The mud analysis logging system, now widely used for exploratory and routine drilling, continuously analyzes and records the oil and gas content of mud returns from wells being drilled by the rotary method. Oil or gas detected in the returning drilling fluid indicates oil or gas in the formation penetrated by the bit. Results of the continuous analyses are instrumentally correlated to the depths and formations from which the showings originated. A second useful phase of the system plots accurately and in detail rate of penetration or drilling speed on the log as a function of depth.Trucks and trailers provide a mobile housing for all of the mud analysis equipment so that it may be moved rapidly from well to well.Applications of the method may be divided into two classes:1. Routine drilling in proved areas where it eliminates unnecessary coring, and locates gas caps and completion zones.2. Exploratory drilling in which it minimizes coring by indicating for testing purposes porous zones containing oil and/or gas.Mud analysis logging can be practiced in areas where the electrical log cannot be used due to high salt content or other local conditions, where dangerous hole conditions make interruption of drilling operations for coring inadvisable, and for evaluating gas zones where cores are difficult to interpret.In addition, each mud analysis logging unit contains equipment to obtain information useful in eliminating washouts, in predicting and preventing blowouts, and in controlling drilling mud characteristics.