Abstract: Subsurface Onondaga Reefs

Arthur M. Van Tyne

Seven subsurface onondaga reefs have been found in southwestern New York (6) and northwestern Pennsylvania (1). These reefs have had a maximum thickness of about 200 feet and cover an area of a few hundred acres. They are similar to nearly 30 smaller reefs in the same geologic section which have previously been found along the onondaga outcrop. The discovery well for onondaga reef gas, although not recognized as such at the time, was the No. 1 Quinlan oil of J. Brooke Reed et al. The well was drilled in 1933 in the Town of Olean, Cattaraugus county, New York near the New York - Pennsylvania State line. The first of the more recent onondaga reef discoveries occurred in 1967 at Wyckoff in the Town of Jasper, Steuben county, New York. This discovery touched off a leasing and seismic exploration boom in this area of New York. Additionally, various companies carried out detailed well cuttings analyses in search of reef material. As a result of these studies, two more reefs were discovered in 1971, two in 1974 and the last so far in 1981. These seven reefs have produced 28 billion cubic feet of gas through 1992. The largest producer, Adrian, has produced 7.1 billion cubic feet of gas. The smallest, Flatstone, has production to date of about 700 million cubic feet.

The Onondaga reefs are of basal Onondaga, or Edgecliff, age. The Edgecliff is a light gray, coarsely crystalline, biostromal limestone. Normally, it varies from a few inches to several feet in thickness. However, where a reef has developed the overlying onondaga units are missing, or lap up against the sides of the reef mound. Onondaga reefs may have begun forming on somewhat higher parts of the sea floor in crinoid thickets. They are mainly composed of colonial and solitary rugose corals. Tabulate corals followed, their skeletons acting as sediment baffles, and helped to build up the mound. Lesser numbers of brachiopods, gastropods and bryozoa completed the reef biota. The main framework of the reefs consists of numerous rugose coral skeletons in a matrix of lime mud and skeletal deb is. Westward and northwestward from the known reefs several presumed onondaga reefs have been discovered. Because the onondaga is considerably thicker in that area these so-called reefs are buried entirely within the total onondaga section. No samples from these wells are available and Gamma Ray logs show nothing unusual for the area. They have been called reefs mainly because gas shows have been encountered in the lower onondaga when it was drilled through by wells aiming for deeper Medina sandstone gas production. Nevertheless, gas production from them has been minimal. The gas trapping mechanism is inter and intra-skeletal porosity. The seal consists of surrounding and overlapping black and gray middle Devonian Hamilton shales. The basal portions are surrounded by onlapping upper onon aga limestones. The source of the gas is believed to be the highly organic Hamilton shale. No water has been encountered so far during production.

AAPG Search and Discovery Article #90954©1995 AAPG Eastern Section, Schenectady, New York