geology · maturity
Sclater & Christie Decompaction
φ(z) = φ0·e^(−c·z); solid thickness conserved: t − (φ0/c)(e^(−cz1) − e^(−cz2)) = t' − (φ0/c)(e^(−cz1') − e^(−cz2'))
click formula to derive ↑
Inputs
ft
ft
fraction
1/ft
Description
Restores a stratigraphic unit's present-day compacted thickness to its decompacted (original, less-compacted) thickness at the paleo-surface, using the Sclater & Christie (1980) exponential porosity-depth decompaction method — the standard first step in backstripping a burial-history or basin-subsidence reconstruction. Applies the same Athy-type exponential porosity-depth model used by the Geomechanics discipline's compaction_pore_pressure calc, but for thickness restoration rather than pore-pressure prediction.
Variables
| Symbol | Unit | Description |
|---|---|---|
| t' | ft | Decompacted Thickness (at Paleo-Surface) |
| t'/t | — | Thickness Expansion Factor |
| z1 | ft | Present-day true vertical depth of the top of the stratigraphic unit being decompacted. |
| z2 | ft | Present-day true vertical depth of the base of the stratigraphic unit being decompacted (must be greater than the top depth). |
| φ0 | fraction | Porosity of the sediment at the time of deposition, before any compaction — Sclater & Christie (1980) give 0.63 for shale and 0.49 for sandstone. |
| c | 1/ft | Exponential compaction decay coefficient for the unit's dominant lithology — same Athy-type exponential porosity-depth model used by compaction_pore_pressure, here applied to restore original (decompacted) thickness rather than infer pore pressure. |
Assumptions
- The unit's lithology (and therefore φ0 and c) is treated as a single, uniform end-member (shale or sandstone) across the interval — mixed-lithology units require a weighted-average or bed-by-bed approach not implemented here.
- Porosity is assumed to depend on depth (effective stress) alone via the exponential model, with no diagenetic cementation or secondary porosity effects distorting the trend.
- This calculator restores the unit to the paleo-surface (top at zero depth) — the standard first backstripping step — not to an arbitrary intermediate paleo-burial depth.
Limitations
- A single-lithology, single-interval decompaction step — a full burial-history reconstruction requires applying this same conservation principle sequentially across every stratigraphic unit and every paleo-time-step in the section, which is not automated here.
- Like compaction_pore_pressure's Athy/Magara coefficient, Sclater & Christie's φ0 and c values are calibrated end-members (shale, sandstone) — mixed lithologies, overpressured intervals, or basins with very different burial/thermal histories may deviate substantially from these constants.
- The exponential porosity-depth decompaction method and Sclater & Christie's (1980) tabulated shale/sandstone constants are corroborated across multiple independent secondary sources (a detailed published derivation citing Allen & Allen's *Basin Analysis* textbook, and multiple basin-modeling technical references), but the primary paper (Sclater & Christie 1980, *Journal of Geophysical Research*) is paywalled and was not directly page-accessed this session. Treat the formula and constants as secondary-source-corroborated, not a page-checked citation from the primary paper directly.
Use Cases
- → Burial-history / subsidence reconstruction: Restore each stratigraphic unit's decompacted thickness as the first step in building a burial-history curve, which then feeds a Lopatin/TTI or other thermal-maturity model.
- → Comparing present versus original depositional thickness: Quantify how much thinner a unit has become due to compaction alone — useful when distinguishing compactional thinning from tectonic or erosional thickness changes.
Related Calculations
Region Notes
North Sea
Sclater & Christie (1980) originally calibrated this method against Central Graben wells — it remains a natural first choice for North Sea burial-history work, though local recalibration is still recommended.
Global
φ0 and c must be calibrated per basin and lithology wherever possible; the shale/sandstone end-member values given here are a widely used starting point, not a universal constant.
References
Primary source
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