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The search for hydrocarbon-bearing Silurian Niagaran pinnacle reefs commenced when exploration technology became sufficiently advanced to accurately locate areally small reefs. Starting in 1947, gravity methods were used to locate 90 reefs in southwest Ontario and southeast Michigan but, as the reef trend was pursued westward, gravity data became unreliable owing to the presence of highly variable, surface glacial till. Initial efforts with single fold seismic were also unsuccessful for the same reason. It required the development of CDP seismic techniques with adequate surface static correction before exploration could successfully be expanded into the rest of the reef trend.
First, it was necessary to define reef seismic characteristics as it was not possible to see the reef directly. A synthetic seismogram study of known reefs in southeast Michigan indicated the initial indirect criteria: (a) thinning of seismic intervals above the reef and (b) pull-up below the reef interval. A CDP seismic survey confirmed these criteria and allowed optimization of field parameters.
The first prospect drilled satisfied the seismic criteria and, in addition, exhibited a salt-solution feature common above reefs in southeast Michigan. However, high-velocity infill material was drilled instead of a reef. A second prospect was selected with the additional criterion of an isochron thick around the reef interval. This feature proved to be a residual salt pillow inside the isochron interval. To reduce the risk of encountering salt features a third prospect was selected south of the salt limit (as defined by geologic mapping). The prospect was modeled with best fit of seismic data for the reef case. This resulted in the Mobil No. 1A Brown Niagaran discovery. Several similar seismic features were drilled confirming the reef seismic characteristics.
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