Measurement of Porosity and Smectite Hydration State Under Deviatoric Stress Conditions: Implications for Water Partitioning Between Pores and Minerals in Sedimentary Basins

Tina Fitts, Kevin M. Brown, Michael Tryon, and Barbara Ransom

Smectite dehydration has been linked to overpressuring in hydrocarbon bearing regions. The hydration state of smectite under subsurface conditions is not well constrained for in situ conditions of temperature and differential stress. We present a technique to determine the partitioning of H₂O between pore and interlayer space within smectite rich sediments. Preliminary data indicates that this technique will also allow changes in hydration state of smectite to be determined at elevated deviatoric stresses and temperatures. In the initial experiments fluid containing a tracer (Br) is pumped through a smectite sample (equilibrated with pore fluid lacking (Br) at a given stress state. The effluent fluid is collected at frequent intervals. The fluids are analyzed for Br, and porosity is calculated by mass balance using concentration and volume data. Chlorinity is measured to detect changes in mineral hydration. In addition to the tracer experiments, pure and natural samples have been equilibrated with a range of synthetic pore fluids and XRD studies undertaken to determine how d-spacings change with fluid composition and concentration, and to observe differences between behaviors of different types of samples. These experiments should allow us to predict the hydration behavior of smectite-rich sediments undergoing compaction and diagenesis in the presence of different pore fluids.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California