Page 344 - Geology and Geochemistry of Oil and Gas

GLOSSARY 305
Covariance A measure of the association between two variables x
and y calculated as the arithmetical mean or the
expected mean value of the product of the deviation
of two variables from their respective mean values.
Symbol: cov(x, y).
Critical point A point at which one phase cannot be distinguished
from another and the material cannot be condensed
regardless of the amount of pressure applied. There is
no volume change when a liquid is vaporized at the
critical point (at critical temperature and critical
pressure).
Critical velocity A velocity at the transitional point between laminar
and turbulent types of fluid flow. This point occurs in
the transitional range of Reynolds numbers of
approximately 2,000–3,000.
Cross-stratification Layers or laminae of sedimentary rock deposited at
angles to the horizontal (not exceeding the angle of
repose in air or water) as a normal consequence of
transport by air or water. Syn: Cross-bedding.
Crown block Pulley installed at the top of the drilling rig, which
raises and lowers the drillstring.
Crude oil Petroleum as it is produced from the formation. The
two categories of crude oils are distinguished: (a)
heavy crude — thick (sticky) oil with an 1API gravity
o 171, and (b) light crude — thin (light) oil with an
1API gravity 4251.
Cryptalgal Term used in reference to a presumed algal or
cyanobacterial origin of certain carbonate rock.
Cryptocrystalline Term used in reference to crystal components (e.g.,
cement or architectural elements of shells) of very fine
size, generally not resolvable without the use of at
least a petrographic microscope; also said of a rock
with such texture. Syn: Microcrystalline.
Crystal silt Internal sediments found in cavities in rocks,
composed of silt-size particles of crystals; generally
form as a result of partial dissolution of host rock/
sediment or boring by organisms.
Cumulative distribution A function giving the probability that a
function [stat] corresponding continuous random variable takes a
value less than or equal to the argument of the
function.
Cumulative frequency A curve drawn to represent the percentage of
distribution [stat] occurrences of a number of observations of a variable
less and greater than any given value for an entire
sample. Syn: Cumulative curve.