T. 0. Bostcorth — Outlines of Oilfield Geology. 



23 



7. Porous Strata Thrust over a folded Oil- generating Formation. — 

 This important class includes a variety of complicated structures, from 

 some of "which great quantities of oil have been obtained. The 

 circumstances belong to regions of intense folding, and are best 

 illustrated by the oilfields of lloumania, where they are well known. 



In Eoumania the source of oil appears to be the saliferous formation, 

 and the places where the oil is concentrated lie along important lines 

 of thrust-folding — "places where there has been maximum pressure 

 and minimum resistance." ^ Along these lines occur very 'pinched' 

 anticlines and overfolds with much overriding of one formation upon 

 another. The saliferous formation contains hard massive salt rock and 

 often forms the core of the fold, in which case oil is generally found. 



Saliferous (Miocene). 

 Fig. 7. Oilfield at Bushterrari. Oil derived from upfolded saliferous Miocene, 

 but accumulated in overfolded Oligocene. 



The oil, however, is not concentrated in the saliferous but in the 

 porous beds of the older (Oligocene, Fig. 7)'or newer (Pliocene, Fig. 8) 



D. 



Fig. 8. Oil derived from upfolded saliferous Miocene, but accumulated in 

 contiguous Dacien (D.). Oilfield at Bacoi (diagrammatic section) . sss indi- 

 cates masses of rock-salt. P. = Pontien ; D. = Dacien. 

 formations thrust over it or faulted against it. The conditions are 

 very favourable, viz., compression of the oil-generating rocks, relief 

 of pressure above, contact with overlying porous beds, the porous beds 

 inclined and generally sealed at their upper ends. 



^ Professor Mrazec, " Excursions dans les regions petroliferes de la vallee de 

 la Prehova," p. 62. Congres International de Petrole, 1907. 

 (To he concluded in our next Number.) 



