Unatego Landowners Association

          The Appalachian Basin in the northeastern United States is an important hydrocarbon province that has been producing oil and gas since the early 1800’s. More than 40 trillion cubic feet (Tcf) of natural gas and

millions of barrels of oil have been produced from reservoir rocks of all ages. Devonian-age shales are a

significant resource in the basin. Their coal-like appearance, wide spread distribution, and stratigraphic

nearness to the surface led to interest and use as an energy source dating back to the 1700’s. The Devonian

Shale of the basin has been estimated to contain up to 900 Tcf of natural gas, and an estimated 120,000

wells have produced roughly 3.0 trillion cubic feet (Tcf) of natural gas in the past 30 years. In addition to

Devonian Shale, other stratigraphically older and deeper black shales are present in the basin, and the

organic-rich Ordovician shales are believed to be a principle source rock for many of the productive

reservoirs in the basin. These shales, though not frequently produced, are often noted in driller’s logs to

have significant gas shows when drilling through them, and may be potential reservoirs.

          New York forms the northern edge of the Appalachian Basin that exists from southern Ontario to

Tennessee. With few exceptions, the state’s bedrock primarily consists of Devonian-age and older

formations. The younger rocks lie to the south and all sedimentary formations outcrop to the north, at the

edge of the Adirondack uplift. The Ordovician and Cambrian become visible again in the St. Lawrence

Lowlands.

          Curiosity about the black shales of New York from a geologic perspective and as a fuel source dates back to the late 1700’s. The black coal-like appearance and slightly combustible nature of the shales were of

interest to the coal industry, and gas seeps in creek beds motivated early explorationists to study the rocks

and find use for them. The first know commercial shale gas well was drilled in 1821 in the town of

Fredonia, Chatauqua County, New York near a gas seep along Canadaway Creek. The well, drilled by

William Aaron Hart, was completed as a gas producer in the shallow Dunkirk shale. The well was connected

to pipeline and provided natural gas to Fredonia’s main street businesses and street lamps in

the 1820’s. Following Hart’s success, the development and use of shale gas proliferated along the south

shore of Lake Erie, eventually spreading southward into Pennsylvania, Ohio, Indiana, and Kentucky. By

the turn of the century hundreds if not thousands of wells had been drilled along the lake shore and in the

basin, and were producing shale gas for domestic and small commercial use. However as exploration

advanced, the development of shale gas wells diminished in favor of more productive conventional oil and

gas horizons. It was observed early on that shale gas was tight, and while successful wells produced

steadily over long periods of time, production volumes were extremely variable and unpredictable, but

usually low (<100 mcfd). The mechanisms controlling production from these wells were not understood,

and the technology to optimize production was in its infancy.

          The rocks in New York have been impacted by at least one of the three major Paleozoic tectonic events. This has left the subsurface folded, fractured, and compressed. Also, numerous sea level changes created significant unconformities including the Knox Unconformity. Studies indicate that the Devonian age and older rocks underwent deep burial before being uplifted to their current elevation. This tectonic history

created the environment for hydrocarbon development and the trapping mechanisms to accumulate

economic quantities of oil and natural gas. Ordovician and younger rocks make up the central and western portions of the state which encompass the Ontario Lowlands and Allegany Plateau. Lake Ontario and the Adirondack Mountains form the northern boundary, the eastern margin is formed by the Hudson Lowlands and Taconic Mountains, and to the west terminates at the shore of Lake Erie. The structure of this region is fairly

simple. Paleozoic rocks overlying the Precambrian crystalline basement outcrop along the northern extent

of the Allegany Plateau, and dip gently to the southwest. In the southern portion of New York, a series of

small-scale folds are present, extending from Chatauqua to Tioga counties. The folds are small anticlines,

dipping less than 2o, which are associated with the Appalachian Fold Belt, an arcuate belt of anticlines and

synclines that extend southward into West Virginia.

          In the last 1 million years, New York has endured significant continental glaciation, with ice thicknesses approaching one mile. Glacial loading and post-glacial isostatic rebound in the gas-producing regions to the south of the Great Lakes appears to have created the fractured pathways for gas to have migrated from black shale source rocks into intercalated brittle silty and sandy reservoirs, as well as to have fractured and enhanced the storage capacity of these reservoirs  The ice at its maximum extent is estimated to have been over 1 mile thick, and theshear weight of the ice sheet caused the region to compress and sag. When the ice melted, ocean water temporarily flooded low-lying areas in the Champlain and St. Lawrence valleys that had been depressed forming the Champlain sea. Many marine deposits of this sea are now found at elevations exceeding 300 feet, indicating rebound of the region occurred. In the south where the glacial ice was thinner the rebound was less, however in the north where the ice was thicker, the rebound is over 400 feet. The uneven rebound is seen throughout northern New York. Glacial lake deposits that were once horizontal are now inclined to the north, and in the Lake Ontario region, the whole are has been tilted north to south. Post glacial rebound is now complete in New York, however the near-surface joint system has been enhanced and opened by the release of the glacial weight. The presence of horizontal fractures in the Devonian is mentioned in well records, and has been attributed to glacial unloading.

Excerpts taken from:  FRACTURED SHALE GAS POTENTIAL IN NEW YORK, By HILL, LOMBARDI & MARTIN (1992)  http://www.pe.tamu.edu/wattenbarger/public_html/Selected_papers/--Shale%20Gas/fractured%20shale%20gas%20potential%20in%20new%20york.pdf