A study of the relationship between hydrocarbon migration and the precipitation of authigenic magnetic minerals in the Triassic Chugwater Formation, southern Montana

摘要

To test the hypothesis that hydrocarbons can cause the precipitation of authigenic magnetite, hydrocarbon-saturated, bleached, and unaltered samples of the Chugwater red beds were examined, using petrographic, rock-magnetic, and paleomagnetic techniques. Samples were collected from the crest of Red Dome in Carbon County, Montana, where the Chugwater is saturated with hydrocarbons and bleached around fractures. The fractures formed during Laramide deformation and provided conduits for oil migration. The red beds contain Triassic and rotated Triassic magnetizations, which reside in hematite. The magnetization of the oil-saturated sandstones predominantly resides in magnetite, and there is some rock-magnetic evidence for pyrrhotite. The intensity of the natural remanent magnetization (NRM) in these sands is at least an order of magnitude less than that observed in the red beds. A stable magnetic direction could not be isolated from the oil-saturated sandstones. Petrographic studies indicate that both detrital and authigenic magnetite are present in the oil-saturated sandstones. The bleached specimens possess intermediate magnetic characteristics and contain abundant secondary authigenic specular hematite. These specimens contain an early Tertiary magnetization that resides in the authigenic hematite. The chemical conditions created by the hydrocarbons caused the dissolution of the hematite, which "uncovered" and relatively enhanced the magnetization residing in detrital magnetite. It is also likely that hydrocarbons caused the precipitation of some authigenic magnetite and pyrrhotite. The early Tertiary magnetization in the bleached sandstones is interpreted to be of chemical origin and a byproduct of hydrocarbon migration at Red Dome. The magnetization was probably acquired when the dissolved iron from the red beds precipitated as specular hematite. The results of this study have implications for understanding hydrocarbon-induced diagenesis and remagnetization mechanisms, and for aeromagnetic hydrocarbon exploration.