T. 0. Bosivorth — Outlines of Oilfield Geology. 



21 



symmetrical (e.g., Yenangyaung Oilfield, Burma), others have the 

 axial plane inclined. In some fields, Trinidad for instance, the crests 

 are sharp and narrow and may be only two or three miles apart, 

 whilst in others they undulate so gently that the dip seldom exceeds 

 a few degrees, and some oil-bearing folds are broad geo-anticlinals 

 many miles across (in the United States). 



Usually the anticlinal axis is not horizontal, but rises and falls 

 repeatedly. This doming of the anticline causes such special con- 

 centration at the domes that sometimes the intervening parts of the 

 anticline are barren. 



When an anticline is asymmetrical the gentler slope generally 

 provides the best well-sites, and gentle anticlines are usually more 

 prolific than steep ones ; indeed, many of the richest fields are on 

 anticlines whose flanks slope at only a few degrees and whose crests 

 are very broad. On very broad anticlinals, several miles across, the 

 hydrocarbons are often concentrated in isolated pools where the 

 circumstances are particularly favourable. Variation in porosity and 

 bedding, faults, minor wrinkles on the fold, etc., have been important 

 in determining these sites. 



3. Salt Domes {^ Saliyies^). — These remarkable structures are 

 abundant in the oilfields of the Gulf Coastal Plane in Texas and 

 Louisiana (Fig. 3). Sometimes the domes are of such recent elevation 

 that they show as circular hillocks at the surface, hundreds or thousands 

 of acres in extent, and upwards of 100 feet in height. 



Fig. 3. Oil accumulation caused by a salt dome. Salt intrusion, sss ; gypsum 

 intrusion, ggg; dolomite intrusion, fft ; sandstone bed, dotted; oil, black; 

 gas shown by circles. 



At depth beneath the upraised strata is found a great plug of 

 crj^stalline salts which in some cases has burst right through the 

 strata and reached the surface. The chief salts are sodium chloride, 

 gypsum, and dolomite, which occur in ascending order. It is supposed 

 that these have been pi'ecipitated from hot saturated solutions 

 cij-culating below, and that the domes have been raised by the force 

 of crystallization. 



The domes apparently occur at places of weakness, such as the 

 intersection of faults. Porous sandstones in such domed Tertiary 

 strata form ideal sites for the accumulation of oil, and have yielded 

 great quantities. 



4. Strata upraised hy Igneous Intrusions. — Probably this case is 

 more common than is yet proved, for many domes and anticlinals may 

 at depth be underlain by laccolitic or other intrusions (though if this 



