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

MINERAL OIL 28g and shale in gentle dipping homoclinal beds. In the Central field, as at McKittrick, the beds have been intensely con- torted and overfolded ; the oil beds reach the surface, but the escape of the oil from some of them has been stopped by deposits of pitch or brea which have plugged the pores (cf. Fig. 63, ¢). In part of the Los Angelos field, the large yields from which in 1923-4 disturbed the oil markets of the world, the oil came from deeply buried domes of thick Miocene sand. Most of the Californian oil has an asphaltic base and is of moderately high density (14°-15° B): but the deep oil is lighter and has probably been derived from the diatom beds of the Lower Miocene. In the Summer- land field the wells are sunk from piers built from the shore : the oil comes from shales beneath the sea and percolates into a fault which there bounds the coast. Mzexico—The Mexican fields are an extension of those of the South Texas. The ordinary Mexican oil has an as- phaltic base, is thick and heavy, with a grade of 11°-124° B., contains much sulphur, and in use is usually mixed with lighter oils. The chief fields lie to the west of the Gulf of Mexico near Tampico and along the Tuxpan River, and they have yielded the most violent gushing wells yet encountered. The rocks of these fields range from the Cretaceous to the Pliocene. They have been folded and fractured by repeated movements, and traversed by many dykes and masses of basalt and dacite (cf. Fig. 63, 7). The chief oil- yielding bed is a thick cavernous limestone, the Tamasopo Limestone of Middle Cretaceous age, which has been frac- tured and oil distilled from it by igneous intrusions. The oil has risen from this limestone into the Upper Cretaceous San Filipe beds, a sheet of thin limestones and shales. These beds are a good oil reservoir as they are capped by 3000 feet of the Mendez Shales, which are Upper Cretaceous to Eocene. Owing to the thick shale cap the oil collects in the San Filipe beds and on the margin of the basalt dykes, where it is under such heavy gas pressure that when tapped by a well the oil may discharge with uncontrollable violence ; after a gusher has flowed for a few months the supply may suddenly cease and be replaced by salt water. The gas pressure of the Dos Bocas well in 1908 led to its eruption with such violence that the whole of its hundred million barrels of oil was lost. 10

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