Vitrinite reflectance measurements from surface samples of mudrock and coal show anomalously high values over the Bell Creek oil field. The average vitrinite reflectance (R_m) increases to a peak of 1.2% over the field against background values of about 0.3%. The R_m anomaly coincides with a geochemical anomaly indicated by ^dgr¹³C in carbonate-cemented sandstones. These samples were taken from the Upper Cretaceous Lance and Paleocene Fort Union formations, which form an essentially conformable sequence. The depositional environment is apparently similar in both formations, and we expect little variation in the source and composition of the organic matter. R_m should be rather constant across the field if conditions of diagenesis w re uniform. The limited topographic relief (< 1,000 ft or 305 m) over the shallow-dipping homoclinal structure of the field and the poor correlation coefficient of R_m regressed against sample locality elevation (r = 0.2) indicate that the R_m anomaly is not due to burial, deformation, and subsequent erosion. Temperature studies over local oil fields with similar geologic conditions suggest the expected thermal anomaly would be less than 10°C (50°F), which is too small to account for the significantly higher rank over the field. Coal clinkers are rare in the vicinity of the Bell Creek and widespread heating by burning of coal seams is unlikely. We suggest that activity by petroleum-metabolizing bacteria is a possible explanation of the R_m anoma y. Microseepages from oil fields support large colonies of these organisms, which could also metabolize aliphatic side-chains on the kerogen molecule. The loss of these side-chains increases the aromaticity of the vitrinite and consequently increases its reflectance.

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