washed off. Some of the larger shale and lime fragments are separated from this sample by running it through sieves and must be considered when estimating percentages. Also, this image view is only a fragment of the total sample and some of the sand at the bottom of the tray can not be seen and must also be considered in the total estimation. With that in mind this sample would be considered to be about 90% shale, 5% sand and 5% lime (In 5% increments). The gas in (Figure 1 & 2) is represented by the green line and is measured in units as the quantity of total gas, but does not represent the actual quantity of oil or gas the reservoir contains. In (Figure 1) the squared-off dash-dot lines just to the right of the sand (in yellow) and left of the gas (in green) represents the heavier hydrocarbons detected. Cyan = C2 (ethane), purple = C3 (propane) and blue = C4 (butane). Detecting and analyzing these heavy gases help to determine the type of oil or gas the formation contains. See also Directional drilling Drilling fluid (mud) Geosteering LWD (Logging While Drilling) MWD (Measurement While Drilling) Well logging References Further reading Chambre Syndicale de la recherche et de la production du petrole et du gaz naturel, 1982, Geological and mud logging in drilling control: catalogue of typical cases, Houston, TX: Gulf Publishing Company and Paris: Editions technip, 81 p. Exlog, 1979, Field geologist's training guide: an introduction to oilfield geology, mud logging and formation evaluation, Sacramento, CA: Exploration Logging, Inc., 301 p. Privately published with no ISBN Whittaker, Alun, 1991, Mud logging handbook, Englewood Cliffs, NJ: Prentice Hall, 531 p. External links Articles and books on mud logging Hand drawn mud logs Geoservices definition of Mud Logging Maverick Energy Lexicon Mud Logging Gas Detectors Well logging Petroleum geology