It will be noted from the foregoing description of a cross sec tion that more detail enters into its construction than is usual in the generalized or ideal cross sections accompanying geological reports.
Peg Models.—The relations existing between a number of wells can not be completely and clearly shown by a single cross sectional drawing, except where the wells are situated along a straight line.
Several cross sections can be made so as to cut the group of wells in all directions and thus present all phases of a situation. However, the separate drawings fail to give a clear and adequate picture such as can be had from a model.
Rods or pegs set up in their proper relative positions both vertically and horizontally, and colored to represent various formations penetrated by bore holes, have for many years been used in connection with mining operations. This method is particularly adaptable to oil wells and justifies a description of some general specifications which have been extensively used.
It must not be understood that models will entirely obviate the necessity for cross sectional drawings. The drawings can be made more accurately than a model, and are therefore abso lutely necessary in many cases.
A convenient scale which will show all necessary details is 100 ft. to the inch. The horizontal and vertical scales should be the same, so as to avoid distortion.
Soft wood, such as sugar pine, is the most suitable material for a peg model on account of the ease with which it can be altered or added to. The wood must be thoroughly seasoned. A base board 1% inch thick will amply support pegs which are ;i inch in diameter.
It is usually more convenient to construct a model in separate pieces so that they can be separated to allow close observation of almost any locality. With the scale of 100 ft. to the inch, a quarter section of land is most conveniently covered by a single base board (26.4 inches square). A light skeleton table support ing four such boards may be moved about the room without greatly disturbing the arrangement of the boards.
The base map, showing well locations and numbers and also property lines can conveniently be drawn directly upon the board. Its appearance will be
improved if the boards are painted white.
Pegs not larger than iz in. in diameter will leave open space sufficient for a general view. Larger pegs are more substantial but give undue prominence to the individual wells and obscure the general effect. Pegs should tightly fit into holes bored through the base board. Care in boring the holes is necessary in order that the pegs will stand vertically and securely.
Each peg should show all the important information furnished by the log of a well. The graphic logs, made according to the foregoing specifications, can be fastened directly upon the pegs. Blue prints will serve all ordinary working purposes, but the general appearance will be improved if white prints are used and the pegs are also painted white.
Blue prints are liable to shrink or stretch from the dimensions of the original draw ings, and some precaution and ingenuity are required to lessen the effect of such distortion upon a model. The use of too much water in the paste for fastening the graphic logs to the pegs will add to the error. Shellac has been used with good results.
After the pegs bearing complete graphic logs have been securely set in the base board, fine strings may be stretched from one peg to another in such a manner as to connect or correlate a stratum. The stringing of a model is the most important step in its con struction. It will sometimes require many trial correlations before one is found which will stand the test of critical examina tion. A geologist familiar with the locality is in the best position to make such a correlation.
When a model is completely and correctly strung, its value will be apparent. All of the persons concerned in the underground condition of the wells will find, after studying the model, new ideas which would not have been discovered otherwise.
Pins with colored heads may advantageously be stuck in the pegs at important places, such as point of water shut-off or bottom of open hole.