Adapting Petroleum Systems Analysis to Evaluate Play Fairways and "Sweet Spots" in Unconventional Resources

Leonard, Jay E.; Heydweiller, Erich C.; Leonard, China O.; Lash, Gary

"…data show that while there are many very active wells, they are often surrounded by vast zones of less-productive wells that in some cases cost more to drill and operate than the gas (sic) they produce is worth."

This quote from the New York Times (June 25th, 2011), recognizes the need for identification of "sweet spots" within unconventional resource plays.

Play analysis in conventional exploration is based on the geological understanding of petroleum system elements (source, reservoir, trap, seal, and charge access) and their respective presence and effectiveness. Geological understanding, either qualitative or quantitative, is translated into a measure of chance adequacy or risk. Unconventional resource plays, despite their differences from conventional, can be evaluated with the same conceptual approach. Whereby, unconventional play analysis will differ is in the choice of failure criteria and the geologic factors used to evaluate the element adequacy; not only in the present but in both the past and future.

Risk in unconventional systems is often more about the producibility of hydrocarbons (i.e. production volumes and rates) than the presence or absence of hydrocarbon accumulations. In other words, in unconventional resources the question is less "will there be hydrocarbons?" and more "will we make money producing these hydrocarbons?".

Similarly, the relative contribution of individual petroleum systems elements to play risk is different between conventional and unconventional plays. In conventional plays, charge access often presents the greatest risk, largely because it is the most difficult to evaluate quantitatively. In unconventional plays, where the coupling of source and reservoir is common, charge access is often taken for granted (i.e. negligible risk). Reservoir effectiveness is often the most critical and difficult to evaluate element in unconventional plays, because of lithologic and diagenetic complexities.

We will develop and refine these ideas with examples from two popular plays: (1) the oil-producing Bakken Formation, and (2) the Upper Devonian shale gas and liquid play of the Pennsylvanian Appalachian Basin. We will show how unconventional resource plays and their complexities can fit into the established conceptual framework of petroleum systems and play fairway analysis and how this approach can reduce risk by identifying and quantifying "sweet spots" within play fairways.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013