Biogenic and thermogenic origins of natural gas in Cook Inlet basin, Alaska.

Two types of natural gas occurrences are present in the Cook Inlet basin. The major reserves (1.8 × 10¹¹m³) occur in shallow (less than 2,300 m), nonassociated dry gas fields that contain methane with ^dgr¹³C in the range of -63 to -56 per mil. These gas fields are in sandstones interbedded with coals of the Sterling and Beluga Formations; the gas fields are interpreted as biogenic in origin. Lesser reserves (1.1 × 10¹⁰ m³) of natural gas are associated with oil in the deeper Hemlock Conglomerate at the base of the Tertiary section; associated gas contains methane with ^dgr¹³C of about -46 per mil. The gases associated with oil in the Hemlock Conglomerate are thermogenic in origin.