Page 346 - Geology and Geochemistry of Oil and Gas

GLOSSARY 307
Density log Measurement of formation porosity. Involves
bombarding the formation with gamma rays, with
detectors measuring the number of gamma rays that
are reflected from the formation.
Depositional karst Term used in reference to various small-scale karst
features (e.g., small dissolution caves and related
speleothems and cave cements, dissolution-etched
erosion surfaces) formed as a result of short periods
of subareal exposure during deposition.
Desiccation Loss of interstitial water from sediments as a result of
drying.
Design of experiments Methods of obtaining mathematical description or
mathematical model of a process under study as a
regression equation based on the limited number of
reproducible experimental data. Syn: Experimental
design.
Detrital Term generally restricted to sediments derived from
the erosion of preexisting rocks. Syn: Terrigenous,
Siliciclastic.
Deviation [stat] The difference between an observation and a fixed
value.
Deviation [drill] Directional change of wellbore from vertical.
Diagenesis All chemical, physical, and biologic changes in
sediments or rocks that have altered their original
textures and mineralogies, operative from the time of
their formation and deposition, up through
lithification.
Diagenetic facies An assemblage of rocks with similar diagenetic
attributes or which have been affected by similar
diagenetic histories.
Differential pressure The difference between two fluid pressures; e.g., the
difference between the pressure in a reservoir and in a
wellbore drilled through the reservoir.
Diffusion (a) The spontaneous movement and scattering of
particles of liquids, gases, or solids.
(b) The migration of dissolved substances from an
area of high concentration to an area of low
concentration.
Directional drilling Intentional deviation of a wellbore from the vertical.
Directional survey A logging method that records drift angle, or
deflection from the vertical, and direction (azimuth)
of the drift.
Discriminant analysis A statistical procedure for classifying subsequent
samples into categories previously defined and