FRACTURE OPENING, BURIAL DIAGENESIS, AND FRACTURE SEALING AS LINKED PROCESSES CONTROLLING PERMEABILITY IN UPPER CRETACEOUS MESAVERDE GROUP SANDSTONES, PICEANCE BASIN, COLORADO

Sandstones of the Upper Cretaceous Mesaverde Group of the Piceance Basin have received renewed interest for domestic natural gas exploration. Deep burial, locally in excess of 6 km, and burial temperatures of 100-200°C, resulted in diagenetically mature sandstones of low matrix permeability on the order of microdarcies. Fluid flow is thus controlled by the presence of open fractures.

This ongoing project combines core and outcrop structural observations, diagenetic analytical techniques, and diagenetic and geomechanical numerical models in an effort to investigate the coupling among fracture opening, burial diagenesis, and fracture cementation as a prerequisite for improved fracture prediction. Fractures in Mesaverde sandstones are frequently cemented by quartz, dolomite, ankerite, and calcite, with quartz consistently the first cement, and calcite, where present, forming a late postkinematic cement. Reconstructions of pore and fracture cement sequences suggest the fracture cementation closely follows the burial diagenetic sequence of pore-filling cement. Quartz either completely occludes fractures or forms cement linings along walls of otherwise uncemented fractures. In several occurrences, fractures lined with quartz cement contain quartz cement bridges, with fracture space between bridges either remaining uncemented, and thus available for fluid movement, or occluded by later carbonate cement. Crack-seal textures of quartz cement bridges indicate synkinematic cement growth, i.e. cement precipitation concurrent with fracture opening. Numerical kinetic models of quartz cement growth demonstrate that bridge growth is controlled by the crystallographic orientation of quartz grains forming the growth substrate along fracture walls, in addition to the fracture opening rate relative to the cement growth rate. In these models, cement bridges form at intermediate fracture opening rates, whereas slow opening favors complete fracture occlusion. Where conditions allow co-precipitation of carbonate and quartz fracture cement, carbonate appears to outpace quartz cementation.

The observed complex relations among fracture opening, host rock diagenesis, and fracture sealing illustrate the need for an integrated approach to fracture prediction.