The Elements of economic geology

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Monograph

Identifikator:: 1773832379

URN:: urn:nbn:de:zbw-retromon-172798

Document type:: Monograph

Author:: Gregory, John W. http://d-nb.info/gnd/11683014X

Title:: The Elements of economic geology

Place of publication:: London

Publisher:: Methuen

Year of publication:: 1928

Scope:: XIV, 312 S.; graph. Darst.

Digitisation:: 2021

Collection:: Economics Books

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Document type:: Monograph

Structure type:: Chapter

Title:: Part V. Mineral fuels

Collection:: Economics Books

280 ECONOMIC GEOLOGY in organic material is most likely to lead to a prolific oil- field. Accordingly those great subsiding areas known as geosynclinals are the seat of the chief oilfields. The tropical and subtropical countries are more favourable than polar regions, where organic matter is less abundant. InpicaTioNs oF Or—The existence of subterranean oil is indicated by several features :— (1) The most obvious are seepages of gas and oil. Gas may escape unseen unless discharged under water; it may supply perpetual gas springs, such as those at Baku which have been burning for thousands of years in the temples of the Fire Worshippers. (2) Escaping oil often forms iridescent films on pools and if set on fire forms a “ burning pool.” Oil films may be dis- tinguished from those of oxide of iron or manganese by their odour, and by stirring with a stick when a metallic film breaks into pieces, and an oil film reunites into a single patch. A film of oil may be a clue to a large deep-seated supply which constantly renews the film as it evaporates; but it may result, as in some deltas from the decay of recent organic matter. (3) A petroliferous sandstone may indicate an underground oil supply; the rock at the outcrop may give no sign of oil, but freshly broken surfaces have a fefid odour, remarked in the name stinkstone. If petroleum or bitumen occur in a rock and a sample be crushed in chloroform its evaporation in a dish leaves a black rim. A bituminous sandstone may act as an impermeable cap and imprison oil in beds below. The Brea of California consists of pitch which has plugged up the outlets from underlying beds, in which the oil has accumulated ; the oil is reached by boring. A bituminous sandstone may however indicate that the oil has evaporated and left only a solid residue. The most famous petroli- ferous residues are the pitch lakes of Trinidad and Venezuela which have been left by the evaporation of large quantities of oil. Bitumen also occurs in veins and * dykes,” such as the albertite veins of New Brunswick, along channels by which oil reached the surface. Limestones often contain bitumen disseminated through the rock or collected in simple or branching gash veins. The bitumen may be the residue of the soft tissues of the organisms that formed the limestone.

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