Smectite Illitization in Pliocene-Age Gulf of Mexico Mudrocks

J. H. Rask1, L. T. Bryndzia2, N. R. Braunsdorf3 and T. E. Murray4
1 Shell Western E&P Company, 200 N. Dairy Ashford, Houston, Texas 77079
2 U.S. EPA/NRMRL, 5995 Center Hill Drive, Cincinnati, Ohio 45224
3 Shell E&P Technology Company, Bellaire Technology Center, PO Box 481, Houston, Texas 77001
4 Shell Development Company, Westhollow Technology Center, PO Box 1382, Houston, Texas 77001

Abstract: Utilizing high-resolution transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD) techniques, we have studied the transition from shallower smectite-rich mudrocks to deeper illite-rich mudrocks in Pliocene-age turbidite sediments from the northern Gulf of Mexico (GOM). Our objective in this work was to better understand how the smectite-illite transition may affect the onset of geopressuring in GOM sediments. The samples studied were sidewall cores from an offshore Louisiana well. In previous studies of GOM sediments, the smectite-to-illite reaction has mainly been documented in considerably older, Miocene-age sediments.

The results of this study elucidate the reaction mechanisms entailed in the transformation of clays in this sediment from smectitic to illitic. We found that illite formed at the expense of smectite in 2 ways: 1) growth of preexisting discrete illite flakes, and 2) creation of new illite layers within mixed-layer illite-smectite. Also, illitization apparently proceeded via a dissolution/precipitation, Al-conserving reaction rather than a solid-state, layer-conserving reaction. Smectite illitization is commonly believed to require input of K from feldspar dissolution. Our XRD results found little correlation between decreases in K-feldspar and increases of illite. However, in 1 instance TEM/EDS analyses indicated the presence of high-charge smectite, which suggests that insufficient K was available for illitization. TEM images also show small packets of authigenic chlorite in illite-rich mudrock. This chlorite may act as a sink for Fe liberated upon smectite illitization.

Key Words: Clay Diagenesis • Core Analysis • Gulf of Mexico • Illite • Smectite • TEM • XRD

Clays and Clay Minerals; February 1997 v. 45; no. 1; p. 99-109; DOI: 10.1346/CCMN.1997.0450112
© 1997, The Clay Minerals Society
Clay Minerals Society (www.clays.org)