The Elements of economic geology

Bibliographic data

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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Chapter

Document type:: Monograph

Structure type:: Chapter

Title:: Part V. Mineral fuels

Collection:: Economics Books

MINERAL OIL 285 clinal dip (cf. Fig. 63, k), which is also described by the hybrid term of uniclinal. Many of the chief oilfields are in homoclinal and not anticlinal areas. The oil may rise along a porous bed in the homoclinal series and collect where the bed is blocked by a fault or dyke, or becomes thinner or denser, or has been plugged by bituminous matter. Oil also occurs in horizontal beds. It has been claimed that the oil really occurs along anticlines which are so gentle that they are not recognizable; but if the beds were anti- clinal, the pools in the successive beds should occur one below another, like the saddle-lodes of a goldfield; but the pools at different levels may not be superposed (Fig. 63, ¢). The pools may be due to lenticles or patches of porous sand in clays, or the concentration of the oil by sur- face tension. If a mixture of oil and water soaked into a bed of irregularly mixed sand and clay, the surface tension would force the oil into the sandier patches, leaving the water in the clays. Many oil pools in horizontal and inclined beds are doubtless due to this process. Oil may also occur in thin seams or gash veins in limestone, due to the gradual production of bitumen from the organic matter of the rock, and its collection in shrinkage cracks. Large accumulations of eil also form where fractured lime- stones are capped by an impermeable bed, as in Mexico; but in these cases as there may be no evidence on the surface as to the distribution of the fissures, boring is very uncertain. Though oil is not formed in igneous rocks it may be forced into them by gas-pressure when distilled from adjacent rocks by heat. Colossal oil pools occur in Mexico beside intrusive igneous rocks (Fig. 63, 7). Estimation oF Or REsourceEs—The estimation of the re- serves of oil in a field is more difficult than that of coal or ore. It is rarely possible for a surveyor to enter the oil bed, while an adequate number of bores to test the reserves might be too costly and even dangerous. Of the two chief methods of estimating oil reserves, the first is determination of the capacity of the oil sands, i.e. their area multiplied by their thickness and by the amount of pore space. The result shows the maximum amount of oil that would be pre- sent if the whole bed were saturated with oil. This “ satur- ation method " is deceptive because the pores may be partly

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