Water Well Journal

November 2015

Water Well Journal

Issue link: http://read.dmtmag.com/i/592194

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Page 32 of 90

FIELD NOTES Introduction to basic field methods for carbonate and sulfate rock identification By Raymond L. Straub Jr., PG I have had the pleasure of working with many young newly minted geo- scientists on a drill site. But one thing I have noticed over the years is the limited training in recogniz- ing rock and soil types in cutting size pieces. The young people tend to find it difficult to move from outcrop size ge- ology to cutting size geology from the borehole. Many times, I have had to en- courage them by saying, "It's the same geology, only in a different scale." In dealing with cutting size geology, sometimes it is difficult to determine rock types in the field by just visual methods. Fortunately—through the use of acids, stains, chemicals, and heat— several field methods can be employed to help identify certain common rock types. Rock Types There are numerous rock types to identify on any given drilling project. However, identification and differ- entiation of limestone, dolomite, and anhydrite in the field simply by visual methods can be challenging. These rock types are all sedimentary rocks that form under lacustrine, marginal marine, or marine settings. As described in the Dictionary of Geological Terms, limestone is a sedi- mentary rock composed primarily of the mineral calcite (calcium carbonate, CaCo 3 ), but may also have impurities of chert, clay, limy mud, sand, and fossils. Dolomite or dolostone is a sedimen- tary rock consisting of more than 50% by weight of the mineral dolomite. Dolomitization is the process where limestone is partially or wholly con- verted into dolostone with the mineral dolomite by which calcium carbonate is replaced by magnesium carbonate, CaMg(CO 3 ) 2 . Anhydrite, a sedimentary rock of an- hydrous calcium sulfate, CaSO 4 , com- monly forms in massive evaporate beds. It can be readily altered into gypsum. (Bates and Jackson 1984) Rationale Accurately identifying samples in the field is an important function of the Working late into the night logging cut- ting samples on a project in West Texas. WWJ November 2015 29 Twitter @WaterWellJournl FIELD NOTES continues on page 30

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