Pennsylvania and New York have, in some ways, paid a price for being such early players in the oil industry. Because little was known about the best way to get oil most efficiently out of the ground in those early days, much oil was left still in place. A big business is to go back to some of these old oil fields and extract the oil using secondary recovery methods. In the 1890's, it was noticed that water reaching the sand by breaking through rusted casing in abandoned wells was increasing the output of producing wells. Oil men began simulating this by injecting water into abandoned wells. This practice, known as "waterflooding", was legalized in New York in 1919 and in Pennsylvania in 1921. Waterflooding "sweeps" the oil that is stuck in the pore spaces, and drives it through the reservoir and into a producing well.

Since the mid 1970's, natural gas fields in northwestern Pennsylvania and western New York (extending into Ohio and southwestern Ontario) have also become productive. The reservoir is the 435 million year old Medina Group sandstones of Silurian age. In parts of western New York near Buffalo, the Medina sands can be as shallow as 1,000 feet (~1,000 meters) below the surface, and can be up to 100 feet (~30 meters) thick. These relatively shallow depths would suggest that these reservoirs would have been drilled and produced years ago. However, the Medina sands generally have very low porosities and permeabilities. To recover the natural gas from these "tight" sands, petroleum engineers artificially fracture the rocks themselves, increasing porosity and permeability, and allowing the gas to flow to the surface. The result is the transformation of these "tight" sands into prolific gas producers in this region. During the energy crisis of the 1970's, natural gas prices rose to the point that drilling these tight sands became economical, marking the beginning of the development of this play.

Natural gas in the Trenton/Black River Formations was first discovered in New York State in the 1880's near near Lake Ontario (Oswego, NY). Over 120 years later, the Ordovician Trenton/Black River Formations are still incredibly important reservoirs in the eastern United States, again coming back into prominence in the last 10 years. 450 million years ago, fractures formed in these limestones, a result of tectonic forces hundreds of miles away. High temperature fluids have since moved into these fractures, locally changing the limestone to a mineral known as dolomite. When this occurs, it actually decreases the volume of the rock, and the weight of overlying formations will cause sagging into this low spot. Therefore, geologists actually look for zones where synclines, not anticlines, are seen in the subsurface. It is these dolomite regions that act as the reservoir for large accumulations of natural gas. In the northeast, the Trenton/Black River today produces mostly from Steuben County in New York State at depths approaching 10,000 feet (~3,000 meters). It is expected that more Trenton/Black River fields will be found in this region near the Finger Lakes. It is also likely that fields will also be found in Pennsylvania, where the depths to this productive reservoir are even greater.

In 2001, New York State produced a fairly insignificant 166,000 barrels of oil, while Pennsylvania produced 1.6 million barrels of oil, down significantly from 3.3 million barrels in 1994. Pennsylvania's "proved reserves" (the oil that is known to still be underground) as of 2001 were10 million barrels, ranking Pennsylvania 23rd in the nation in this category. Maryland no longer produces significant quantities of hydrocarbons.

Oil and natural gas to the east of Maine in Nova Scotia, Newfoundland and New Brunswick has been known since the 1800's. Sable Basin, to the east of Nova Scotia in the North Atlantic Ocean, contains some oil, and is currently being developed as a major producer of natural gas. So far, no petroleum resources have been found off the waters of New England. Test wells have shown oil and gas to be present off the coast of New Jersey, but the resources are slim. Due to the small amounts of petroleum located here, and the high cost of offshore drilling and transport, further exploration is unlikely in the near future. Most of the rocks of New England are metamorphic and igneous. These rock types do not contain petroleum. And while a relatively thin veneer of sandstone blankets much of Delaware, eastern Maryland and New Jersey, the rocks below are the same type of Precambrian rocks that are exposed in New England. Therefore, no source rocks exist in significant-enough quantities to create hydrocarbons.

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