Page 380 - Geology and Geochemistry of Oil and Gas
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GLOSSARY                                                             341
             Turbulent flow             The erratic, nonlinear flow of a fluid caused by high
                                       velocity. Cf: Laminar flow.
                                       U
             Ultraviolet light         Light waves shorter than the visible blue violet waves
                                       of the spectrum. Crude oil, colored distillates,
                                       residuum, a few drilling fluid additives, and certain
                                       minerals and chemicals fluoresce in the presence of
                                       ultraviolet light.
             Unconformity              A substantial break or gap in the geologic record
                                       where a rock unit is overlain by another that is not
                                       next in the stratigraphic succession.
             Unconformity trap         A stratigraphic trap associated with an unconformity.
             Underbalanced             Of or relating to a condition in which pressure
                                       in the wellbore is less than the pressure in the
                                       formation.
             Uninvaded zone            The area far enough away from the borehole so as not
                                       to have been invaded by the filtrate of drilling mud.
                                       Cf: Flushed zone.
             Upward-shoaling           A vertical section of deposits that records continually
                                       decreasing paleowater depths.

                                       V
             Vadose                    That zone in an unconfined groundwater lens wherein
                                       the pores in the sediments are filled mostly with air.
                                       Cf: Phreatic.
             Viscosity                 Measure of the internal resistance of a fluid to flow.
                                       Viscosity is equal to the ratio of shearing stress to the
                                       rate of shearing strain. There are two kinds of
                                       viscosity: (1) absolute or dynamic viscosity and (2)
                                       kinematic viscosity. Cf: Fluidity.
             Vitrinite                 A coal maceral group that is the dominant organic
                                       constituent of humic coals. Vitrinite forms the
                                       familiar brilliant black bands of coal. Vitrinite
                                       particles are found in about 80% of the clays and
                                       sands.
             Vitrinite reflectance      One of the petrographic components of coals used for
                                       the determination of paleotemperatures based on the
                                       thermogenic changes in optical properties of the
                                       organic matter. The reflectance of vitrinite is
                                       determined by microscopic examination of polished
                                       sections, measuring and comparing the electric
                                       currents generated in a photomultiplier under the
                                       influence of light reflected from the polished surface
                                       of the rock sample and from the standard with a
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