Abstract

Ninilchik gas field is a significant gas producer in southern Alaska. Located 90 mi (144 km) southwest of Anchorage on the eastern margin of the Cook Inlet, Ninilchik was discovered in 2001 with the drilling of the Grassim Oskolkoff #1 and Falls Creek #1-RD wells. Commercial gas production commenced in 2003 and has totaled 102.7 BCF from 21 productive wells.

The primary reservoir interval is the Oligo-Miocene Tyonek Formation. Fluvial deposits of interbedded sandstone, siltstone, and coal comprise a gas-prone section up to 3500 ft (1067 m) thick. Discrete sandstone reservoirs averaging 30 ft (9 m) thick are irregularly distributed laterally and vertically throughout the section and can total up to 300 ft (91 m) gross thickness in a single well. Reservoir quality is reduced by the presence of clays. Core samples from gas-productive sand intervals reveal a range of porosity values from 10 to 25% and permeability between 3 and 250 md.

Structurally, Ninilchik field is an asymmetric anticline bounded on the west by an eastward-dipping reverse fault that roughly parallels the present-day coastline. The structure is segmented by northward-dipping normal faults striking perpendicular to the controlling thrust fault. These normal faults also exhibit apparent dextral shear, reflecting the wrench-compressional tectonic setting of the Cook Inlet Basin. The cross faults segment the field into three distinct units from southwest to northeast: Susan Dionne/Paxton, Grassim Oskolkoff, and Falls Creek. The structures do not share common reservoir intervals or fluid contacts and are interpreted as discrete gas accumulations. Traps result from both structure and stratigraphy with discontinuous fluvial reservoirs bounded laterally and vertically by impermeable siltstone and coal beds. Dry gas of biogenic origin sourced from coal strata charges the adjacent fluvial reservoirs.