Fields producing oil from the Rogers City dolomite in the Michigan basin are becoming more difficult to discover. To aid in discoveries, a method of sample analysis based on the Versenate method, which quickly gives the magnesium-calcium ratio of well samples, has been devised. A study of these ratios shows that dolomitization of the Rogers City limestone occurs at two places: (1) on the apices of folded structures, as a result of solution and replacement by ascending waters along tension fractures; and (2) in long narrow bands, the result of high-magnesium waters ascending faults and master fissures. These dolomitized areas have distinct flank-zones of decreasing magnesium-calcium ratios, which aid materially in predicting the location of dolomites, and their distance fr m control points. Secondary dolomites in the Rogers City limestone are commonly porous and serve as good reservoirs for the possible accumulation of oil and gas.

This method of magnesium-calcium ratio analysis should aid materially in predicting the presence of local secondary dolomitization in any calcareous formation and in any area, and thus help to locate more easily productive reservoirs in secondary dolomite.