TO WHAT EXTENT SHOULD THE DEVONIAN-MISSISSIPPIAN BLACK SHALES OF THE APPALACHIAN BASIN SERVE AS A MODEL FOR GLOBAL EXPLORATION OF SHALE GAS?

Shale gas occurs in mudstones with common characteristics including a minimum TOC buried to depths sufficient to achieve a certain thermal maturity. Various mudstones around the globe differ in thickness and regional extent but these characteristic are often not the most important in controlling economic viability, particularly when the shales are developed with horizontal drilling. Rock properties and tectonic histories are quite distinct from shale play to shale play and it is these parameters that can make all the difference in the evolution of an economic play. Rock properties are a function of conditions at the time of deposition and have a strong influence on such engineering considerations as drilling and completion. Tectonic history is the final step in the evolution of gas shales toward or away from economic viability, depending on structural complexity. Devonian black shale of the northern Appalachian Basin developed through two periods of tectonic loading. The earlier (Acadian loading) affected depositional style whereas the later (Alleghanian loading) affected tectonic fabric. As a consequence of the Alleghanian orogeny black shales of the northern Appalachian Basin (mainly the Marcellus) carry tectonic structures ranging from microscopic to seismic in scale. These Appalachian structures present some challenges to horizontal drilling and completion that may or may not appear in other basins of the world. Nevertheless, lessons learned in dealing with structures of the Appalachian Basin will undoubtedly provide valuable models as the global gas shale play continues to expand to new basins of the world.