GEOLOGICAL MAPS Maps dealing with the various phases of geology, such as areal geological maps, structure maps, isobath, isochore, iso pachous and other similar maps are considered under this heading.
Areal Geological Maps. These maps show the location of the geological formations that outcrop at the surface; the value of which is to acquaint one with the various rocks that will be encountered in the field, also it may give certain hints as to the presence of folds and faults. (Fig. 38.) Structural Maps. Structural geology deals with the deforma tion and attitude of the strata and the relation of the rocks to one another. There are various ways to depict structural features, but the best and simplest method is by means of contour lines used to denote the elevations and depressions of the strata.
Such maps are known as isobath maps or sometimes structure contour maps, and they show graphically the various folds and faults over the territory that has been examined.
The use of contour lines becomes very valuable in this con nection as by means of it the various facts may be shown. Thus we may indicate the dips of d stratum, the thickness of the stratum as well as convergence or lateral variation between strata. For this reason a thorough understanding of contour lines must be had, as well as the distinction between the surface contours and structural contours.
Isobath Map. The object of this map is to show the dips of a stratum. (Fig. 39.) Such a map is constructed from the data obtained in the field during the course of the geological work. The elevations of a key-horizon are determined, and if more than one key-horizon has been used, they are to be reduced to a common one by the addition or subtraction of the interval between them. The elevations thus determined are plotted on a map in their proper places, and contour lines or isobaths are drawn through the points having the same elevations. It is quite often necessary that certain mechanical computations are to be made to obtain all the points through which the lines are to be drawn; thus if we have two figures of 520 and sso feet and we intend to use to feet contour interval, it will be necessary to determine the points where the 53o and 540 feet lines are to pass through, so their position is to be interpolated between the known points of 520 and 550. (Fig. 41.) From this it will be seen that the accuracy of the map depends on the number of elevations of outcrops and their proximity to each other. An isobath map that is constructed in this way will show the various dips of the strata in a graphical way, it will be correct for the various deformations of the key-horizon, and it will give us an idea what such a key-horizon would look like if the entire surface soil had been stripped off. The completed map will show all anticlines, synclines and other structures existing in the territory so mapped. (Fig. 4o.) Isochore Map. The isochore map or convergence sheet is used to assist in making an isobath map of the producing horizon from an isobath map of a surface or key-horizon. In most places, the interval or the distance between a key-horizon and the oil or gas-bearing stratum may not be the same, and will vary from place to place, thus at one point, for example, the interval will be 15co feet, and about four miles further east the interval between the same two strata may be 1540 feet, in other words the lateral variation between the two points will be 40 feet. The informa
tion can be obtained from well records only, and the more numer ous the records the more correct will be the convergence sheet, which is constructed as follows: The position of each well whose record is used is plotted in its proper place and the interval between a key-horizon and the producing horizon is noted alongside the well, with these figures as a basis a contour map is constructed, which when completed will show the points along which the interval is the same, in other words each line or isochore will represent equal intervals. (Fig. 41.) Such a map should be made on tracing cloth or other transparent material. The next procedure is to combine the isobath map of the surface key-horizon and the isochore map, which is as follows: the isochore map is superimposed over the isobath map, and the figure of each isochore is subtracted from the figures of the isobath at every point where they cross each other, obtaining a new set of figures, which, when contoured, will be the isobath map of the producing horizon, as the figures resulting from such operation will be the elevation of the producing sand at that point with reference to the chosen datum plane. (Fig. 43.) Lateral variation in most instances is quite regular in a certain direction, that is, it may be that the intervening strata causing such a variation thicken in a certain direction at a certain rate, or as may best be expressed by the statement that a certain stratum has a lateral variation of io feet per mile southeast.
Lateral variation may be so strong that it may entirely obliterate any surface structure, and where small structures are known a strong lateral variation may obliterate it in such a way that the producing horizon may not show such a structure. This is the case in the "Clinton" sand pools of Ohio, where all the forma tions thicken towards the east, due to the fact that deposition has taken place along the east flank of an anticlinal fold, where the folding and deposition took place at about the same time, thus the deposits further from the axis will be in all cases thicker. Many lenticular as well as many new formations and sands may make their appearance under such conditions, away from the axis.
Of course, the value of a convergence sheet is increased with the number of well records that are obtainable. Wherever well logs are numerous, it is possible to construct an isobath map directly from such records, simply by taking the elevation of the surface at each well, the elevation of the sand computed and the isobath map constructed from those figures.
Another valuable use to which contour lines may be put to is to show the general variation of thickening of a certain stratum, generally that of a "pay-sand." Thus if the figures showing the thickness of a stratum are properly plotted and a contour map made from those figures the resulting contour map will show the direction and the amount of variation in the sand. Such maps are known as iso-pachous maps, and are of great importance in observing the pinching and swelling of a sand, and as the varia tions take place with some regularity, it becomes possible to determine the points where the sand may be expected to "pinch" out entirely. Lenticular sands, as well as porous portions of sandstones, may be mapped in this manner, the data being obtained from well records. (Fig. 44.)