Abstract: Geomechanical Model for Fault Sealing in Sandstone Reservoirs

Peter D'Onfro, Michael Fahy, William Rizer

Faults in sandstone can act as either permeability conduits or barriers (seals). Sealing faults can trap hydrocarbons, compartmentalize reservoirs, and affect diagenesis. In this poster we describe a geomechanical model for the formation of sealing faults in porous sandstones that is based on physical characteristics of natural faults, rock mechanics experiments, and critical state concepts as adapted from soil mechanics.

Laboratory experiments and field observations indicate that faulting in porous sandstone involves the process of cataclasis, the fracture, crushing, and rotation of mineral grains. Porosity measurements during deformation under confined compression indicate that faulting in porous sandstone is either dilatant (pore volume increases) or compactive (pore volume decreases) depending on the effective confining pressure and initial porosity of the sandstone. Flow measurements during deformation indicate compactive failure is accompanied by drastic permeability reduction relative to dilatant failure.

We relate these laboratory observations to natural faulting through critical state concepts to create a predictive model for fault sealing. The model demonstrates that porosity, burial stress (effective confining pressure), type of fault (stress path), and changes in stress state along a fault system have major control on whether faults in porous sandstones act as permeability conduits or seals. When combined with knowledge of burial history, porosity-depth relationships, and tectonic stress path, the geomechanical model can be used qualitatively to evaluate fault sealing potential in sandstone reservoirs.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994