Tectonic dolomitization in the Gavilan Mancos oil pool, Rio Arriba County, New Mexico
— Alan P. Emmendorfer

The Gavilan Mancos Oil Pool is one of several naturally fractured oil reservoirs producing from the Mancos Formation on the eastern side of the San Juan Basin. Commercial production is the result of the concentration, on structure, of natural fractures within carbonate-cemented sandstone and calcareous shale intervals of the Mancos Formation. Fracturing of the rock column resulted from the formation of the Gavilan dome by compressional forces associated with right lateral movement along the Nacimiento fault zone. Accompanying the fracturing process was the metasomatic formation of ankerite within calcareous portions of the rock column by tectonic dolomitization. Sulfide mineralization also formed by this mechanism. Detailed sample examination and thin section analysis of drill cuttings enabled recognition of the correct relationship between fracturing and the dolomite content of the rock. An understanding of the influence of tectonic dolomitization on the Gavilan Mancos Oil Pool will aid in additional exploration efforts on the eastern side of the San Juan Basin.