geology · stratigraphic
Isopach from Isochore Thickness
TST = TVT × cos(δ)
click formula to derive ↑
Inputs
ft
°
Description
Converts an isochore (true vertical thickness) value to an isopach (true stratigraphic thickness) value for a uniformly dipping bed. Isochore and isopach maps are frequently confused in practice — this calculator makes the geometric correction between the two explicit.
Variables
| Symbol | Unit | Description |
|---|---|---|
| TST | ft | Isopach Thickness (TST) |
| TVT | ft | The true vertical thickness of the unit — measured straight down, regardless of bed attitude — as contoured on an isochore map. |
| δ | ° | The true dip of the bedding over the interval being converted. |
Assumptions
- The bed has a uniform, planar dip across the interval being converted — no internal folding within the unit.
- The input isochore thickness is already a true vertical thickness (i.e., corrected for any wellbore deviation), not a raw measured/logged interval in a deviated well.
- The dip angle used is the true dip of the bedding, not an apparent dip seen in an oblique cross-section.
Limitations
- The terms isopach and isochore are frequently used interchangeably (and often incorrectly) in industry practice — always confirm which thickness convention a given map or dataset is actually using before applying this conversion.
- Becomes highly sensitive to small dip errors at steep dip angles, where cos(δ) changes rapidly.
- A purely geometric correction — it does not account for compaction or facies-related thickness changes across the interval.
Use Cases
- → Volumetric mapping: Convert a vertical-thickness (isochore) map built from well TVT picks into a true stratigraphic thickness (isopach) map for accurate rock-volume calculations in dipping reservoirs.
- → Map-type quality control: Check whether a thickness map's stated values are consistent with being an isochore or an isopach, given the field's known structural dip.
Related Calculations
Region Notes
Global
The distinction matters most in structurally dipping settings (fold-and-thrust belts, salt-flank plays, steeply dipping fault blocks) — in gently dipping basins (under ~5° dip) isochore and isopach thicknesses differ by less than 0.5% and the distinction is often immaterial.
References
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