Numerical modeling of hydrocarbon generation in the Douglas Formation of the Athabasca basin (Canada) and implications for unconformity-related uranium mineralization

Abstract

The Proterozoic Athabasca Basin in Canada is known for its world-class, high-grade uranium deposits developed near the unconformity between the basin and the basement. The Douglas Formation in the upper part of the preserved strata in the basin contains total organic carbon (TOC) of up to 3.56wt.%. Previous studies of organic matter maturation and hydrocarbon inclusions indicate that oil and gas have been generated from this formation, and that some hydrocarbons found in the unconformity-related uranium deposits were derived from the Douglas Formation. This study aims to evaluate how the oil and gas generation processes may have affected the fluid overpressure development in the basin, and whether or not the hydrocarbons generated in the Douglas Formation could migrate downward to stratigraphically lower intervals and eventually to the sites of mineralization near the unconformities. We carried out a series of numerical experiments to examine fluid overpressures, flow directions, temperatures, and oil and gas generation processes using a two-dimensional conceptual model derived from a geologic cross-section from the basin center to the eastern margin. An additional 5km strata were added to current basin stratigraphy to account for observed paleogeothermal data. Variation studies were undertaken to account for uncertainties in the lithologies of the eroded strata, and the wide ranges of possible permeabilities of different lithologies and kinetic parameters of oil and gas generation. It is found that, if moderate permeabilities are used in the modeling for each lithology (known as the base model), oil and gas generation processes contribute little to the development of fluid overpressure, and fluid pressure in the basin is close to hydrostatic regardless of whether or not hydrocarbon generation in the Douglas Formation is included in the modeling. However, if permeabilities are assigned values one order of magnitude lower than in the base model, significant fluid overpressures are developed in the eroded strata in the upper part of the model. In the base model, oil generated in the Douglas Formation may migrate downward, driven by an overpressure zone situated above the Douglas Formation, but gas migrates upward. In the low-permeability model, however, the overpressures developed above the Douglas Formation are so high that both oil and gas generated in the Douglas Formation migrate downward. The numerical modeling results thus indicate that it is hydrodynamically possible for oil and gas generated in the Douglas Formation to migrate to the base of the basin and reach the sites of the unconformity-related uranium deposits.