BURIAL AND THERMAL HISTORY MODELS TO EVALUATE HYDROCARBON SOURCE ROCK MATURITY IN THE YUKON FLATS BASIN, CENTRAL ALASKA

1D burial/thermal history modeling in the Yukon Flats basin, east-central Alaska, was undertaken as part of a U.S. Geological Survey assessment of undiscovered oil and gas resources. The subsurface geology of the Yukon Flats basin was inferred from geophysical imaging using seismic reflection, gravity, and magnetics, but is otherwise poorly known because no deep exploratory wells have been drilled. The burial/thermal history models therefore represent the sole source of information on the maturity of potential source rocks at depth.

We developed 4 models that represent alternative geologic scenarios for a hypothetical well in the deep, central portion of Yukon Flats basin. The models vary the thickness of Cenozoic-Late Cretaceous non-marine strata, as well as the timing and duration of inferred unconformities. The primary sources of uncertainty in these models are the lithologies, thicknesses, and ages of stratigraphic units. Lithologies are based on outcrop samples from the basin margins, with the assumption that similar units extend into the basin's subsurface. Unit thicknesses were estimated from seismic reflectors. Unconformities, also inferred from seismic profiles, are treated as hiatuses in deposition. Stratigraphic units in our models have not been directly dated, but provisional ages were assigned based on extrapolation from regional stratigraphic information derived mainly from outcrop studies in the Yukon Flats region.

Model calculations in all 4 geologic scenarios indicate the depth to the top of the oil window to be within the Tertiary at 7,500 – 8,100 ft. Only Tertiary units were considered to be potential source rocks. To summarize maturation timing: 1) Miocene units are generally immature, 2) Oligocene units are immature (2 scenarios) or entered the oil window between 15 and 18 Ma (2 scenarios), 3) Eocene units entered the oil window between 30 and 46 Ma, and 4) Paleocene units entered the oil window between 42 and 55 Ma. At present day, Eocene units are generally still in the oil window, and Paleocene units are in the gas window. The models therefore suggest that potential source rocks throughout most of the pre-Miocene stratigraphic section in the Yukon Flats basin have reached sufficient thermal maturity to have generated hydrocarbons and that generation may still be occurring at present day.