EVALUATION OF PETROLEUM SOURCE ROCKS FROM YUKON FLATS, EAST-CENTRAL ALASKA

In preparation for the 2004 USGS assessment of undiscovered oil and gas resources of Yukon Flats, east-central Alaska, petroleum source rocks from shallow core holes and outcrops in the area were evaluated using Rock-Eval pyrolysis, maceral composition, organic carbon content, rock extract composition, and organic maturity parameters such as vitrinite reflectance. Approximately 250 of nearly 700 Rock-Eval analyses were considered acceptable quality for interpretation based on T_max values between 380 and 520˚C and S₂ values greater than 0.2 mg HC/g rock. The results show that most of the potential source rocks (Tertiary nonmarine mudstone and coal) in the Yukon Flats area are gas-prone (Type III kerogen), although some of the Paleocene, Eocene and Oligocene rocks contain oil–prone (Type II) kerogen. Macerals in most of the core-hole samples consist mainly of land plant debris but at least two samples reportedly contain fresh-water algae. Gas chromatograms of 10 rock extracts indicate predominantly land plant wax and possibly resin compositions. Excellent oil-prone Paleozoic and Mesozoic source rocks exist southeast of Yukon Flats, especially the Triassic Glenn Shale, but these strata are not known to be present within the Yukon Flats assessment area. At least two tasmanite oil shale localities exist north of Yukon Flats but the extent and volume of tasmanite within the basin are unknown. The only well in the basin penetrated 2,287 feet of nonmarine strata and lignite coal with low gas saturation. Neogene coal beds in this well are immature and generally gas prone (Type III kerogen), and more likely contain microbial gas than thermogenic gas. In the deeper portions of the Yukon Flats basin, mature Paleogene and possibly Cretaceous nonmarine source rocks likely generated thermogenic gas and possibly some oil, which may be trapped in sandstone and conglomerate reservoirs.