Abstract

The Granite Point field was discovered by the Pan American Tyonek State 18742-1 well in July of 1965. The well encountered more than 300 ft (91 m) of net pay in the Tyonek Formation between 8000 and 9000 ft (2438 to 2743 m)TVD. The well produced 2455 BOPD from 40 ft (12 m) of perforations within this interval on a drill stem test. Appraisal drilling proceeded over the following 14 months, confirming the downdip extent of the field and testing oil from the Hemlock Formation. Shallow gas has been detected in the Beluga Formation and produced from the Tyonek at less than 1000 MCFGPD for use in providing fuel to the field facilities. Original oil in place is estimated to be 730 MMBO. Field development began with the setting of three oil production platforms by early 1967. Production peaked at close to 53,000 BOPD then began a rapid decline until a continuous water injection program was instituted. Since that time, the field has been on roughly a 10% decline, having produced more than 145 MMBO with a current rate of more than 2000 BOPD.

The Granite Point field is located on the west flank of the Cook Inlet Basin, a forearc basin associated with the subduction of the Pacific plate beneath the accreted terranes of Alaska. The field is characterized by an NNE-SSW elongated sharp asymmetric fold bounded on the west by a reverse fault that is interpreted to extend into the basement. A number of seismically defined normal faults cross-cut the field. The main phase of structural development occurred in the Middle to Late Miocene as the Cook Inlet Basin underwent a period of increased transpression.

Source rock analysis and oil geochemistry studies have identified the Jurassic Tuxedni marine shales as the source for the oil. Basin modeling and source rock maturity studies indicate that the oil was generated to the east in the deeper portion of the basin and migrated up faults and along laterally continuous sands in the lower Tertiary. The gas accumulation in the shallow interval is considered biogenic, sourced from the numerous interbedded, laterally continuous subbituminous coals found throughout the Tyonek and Beluga.

The oldest rocks penetrated in the field are the moderately metamorphosed marine clastics of the Lower Jurassic. A significant erosional unconformity developed at the end of the Mesozoic followed by deposition of a thin Eocene West Foreland, Oligocene Hemlock, and the Oligo-Miocene Tyonek Formations. The Beluga Formation overlies the Tyonek with Holocene glacial deposits extending up to the seafloor. The entire Tertiary section is characterized by fluvial deposits of conglomerate, sand, and silt separated by overbank claystones and coal. The deep gravel-bed braided to sandy meandering depositional environments produced reservoirs of poor to fair quality with fair to moderate lateral continuity.