North American Shale Reservoirs - Similiar, Yet So Different

Roth, Murray ¹
(1)Transform Software and Services, Littleton, CO.

Recent estimates of recoverable gas from unconventional shale reservoirs in the US exceed .5 qcf (quadrillion cubic feet) (USGS 2009) with potential for another .1 qcf in Canada (NEB). Large amounts of oil and natural gas liquids further enhance the appeal of unconventional shale reservoirs. While broadly distributed, North American shale oil and gas basins generally follow a trend of thrust belts and a Mississippian/Devonian shale fairway from Western Canada and into the Western, Southern and Eastern United States.

North American shale gas reservoirs currently rank as 6 of the largest 22 global gas fields, based upon estimated recoverable reserves, with average recovery factors of about 20%. Innovations in horizontal well drilling and completions, supported by 3D seismic, microseismic, FMI/FMS and other measurements, are unlocking North American gas supplies for the decades ahead. However, volatile commodity markets and dramatic variability in well production rates make economic shale gas production a challenge, driving the current trend to develop the “wet gas” and oil-bearing legs of unconventional shale plays.

With a motivation to understand why "all shales reservoirs are not created equal" - this study integrates published data, type logs, accessible seismic and microseismic data along with 5 years of experience across most significant North American shale basins. Our tabulation of shale reservoir characteristics and well log analysis highlights key production differentiators including depth, thickness, porosity, pressure and TOC. While basins and reservoir characteristics clearly vary - this does not explain significant well-to-well production variations. Part of this variability in production performance is related to evolutionary and company-to-company differences in fracturing "best practices".

It is our work with 3D seismic and microseismic, however, that clearly supports the concept of shale oil and gas "sweet-spot" fairways and converse "dead zones". Whether it is faulting in the Woodford, karst collapse chimneys in the Barnett, natural fracturing in the Marcellus or clay/silica content in many plays - seismic and microseismic data provide valuable calibration and prediction tools for mapping productive/non-productive fairways. Multiple data examples from key North American shale plays will be used to illustrate the unique characteristics of the most and least prolific oil and gas producing regions

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.