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. 
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