Hydrothermal petroleums from Yellowstone National Park, Wyoming, U.S.A.

Abstract

In Yellowstone National Park, oil is actively discharging with thermal waters at several widely separated thermal areas. Samples from two locations were studied. Petroleum from Calcite Springs occurs as vapor condensates in steam vents. Detailed geochemical analyses show a complex origin for the petroleum fluid. Deeply buried, hydrothermally altered sedimentary rocks are believed to have contributed a high temperature pyrolysate consisting largely of polynuclear aromatic hydrocarbons and heterocyclic compounds. Shallow sedimentary rocks, which are also hydrothermally altered, have contributed large amounts of aromatic and polar compounds, as well as minor amounts of hydrocarbons. The Permian Phosphoria Formation is suggested as a likely source rock for the bulk of the oils. Autochthonous pyrolysates and lipids from thermally altered surface and near surface bacterial and land plant debris have also been introduced to the vents. The latter may have been introduced by downward percolating meteoric water. The oil from Rainbow Springs is a highly paraffinic, low-S crude oil, depleted in light hydrocarbons. Unlike the Calcite Springs samples, the oil from Rainbow Springs is believed to have originated from a single sedimentary source rock with little or no hydrocarbon contributions from surface biota. The oil contains a unique molecular marker assemblage that suggests both gymnosperm and angiosperm input and places the age of the source unit in the Tertiary. Consideration of the local stratigraphy and water temperatures limits the probable source zone to the Eocene Aycross Formation, an intra-volcanic lacustrine mudstone. Calculation of deep water temperatures indicates that this unit is exposed to thermal waters in an extensive near surface aquifer. Such conditions are consistent with the widespread surface manifestations and biomarker kinetic models. Consideration of the petroleum chemistry and regional hydrodynamics indicates that the Yellowstone Park oils did not migrate into the Park from distant sources. The chemical data are also inconsistent with the oils being generated and trapped prior to Tertiary volcanism. At both Calcite Springs and Rainbow Springs, oil generation appears to be contemporaneous and is attributed to thermal alteration of relatively shallow sedimentary rocks by hydrothermal fluids.