Abstract

East-west crustal shortening during Laramide (Late Cretaceous-early Tertiary) deformation resulted in north-trending thrust faults separating the Raton Basin from the Sangre de Cristo Uplift. Westerly striking, steep Tertiary dikes in Cretaceous strata in the basin suggest the dikes occur along extensional fractures parallel to the inferred direction of maximum horizontal compression.

Minor hydrocarbon production and at least 55 shows of gas and oil are recorded from Cretaceous strata in the New Mexico part of the basin. Hydrocarbon generation from Cretaceous shales probably occurred during Laramide deformation and burial. Fluid overpressuring in rocks with low indigenous permeability during hydrocarbon generation may lead to fracturing of source rocks. Open, steep, extensional fractures tend to form in planes parallel to the direction of maximum horizontal stress. These fractures can form with high lithostatic pressure if there is fluid overpressuring and differential tectonic stress.

Regional tectonic evolution of the Raton Basin may be a major factor in the orientation of fractures in source beds unaffected by local folding and faulting, suggesting that steep, open fractures in the center of the basin probably trend westerly. Drill holes should be horizontal and oriented north-south to efficiently intersect inferred open fractures in Cretaceous shales of the Raton Basin.