Northstar Oil Pool as the accumulation of hydrocarbons common to and correlating with the interval between measured depths of 12,418 feet and 13,044 feet the Seal A-01 well. The reservoir interval consists of the Sag River, Shublik, and Ivishak Formations. The Sag River Formation is typically 100 feet thick in this area, and it was deposited as Triassic-aged, transgressive marine sandstone, siltstone and shale. The sandstone mineralogy is mature: quartz with minor amounts of feldspar and authigenic clay. The primary cementing agents are calcite, silica and siderite. Sag River Formation porosity ranges from 6.8% to 22.8% and averages 13%. Permeability ranges from 0.01 to 28.0 millidarcies and averages 0.86 millidarcies. Calculated water saturation values range from 50% to 65%. The net-to-gross ratio for the Sag River Formation varies from 15% to 20%.
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The Shublik Formation is also Triassic-aged, and is stratigraphically complex, being characterized by marine siltstone, shale, sandstone and phosphatic limestone. Within the Shublik Formation, reservoir quality rock is limited to a basal sandstone member, the Shublik D. Porosity and permeability for the Shublik D are typically less than 10% and 1 millidarcy, respectively. The net-to-gross ratio for the Shublik D is about 50%.
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The Ivishak Formation comprises a series of Permian and Triassic-aged delta-front sandstones and shales that grade upward to fluvial sandstone and medium to coarse-grained pebbly conglomerates. Within the pool, the Ivishak is about 325 feet thick, and it is more proximal, coarser grained, more deeply buried and cemented than at the Prudhoe Bay Field. Here, the Ivishak is primarily cemented with calcite, silica and siderite. The lower sand unit of the Ivishak Formation has an average porosity of about 18%, and the overlying conglomerate unit averages about 14%. However, laboratory studies show that about 40% of the total porosity within the lower sand unit is non-effective micro-porosity, and within the conglomerate unit, about 50% of the total porosity is non-effective micro-porosity. Core permeability ranges from 0.01 to 808 millidarcies, with a stress-corrected mean value of 53 millidarcies. The net-to-gross ratio for the Ivishak is 93% to 95%. The average oil saturation of the Ivishak is 42%, and the maximum oil column is estimated to range from 270 to 300 feet.
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The pool structure consists of a faulted anticline defined by three-way dip closure on west, south and east. Closure to the north is obtained through fault seal or structural dip. Faults within the pool have interpreted maximum vertical displacements of less than 200 feet, and they are not expected to significantly affect reservoir performance. The oil-water contact is placed at 11,100 feet true vertical depth subsea, based on core, RFT, MDT and well test data.
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