Page 376 - Geology and Geochemistry of Oil and Gas

GLOSSARY 337
Squeeze cementing Placing cement by squeezing it under pressure.
Standard deviation A statistical measure of dispersion equal to the
square root of the ratio of the sum of the squared
deviations from the mean of a set of observations to
either the sample size (the population standard
deviation) or the sample size minus one (the sample
standard deviation); i.e., the square root of the
variance. Syn: Root Mean Square (RMS) deviation.
Symbol: s.
Statistics A branch of mathematics concerned with the
collection, classification, tabulation, and study of
numerical facts or the mathematical study of methods
of summarizing or describing data (descriptive
statistics) and then drawing inferences from the
summary measures (inferential statistics).
Strain The dimensional change in the shape or volume of a
body as a result of an applied stress.
Strand, strandline The zone of contact between the sea and land
commonly represented by beach deposits.
Stratigraphic trap The sealing of a reservoir bed as the result of litho-
logic changes rather than through structural trapping
conditions. See also: Pinch-out trap, Shale-out trap.
Stress A measure of the intensity of a force (F) acting upon
an area A of a body. Stress ¼ F/A can be expressed in
2
units of N/m . Stress can be resolved into two
components: (1) compressive (tensile) stress acts
normal to the surface and changes the volume of the
body, and (2) shear stress acts parallel to the surface
and changes the shape of the body.
Stromatolite A laminated organo-sedimentary deposit, either
planar or dome-shaped, constructed by the sediment
trapping and binding activities, together with some
amount of syndepositional lithification, of blue-green
algae (cyanobacteria).
Structural trap The containment of oil or gas within a reservoir bed
as the result of bed bend or flexure. Syn: Anticlinal
trap.
Stylolite A pressure-solution feature, generally formed in
moderately to deeply buried rocks, characterized by a
thin seam or suture of irregular, interlocking, saw-
toothed appearance.
Subaerial Referring to exposure on land, to meteoric fluids.
Subsidence Local or regional downwarping of a depositional
surface due to tectonism or sediment loading.
Subsidence of land surface can occur as a result of