58 T. 0. Boswortli — Outlines of Oilfield Geology. 



Limestones, sandstones, and conglomerates are often heavily charged 

 with asphaltic bitumen. 



The laitumens derived from paraffin oils are soft waxy substances 

 of pale colour, the most important being ozokerite, which is mined 

 in the strata overlying the oil rocks at Boryslav in the Galician 

 Oilfield, and also occurs in quantity on the island of Cheleken. This 

 ozokerite occurs in fissures sometimes as lumps and cakes weighing 

 several pounds, but the main production is obtained on treating the 

 clay shales with hot water. 



Gas. — Exudations of gas are numerous in most oilfields, ascending 

 through the surface soil from outcrops and fractures. Generally they 

 can be detected by the smell, and often also they can be ignited and 

 may burn continuously. In wet ground the noise of the gas bubbling 

 up through pools of water assists detection. Occasionally violent 

 outbursts hav« been known to blow out cavities in the rocks. 



Sulphur. — Sulpiiuretted hydrogen and sulphur-dioxide frequently 

 accompany the escaping gas, and deposits of sulphur in cracks in the 

 rocks or on tlie surface are not uncommon indications of hydrocarbons 

 below. 



Sandstones in Peru, and thick limestones in Texas and Louisiana, 

 are impregnated with sulphur to such an extent tliat when they are 

 heated melted sulphur runs out. Hot sulphurous springs of water 

 are of frequent occurrence in oil territory, and often when a well 

 approaches exhaustion sulphur water ascends. 



Salt. — The complete relationship between salt and petroleum is not 

 understood, but it is a conspicuous fact that salt is nearly always 

 in some way associated with petroleum either as crystalline masses of 

 rock-salt, or salt water, or saliferous strata. It is true that the 

 conditions favourable for the preservation and storage of hydrocarbons 

 are also those favourable for the presence of bodies of imprisoned 

 sedimentation water, but the salt water which accompanies petroleum 

 is usually much more saline than the water normally present at depth 

 in sedimentai'y rocks, and suggests some further relationship. 



Mud Volcanoes. — These occur in almost all parts of the world where 

 oil-rocks approach the surface. They are built up over springs which 

 exude mud, gas, and hot water often with a little oil. Sometimes 

 mud volcanoes form hills a hundred feet or even several hundred feet 

 high, but most numerous are small cones rising onlj- a few feet. 

 Common also where much water accompanies the mud are volcanoes 

 of larger area and very slight elevation. A typical example observed 

 by the writer in Trinidad had a soft mud crater of 35 yards diameter, 

 in the middle of which was a salt pool 1 2 ^-ards across. 



The noise of the great gas bubbles, which burst up every minute or 

 two, could be heard two or three hundred yards away. The soft mud 

 was surrounded by harder mud on which one might cautiously walk, 

 the whole volcano forming a nearly flat, bare mud patch of 100 yards 

 diameter, in the midst of thick forest. The volcano was surrounded 

 by a little moat into which streams flowed from the crater. Outside 

 this, sandy ground rose 5 or 10 feet, but was cut through in one place 

 by a miniature gorge where the muddy salt water found egress from 

 the moat as the source of a stream. 



