Petrographic and Magnetic
Characteristics of
Hydrocarbon-Saturated Red Beds: Implications for Diagenesis and Hydrocarbon
Exploration
B. Kilgore, R. McCollum, R. Douglas Elmore
To test the hypothesis that there is a relationship between hydrocarbons and
authigenic magnetic
minerals, both hydrocarbon-saturated and unaltered samples
of the Triassic Chugwater Formation were examined using petrographic, rock
magnetic
, and paleomagnetic techniques. Samples were collected from the crest of
Red Dome in Montana where the Chugwater is saturated with hydrocarbons and
bleached around fractures. The fractures, which formed during Laramide
deformation, provided conduits for oil migration. The magnetization in the
unaltered red beds resides in hematite. The magnetization in the oil saturated
sandstones, however, resides in magnetite, although the
magnetic
intensity is at
least an order of magnitude less than that observed in the red beds. The
bleached samples possess intermediate
magnetic
characteristics and contain
abundant authigenic specular hematite. Although a
magnetic
direction could not
be isolated from the hydrocarbon-saturated sandstones, the bleached samples
contain an early Tertiary magnetization that resides in authigeneic hematite.
The magnetic
found in the hydrocarbon-saturated sandstones is probably a
combination of detrital and authigenic magnetite. The chemical conditions
created by the hydrocarbons caused the removal of the hematite, which uncovered
the detrital magnetite, and may also have caused precipitation of some
authigenic magnetite. The magnetization in the bleached sandstones was probably
acquired when iron removed from the red beds precipitated as specular hematite.
This magnetization constrains the timing of oil migration and bleaching.
The results of this study, particularly the intensity differences between the unaltered and hydrocarbon-saturated red beds, also have important implications for aeromagnetic hydrocarbon exploration.
AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.