Characterization and Implications of Dickite-Mineralized Fractures in Mesaverde Core from Piceance Creek Basin

Sharon J. Finley, John C. Lorenz

The presence of authigenic dickite, a polymorph of kaolinite, in fractures in the gas-productive Mesaverde Formation of the Piceance Creek basin may indicate a locally interconnected fracture system. Core used in this study is from the United States Department of Energy's Multiwell Experiment located in the Rulison field in northwestern Colorado. The Mesaverde is fractured throughout the 4,000-ft section cored at this location. Although sparry calcite is the most common fracture mineralization throughout the section, dickite mineralization commonly occurs as a secondary phase in fractures within a roughly 400 ft-thick interval. Moreover, a specific type of dickite-mineralized fracture is concentrated in a 75-ft section within the 400-ft dickite mineralization interval. Sl ghtly more than 70% of the fractures cemented with dickite are anomalous extension fractures that imparted an expanded rock texture to the core, implying high pore pressures and/or a locally complex paleostress regime. These anomalous fractures vary from undulatory to planar and are locally coincident with syndepositional sedimentary structures and bedding planes. The dip angle of what appears to be single fractures may vary from horizontal to vertical.

The petrography of the host rock where these fractures are concentrated is similar to the petrography of other core intervals where dickite mineralization and/or irregular fractures were not detected. No apparent difference is seen in the mineral constituents or the diagenetic alteration of the minerals present, suggesting that the source of the dickite may have been extraformational. If the dickite is extraformational, the variation in the orientations of these anomalous dickite-mineralized fractures, coupled with the presence of dickite on some other types of fractures, implies that at some time in the depositional and burial history of the rock the dickite fractures represented an interconnected fracture system. Microporosity is associated with the dickite mineralization, and this racture system may still be locally interconnected and permeable making this a potentially gas-productive interval. Special consideration of drilling and/or stimulation fluid interaction with dickite may be necessary.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.