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K-Ar Dating of Authigenic Illites: Integrating Diagenetic History of the Mesa Verde Group, Piceance Basin, NW Colorado

Stroker, Trevor 1; Harris, Nick 1
1 Geology and Geological Engineering, Colorado School of Mines, Golden, CO.

Tight gas sands represent a significant portion of the U.S. domestic petroleum reserves. The ability to date diagenetic reactions that significantly influence reservoir quality will enhance our ability to characterize and produce these fields. Though diagenetic clays form only a small percent of the sandstone, they have a disproportionately large impact on reservoir properties because of their high surface-to-volume ratio. Using thin-section petrographic analysis, X-ray diffraction, scanning electron mircoscopy, and K-Ar dating, systematic trends in feldspar alteration and illitization can be seen in the fluvial section of the Williams Fork Formation (Upper Mesaverde), Piceance Basin, Colorado.

Samples taken from wells in Rulison and Parachute fields between 4500 to 7000 feet below surface were investigated for whole rock mineralogy and also treated to isolate the clay- sized fraction (<1µm). Illite from the extracted clays provide K-Ar ages for fifteen samples, revealing a linear increase in age from approximately 37 Ma to 55 Ma with depth. These ages incorporate two different types of diagenetic illite. The largest constituent, as determined by SEM microscopy, is a highly illitized, pore-lining, mixed layer illite/smectite; smaller quantities of later stage, pore-filling fibrous illite are also present. Samples with relatively high concentrations of fibrous pore-filling illite compared to MLIS exhibit age dates 4-7 Ma younger than those of the same depth with little to no fibrous pore-filling illite, corroborating the relative age relationship between the two illite polytypes based on petrographic observations.

The illite age dates correspond to entry into the calculated 100°C window during initial burial as determined by Crossey and Larsen (1992) and Johnson and Nuccio (1983). This temperature regime is often associated with a transition to highly ordered MLIS from early smectite (Lee, 1989) with water released from smectite creating overpressure conditions and aiding in the generation of hydrocarbons in nearby organic rich stratum (Ko and Hesse, 1998). Ages recorded by illite K-Ar dating are approximately 20 Ma younger and correspond to a depth 4000 feet shallower than the maximum burial for the fluvial interval of the Williams Fork Formation, possibly indicating a mechanism for early over-pressurization and fracturing of this interval.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009