Stratigraphy and Source Rock Potential of Mowry Shale (Lower Cretaceous), North Park Basin, Colorado: ABSTRACT

Abstract

The Lower Cretaceous Mowry Shale in North Park basin, Colorado, consists of hard, dark-gray to black, siliceous shales containing abundant fish scales and interbedded bentonite layers. The formation has a uniform thickness, averaging 115 ft basinwide. The shales were apparently deposited in quiet, anaerobic, shallow marine waters. Crude-oil geochemistry indicates that North Park basin Cretaceous oils are genetically related and have been derived from a common source rock or source interval. Source rock geochemistry indicates that the Mowry shale in North Park basin has above-average amounts of organic matter that is sapropelic and humic in content with oil and gas generative capability. The onset of oil generation occurred 50 Ma (early Eocene) at temperatures as low as 100/sup 0/C. Intense oil generation occurred in shales with present-day burial depth between 7000 and 9550 ft. Burial and thermal conditions were not sufficient to cause gas generation in the Mowry Shale. Migration of oil was probably controlled by Laramide structures. Oil fields are located within, adjacent to, or updip from areas of rich Mowry Shale oil source rocks. Geochemical correlation of shales and oils indicates the molecular distribution of hydrocarbons in source rocks and reservoirs is similar, and the Mowry Shale ismore » at least one of the source rocks for North Park Cretaceous oils. Volumetric calculations indicate the Mowry Shale has generates sufficient quantities of oil to be an effective source rock and possibly the main source rock for Cretaceous oils in North Park basin.« less