Abstract

The Lower Silurian "Clinton"-Medina sandstone play is one of the most significant gas plays in the Appalachian basin. Producing fields are located in a broad trend extending from northeastern Kentucky north into Ohio to the Canadian portion of Lake Erie and southwestern Ontario, and into northwestern Pennsylvania and western New York. More than 100,000 "Clinton" wells have been drilled in over 400 fields and have produced an estimated 6 tcf of gas.

In New York and Pennsylvania this play is known as the Grimsby or Medina Group while in Kentucky and Ohio the name "Clinton" is used. Ontario uses the designation of Cataract Group. In early correlations, the name "Clinton" was mistakenly applied to the sandstones of the Medina Group. By 1916 it was felt that the name "Clinton" was too firmly established to be corrected. Since then the name has become thoroughly entrenched within the industry and the literature.

The "Clinton"-Medina is not a "blanket" sand but rather a series of interbedded sandstones, siltstones and shales. These rocks do not crop out in Kentucky, Ohio or Pennsylvania. The closest outcrop reference is in Niagara County, New York and Welland County, Province of Ontario along the Niagara escarpment. The sandstone interval thins westward and eventually pinches out in central Ohio at a depth of approximately 1,000 feet below sea level. The Medina Group is subdivided in descending order: the Thorold Sandstone (Stray "Clinton"), the Grimsby Sandstone (Red and White "Clinton"), the Cabot Head Shale and the Whirlpool Sandstone. Each Red and White unit may consist of a single body of sandstone or up to five separate sandstone beds with interbedded shales. They are overlain by the shales and carbonates of the Middle Silurian Clinton Group and unconformably underlain by the Ordovician Queenston shales.

This play is bounded by shallow marine carbonates to the west and northwest and a continental sandstone to the east and southeast. Source area of the sands is from the east-southeast and may be the result of a dying pulse of the Taconic orogeny which, with ensuing erosion, caused a fresh influx of elastic materials into the depositional basin. Medina Group reservoir sandstones were deposited in a complex deltaic to shallow marine environment during an overall regressive sequence. This depositional environment includes various sub- environments such as fluvial, delta-front, barrier island and shelf sands. The relative influence of these sub- environments varies geographically and stratigraphically because of the complex interplay of the depositional settings and the effects of marine reworking.

Although the "Clinton"-Medina play is considered primarily stratigraphic, locally structure has been shown to influence production. Interbedded shale permeability barriers have created vertical compartmentaliza- tion of the "Clinton" sandstone reservoirs. In general, porosity and permeability increase in the updip direction. Fields developed prior to hydrofracturing tend to delineate trends of maximum porosity and permeability. Most of the early gas fields were discovered in a narrow zone parallel to the updip pinchout in central Ohio and southern Ontario.