ABSTRACT: Thermal Maturity and Petroleum Generation History of Pennsylvanian Source Rocks, Paradox Basin, Utah and Colorado

NUCCIO, V. F., B. L. CRYSDALE, and C. E. BARKER, U.S. Geological Survey, Denver, CO

Several hundred samples of black shale from the Pennsylvanian Hermosa Formation, Paradox basin, Utah and Colorado, have been analyzed by Rock-Eval pyrolysis to determine their thermal maturity and petroleum source rock potential. Thermal maturity maps have been constructed for selected horizons to illustrate the variation in burial and thermal histories across the basin. For example, a map of the Ismay and Desert Creek production zones shows that production indices increase from <0.10 (immature) in the south-central part of the basin, near Blanding and Bluff, Utah, to >0.40 (mature) around an area extending from Arches National Park in southeastern Utah, southeastward to Cahone and Cortez, Colorado. Source rocks from this same interval contain types I, II, and III kerogen, and h ve total organic carbon contents that range from 0.30 to greater than 4.0 %, making them excellent sources for both oil and gas.

Models illustrating the burial and petroleum generation history of Hermosa Formation source rocks have been constructed for several areas in the basin. In the south-central part of the basin near Blanding, Utah, the Hermosa at the Ismay and Desert Creek level was buried to 10,000 ft at maximum burial 50 Ma, and little oil generation

has taken place. Source rocks at the base of the Hermosa in this area were buried to 12,000 ft, at maximum burial 50 Ma, and oil generation began around that same time. Assuming no migration, oil fields in this area are likely to contain petroleum from source rocks in the lower part of the Hermosa. By contrast, near Moab, Utah, the upper part of Hermosa at the Ismay and Desert Creek interval was buried to 12,000 ft at maximum burial 50 Ma, and oil generation began at that time. The lower part of the Hermosa was buried to 17,500 ft at maximum burial 50 Ma, but because of an additional 5500 ft of burial during the Pennsylvanian, oil generation began as early as 225 Ma. Much more oil could have been generated in the Moab area than in the Blanding area, and the fields are likely to contai petroleum from the entire Hermosa section. It is also likely that source rocks in the deeper parts of the Paradox basin, where temperatures are high enough and kerogen in the source rocks is available (not "cooked out"), could be generating oil and gas at the present time.

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)