Tectonic dolomitization in the Gavilan Mancos oil pool, Rio Arriba County, New Mexico

-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 slructure, of natural fractures wi1hin carbonale-cemenled sandstone and calcareous shale intervals of the Mancos Formalion. Fracturing of lhe rock column resulted from the formalion of the Gavilan dome by compressional forces associated with right lateral movement along the Nacimiento fault zone. Ac companying 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 lhe influence of tectonic dolomitization on the Gavilan Mancos Oil Pool will aid in additional exploralion efforts on the ea,tem side of the San Juan