Critically-Stressed Faults, in Situ Stress and Fluid Flow - Contrasting the Gulf of Mexico with Other Regions of Active Faulting

Mark D. Zoback

In situ measurements in many regions of the world indicate that the magnitude and orientation of in situ stresses are in accordance with simple Coulomb faulting theory, laboratory-derived coefficients of friction and in situ pore pressure. In several boreholes in the western U.S., we have recently demonstrated that the fractures controlling bulk permeability are optimally-oriented potentially-active shear faults. Over the past year, we have begun to investigate whether this might also apply in the Gulf of Mexico, an area of both active faulting and fluid migration. Our preliminary results indicate that at relatively shallow depth (where pore pressure is approximately hydrostatic), the state of stress is in agreement with Coulomb theory and laboratory- derived coefficients of friction. However, at greater depths where elevated pore pressures are encountered, anomalously high values of least principal stress are observed. The unexpectedly high values of least principal stress may be the result of the compaction rate exceeding the rate of pore fluid diffusion, resulting in normal faulting under relatively "undrained" conditions. The importance of this for fluid flow along faults in overpressured areas of the Gulf Coast is that if deformation was occurring under drained conditions, only very minor increases of pore pressure could occur prior to hydraulic fracturing. Under undrained conditions, however, the higher value of least principal stress allows for higher pore pressure perturbations which could potentially drive fluid flow along faults. It is not yet c ear which of these two mechanisms is dominant in the Gulf of Mexico.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California