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Pressure-Temperature-Composition of Illite/Smectite Mixed-Layer Minerals: Niger Delta Mudstones and other Examples

Published online by Cambridge University Press:  02 April 2024

B. Velde ,
T. Suzuki  and
E. Nicot
B. Velde
Affiliation:
Laboratoire de Géologie, ER 224 CNRS, Ecole Normale Supérieure, 46 rue d'Ulm, 75230 Paris, France
T. Suzuki
Affiliation:
Laboratoire de Géologie, ER 224 CNRS, Ecole Normale Supérieure, 46 rue d'Ulm, 75230 Paris, France
E. Nicot
Affiliation:
Laboratoire de Géologie, ER 224 CNRS, Ecole Normale Supérieure, 46 rue d'Ulm, 75230 Paris, France
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Abstract

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X-ray powder diffraction (XRD) studies of the clay fraction of Upper Cretaceous mudstones from a shallow (2.5 km) drill hole in the Niger delta indicate a high geothermal gradient (about 100°C/km) during diagenesis. The mineralogy of the clays is similar to that observed elsewhere and consists of interstratified illite/smectite (I/S), kaolinite, and chlorite. Detrital mica and K-feldspar are also present throughout the section. The composition-depth relationship of the I/S is different from that observed in deeply buried Gulf Coast shales. The Niger delta rocks show a linear change in composition of the I/S as a function of depth in the drill hole from 60 to 10% smectite layers. The I/S ordering is R=0 in shallower samples and progresses to R=1 in deeper samples. The I/S in the deepest samples has R=3 ordering. No R=1 I/S showing a first-order 27-Å XRD reflection was found. The Niger delta sequence differs from the Gulf Coast sequence b. (1) a lack of R=1 I/S showing a 27-Å XRD reflection (which are common in Gulf Coast samples and contain 20% smectite layers), an. (2) the existence of a simple, continuous linear relation between the composition of the R=1 and R=0 I/S and depth.

A comparison of composition-temperature curves for I/S formed under different diagenetic regimes shows different types of I/S ordering in which the presence of R=1 I/S showing a 27-Å reflection and R=3 I/S types changes the composition-depth (and thus, composition-temperature) relations. These changes suggest a difference in the energy necessary to form the various ordering types. Also, geothermal gradient during burial appears to be responsible for different composition-temperature gradients found for the same type of I/S ordering.

Keywords

DiagenesisGeothermal gradientIllite/smectiteOrderingSmectite
Type
Research Article
Information
Clays and Clay Minerals , Volume 34 , Issue 4 , August 1986 , pp. 435 - 441
Copyright
Copyright © 1986, The Clay Minerals Society

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