Major Petroleum Systems of the San Juan Basin, New Mexico and Colorado – Their

Relation to Framework Geology and Resource Assessment

By

J.L. Ridgley

U.S.Geological Survey, Lakewood, CO

Most oil and gas resources in and on the periphery of the San Juan Basin, New Mexico and Colorado, can be assigned to one of four major petroleum systems. A petroleum system consists of source rocks, reservoir rocks, seals, genetically related petroleum accumulations, and the distribution network of migration paths that connect the source rock with the accumulations. Pennsylvanian source rocks and reservoirs, as the oldest petroleum system, underlie the northern part of the basin; to date, oil produced from these source rocks has been confined to Pennsylvanian reservoirs in local structures and carbonate mounds on the west margin of the basin. Three Cretaceous source rocks, part of the other three petroleum systems, account for most oil and gas resources in the basin. These are the 1) Mancos Shale, which is the source of petroleum in reservoirs above the Jurassic Todilto Limestone Member of the Wanakah Formation and below the Cretaceous Mesaverde Formation; 2) Lewis Shale, the likely source of most petroleum in reservoirs between the Mancos Shale and the Fruitland Formation; and 3) Fruitland Formation coals, which are the source of petroleum in the Fruitland Formation and some underlying reservoirs in the Pictured Cliffs Sandstone. Hydrocarbon-producing Cretaceous strata were laid down as a series of transgressive, highstand, and lowstand deposits associated with episodic incursion of the sea. The U.S. Geological Survey is currently reassessing oil and gas resources that have the potential for additions to reserves in the next 30 years in the basin, using a petroleum system approach.

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