Figure 5. Diagenetic shrinkage at a minor fault in anhydrite.

Left - The surface of a minor fault in the anhydrite layer shows polygonal extension cracks that are widest at the fault surface.

Right - Photomicrograph of thin-section cut at right angles to both the fault and one of the extension cracks. Calcite is stained pink; anhydrite is white; epoxy is blue. The fault surface (left side) is completely altered to calcite. Both calcite alteration and the crack die-out away from the fault surface. Note residual bitumen layer parallel to the fault surface, resulting from oil movement along the fault. Bacteria consume the hydrocarbon, convert sulfate to sulfide, generate bicarbonate, and back precipitate calcite in the presence of calcium. Conversion of anhydrite to calcite results in a 20% molar volume reduction. This volume change shrinks the caprock and produces extensional fractures that contribute to the fracture system.