Lewis Shale Petroleum System, Eastern Greater Green River Basin, Wyoming and Colorado

Ira Pasternack
Northern Rockies, EnCana Oil & Gas (USA) Inc., Denver, CO

Sandstones within the Lewis Shale have produced approximately 892 billion cubic feet of gas and 8.7 million barrels of condensate. Recent assessments of undiscovered gas indicate that resources range from 9.7 to 13.7 trillion cubic feet of gas, with significant potential for additional reserve development in the next 30 years. Data from published studies and investigations conducted by Colorado School of Mines Lewis Shale Consortium students are integrated utilizing a petroleum-system approach, with emphasis on stratigraphic framework and characterization of reservoir and source rocks. The stratigraphic framework was established from analyses of marker horizons in over 1,000 wells and outcrop localities. Isopach map anomalies indicate syndepositional tectonics played a major role in influencing sediment dispersal patterns and impacted distribution of both reservoir and source rocks. Reservoir characterization focused on Hay Reservoir field because of the availability of modern logs and abundant core data. Production variations in the field may be explained by a new model that suggests early charging preserved reservoir quality in paleostructurally high positions, which differ from the present-day structural configuration. The model is applicable to future field development and exploration programs. The Asquith Marker condensed zone contains the highest concentration of total organic carbon in the lower part of the Lewis Shale. Core-calibrated gamma-ray logs are used to develop a regional Asquith Marker average TOC map. Thermal maturity of the Asquith Marker is estimated from vitrinite reflectance analyses of carbonaceous shales in the Lewis Shale and coals in associated intervals. The average TOC and thermal maturity maps suggest that the Asquith Marker interval contains sufficient organic matter at appropriate levels of thermal maturity to have generated petroleum.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado