FREED, ROBERT L., Trinity University, San Antonio, TX; and DONALD R. PEACOR, The University of Michigan, Ann Arbor, MI

ABSTRACT: Feldspar Diagenesis in Texas Gulf Coast Shales: Parallelism with the Smectite to Illite Transition

Well-cuttings from 305 m intervals in a single well in DeWitt County, Texas, were examined by XRD, SEM, and microprobe techniques in order to characterize clays and feldspars in shale as a function of depth. The smectite to illite (S-I) transition occurs abruptly over a narrow depth interval near 2,134 Sm. Feldspars from pre-transition depths (< 2,134 m) retain detrital outlines and are notable for their lack of major alteration features. Both alkali and plagioclase feldspars display wide ranges in composition. Also, significant Ca is present in alkali feldspar and significant K is present in plagioclase feldspar. In contrast, feldspars from post-transition depths (> 2,438 m) show a variety of alteration features: K-feldspar dissolution textures; K-feldspar replacement of plagio lase; and albitization of K-feldspar. Chemical data suggest complete albitization of plagioclase. Essentially the same examples of feldspar diagenesis were observed at all post-transition depths, suggesting that feldspar diagenesis was not progressive as a function of depth. Feldspar compositions cluster near end-member Or and Ab, with marked absence of both Ca in alkali feldspar and K in plagioclase feldspar.

The data imply that significant feldspar alteration only occurred in samples from post-transition depths and that feldspar alteration at these post-transition depths is frozen in place, i.e. there is no progressive change or development in alteration with depth. K-feldspar proportion (ca. 4 wt%) is similar in all samples, including those from post-transition depths. These observations are consistent with a dissolution/crystallization model for the Gulf Coast S-I transition in which significant fluid is available over large shale volumes, leading to the development of illite-rich material that inhibits the further flow of fluids.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.