Page 336 - Geology and Geochemistry of Oil and Gas

GLOSSARY 297
Analysis of covariance The partition of the total variation in a set of
observations into components corresponding to
differences between and within sub-classifications of
the data, after adjustment for the effect of other,
possibly related, variables (covariates).
Analysis of variance A statistical technique for simultaneously partitioning
(ANOVA) the total variance of a set of the data into components
which can be attributed to different sources of
variation, and which can be used to test for
differences among several samples.
Anisotropic medium A medium whose physical properties vary in different
directions. Cf: Isotropic medium.
Annular space Space between the outside of the casing and the
wellbore.
API Gravity The standard method of expressing the specific weight
o
of oils: API ¼ 141.5/SG – 131.5, where SG is the
o
specific gravity of oil at 60 F compared with water at
o
60 F.
Aquiclude A body of relatively impermeable rock functioning as
an upper or lower boundary of an aquifer and does
not transmit water. Cf: Aquifer.
Aquifer A permeable reservoir or portion of reservoir
containing formation water or groundwater. Cf:
Aquiclude.
Argillaceous Containing clay minerals as impurities in carbonate,
siliciclastic, or evaporitic sediments.
Aromatics Group of cyclic hydrocarbons found in oils. Contain
a benzene ring nucleus in their structure, with a
general formula C n H 2n 6 .
Artificial lift Any mechanism, other than natural, that lifts fluid
from the wellbore to the surface.
Asphalt Black to dark-brown solid or semisolid bitumens,
which gradually liquify when heated. Composed
principally of carbon and hydrogen, but contain
appreciable quantities of nitrogen, sulfur, and oxygen;
largely soluble in carbon disulfide.
Asphaltenes Any of the solid, amorphous, black to dark-brown
dissolved or dispersed constituents of crude oils and
other bitumens, which are soluble in carbon disulfide.
They consist of carbon, hydrogen, and some nitrogen
and oxygen, hold most of inorganic constituents of
bitumens.
Atoll A ring-shaped reef, circular or elliptical or horseshoe-
shaped, generally encircling an interior lagoon, and
surrounded by deeper water.