Early Clay Diagenesis In Gulf Coast Sediments: New Insights From XRD Profile Modeling

EARLY CLAY DIAGENESIS IN GULF COAST SEDIMENTS:
NEW INSIGHTS FROM XRD PROFILE MODELING

DOUGLAS K. MCCARTY1,*, BORIS A. SAKHAROV2
, AND VICTOR A. DRITS2
1 Chevron ETC, 3901 Briarpark, Houston, TX, 77063 USA 2 Geological Institute of the Russian Academy of Science, Pyzevskij per. D.7, 119017 Moscow, Russia

Abstract—Samples from different depths in the Oligocene Frio formation (offshore Gulf of Mexico) were
studied by X-ray diffraction (XRD), thermal analyses, and scanning electron microscopy. The
experimental XRD patterns recorded from oriented and ethylene glycol (EG) solvated clay fractions of
the samples were similar to those typical of random, mixed-layered illite-smectite (R0 I-S). The
experimental XRD patterns recorded in air-dried (AD) and EG states were simulated using three different
models. One of them corresponds to R0 I-S for which thickness and content of the interstratified layers
were determined by the S´rodon´ technique. The second model is represented by a single homogeneous I-S in
which illite and smectite layers are interstratified with a tendency to segregation. The expandability of the
segregated I-S model varies from 48% to 75% without any rational relationship between the smectite layer
content and depth.
The third model assumes that the clay fraction is a physical mixture of smectite and an R0 I-S. In this
model the I-S contains 65% illite and 35% smectite layers independent of depth, whereas the smectite
content varies from 28% to 63%. This model has consistently smaller profile factors, Rp, for both EG and
AD XRD scans compared with the Rp values determined for the other two models.
The mineralogical association, volcanic origin, narrow stratigraphic interval (427 m), and low maximum
temperature (42ºC) of the studied Frio Formation are considered. These features are completely consistent
with the two-phase model and so the segregation model must be rejected. An authigenic origin of the pure
smectite and an alternative detrital or authigenic origin of the R0 I-S are discussed.
Key Words—Diagenesis, Gulf Coast, Illite-Smectite, Frio Formation, Smectite.