Regional Tectonic Control of Laramide (Late Cretaceous-early Tertiary) Fractures in the Raton Basin, New Mexico - Implications for Horizontal Drilling in Cretaceous Shales

Lee A. Woodward

East-west crustal shortening during Laramide deformation resulted in north-trending thrust faults separating the Raton Basin from the Sangre de Cristo uplift on the west. Narrow, scattered, westerly striking, nearly vertical Tertiary dikes occur rn Cretaceous and lower Tertiary strata in the Raton Basin, suggesting that the dikes were emplaced along extensional fractures parallel to the direction of maximum compressional stress.

Only minor amounts of hydrocarbons have been produced from Cretaceous rocks in the Raton Basin, but at least 40 shows have been recorded from Cretaceous strata, mainly shales and calcareous shales, in the New Mexico part of the basin. Hydrocarbon generation is inferred to have occurred during Laramide burial and deformation of the basin. Fluid overpressuring generally occurs in rocks with low indigenous permeability during hydrocarbon generation and may lead to fracturing of the source rock. Open, extensional fractures tend to form in planes parallel to the direction of maximum horizontal stress and are approximately perpendicular to the direction of least compressive stress. These fractures can form at considerable depth with high lithostatic pressure if there is fluid overpressuring and differential tectonic stress.

Cored Cretaceous shale in one well has vertical fractures with oil stain. Minor gas was produced from fracture reservoirs in the Niobrara, Carlile, and Greenhorn formations at the Garcia field in the Raton Basin in southern Colorado.

Thus, the regional tectonic development of the Raton Basin may have been a major factor in the orientation of open fractures. This suggests that open fractures probably trend westerly and are steep. Horizontal drill holes should be oriented north-south in order to efficiently intersect the inferred open fractures in Cretaceous shales of the Raton Basin.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995