MINERAL OIL 
283 
with suitable isovols are barren of oil. It is only applicable 
where the general distribution of the coals is well known, 
so that allowance can be made for those with an especially 
high proportion of volatile matter. Moreover, in the absence 
of porous beds in which oil can accumulate or of suitable 
structural conditions a high ratio of volatile to fixed carbon 
may occur without supplies of oil. 
EsseNtiaLs of OiLrieLps AND OILFIELD STRUCTURES— 
The geological conditions essential to a great oilfield are 
first the presence of sedimentary rocks, with or without 
igneous rocks. Second, the presence of beds, such as sand, 
sandstone, or jointed limestone, which contain sufficient 
pores or other spaces to hold a considerable supply of oil. 
Third, the absence of extensive metamorphism later than the 
date of the possible oil-producing bed. Fourth, local ma- 
terial rich in organic matter. Fifth, an impermeable cover to 
prevent the oil escaping at the surface. Sixth, water con- 
ditions favourable to the concentration of oil into pools. 
{c) Oil pools ir horizontal beds of sandstone. The oil is concentrated by 
the surface tension of the water. The fact that in the successive 
oil-bearing beds the pools are not superposed shows that the oil 
concentration was not due to an undetected anticlinal, 
{d) The oilfield in the rift-valley of the Upper Rhine. The oil occurs in 
Kainozoic beds which have been slightly folded by compression 
between faults (F) which bound the Vosges to the W. and the old 
rocks of the Black Forest to the E. 
(e) Oil distribution in isoclinally folded beds in California. The under- 
ground oil pools are shown in black. The outcrops are plugged by 
patches of Brea, 
{f) Oil pools in beds due to over-folds and overthrust faults as in Galicia. 
The beds with dots are Oligocene and those with bars are Eocene. 
(2) Oil pool formed beneath a thrust-plane, TP., which has thrust im- 
permeable beds over the end of an oil-bearing layer; the outcrop is 
sealed by a patch of Brea. 
(h) An oil pool due to an unconformity, The oil-bearing bed is capped by 
a bed of shale in the upper series. 
{i) Oil accumulations due to igneous concentration as in Mexico. An 
igneous block in the centre has invaded a series of Cretaceous lime- 
stone and Eocene shales. The distillation of the organic matter of 
the limestone has formed pools of oil shown by the black bars along 
the faults (F) and in permeable patches of the limestone. 
(j} An oilfield formed at a salt dome. The salt dome, has formed by salt 
solutions rising from the basal red sandstones with salt patches, up 
the fault, F. The ascent of the salt plug has contorted the over- 
lying clays.