Among various structural forms with which oil fields may be connected, those of diapiric structure, known in many oil-bearing areas in Roumania, in the Caucasus, and elsewhere, are the least favorable for prospective drilling. They are places of mere manifestations of crude oil at the surface. Oil deposits of commercial value are found generally on different forms of structure, and may occur in more deep-seated forms, adjacent to, or otherwise associated with, diapiric forms of more superficial nature.

Folds, as they occur commonly in the northwestern part of the Median zone of the Polish Carpathians, showing different stages of diapirism, are disposed en echelon, and areas intervening between such fan-like diverging folds might be favorable locations for exploration for deep-seated oil horizons in the lower part (Cretaceous) of the Flysch. This possible oil reserve has not been investigated.

The longevity of Polish oil deposits depends on the primary or secondary nature of the reservoirs (whether differentiated by original process of sedimentation, or degree of subsequent cementation), on the content of gas in the reservoirs, and on the intensity of reservoir pressure.

In the sub-Carpathian province, comprising the foreland depression filled with Miocene formations, only three gas fields are being exploited, and it was only in 1932 that the first test well for oil was located there; locations for two more wells have been selected on the basis of preliminary geophysical surveys.

The Marginal zone of the Carpathian petroliferous province, consisting of a complex of strata in many places repeatedly superimposed one upon another as thrust slices (skiba in Polish terminology), is not promising for test drilling, with the exception of that part that has been thrust bodily, by a deep-seated thrust plane, upon the sub-Carpathian Miocene formation. At present two test wells have been commenced with the object of searching for this deep possibility, outside of the few productive areas of this type already known in Boryslaw, Rypne, Majdan, and Bitkow.

In Poland there are no reserves of proved oil areas, but there exist widespread and reasonable possibilities for future discoveries. With steady progress in efficiency

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and technical perfection of exploitation methods in old fields, with the inauguration of oil mining, and with general policies in the industry improving, it will be possible to lengthen the producing life of oil fields.