T. 0. Boaworth — Ontlinea of Oilfield Geology. 19 



Paet II. The Accumulations op Petroleum. 



JSText will be given an epitome of the circumstances which are found 

 to have been favourable to the accumulation of petroleum. 



The chief rocks in which tlie entrapped oil finds accommodation are 

 beds of uncemented sandstones and dolomitized limestone. Of these 

 the oil sandstones are the more usual, but oil in dolomites is also 

 common, e.g. in the Palteozoic oil-formation of the United .States and 

 Canada and in the Tertiary oil-rocks of Mexico. Other porous rocks 

 occasionally contain oil, e.g. tuffs and much jointed shales. 



Practical men have often classified the occuri'ences into two 



divisions — ^-i . ,. .■ 



Oil m anticlines. 



Oil in monoclines. 



The latter term has been used to include almost all cases where the 



oil strata are not now in the form of an arch (which was once thought 



to be essential). To geologists this is unsatisfactory, because the 



' monocline ' is so often simplj' one limb of a denuded anticline. 



Here the accumulations will be classed in three main groups, 



according as they have been determined by — 



A. Geological Structure. 



B. Denudation Effects. 



C. Petrographical Circumstances. 



A. Oil Accumulations determined by Geological Structure. — 

 Where the strata are not horizontal the geological structure has been 

 the main factor in deciding the distribution of oil. Under the action 

 of gravity oil tends to move upwards in wet rocks and downwards in 

 dry rocks, and accumulation takes place wherever the geological 

 structure provides an obstruction or a terminus to the path in Avhich 

 the oil is moving. 



1. Terrace Striicture. — Many varieties of this structure occur in the 

 oilfields of Kentucky, East Ohio, Pennsylvania, iSTew York, and West 

 Virginia. 



riG.l. Oil accumulation due to terrace structure. Sandstone , clotted ; oil, black. 



The strata have a prevalent slight dip in one general direction, but 

 here and there the dip abruptly increases or decreases for a short 

 distance and then continues as before. Often where these terraces 

 are narrow and almost horizontal, accumulations of oil and gas are 

 found. It would seem that the oil has been migrating in one 

 direction — up the dip if the rocks contain water, or down the dip if 

 they be dry — and that any sudden change in the inclination of the 

 beds (especially a diminution) has acted as a slight check, causing 

 local accumulation (Fig. 1). 



