Aspects of the exploration phase are entertained in Lesson 1. The U.S. Geological Survey now routinely assesses petroleum potential for various basins or "plays" worldwide and publishes their assessments with estimates for total reserves of different types including the Marcellus Shale (U.S. Geological Survey Information Relevant to the U.S. Geological Survey Assessment of the Middle Devonian Marcellus Shale of the Appalachian Basin Province, 2011).
Industry exploration teams acquire all data available for a certain geologic basin in order to assess the prospects for economic deposits of hydrocarbons (oil, natural gas liquids, and/or natural gas). In the continental U.S., most basins have been explored and drilled for decades, if not longer, and the Appalachian Basin is no exception. In fact, shallow Appalachian Basin strata in Pennsylvania were first drilled and produced in 1859. In addition to compiling and analyzing drilling and production records and geophysical (downhole) logs available from individual wells, industry will also acquire seismic surveys or contract with service companies to run such surveys for the purpose of understanding the complexities of subsurface structure and stratigraphy. For the Marcellus Shale Play, note that these seismic surveys are not done for the purpose of locating the Marcellus Shale horizon or detecting hydrocarbons per se; the most important reason such surveys are done is to identify impediments to drilling, including through-going faults or other features that could possibly cause problems with extending long lateral production strings (so-called "horizontals" that could extend over 2 miles away from the drill site; this will be discussed a bit later in this lesson) in the subsurface unit from which hydrocarbons will be produced. These seismic surveys are now typically 3-D, providing large swaths of coverage of the subsurface for interpretational significant cost (e.g. can exceed $300,000 per square mile in rough terrain).
After assessing the available data, industry will then decide whether to lease in the "play." Sometimes, however, leasing precedes careful assessment if it appears that delays will cause lease costs to increase significantly and/or availability of territory will be limited as the result of a frenzy of leasing by other companies. Of course, in the latter case, a company may lease territory that they may never drill should it not be prospective economically.