50 GRIMES : MYINGYAN, MAGWE AND PAKOKKU DISTRICTS. 



occurrence of petroleum at Yenangyat there is little to add to what 

 has been previously written, as to the oil being accumulated in the 

 higher parts of the anticlinal arch— a rule which applies also to 

 occurrences in other places. There is, however, a slight modifica- 

 tion of this at Yenangyat, as the wells a short distance, about 400 

 feet, west of the axis give a greater yield than those at the very 

 crest of the arch, this possibly being due to the beds at the centre 

 being somewhat broken up, so that the oil is partially diffused 

 through the adjoining beds. The wells too are mostly bored from 

 sites on the hills, as it was found that those sunk in the valleys 

 gave a poor yield. With the oil there is often gas at a considerable 

 pressure, which, especially when the oil sand is first pierced by the 

 boring tools, causes the oil to rise to a considerable height up the 

 bore-hole, often some hundreds of feet, and sometimes up to the 

 surface. In most cases this rise ceases in two or three days, but in 

 two of the wells it has been continuous ever since the bore-holes 

 were made. Owing to the great variability in the thickness of 

 the oil-bearing sandstones and to the enclosing beds of imperme- 

 able soft shale, which often completely encases the sands, so 

 that they are merely lenticular masses embedded in shale, the 

 separation of the gas, oil and water, and the collection of the 

 gas at the top of the anticlinal arch, then oil at a lower level and 

 below this the water, does not take place- in the neighbourhood 

 of Yenangyat ; and in the case of well No. 6, which is the one 

 farthest back from the crest of the anticlinal arch, and which 

 yields only a small quantity of oil, this oil is forced up by gas from 

 an oil sand 1,100 feet from the surface, whereas the oil from wells 

 close by and nearer the centre, has to be pumped. This encasing 

 of the sandstones in the shales, and the presence of bands of shale 

 or shaly sandstone in the sands themselves, also impede any flow 

 of the oil within the beds, so that neighbouring wells do not 

 interfere with or draw oil from one another, at least not to any 

 extent. 



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