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

Trevor Stroker and Nicholas B. Harris
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. Information from this study will help provide a chronology for reservoir alteration and hydrocarbon charge for several fields in the Williams Fork Formation, Piceance Basin. Correlation of secondary mineral alterations with tectonic events such as burial and uplift will provide a better understanding of the factors controlling diagenesis during these times.

Core samples from fields located in the central basin area will be examined for clay minerals, in particular for the presence of fibrous illite. This process includes freeze-thaw disaggregation to preserve the size and character of the clays followed by XRD and SEM to classify clay composition and habit; fibrous illite will be separated using centrifuge and reprocessed for K-Ar dating.

Illite dating will provide fixed reference points for the digenetic history of the Piceance Basin, which until now has been limited to relative order. 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. It has been found in other basin-centered gas systems that fibrous illite typically precipitates from brine. As gas replaces this brine, growth is halted, recording the time of significant gas saturation. Mapping the history of latest growth should provide a chronologic depiction of gas charge through the reservoir. Integrating gas charge with the burial and uplift history may add support to the proposed diagenetic fracturing of sandstones due to increased pore pressure associated with gas charge.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado