Abstract: Hydrocarbon Potential of Archuleta Anticlinorium, Brazos Uplift, and Chama Basin in Southwestern Colorado

Robert T. Ryder

The Archuleta anticlinorium, Brazos uplift, and Chama basin are primarily Laramide features that have developed adjacent to, and concurrent with, the northeastern rim of the San Juan basin. Oil production has been sparse in these provinces, except in the Gramps oil field (6 to 8 million bbl) and the abandoned Chromo oil field (< 1 million bbl). Between 180 and 200 test holes have been drilled in the area, and nearly half of them have yielded either commercial quantities of oil and gas or good shows.

Although no published geochemical measurements are available, the numerous oil shows from the Mancos Shale in the vicinity of Gramps and Chromo and its documented organic richness in adjacent parts of Colorado and New Mexico suggest that the Mancos Shale is volumetrically the most important source rock in the area. Moreover, Btu values between 12,500 and 13,500, obtained from coal samples in the Fruitland Formation (1,500 m up section), indicate that the Mancos Shale in the area is probably thermally mature. The organic-rich shales of the Mancos do not appear to have generated and expelled significant quantities of oil until they were blanketed by 1,500 to 2,000 m of volcanic rocks in middle to late Oligocene time. If correct, this time of generation and migration of hydrocarbons from the Mancos Shale is significantly later than the approximately middle to late Paleocene time hypothesized for equivalent units in the adjacent San Juan basin. Migration from the Archuleta, Brazos, and Chama provinces probably was rather local; also, a large proportion of the hydrocarbons generated and expelled may have escaped into the overlying volcanic cover.

The best reservoirs appear to be porous sandstone units in the Dakota Sandstone and fractured intervals in the Mancos Shale. Faulted anticlinal structures--several of which are untested--provide potential traps. Although many of these structures have been breached or invaded with fresh water, small hydrocarbon accumulations may exist in regions isolated from groundwater flow.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado