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Abstract

Paleomagnetic, rock magnetic, petrographic, and geochemical studies indicate that hydrocarbons can cause either an increase or decrease in the magnetization in sedimentary rocks. For example, hydrocarbon-impregnated Permian calcite speleothems in southwestern Oklahoma contain a Permian chemical remanent magnetization (CRM) that resides in magnetite. A positive relationship between extractable organic matter and the natural remanent magnetization (NRM) suggests that the chemical conditions created by the hydrocarbons caused precipitation of the magnetite and acquisition of the associated CRM. There is no correlation, however, between percent asphaltenes and NRM in the speleothems. In addition, bitumen speleothems with high NRMs are, in general, less extensively biode-graded. These results suggest that a chemical process, and not biodegradation, is the mechanism for magnetite authigenesis in speleothems. The results from the speleothems suggest that hydrocarbons can cause acquisition of magnetization that can be dated using paleomagnetic analysis. Development of this dating approach, however, requires more work to better understand the mechanism(s) of magnetite precipitation.

Studies of red bed and hydrocarbon-impregnated samples from cores of the Lyons Sandstone in the Denver Basin and from outcrops of the Maroon Formation (Schoolhouse Member) in northwest Colorado indicate that while hydrocarbons can cause precipitation of magnetite, they can also reduce the NRM by dissolution of hematite. This has implications for the various types of magnetic prospecting techniques that have been proposed.

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