Producing conditions, such as amounts of oil and of water can be conveniently shown by various colored labels or cards attached to the pegs.
Contour Maps of Underground Surfaces.—Upon an accurately constructed peg model, it is possible to see the various phases of geological structure. It is also possible to make direct measure ments of depth, thickness and dip, at any point covered by the model. A peg model takes up considerable space and is not conveniently moved about, and, therefore, may not be readily available for all demands.
Contour maps of various strata furnish most of the information available on a peg model and such maps should be kept up to date.
A disadvantage in the use of contour maps is that their mean ing is not self-evident, except to a person who is thoroughly accustomed to their use. Even in the hands of an experienced person, a contour map fails to impart as complete a mental picture as a model does. One advantage a contour map has is that it can be more accurately constructed than a model.
The method of constructing an underground contour map depends upon the simplicity or complexity of geological condi tions. Where definite marker-strata are penetrated by wells, the only preliminary information necessary is the accurate location and elevation of each well and the depth to the marker stratum. The foregoing data furnish the elevation of the marker-stratum at each well and fix the position of contour lines.
Where geological relations from well to well are difficult to trace it is necessary, before making a contour map, to make complete cross sections along many intersecting lines. Correct correlations may in some cases be impossible until after the stringing of a model. After complete cross sections have been correlated the elevations of various points in any given stratum are readily determined, and a contour map can be constructed.
The scale of contour maps will ordinarily be the same as that of the regular working map of the field.
A convenient procedure is to draw the contour lines directly upon a blue line print of the field map.
The contour interval will be determined by the dip of the formations, being smallest for flat dips.
The general methods of constructing and using topographical contour maps are applicable to underground maps.
Chart of Drilling Progress.—As wells are drilled, one after another, into productive formations changing conditions occur which affect production. In order to determine whether certain specific wells have caused a change it is necessary to devise some means of showing quickly and clearly the degree of completion of all the wells at any time. The following method devised by R. E. Colloml has been found most useful.
In order to compare the effect of wells which are in process of drilling, upon neighboring wells a chart showing drilling progress will be found more convenient than written records. The chart here presented differs from those ordinarily used in the fact that it refers to distances from known strata rather than from the ground surface. It therefore directly compares geological in formation with drilling and production data.
Referring to the accompanying cross-section of a group of wells, (Fig. 23) it will be noted that a line of correlation, "B ", has been "In certain localities where some definitely known stratum or forma tional marker exists—such as "Red rock," in the Coalinga East Side field, California, or "Bottom of blue shale," in the Casmalia field, California,—the line of correlation of this stratum, in the various wells may be used as stratigraphic datum.
" When such a stratum, as the one referred to, exists in a group of wells, one progress chart can be made for the entire group, irrespective of their location.
"On the accompanying cross-section stratigraphic datum is drawn through zero depth, that is derrick floor at Well No. 6, so that all correc tions for differences from surface to stratigraphic datum in each of the wells will be plus. In this position also the drilling records with respect to the principal upper water strata and other formations of importance can be plotted.