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Sankar K Muhuri1, Satavisa Sarkar1, Thomas Dewers1
(1) University of Oklahoma, Norman, OK

Abstract: Reaction and lithification of calcite aggregates under diagenetic conditions

We have investigated rates of porosity loss and dissolution/precipitation of calcite aggregates under diagenetic conditions, including temperature, pressure, and effective stress. Volume creep experiments using hydrostatically loaded granular aggregates of sparry calcite and oolites in presence of water, test rates of porosity loss as functions of effective pressure, temperature, grain size, volume strain, and clay content. In all cases, pore volume decreases with logarithm of time, however, at least three different mechanistic responses can be identified. Loss of porosity is rapid initially due to mechanical rearrangement of grains, microfracturing, and twinning, especially at larger grain sizes, higher stresses, higher porosities, and lower temperatures. Subsequent time dependent loss in porosity (so called chemical compaction) is via pressure solution at grain contacts or intragranular sub critical crack growth and healing. The presence of clay enhances pressure solution compaction rates over clay-free experiments for similar porosity. These observations have been used to generate "creep laws" for calcite compaction.

In conjunction with the above we have also measured kinetics of dissolution and precipitation of calcite over diagenetic P-T ranges as functions of fluid saturation, pH, pCO2, and salinity. A rotating disc/mixed flow reactor design allows the assessment of transport versus surface control of rates. Rate laws for calcite dissolution and precipitation under these conditions are developed and used to test the validity of a local equilibrium assumption in a given basin setting. The identification of different reaction mechanisms aids in the interpretation of creep behavior under the same P,T and fluid chemistry conditions.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana