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ABSTRACT: Utilization of Micromagnetic Surveys in Exploration

S. E. Sommer, B. K. Jain

Analyses of field and laboratory data demonstrate that surface and subsurface magnetic mineralization can be related to hydrocarbon proximity in favorable geological contexts and that such mineralization may be detected as micromagnetic anomalies in aeromagnetic data. Pyrrhotite, a magnetic iron sulfide mineral, is associated with micromagnetic anomalies in Pecos County, Texas, and with anomalies at Cement field, Oklahoma. Pyrrhotite has also been identified within solid hydrocarbon in samples from Damon Mound, Texas, and tar sands from Cold Lake, Canada. A geochemical pathway leading from an iron-rich substrate to pyrrhotite and then to pyrite has been identified and is used to illustrate a likely mechanism for hydrocarbon-mediated pyrrhotite formation.

Siderite- and chlorite-sourced anomalies in Trinity County, Texas, have been assigned a detrital origin, although it is possible that these minerals may provide an authigenic source for micromagnetic anomalies elsewhere. The most abundant magnetic "minerals" obtained from boreholes in this study are determined to be produced by contamination during drilling. Magnetic minerals of detrital and authigenic origin are either absent or minor components. A unique texture has been identified in hydrocarbon-associated pyrite that may be a clue to a pyrrhotite precursor, suggesting that magnetic mineralization may be destroyed by conversion to the more chemically stable sulfide, pyrite. Pyrrhotite has limited occurrence and must be carefully documented to support the interpretation of micromagn tic surveys as correlated with hydrocarbons.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990