Fault analysis: throw, heave, dip, and seal

The four fault measurements

• Throw (T) is the VERTICAL component of displacement. On a seismic cross-section, if a bed at depth Z in the footwall is correlative with the same bed at depth Z + T in the hanging wall, then T is the throw. Units: meters (or milliseconds if you haven't depth-converted yet).
• Heave (H) is the HORIZONTAL component of displacement — how far the hanging wall has moved horizontally from its original position above the footwall. Often harder to measure directly from a seismic section; usually computed from throw and dip.
• Slip (S) is the TOTAL displacement along the fault plane — the distance a given point on the fault surface has moved from its pre-deformation position. The hypotenuse of the right triangle with throw and heave as legs.
• Dip (θ) is the angle of the fault plane measured from horizontal. Normal faults usually dip 50–70°; thrusts dip 20–40°; strike-slip faults are nearly vertical (80–90°).

The four quantities are related by trigonometry of the fault-displacement right triangle:

$T = S \sin\theta$     $H = S \cos\theta$     $\tan\theta = T/H$

Given any two, the third follows. Real interpretation usually measures throw directly from the seismic (vertical reflector separation) and measures dip from the fault’s angle on the section, then computes heave and slip from the equations above.

Planar vs listric faults

Real faults are not always straight in cross-section:

• Planar faults maintain a constant dip with depth. The dip angle you measure at the top of the fault is the same at the bottom. Typical of small-scale faults (< 1 km offset) in competent rock and of basement-involved faults.
• Listric faults curve from steep at the top (typically 60–70°) to shallow or flat at depth, where they merge into a horizontal detachment. Typical of large-scale extensional systems and gravity-driven growth faults. The transition from steep to shallow happens over a few kilometers of depth.

The geometry matters kinematically:

• A planar fault moves the hangingwall as a RIGID block — throw is roughly uniform along the fault.
• A listric fault requires the hangingwall to DEFORM internally as it slides down the curved surface — this is why listric faults produce rollover anticlines.

Production interpretation uses both. If you interpret a fault as planar but it is actually listric, your throw maps will be wrong; the throw changes systematically with depth on a listric fault because the fault's flat bottom doesn't transmit the same shear as its steep top.

Shale Gouge Ratio (SGR): the fault-seal calculation

A fault that offsets a stratigraphic section produces a fault rock — the pulverized material smeared along the fault plane as the rocks slid past each other. The composition of this fault rock determines whether the fault SEALS hydrocarbons (retains them on one side) or LEAKS (lets them migrate through).

Shale Gouge Ratio (SGR, Yielding et al. 1997) is the simple and widely-used approximation for fault-rock composition:

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where the sum is taken over all layers that slid past the point on the fault at depth z, $f_{sh,i}$ is the shale fraction of layer i, and $t_i$ is the thickness of layer i. In words: SGR is the VOLUME-WEIGHTED average shale content of the rocks that slid past the fault point. It is a percentage from 0 (all sand) to 100 (all shale).

The sealing threshold is empirical and basin-specific but typically:

• SGR < 15–20%: fault rock is sand-rich and porous — the fault LEAKS.
• SGR 15–20% (threshold zone): marginal, use caution.
• SGR > 20%: fault rock is clay-rich and impermeable — the fault SEALS.
• SGR > 30%: robust seal, high confidence.

Why this works geologically: shale minerals (mostly clay particles) are soft and deform by smearing, producing a continuous clay-rich layer along the fault. Sand grains fracture rather than smear, leaving the fault rock granular and permeable. A high-SGR fault has continuous clay drape; a low-SGR fault has interrupted, sand-rich granular fault rock.

Exercise — use SGR to evaluate a fault

• The widget starts with a 6-layer stratigraphy and a 60°-dipping planar fault with 150 m throw. Below the canvas the stats panel shows throw, heave, slip, and the SGR calculation. The verdict line summarizes whether the fault seals overall.
• Slide the Throw slider from 150 m up to 300 m. Watch the SGR band along the fault evolve. At larger throws, more layers are juxtaposed across the fault, and the SGR values tend to drop (the volume-weighted shale fraction decreases as more sand layers contribute). Large-throw faults are HARDER to seal than small-throw faults in a given stratigraphy.
• Drop the throw back to 80 m. Notice most of the fault is now in the SEAL zone (green), because at small throws only nearby layers juxtapose and the shale fractions of those layers dominate the SGR.
• Change the geometry from Planar to Listric. Notice the fault curve bends at depth, and the sealing pattern shifts — the flat portion of the fault juxtaposes different stratigraphic intervals than the steep portion did. Real listric faults in the Gulf of Mexico and Niger Delta show exactly this depth-dependent sealing.
• Slide the Dip slider. Notice the throw stays constant (the slider is set independently), but the heave and slip change. At shallow dip, heave is large — slip partitions more into horizontal motion. At steep dip, heave is small — slip is mostly vertical.
• Try the Seal SGR slider. Lower it to 15% and see more of the fault classified as "sealing". Raise it to 30% and much less is sealing. This slider represents BASIN CALIBRATION — North Sea basins often use ~20%, Gulf of Mexico sometimes 30%, depending on how well-calibrated the fault-seal threshold is from local wells.

Common fault-analysis pitfalls

• Throw in time vs throw in depth. If you pick a fault in two-way time on seismic, your "throw in ms" is NOT the depth throw. Converting requires velocities at both sides of the fault and can change the apparent throw by 20–40%. Report throws in meters (depth-converted) whenever possible.
• Picking the fault dip from a section that is not perpendicular to fault strike. Any seismic line that is oblique to the fault strike shows APPARENT dip — always shallower than the true dip. True dip requires a line perpendicular to the fault strike, or a correction using the strike angle.
• SGR is a GROSS simplification. Real fault rocks have more complex fabric, mineralogy, and cementation history than the shale-fraction-weighted average captures. SGR is a useful screening tool, not a definitive seal predictor. Rely on well-calibration wherever possible.
• Small faults matter. A 10 m throw may be below seismic resolution, but it can be critical for field compartmentalization. Production reservoir models often include sub-seismic fault populations derived by extrapolation from the observable larger faults (using fractal displacement-length relationships).
• Fault seal is a GEOLOGICAL property, not just a geometric one. SGR alone doesn't capture the stress state acting ON the fault, which affects current-day permeability. A fault that was sealing at the time of hydrocarbon migration might leak today if stress changes have caused it to reactivate. Integrate with in-situ stress analysis for critical decisions.
• Throw is sometimes MEASURED wrong. The common mistake: measuring the vertical distance between two reflectors that are NOT correlative across the fault. Always verify correlation using adjacent lines or distinctive marker horizons; a miscorrelation introduces a throw error equal to one bed spacing.

From fault analysis to trap evaluation

A quantitative fault analysis feeds directly into prospect evaluation:

• Measure throw and dip on the fault from seismic sections — creates the fault geometry.
• Tie fault throws to stratigraphy at nearest wells — identifies which layers juxtapose across the fault at each depth.
• Compute SGR along the fault plane — identifies sealing and leaking segments.
• Map fault throw and seal status in 3D — produces fault-seal maps for the prospect.
• Compare with structural trap outlines — identifies which portions of the closure are fault-sealed and therefore capable of trapping hydrocarbons.
• Estimate hydrocarbon column height supported by the seal — uses the buoyant pressure a fault can hold (calibrated from capillary entry pressure of the fault rock).

This pipeline is the modern standard for structural-trap prospect evaluation. Section §3.6 will integrate fault-seal analysis with structural closure definition to produce complete prospect assessments.