This map shows unconventional oil and gas well density per county in Pennsylvania. The cross-hatched area represents the extent of the Marcellus Shale Region. Counties that are white have zero wells. Each county that contains wells is represented by different shades of orange to brown. The darker a county the higher density of wells it has.

Oil and gas resources are present in Pennsylvania because millions of years ago the areas of the state that have these resources were warm, shallow ocean waters along tropical coasts. The early forms of sea life in those waters were trapped in sandy sediments when they died. Gradually those sediments were covered by overlying layers of sediment and eventually hardened into solid sedimentary rock. The oceans receded and our continent was pushed and pulled into the northern hemisphere. Natural gas separated from those deposits and migrated into more distant and often deeper sedimentary deposits.

Conventional wells access the oil and gas resources that can be developed by drilling technologies and techniques that have been employed for over a hundred years. These technologies employ drill bits and well casings (linings) that clear a narrow path into an existing oil or gas deposit. The oil or gas is typically under natural compression and flows easily through the sedimentary rock, into the deep end of the well and up to the surface. In some deposits, the sedimentary rock is less porous, so that flow must be stimulated using pumping or using compression (also called hydraulic fracturing, or fracking) methods. Most of these wells are hundreds of feet deep.

Unconventiional development of natural gas, especially from the Marcellus Shale bedrock layer, requires a different approach. First, the Marcellus Shale bedrock is much deeper than the bedrock tapped by conventional wells, much of it well over a mile below the surface. The cost of drilling so deep is many times greater than conventional wells. Second, the Marcellus Shale bedrock layer is hundreds of feet thick, many times thicker than the shallower conventional wells. Third, the Marcellus Shale bedrock is more dense and requires fracking in every instance. Fourth, drilling companies in southern US states developed the technological ability to guide their drilling bits along a curving path deep underground, so a well that starts out vertical eventually becomes horizontal. To make the drilling profitable, instead of fracking and drawing in the gas only through the deep end of a linear well, the horizontal section of the well casing is lined with many holes, each of which serves as an opening through which fracking occurs and from which natural gas can be extracted.

Marcellus shale is a sedimentary rock formation containing vast deposits of natural gas that was not economically viable until these advancements.