Shale Gas Potential of Fine-Grained Cretaceous Source Rocks, Raton Basin, South-Central Colorado and Northeastern New Mexico

Edmund R. Gustason¹ and Mark H. Tobey^2
1El Paso Exploration & Production, Denver, CO
²Independent Petroleum Geochemist, Castle Rock, CO

The Raton Basin is an asymmetrical Laramide basin west of the Sangre de Cristo Mountains in south-central Colorado and northeastern New Mexico. Since 1906, numerous exploration wells have been drilled within the basin. Except for coalbed methane and CO₂ at Sheep Mountain gas field, the basin has yielded little oil and gas. Although numerous gas shows were reported from the Dakota Sandstone and overlying Cretaceous “shales”, gas has only been produced from the small Garcia (Niobrara) and Wagon Mound (Dakota) structural fields.

In 2004, the gas potential of Cretaceous fine-grained source rocks was evaluated by EnCana Oil & Gas, USA. In ascending order, these “shales” include the Graneros, Greenhorn, Carlile, Niobrara and Pierre formations. To assess the rock type, kerogen type and quality, regional thermal history, samples from outcrops, cores and cuttings were analyzed for CaCO₃, TOC, and Rock-Eval pyrolysis. Processed samples were examined optically to determine the degree of thermal alteration and vitrinite reflectance, and kerogen type.

The Graneros, Greenhorn, Niobrara and Pierre intervals have good organic enrichment. Residual TOC contents range from 1.5 to 2.5% for thermally mature sediments, and up to 5.0 % for adjacent immature / early mature Niobrara sediments. They all contain mixed Type II/III kerogens. However, the Niobrara contains intervals dominated by Type II oil-prone organic matter. The Greenhorn and Niobrara have the greatest hydrocarbon generative potential per unit rock and because of its thickness (>1500 feet thick), the Niobrara is the most potentially prolific source rock in the basin. The paleo-thermal gradient is steep and consistent with published data from coal studies, indicating high heat flow in the basin. These fine-grained source rocks pass into the dry gas thermal maturity window between approximately 4000 and 5000 feet TVDSS.

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