There is no formal definition of “unconventional resources” despite the fact that unconventional resources are the most active petroleum play in North America. Meckel and Thomasson, 2008, defined unconventional resources using purely a permeability threshold (< 0.1 md). Yet, coal bed methane plays are considered unconventional and many have permeabilities exceeding 1 md over large portions of the fairway (ex: San Juan Basin, Powder River Basin). Other workers have defined unconventional resources based on an interpretation of the petroleum system and have stated that unconventional resources are “continuous” or “basin centered” and lack traditional traps. While some have restricted the term to product type (i.e. unconventional gas), many shale and tight sand plays have gas, wet gas, and oil fairways and all can be considered unconventional. Heavy oil and oil sands are also unconventional resources and many of these deposits are in reservoirs with permeability exceeding 500 md. Thus, unconventional resources include both low and high permeability reservoirs with both low and high viscosity fluids. Previous definitions have not accounted for all phases of petroleum in all types of reservoirs in all types of petroleum systems.
This paper proposes a simple graphical definition that incorporates properties of both the rocks and their fluids. All petroleum reservoirs can be plotted on a graph of viscosity versus permeability (both in log scale). On this graph, conventional resources all plot in the lower right quadrant, regardless of fluid phase. All unconventional resources plot outside this quadrant due to a low ratio of permeability to viscosity. Unconventional resources are thus defined as those petroleum reservoirs whose permeability/viscosity ratio requires use of technology to alter either the rock permeability or the fluid viscosity in order to produce the petroleum at commercially competitive rates. Conversely, conventional resources are those that can be produced commercially without altering permeability or viscosity. This simple graphical definition avoids the pitfalls inherent in a petroleum system interpretation (i.e. basin centered or self-sourced versus migrated petroleum). The graphical definition accommodates and delineates tight gas, tight oil, shale gas, shale oil, heavy oil, coal bed methane, and even offshore reservoirs with low k/viscosity ratios.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California