Man's physical universe

xanabras

190 THE EARTH AS MAN'S ABODE

sedimentary rocks of marine origin, it is believed that they originated

from marine plant and animal life. In the Galician petroleum fields

the abundance of fossil fishes indicates the probability that this petroleum

was formed from fishes. Deposits in the Caucasian fields were

probably formed from moUusks, while the unusually high iodine content

of the brines from the oil wells in southern California point to

seaweeds as the origin of these petroleum deposits. (During World

War I seaweeds were burned in order to obtain the iodine which they

extract from sea water.)

It has been suggested that petroleum deposits resulted from inorganic

chemical reactions or from algae and diatoms which constantly rain

down on the ocean floors from their breeding-grounds in the surface.

Fig. 58.

This diagram of a dome-like formation shows the proper locations for

gas or oil wells.

However, most scientists agree on an organic origin, although it is

uncertain in what manner natural gas, petroleum, and asphaltum

were produced from these organic bodies. Perhaps bacteria played an

important role, and it is quite possible that heat produced by slow

oxidation or other chemical reactions, and pressure produced by

superimposed layers of sedimentary rock aided in their transformation.

Once formed, oil and gas would migrate through porous layers of rock,

always moving upwards because of their low density, until they were

trapped beneath an impervious stratum of clay, where a reservoir

would thus be formed.

Modem Oil-prospecting Is Quite Scientific.

The knowledge of the type of geological formations which are likely

to contain oil enables the oil geologist to predict many oil discoveries,

although "wild-cat" {i.e., unscientific, "drill-and-see") wells often

bring in equally important new fields because the knowledge of forma-

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