Hydrocarbon Potential of Bellingham Basin, Northwestern Washington and Southern British Columbia

Shannon Fitzgerald, Paul Willmette

Good source rocks, sufficient maturation, potential reservoir rocks, and numerous traps indicate that hydrocarbon accumulations may be found in Bellingham basin, which occupies over 2,400 mi² (6,200 km²). The primarily continental sediments were probably deposited in a rapidly subsiding pull-apart basin. Two gravity lows occur within the basin, one of which is at least 15,000 ft (4,600 m) deep.

Source rocks to the north include nearly 5,000 ft (1,500 m) of a Lower Cretaceous marine section that contains oil-prone shales. Eocene continental, carbonaceous, gas- and condensate-prone shales and coals occur throughout the basin. The coal rank is high volatile C bituminous, which is within the oil and gas generation window. Hydrocarbon maturation and migration should have begun before the Pleistocene, with 20,000 ft (6,100 m) of Eocene section and a present geothermal gradient of 1.7°/100 ft, which probably doubled during Cascade volcanism.

Potential reservoir rocks are: (1) Cretaceous marine and continental sands; (2) Eocene continental, multistoried, channel sands up to 120 ft (37 m) thick and 1-2 mi (1.6-3.2 km) long; and (3) Oligocene and Miocene continental sands. Core analyses of Eocene sandstones reveal up to 16% porosity and up to 45-md permeability. Calcite and clay cements occur throughout the basin; however, unconsolidated sands have been reported from wells in the northwest.

Traps within the basin include multistoried channel sands capped by siltstones and mudstones, pinch-outs, intraformational unconformities, normal and thrust faults, and anticlines.

Drilling activity has been limited primarily to shallow wells, most of which were drilled before the mid-1930s. Fifteen of these wells yielded domestic methane gas production. Detailed gravity, magnetic, and seismic studies are needed because of extensive Pleistocene sediment cover, complex tectonism, and rapid facies changes.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.