WorldWide Drilling Resource

Frequently-Used Geological Terms Part 51 Glossary Adapted from the Dictionary of Geological Terms M Microlitic - The texture of a porphyritic igneous rock in which the groundmass is composed of an aggregate of differently oriented or parallel microlites in glassy or cryptocrystalline interstitial material (material within a fissure). Microlog - A well log designed to measure the resistivity of a small volume of rock next to the borehole. Response is dominated by the presence of drilling mud caked on the walls of the hole, which causes separation between curves indicating porous zones. Micrometeorite - Ameteorite particle so small it can pass through the earth's atmosphere without vaporizing or becoming intensely heated. Microperthite - A fine grained intergrowth of potassic and sodic feldspar in which the lamellar structure is only visible with the aid of a microscope. Microphyric - Said of the texture of a pory- phyritic igneous rock in which the phe- nocrysts are microscopic. Also used to refer to a rock with such texture. Migma - Mobile, or potentially mobile, mix- ture of solid rock material and magma, the magma having been injected into or melted out of the rock material. Migmatite - A rock composed of igneous, or igneous appearing, and/or metamorphic materials, which can be seen with the naked eye. Migration - The movement of oil and gas from their source beds through permeable formations into reservoir rocks. Miller Indices - A set of three or four sym- bols used to define the orientation of a crys- tal face or internal crystal plane. The indices are determined by expressing, in terms of lattice constants, the reciprocals of the intercepts of the face, or place on the three crystallographic axes, and reducing if necessary, to the lowest integers retaining the same ratio. Milling Ore - An ore which needs pre- liminary treatment before it is acceptable for shipment or market. Mine - An underground excavation for the extraction of mineral deposits. The term is also used to describe aboveground open-pit workings. The area/property of a mineral deposit being excavated. To excavate and extract mineral deposits. Look for more terms next month! WWDR photo of the Kennecott open-pit mine in Utah. 43 WorldWide Drilling Resource ® JUNE 2016