X-ray diffraction, thin-section, and titrimetric analyses have been performed on samples from wells completed in structurally induced fractured zones of the West Puerto Chiquito pool in New Mexico. Results indicate that production is restricted to fractures in dolomite-rich sandy layers in the Upper Cretaceous Niobrara Member of the Mancos Shale rather than to fractures in competent limestone layers interbedded with more plastic shales, as has been reported for similar Niobrara Member pools. The difference between limestone and dolomite in susceptibility to fracturing is offered as an explanation for the fractures in the West Puerto Chiquito pool. This difference is related to differences in the crystal structures of calcite and dolomite, and consequently to the gliding m chanism of the two minerals, and their relative brittle or ductile behaviors in a common strain environment.

The West Puerto Chiquito pool is considered commercially significant because of the unusual degree of inter-well communication through the fractured zones, as a few widely spaced wells are efficiently draining a reservoir of 25,000 acres or more. The hypothesis developed herein also may apply to Niobrara Member pools in northwestern New Mexico and the northwestern corner of Colorado which display many similarities to this pool.