ABSTRACT: Rates of pore volume loss in shales due to chemical compaction: implications for fluid pressures

The potential for clay mineral reactions in shales to generate over pressures has been discussed for the past three decades. We have been evaluating the potential effects of clay mineral reactions in shales on pore volume change and fluid volume change in an attempt to better constrain the impact of shale diagenesis on fluid pressures in sedimentary basins.

Shales, like sandstones, loose pore volume due to both mechanical compaction and chemical compaction (i.e. diagenesis). Modeling results using synthetic examples and rocks from various sedimentary basins indicate that the rate of pore volume loss due to clay diagenesis can reach the same order of magnitude as the rate due to mechanical compaction. Rates of pore volume loss due to mechanical compaction decrease exponentially during the first kilometer of burial. Clay mineral reaction rates are highest during the early part of the reaction, once temperatures reach approximately 70°C. This is typically after rates due to mechanical compaction have been reduced significantly. In addition, during periods of non-deposition or erosion, mechanical compaction essentially stops. Whenever effective stress stops increasing and temperatures are appropriate, rates of pore volume loss due to diagenesis can surpass those due to mechanical compaction. The impact of chemical compaction and the release of bound water from clays into pore space on fluid pressures will be discussed.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia