Glossary

Z/A	The ratio of atomic number (Z) to atomic weight (A). Nuclei of the same Z but different A are different forms of the same element and are called isotopes.

Z/A effect	In density logging. Common rock elements of usual interest in oil exploration, C, O, Si, and Ca all have a Z/A = 1/2, to a very high degree of approximation. For these elements and their molecules, the electron population is closely related to the unit mass or density of the material. In rocks containing elements in which Z/A differs from the Z/A value selected as a standard, the measured response will be different from the expected value. The amount that the measured bulk density will differ from the true bulk density of the rock material in question is related to the amount that Z/A differs from the standard. This influence on the interpretation of the density logging response is referred to as the Z/A effect. See also density log and compensated density log.

Z-axis	(1) A third dimension added to a crossplot of two parameters in an X-Y plane. The z-axis is perpendicular to both x- and y-axes. (2) In well logging, variation of the intensity of the recording spot is called z-axis variation, as in Micro-Seismogram.

zero	(1) mechanical zero or electrical zero. (2) To place the measure point on a downhole logging tool at the depth datum and adjust the depth-meter to read zero depth.

zonal isolation	Refers to the state or quality with which the fluids in one permeable zone can be kept separate from those fluids of another. Zonal isolation is created and maintained in the well bore by cementing the production string in place and by appropriate use of casing plugs and packers. Beyond the well bore radius and inside the formations, zonal isolation is a function of the existance of permeable barriers such as impermeable shale members, dense sediments, or cap rock. The quality of zonal isolation is directly related to the quality of the cement and bonding which is found between casing, cement sheath, and formation; and to the degree of or lack of permeability of the members which can serve as permeability barriers.

zone	A rock stratum which is singled out for reference because of its different character or fluid content from other strata.

zone of aeration	(1) A subsurface zone containing water under pressure less than that of the atmosphere, including water held by capillarity; and containing air or gases generally at atmospheric pressure. Extends from the ground surface to the water table. (2) Also refers to zones shallow enough that they are oxidizing, since they are replenished by rain water.