THERMAL MATURATION AND ORGANIC RICHNESS OF POTENTIAL PETROLEUM SOURCEROCKS IN PROTEROZOIC RICE FORMATION, NORTH-AMERICAN MIDCONTINENT RIFTSYSTEM, NORTHEASTERN KANSAS

A recent well in northeastern Kansas penetrated 296 ft (90.2 m) of dar k gray siltstone in the Precambrian Mid-Continent rift (Proterozoic Ri ce Formation). Correlations indicate this unit may be as thick as 600 ft (183 m) and is possibly time-equivalent to the Nonesuch Shale (Midd le Proterozoic) in the Lake Superior region. The upper half of this un it qualifies as a lean source rock (averaging 0.66 wt. % TOC), and org anic matter in it is in the transition stage between oil and wet gas g eneration. The presence of the gray siltstone in this well and similar lithologies in other wells is encouraging because it indicates that s ource rock deposition may be common along the Mid-Continent rift, and that parts of the rift may remain thermally within the oil and gas win dow. Microscopic examination of calcite veins penetrating the dark gra y siltstone reveals numerous oil-filled and subordinate aqueous fluid inclusions. Homogenization temperatures indicate these rocks have been subjected to temperature of at least 110-115-degrees-C (230-239-degre es-F). Burial during the Phanerozoic is inadequate to account for the homogenization temperatures and thermal maturity of the Precambrian ro cks. With the present geothermal gradient, at least 8250 ft (2.5 km) o f burial is necessary, but lesser burial may be likely with probably h igher geothermal gradients during rifting. Fluorescence colors and gas chromatograms indicate compositions of oils in the fluid inclusions v ary. However, oils in the fluid inclusions are markedly dissimilar to the nearest oils produced from Paleozoic rocks.