WorldWide Drilling Resource

Frequently-Used Geological Terms Part 57 Glossary Adapted from the Dictionary of Geological Terms O Obsidian - A dense volcanic glass, usually rhyolite in composition and typically black in color. Obsidian forms in lava flows where the lava cools so fast crystals do not have time to grow. Ocellar - The texture of an igneous rock when aggregates of smaller crystals are arranged radially or tangentially around larger euhedral crystals. Ocher - An earthy, powdery, red, yellow, or brown iron oxide used as a pigment. Yellow or brown ocher (limonite) and red ocher (hematite) are a few examples. Also refers to various clays strongly colored by iron oxides. Octahedral Cleavage - Mineral cleavage parallel to the faces of the octahedron as in fluorite. Octahedron - A crystal form consisting of eight triangular faces each having equal intercepts on all three crystallographic axes. Oil Pool - A subsurface accumulation of petroleum which will yield crude oil in economic quantities. The oil occurs in the pores of the rock. Oil Sand - Term applied to any porous stratum containing petroleum or impreg- nated with hydrocarbons; specifically a sandstone or unconsolidated sand. Oil Shale - A kerogen-bearing, finely laminated brown or black sedimentary rock which will yield liquid or gaseous hydrocarbons when distilled. Oil Trap - Any barrier to the upward movement of oil or gas, allowing either or both to accumulate. A trap includes a reservoir rock and an impermeable roof rock. Oil Red Sandstone - A thick sequence of nonmarine, predominantly red sedimentary rocks (primarily sandstones, conglomer- ates, and shales) which represent the Devonian System in parts of Great Britain and elsewhere in Europe. Oligoclase - A mineral of the plagioclase feldspar group. It is common in igneous rocks of intermediate to high silica content. Olivine - A green or brown orthorhombic mineral. It is a common rock-forming min- eral of basic, ultra basic, and low silica igneous rocks such as gabbro, basalt, peridotite, and dunite. It crystallizes early from a magma, weathers readily at the earth’s surface, and changes into serpen- tine. Look for more terms next month! Obsidian sample photo courtesy of the U.S. Geological Survey. 41 WorldWide Drilling Resource ® JANUARY 2017