WorldWide Drilling Resource

Frequently-Used Geological Terms Part 63 Glossary Adapted from the Dictionary of Geological Terms P Penetration Twin - A twinned crystal where the individuals appear to have grown through one another. Pentlandite - A pale bronze isometric mineral commonly intergrown with pyrrhotite. It is also the principal ore of nickel. Percentage Log - A sample log in which the percentage of each type of rock present in each sam- ple of well cuttings is estimated and plotted. Percolation - Slow movement of water through small openings within a porous material. Perforation - The puncturing of well casing opposite an oil- or gas-bearing zone to permit gas or oil to flow into a cased borehole. Peridot - The gem variety of olivine and the birthstone for August. Peridotite - A coarse-grained plutonic rock composed primarily of olivine with or without other minerals or micas, and containing little or no feldspar. Perlite - A volcanic glass having the composition of rhyolite, and a generally higher water content than obsidian. Permeability - The capacity of a porous rock, sediment, or soil for transmitting a fluid. Permeability Trap - A trap for oil or gas formed by lateral variation of permeability within a reservoir bed. Permeable - Said of a rock or sediment which allows water, oil, or gas to move through it at a rate which can be perceived or measured. Perthite - A feldspar rock consisting of orthoclase or microcline in which is interlaminated albite. Petrochemistry - The study of the chemical composition of rocks. It is an aspect of geochemistry and not equivalent to petroleum chemistry. Petroleum - A naturally occurring, complex liquid hydrocarbon which after distillation and removal of impurities yields a range of combustible fuels, petrochemicals, and lubricants. A general term for all naturally occurring hydrocarbons, whether gaseous, liquid, or solid. Look for more terms next month! Photo of a gypsum penetration twin from Kansas, courtesy of Rob Lavinsky, iRocks.com 14 JULY 2017 WorldWide Drilling Resource ®