Early Diagenetic Kaolinite Precipitation: A Widespread Phenomenon in Organic-Rich Mudrocks

Kitty Milliken¹, Joe Macquaker², and Kevin Taylor³
¹ Bureau of Economic Geology, University of Texas at Austin, Austin, Texas
² ExxonMobil Upstream Research Company, Houston, Texas
³ School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, Great Britain

Kaolinite occurs in both detrital and authigenic forms in mudrocks. SEM imaging of numerous organic-rich mudrocks reveals relatively coarse (silt-size) crystals of euhedral kaolinite filling the larger pores, for example, within the tests of foraminifers and in packing flaws around sand-size particles. In some cases, pore-filling kaolinite is accompanied by kaolinite in the form of massive replacement of carbonate skeletal allochems and also of silicate grains. Such diagenetic kaolinization has been observed in the Pearsall and Eagle Ford Formations (South Texas), the Tuscaloosa Formation (Mississippi), the Mancos Shale (Colorado), and many other units recognized as oil and gas shales. The exact mechanisms of kaolinite precipitation remain uncertain, but possible mobilization of silica and aluminum from the detrital components by microbiallymediated acid-generating processes deserves further investigation.

AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012