geology · structural
Shale Gouge Ratio (SGR)
SGR = (Σ shale thickness in slipped interval / fault throw) × 100%
click formula to derive ↑
Inputs
ft
ft
Description
Estimates the Shale Gouge Ratio (SGR) — a simple, widely used proxy for the clay content entrained into a fault zone, and therefore for its likely sealing capacity — from the total shale/clay bed thickness within the slipped stratigraphic interval and the fault's throw. Higher SGR generally indicates a fault zone richer in fine-grained material and more capable of sustaining an across-fault pressure difference.
Variables
| Symbol | Unit | Description |
|---|---|---|
| SGR | % | Shale Gouge Ratio |
| Σsh | ft | The total thickness of shale/clay-rich beds within the stratigraphic interval that has slipped past the point of interest on the fault (the window defined by the fault throw). |
| T | ft | The vertical displacement (throw) at the point of interest on the fault — defines the stratigraphic 'window' of section that has slipped past that point. |
Assumptions
- The input shale thickness sum was correctly restricted to the stratigraphic window defined by the fault throw at the point of interest, not the whole well or field section.
- Shale/clay content is a reasonable proxy for fault-gouge composition and sealing capacity at this location — SGR does not account for diagenetic cementation, fault-rock mineralogy, or fault-zone architecture beyond simple clay-content mixing.
- The fault throw value used is the local throw at the specific point being evaluated, not an average or maximum throw for the fault as a whole.
Limitations
- SGR is a simple lithology-mixing proxy, not a direct measurement of fault-rock capillary entry pressure — it should be calibrated against known across-fault pressure differences in the same basin/play wherever possible, exactly as Yielding, Freeman & Needham (1997) did.
- Does not distinguish how the shale/clay is distributed within the slipped interval (e.g., one thick shale versus many thin interbedded shales) — the smear-factor family of methods addresses that distribution question and is not implemented here.
- This formula and the ~15-20% typical threshold are corroborated across multiple independent secondary structural-geology and fault-seal-analysis sources (AAPG Wiki's dedicated pages, technical conference papers, the paper's own OSTI abstract record), but Yielding, Freeman & Needham's original AAPG Bulletin paper (1997) is paywalled and was not directly page-accessed this session. Treat the formula and threshold as secondary-source-corroborated, not a page-checked citation from the primary paper directly.
Use Cases
- → Fault-seal risking for a cross-fault trap: Screen whether a fault juxtaposing reservoir against reservoir is likely to hold a hydrocarbon column, before committing to more detailed capillary-pressure-based seal analysis.
- → Ranking multiple fault segments by seal likelihood: Compare SGR across several fault segments or compartments in a field to prioritize which are most likely to need pressure communication or compartmentalization studies.
Related Calculations
Region Notes
Global
SGR thresholds calibrated in one basin (e.g., the Brent province, Niger Delta, or Columbus basin examples in Yielding, Freeman & Needham 1997) do not automatically transfer to a different basin's lithology and burial history — recalibrate against local well pressure data wherever possible.
References
Primary source
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