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Geochemical Assessment of Unconventional Shale and Tight Gas Sands Plays

Daniel M. Jarvie, Tim E. Ruble, Richard Drozd, Wayne Knowles, Hossein Alimi, Valentina Baum, and Brian M. Jarvie, Humble Geochemical Services, P.O. Box 789, Humble, Texas 77347

Pre-lease and pre-drill assessment of resource plays requires a detailed understanding of the hydrocarbon system. Geological information and knowledge must be combined with geochemical characteristics to assess the likelihood of commerciality. Geological and geochemical reconnaissance requires evaluation of publicly available information and also requires further evaluation of nearby outcrop or well samples, whether archived cuttings or cores, well head or production gases, or oils.

The first step in assessing unconventional plays is to determine the gas type, i.e., whether the gas is biogenic or thermogenic. Biogenic gas plays will have low initial flow rates, but long-lived production of low calorific gas. Unconventional thermogenic gas systems will yield varying flow rates depending on the system type (shale or tight gas sand), and organic richness and thermal maturity of the shale source rock. In such tight shales or sands, flow rates are restricted by the presence of even low levels of oil saturation. Unconventional thermogenic gas production may be developed from early mature or highly mature shales and economics are dependent upon projected recoverable gas versus development costs. The New Albany Shale, Illinois Basin and the Barnett Shale, Ft. Worth Basin, respectively, are examples of these shale gas system types. However, thermogenic gas derived from shales may also be produced from intraformational tight sands (e.g., the Bossier gas system of Central-East Texas), another unconventional gas system type.

Comparison of these unconventional play, system, and gas types is made among known and prospective hydrocarbon systems.

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York