5. Shale Properties as a Function of Geologic Burial and Compaction

Lecturer

Content

Clay-rich sediments at the seafloor have porosity over 70%. As these sediments are buried they become stiff muds, claystones, and eventually shales. This burial and compaction (a geologic rather than soil mechanics definition) reduces the porosity to extremely low values, with 10% (or less) porosity not uncommon for deeply buried shales. This is mostly a function only of effective stress increase, although geologic burial also includes very long time scales and increases in temperature. This natural compaction changes a myriad of properties, but in a systematic way.

This lecture will present observations collected over many years on an extensive library of preserved shales with burial depths ranging from <200m to almost 4000m. Certain properties are found to correlate almost exclusively with porosity, independent of clay type and exact clay content, as long as clay content is greater than ~50%. Other properties are a function not only of porosity but also of clay type (smectite vs. illite vs. kaolinite). A brief overview will be provided of the effects of increasing silt content. The properties and behaviors of resource shales will be introduced and contrasted with those of clay-dominated shales.

The following topics will be covered:
• Physical structure of shales and pore space
• Changes in physical structure, from seafloor to great depth
• Porosity and pore size distribution
• Void ratio vs. effective stress (to 40 MPa); similarities and differences to soil mechanics
• Definition of ‘compaction state’
• Permeability; pressure diffusivity
• Entry pressure for non-wetting phase (air, CO2)
• Stiffness (modulus)
• Shear strength and undrained stress paths; brittle vs. ductile, dry-side vs. wet-side
• Apparent OCR and apparent maximum past stress; diagenesis
• Fabric alignment; development of anisotropy
• Response to water and brine contact: swelling, osmotic effects
• Influence of clay type
• Influence of silt content
• Resource shales (gas shales) vs claystone shales; similarities and differences