Knowledge of the hydration state of smectite under burial diagenetic conditions is necessary for the interpretation of the smectite to illite transformation, abnormal fluid pressure development, and reservoir properties. Simple changes in fluid composition, pressure, or temperature have been thought to affect the number of layers of water associated with smectite at depth.

The hydration state of smectite while in contact with brine, at pressures and temperatures that simulate burial diagenetic conditions, has been determined by X-ray diffraction analysis. A heated high-pressure X-ray cell was used to determine the basal spacing from oriented mounts. The hydration state of the smectite was monitored by determining the 2^Thgr positions of the 003 and 005 reflections.

Two layers of water (d₀₀₁ = 15.8A) are retained by Cheto montmorillonite in 1 molal NaCl solution under 15.1 MPa (2,200 psi) pressure, to at least 175°C, and in the same solution under 3.4 MPa (500 psi) to 125°C. Two layers of water are also retained by Cheto montmorillonite in 5 molal NaCl solution to 200°C under pressures of 4.83 MPa (700 psi) and 46.2 MPa (6,700 psi).

This experimental evidence contradicts the previously suggested mechanism for generating fluid overpressuring in shales by simple thermal dehydration of smectite under burial conditions.

Experiments done with dilute NaCl solutions demonstrate the effect of concentration on the swelling of smectite at depth, while experiments with KCl solutions illustrate the effect of the interlayer cation on the hydration state of the clay.

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