Lewis Shale (Wyoming and Colorado) Exploration and Production Issues

Related to Stratigraphy

By

Roger M. Slatt

University of Oklahoma, Norman, OK

Neil F. Hurley

Colorado School of Mines, Golden, CO

David R. Pyles

University of Colorado, Boulder, Boulder, CO

The Lewis Shale of the eastern Greater Green River Basin is a significant potential gas exploration target for the foreseeable future. Issues related to the stratigraphy of the Lewis Shale are important to exploration, as well as to production once discoveries have been made.

The Lewis Shale comprises a deep-water (turbidite) depositional system with component sheet, channel, and levee facies as potential reservoirs. Each facies exhibits unique external geometries and trends, as well as internal stratigraphic variability.

Concerning exploration, local tectonics, regional eustacy, and the location of contemporaneous, shallower water (Fox Hills Sandstone) deltaic depocenters governed the focusing, distribution, and trends of sandstones, as well as hydrocarbon source rocks within the deeper Lewis Seaway.

Concerning production, sheet sandstones are areally extensive, and relatively “clean”. However, continuous shales can compartmentalize reservoir sandstones and prevent good vertical connectivity. Channel sandstones are sinuous and lenticular, internally complex, of lesser reservoir quality, and often are separated by shale breaks, resulting in complex connectivity and horizontal continuity. Levee sandstones are thin-bedded, and of variable continuity, and can be easily overlooked on conventional well logs. All three facies can be gas saturated, but well spacing and orientation, as well as perforation and fracture strategies, should differ for each facies in order to maximize production.

These three facies can be identified with core or borehole image logs with a higher degree of certainty than they can be with conventional well logs. Also, a well-to-well correlation strategy that incorporates sequence stratigraphic, rather than lithostratigraphic concepts, and which utilizes dip or borehole image data, can improve both exploration and production performance within the Lewis.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming