PETROLEUM AND NATURAL GAS IN NEW YORK 425 



sition fairly represented in the following analysis (Trenton 

 limestone — northern Michigan). 



Carbonate of lime 82.00 



Carbonate of magnesia 3.00 



Insoluble residue 14.50 



The composition of the dolomite, on the other hand, is shown 

 in the following figures (Findlay gas rock). 



Carbonate of lime 53.50 



Carbonate of magnesia 43.05 



Insoluble residue 2.96 



Where the composition of the rock corresponded to the first 

 table there was nothing to justify the application of the term 

 oil rock to the Trenton limestone. It was seen that the dolomitic 

 metamorphosis had taken place in only those portions of the lime- 

 stone that were originally exceptionally pure in composition. 



The conclusion seemed therefore justifiable that the storage 

 quality of the Trenton limestone could be safely determined from 

 its chemical composition. This conclusion applied to its great 

 petroleum fields, namely, Ohio and Indiana, without qualification 

 and holds good with respect to Michigan and Illinois as far as 

 facts from these states have come in. 



But subsequent observations and specially those to be recorded 

 in the present report show that the conclusion must not be made 

 general in its terms. It appears that the Trenton limestone holds 

 considerable petroliferous accumulation in the form of natural 

 gas in regions where no trace of the dolomitic replacement has 

 occurred. In such districts it does not appear to be a true reser- 

 voir rock. It contains but little salt water and no continuity is 

 apparent in such occurrences of the latter as are found. Nor 

 does the water give indication of artesian pressure as in the oil 

 fields of Ohio and Indiana. 



Two principal modes in which gas is stored in rocks come into 

 view in this connection. It is either stored in porous rocks, as 

 sandstones, conglomerates or dolomites, in conjunction with 

 other fluid, in which case it can be styled reservoir gas, or it is 

 held in small spaces intervening between the leaves of shale or 



