Organic-rich shales form when ancient organic matter such as marine algae, plankton, diatoms and spores form in the water column, fall to the sea floor and are buried in clay and other fine grained material before they have a chance to decompose. As this organic-rich clay is buried and compacted it becomes harder and turns into shale. Continued burial leads to heating of the rock and maturation of the organic matter. The organic matter first turns into kerogen, which at temperatures of about 75ºC starts to expel oil. The source rock is in the “oil window” until about 125ºC when it enters the “gas window” and most of the oil and some of the remaining kerogen turns into natural gas. In the far western part of the state, the organic-rich Marcellus is in the oil window, but from Allegheny County east it is in the gas window. Eastern New York was once the home to mountains that may have been as big as the Himalayas at one time. When these great mountains existed, the Marcellus may have been buried more than 15,000 feet deeper than it is today. At that depth it was much hotter than it is in the surface -- probably more than 200ºC.The organic-rich part of the Marcellus ranges in thickness from 10 feet in south-western New York to more than 200 feet in the southern Catskill Mountain area (Figure 1).
Although the organic-rich part of the Marcellus Shale likely contains gas across most of that area, it is most likely to be developed with long horizontal wells where it is at least 3000 or 4000 feet deep and where it is at least 50 feet thick.
Figure 2 shows the thickness map in colored contours and the burial depth to the top of the most organic-rich section in the black contour lines. The organic richness decreases from west to east and it is possible that there is not enough ancient organic matter in some of the easternmost areas.
Figure 2. Thickness of Marcellus Shale in feet (colored contours) with burial depth to top of most organic-rich interval in the Marcellus (black lines).
The Geological Survey will continue to conduct research on the Marcellus Shale in order to provide citizens, other agencies and policy makers with geological information necessary to make informed decisions.
The Reservoir Characterization Group has also done extensive work on analysis of the Utica and Marcellus Shales and other natural gas reservoirs in the State. Some of their posters and presentations are available here: http://esogis.nysm.nysed.gov/Talks_TOC.cfm