Page 368 - Geology and Geochemistry of Oil and Gas

GLOSSARY 329
to thicken drilling mud, fracturing fluids, acid, water,
and other liquids.
Polymineralic Composed of more than one mineral.
Polymorph A mineral species with more than one crystal form,
e.g., CaCO 3 , calcite (hexagonal) and aragonite
(orthorhombic).
Population [stat] Any theoretical group of items or samples all of which
are capable of being measured statistically in one or
more respects; all possible values of a variable, either
finite or infinite, or continuous or discrete.
Pore A hole, opening, or passageway in a rock. Syn:
Interstice.
Porosity Absolute or total porosity is the total of all
void spaces present within a rock, but not all these
spaces will be interconnected and thus able to
contain and transmit fluids. The effective porosity
is thus defined as the proportion of the rock,
which consists of interconnected pores. Porosity is
expressed as a percentage of the bulk volume of
the rock.
Porosity terms (a) Fabric-selective porosity: pores that occur in
regard to specific elements in the rock. Cf: Non-
fabric-selective.
(b) Framework porosity: porosity in the matrix of
rocks, exclusive of fractures. Syn: Matrix porosity.
(c) Porosity-specific: porosity occurrence within a
given rock type or paleodepositional facies.
(d) Pore system: the total petrophysical attributes of a
porous unit.
(e) Primary porosity: porosity inherited from the
depositional environment. Cf: Secondary porosity.
(f) Secondary porosity: developed after deposition as
a result of dissolution, for example. Cf: Primary
porosity.
Postdepositional Physical or chemical changes in sediments or rocks
after final deposition and burial.
Pour point The temperature at which a liquid hydrocarbon
ceases to flow, or at which it congeals.
Precision (a) The degree of agreement or uniformity of repeated
measurements of a quantity. Precision relates to the
quality of the operation by which a result is obtained,
as distinguished from accuracy. (b) The deviation of a
set of estimates or observations from their mean.
Pressure gradient The difference in pressure at two given points, divided
by the distance between these two points.