The dip of a measure is of course not a constant factor, and as it falls away from the summit it tends to approach a horizontal position. When sufficient wells have been drilled along a line to establish the relation between the dip and the surface gradient, it is an easy matter to plat them to scale and to predict within narrow limits the depth of a well at any given point (Fig. 28). Such platting when carefully done helps to bring out the presence of minor folds or waves and irregularities in the measure, if such be present.
Surface Indications of Oil.
Aside from the study of geological structure and the applica tion of such information to the question as to whether or not oil may be found in underlying strata, there are certain occurrences of surface phenomena which often suggest the presence of oil and which, in fact, are what usually lead to the first hope or belief that oil may be present.
The first, and most commonly observed, of these are the seep ages of oil found in districts all over the world. They are usually detected by the light iridescent film or play of colors on top of the water emerging from springs in ravines. Although the actual amount of oil present is apt to be very slight, occasionally it is present in greater quantities, but ju any case the characteristic odor of petroleum readily identifies it and distinguishes it from some of the compounds of iron that also form the colors on water and are often mistaken for oil indications. It may also be dis criminated by breaking the film.
Seepages may result from fracture planes in the earth supplying a passage way for the oil from the point of origin to the surface, or by direct mixing at or near the surface of water with the oil from measures outcropping nearby. The nature of the oil may fre quently be learned by observing it carefully. Asphalt oil tends to dry and form small deposits of solid asphalt, while that with a paraffin base will flow for a longer period, eventually forming small particles of a brown substance that often takes a reddish tinge.
In other occurrences the oil in its upward migration has been sub jected to filtering processes which have removed from it the greater portion of its heavier constituents, leaving it light and clear, and it is evident that samples of such will be misleading if accepted as indications of the quality of petroleum that will be encountered with drilling. In any case, when oils reached the surface the more volatile varieties will tend to disseminate more readily while the heavier ones will thicken and gather locally. Often a seepage of
gas will lead to the discovery of petroleum when no signs of the oil itself may be found.
Other indications of the presence of oil, commonly observed, are the outcrops of oil-bearing strata. These may be detected by their appearance and discoloration, their odor, and by the test of placing a few grains in a test-tube containing chloroform and watching for the brown color that will appear if these hydro-carbons are present. Slight showings of sulphur flakes may be found in them also, and their effect on vegetation is often so pronounced in contrast with that supported by the neighboring measures, that, at some seasons of the year, such an outcrop may be traced across the country for considerable distances by observing only the marked difference in the appearance of the grass or other growths. All these indications, however, are much more apparent with out crops bearing an asphalt oil than when the oil is the lighter and more volatile variety with a paraffin base. In the latter case, the faint odor of vaseline may be the only means of its identification. Outcrops of measures heavily impregnated with asphalt oil make excellent road-building material and are frequently quarried for this purpose.
A third form of indication occurs when neither oil nor gas may be definitely found but when the evidence of their action on other materials may be observed, as in the case of the presence of small flakes of sulphur and the foul-smelling gas hydrogen sul phide, associated with the fields where limestone is the source of the oil. In these districts the outcrops of the oil-bearing strata rarely carry direct indications, but the sulphur deposited along small stream courses and the hydrogen sulphide, detected particu larly in daily weather, are suggestive guides.
It must not be thought, however, that every petroleum seepage or outcrop of an oil sand is indicative of the presence of oil in abundant quantities. Many seepages are found but few develop into oil fields, because the oil may never have been present in the strata except in minute quantities, or, if there at one time, it may have escaped because of any one of a number of geological changes and the resulting alterations in underground conditions and structure.