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Experiments made by the geological departments of the Marland Oil Company of Oklahoma and the Gypsy Oil Company, respectively, have shown that a satisfactory log can be made from a continuous set of rotary cuttings.
To be satisfactory for this work the samples must be collected by a device capable of making a continuous, automatic separation of a representative sample of the cuttings. Most sampling devices employ one or the other of two basic principles--straining or settling. The settling devices have proved the more satisfactory. Where a wooden flume carries the returns direct to the settling pit, the settling of cuttings can usually be accomplished by installing a weir or dam in the flume itself. This checks the velocity of the mud stream and causes the cuttings to be deposited above the weir. Where the use of the flume weir is impracticable, the samples can be obtained by diverting a portion of the returns to a specially constructed collecting box in which the separation is also accomplished y the use of a weir to reduce the rate of flow.
Especial care is necessary in the washing of rotary cuttings. Since each individual fragment of sand, shale, or limestone, as it comes from the well, is coated with a thin film of rotary mud, it is absolutely impossible to distinguish one type of rock from another unless this coating of mud is removed by a thorough washing of the sample.
Reasonably accurate depth measurements are essential if the log is to be of any value for structure mapping. There are two possible sources of errors in determining the depth which is represented by the sample; those due to inaccuracies in determining the depth at which the drill is working and those due to the lag in the return of the cuttings.
The errors in the determination of the drilling depth are by far the more serious. The only satisfactory method of obtaining depth measurements is to measure, with a steel tape, the length of each joint of drill pipe as it is first run into the hole. Since the errors due to lag are only excessive where the rate of progress is great, they can usually be disregarded.
The preliminary description of rotary cuttings is simply a record of the approximate percentages of sand, lime, shale, gypsum, and other constituents, with appropriate descriptions as to color, texture and fossil content, of the material present in the sample. It is the abrupt changes which appear in such a sequence of samples that are significant rather than the gross material in the individual sample. The contacts between the different formations can be located with reasonable accuracy by noting the depths at which these abrupt changes take place. The final log is simply a record of these contacts and is identical in form with the ordinary log made by the driller.
In addition to the data contained in the log, fragments of the sands may be tested for possible oil content by using some of the ordinary solvents and noting the
discoloration, or subjecting a small charge of fragments of the sand to a simple distillation test.
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