ABSTRACT

The exceptional production that is being realized from the Atwater oil pool in Atwater Township, Portage County, is the product of several factors. It is evident that the wells in the heart of the pool are overproducing any reserves that could be calculated from simple volumetrics. The flow rates and the cumulative production both suggest that the pool is drawing on an extensive fracture system. The fracture system in turn suggests faulting. The most convincing evidence of faulting is seen on a Beekmantown isopach map that portrays Rose Run structure at the time of Knox erosion. This structure is also demonstrated on cross sections hung on the top of the Beekmantown dolomite. Faults are readily interpreted for the south side of the pool, strongly inferred on the north and east sides of the pool, and are suspected on the west side. The 3B sand is the best developed of the Rose Run sands and is considered the primary reservoir sand. A porosity-feet map of this sand precisely overlays the heart of the Atwater pool.

From these observations it is concluded that the area of the Atwater pool was one in a series of upthrown, en echelon fault blocks. Though the relief on this fault block was small at the time of Rose Run deposition, probably a matter of a few feet, it placed the sand in a more shallow, wave-washed environment that provided for a locally clean, well-sorted and well-developed sand. At the same time, lateral equivalents of this sand on downthrown blocks were less well developed. Growth of the area's faults during the deposition of overlying sediments continued to enhance the relative structural position of the Rose Run. By the time the area was flushed with migrating hydrocarbons, likely during the middle and late Ordovician, the Atwater pool was a structurally high reservoir that had enhanced primary and secondary porosity and lateral and overhead stratigraphic seals in place.