Claystone Porosity and Mechanical Behavior vs. Geologic Burial Stress

A dataset was assembled resulting from studies on ~35 preserved claystone cores from different locations around the world and burial depths from <200m to ~3700m. Combining all data together, within a small amount of scatter there appears to be a single common trend for bulk density (or porosity) vs. depth (or vs. effective stress), irrespective of location, clay type or the exact clay content, as long as the formation is clay-supported. High-pressure mercury injection porosimetry shows that the modal pore size reduces in a systematic way with increasing burial stress. This loss of porosity and closer packing of the clay aggregates, perhaps combined with geologic time and/or temperature, causes mechanical properties (e.g. stiffness, strength) to also change in a systematic way. Whether or not a claystone behaves in brittle vs. a ductile manner generally depends on the value of confining stress relative to the maximum past burial stress, with young high-porosity claystones behaving quite similar to soil mechanics critical-state theory and deeper, low-porosity claystones deviating from several aspects of this theory and having an apparent maximum past stress higher than actual. In some cases there is evidence of a clay-type effect (smectite vs. kaolinite) on mechanical behavior.