A high magnification SEM image of a purely diagenetic reservoir pore system. The micro-rhombic, microcrystalline, low-Mg calcite crystals are diagenetic in origin after a poorly-understood process of mineralogical stabilization changed the precursor mineral (interpreted to have been high-Mg calcite) into this microporous network. All concentricity and radial structures in the ooids are lost, but porosity is continuous through each grain. The scale is 10 micrometers, so the pore-throat size is in the micrometer range - too small for the passage of oil, but excellent for storing gas. This example is from the Cotton Valley (Jurassic) Formation at Overton gas field, Texas. The image is from a study by Ahr student Tom Covington.