Geochemical reconnaissance of Cretaceous inliers in north-central Oregon

Cretaceous inliers in north-central Oregon represent isolated exposures of potential source rocks in the region. Interpreted neritic to upper bathyal mudstones of the Hudspeth Formation represent late deposition within the Mesozoic fore-arc terrane. Accretionary terranes that are represented by the Blue Mountain province are in part inundated by voluminous Tertiary volcanic sequences, most notably the Columbia River Group. Both pyrolysis and solvent-extraction source rock data from the Hudspeth Formation present in local Cretaceous inliers support a mature, oil-prone, mixed-marine source rock environment. A trend reflected in these data indicates that the western part of the study area is more mature. If these same strata and conditions prevail beneath the volcanic cover, the potential for the presence of paraffinic oil accumulations exists. Near-surface soil-gas investigations have shown the occurrence of light hydrocarbon microseep gases, which cluster in the vicinity of the Mitchell inlier. The soil-gas signal received in the near-surface, far removed from the inliers, is identical to the soil-gas signature measured on the Cretaceous sediments. It is suggested that the application of reconnaissance surface geochemical methods may offer additional insight in defining potentially favorable source rocks peripheral to the Columbia River Plateau, as well as projecting their presence into the interior of the plateau.