UPPER CRETACEOUS TIGHT-GAS RESERVOIRS AND SOURCE ROCKS OF THE GREATER GREEN RIVER BASIN, WYOMING AND COLORADO
by Ira Pasternack
EnCana Oil and Gas (USA) Inc. and
Dept. of Geology and Geological Engineering, Colorado School of Mines
Regional stratigraphic analyses indicate that syndepositional tectonics played a major role in influencing lower Lewis Shale sediment dispersal patterns and impacted the distribution of both reservoir and source rocks. Lewis Shale sandstones were deposited in shallow- to offshore-marine (turbidite) environments during the final transgressive-regressive cycle of the Cretaceous Epeiric Seaway. Hay Reservoir Field is discussed in detail because of the availability of the most complete set of modern log and core data from any Lewis field. Production variations at Hay Reservoir are explained by a model that suggests early charging preserved reservoir quality in paleo-structurally high positions, which differ from the present-day structural configuration. The model is applicable to future Lewis Shale field development and exploration programs.
A condensed zone, informally known as the “Asquith Marker,” contains the highest concentration of total organic carbon (TOC, 0.7 to 3.3 weight percent) 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 (%Ro = 0.5 to 2.3%), is estimated from vitrinite reflectance analyses of carbonaceous shales in the Lewis Shale and coals in associated intervals. Mature organic-rich shales are also found closely associated with the key productive sandstone interval at Hay Reservoir Field.
GGRB Lance Formation production occurs primarily in the Jonah-Pinedale anticline area where active development programs are currently underway. The Lance ranges from about 2,500 to 5,000 feet in gross interval thickness, with fluvial and associated floodplain sandstone reservoirs comprising 20 to 40 percent of the total thickness in the productive areas. Source rocks for the Jonah-Pinedale anticline area production are not well established, but probably include organic-rich lower Lance mudstones.
The discontinuous nature of Jonah sandstone reservoirs, attributable to their fluvial origin and fault compartmentalization, presents a significant challenge to optimizing field development. Better reservoir understanding, evolving fracture-stimulation completion methods and drilling techniques focused on maximum efficiency have all contributed to enhancing Jonah Field economics—the same ingredients that have played critical roles in the successful development of all GGRB Cretaceous tight-gas reservoirs.