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ABSTRACT: Coal Maturation and the Potential for Natural Gas Accumulation in West-Central Washington State

Timothy J. Walsh, William S. Lingley, Jr.

New isopach maps show that Paleogene sedimentary and volcanogenic rocks ranging in thickness from 12,000 to approximately 30,000 ft accumulated in five depocenters within the Puget Basin. From the north, these are the Bellingham, Everett, Seattle, Tacoma, and Chehalis subbasins. Shallow coals near these depocenters are mostly lignite and subbituminous, but rank increases eastward to anthracite in the Cascade mountains. High volatile B bituminous coal cropping out near the Bellingham depocenter is the exception.

In mining districts where coal rank is greater than high volatile C bituminous, isorank contours increase monotonically toward the Cascade crest and cut across structural contours mapped on Eocene coal seams. In places, Oligocene and probable Miocene rocks are folded together with older coal seams. These relations suggest that coal rank in the eastern Puget basin results mainly from post-Oligocene thermal overprinting associated with plutonic and volcanic activity.

Thermally mature, gas-generative sequences containing 8-11% coal are common in the eastern Puget Sound lowland and Cascade foothills. These observations, together with the presence of sandstones having porosities >30% and permeabilities >1 darcy, suggest that natural gas should be present in commercial quantities. Complex structure and difficulty in acquiring seismic data hinder exploration; only coalbed methane exploration has been aggressively pursued in recent years in the Puget basin.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990