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Ascertaining the location and properties of subsurface lithological layers requires acquisition and interpretation of well logs. Logs can be drawn by hand to reflect lithology as cuttings are pumped to surface; or they can be acquired by wireline logging tools lowered into the well, or while drilling, using specially instrumented drill collars that make up part of the drilling assembly. Interpretation can be carried out by hand, using established log analysis formulae, or by sophisticated computer routines. Formation evaluation is the key to determining whether a well has struck pay, or is just a dry hole. Terms in this discipline range from Archie equation to logging, and from permeability to Z/A effect. All definitions have been reviewed by formation-evaluation experts, and many are accompanied by high-quality photographs or illustrations. |