Do differently oriented faults imply inhomogeneous stress states?

Orientation of faults/fractures created by slip can be explained by Anderson’s faulting regime and the Mohr-Coulomb failure envelope. Faults/flaws typically slip when they are preferentially oriented with regard to the stress regime. A safe assumption for the coefficient of sliding friction, mu, is 0.6, which would be the slope of the failure line on the Mohr-Coulomb failure envelope. If so, the angle 2*beta at which the Mohr’s circle touches the failure line is 120 deg (beta=60 deg), i.e., the angle between the normal of the fault that slips & direction of maximum stress is 60 deg (Fig 1).

Fig 1: Mohr Circle showing Failure line and angle beta (From Reservoir Geomechanics, Stanford Online)

According to Anderson’s faulting regime there are typically 3 cases – normal faulting, strike-slip faulting and reverse faulting. In the normal faulting regime Sv > SHmax > Shmin. For, strike-slip SHmax > Sv > Shmin, while for reverse faulting, SHmax > Shmin > Sv. In the normal faulting regime faults typically strike parallel to SHmax and dip at 60 deg while in the reverse faulting regime the faults typically strike perpendicular to SHmax and dip at 30 deg. For a strike-slip faulting regime expect to see conjugate faults 30 deg clockwise and counterclockwise to SHmax (Fig 2).

Fig 2: Anderson's Faulting Regimes - Map View (Left) & CX-Section (Right) (From Reservoir Geomechanics, Stanford Online)

However, these aren't the only possible stress states since one needs to consider the limiting cases of the stress states also. If Sv = SHmax > Smin then both normal and strike slip faulting can occur, likewise, if SHmax > Sv = Shmin then both reverse and strike-slip faulting can occur (Fig 3) – both of these are fairly common in oil and gas fields.

Fig 3: Limiting case of Normal/Strike-Slip & Reverse/Strike-slip Faulting

Slightly less common are cases where Sv >> SHmax = Shmin due to which radial extension can occur and SHmax = Shmin >> Sv due to which radial compression can occur (Fig 4) – both of these create faults of various orientations.

Fig 4: Limiting Case of Radial Extension and Radial Compression

In summary, in limiting cases of stress field the appropriate faults - strike/slip & normal - or strike-slip & reverse - can occur simultaneously. It is also to be noted that the stress regime when the observed faults occurred could be very different from the stress regime in existence today.

Note: The images in this article are modified from Reservoir Geomechanics, Stanford Online - course taught by Dr. Mark Zoback.