Abstract

The Grayling Gas Sands (GGS) reservoir of the McArthur River field (MRF) (Union Oil Company of California 48.8% operator*, Marathon 51.2%), is the fourth-largest gas field in the Cook Inlet, Alaska, with an original gas-in-place (OGIP) of 1.35 to 1.80 TCF and a cumulative gas production of 1.14 TCF and 556 MBW. The GGS, also referred to as the Middle Kenai Gas Pool, produces sweet, dry gas from a 7200+ ft-thick sequence of continental forearc stacked fluvial sands of the Tertiary Tyonek Formation and overlies older, unassociated Tertiary and middle Jurassic oil reservoirs of the MRF. The GGS was established as significant gas resource in 1968 with the drilling of the TBU G-18 well from the Grayling platform, and initial GGS gas production began that year to provide fuel gas to the existing MRF Dolly Varden, Grayling, and King Salmon oil platforms. Sales gas production from the GGS began in 1988 after the installation of the Steelhead platform. Production peaked at 225 MMSCFGPD in 1997, and by late 2011 field production capacity was approximately 65 MMSCFGPD from 23 active wells. The GGS is trapped by structural and stratigraphic components associated with a NNE-SSW fault-propagated anticline. GGS gas is biogenic, sourced from numerous interbedded, often laterally continuous subbituminous to bituminous coal beds. Drive mechanisms for the GGS are natural depletion and aquifer drive. There are 42 proven gas pay sands in the GGS, most of which have independent original gas-water contacts, and none are filled to spill. The originally normally pressured gas sands are vertically isolated and differentially depleted due to more than 40 years of production. Framework grains reflect the influence of the magmatic arc bounding the west and accreted mélange and turbidite metasediments bounding the east of the Tertiary Cook Inlet Basin. Reservoir quality is excellent, with core porosity ranging from 15 to more than 30%, and permeability from less than 10 to hundreds of millidarcies. Compaction effects, authigenic montmorillonite (smectite) and kaolinite clays, and primary and secondary porosity are present. GGS lithofacies are highly cyclical, with fluvial single-story to channel-belt-complex sands ranging from tens to more than 200 ft (61 m) thick. Many of the GGS sands are lenticular, thinning on the crest of the field, suggesting that the MRF structure was likely active during GGS deposition. Since 2007, the mature GGS reservoir has been undergoing a renewed phase of development with extended reach drilling to access underdeveloped sands, infill drilling targeting prematurely abandoned zones, horizontal wells to optimize inflow performance and recovery, well workovers, and facility improvements.