geology · depth conversion
TVT/TST Correction (Wellbore Deviation)
TVT = MLT[cos(φ) − sin(φ)cos(α)tan(θ)]; TST = TVT × cos(θ)
click formula to derive ↑
Inputs
ft
°
°
°
Description
Corrects a logged interval thickness measured along a deviated wellbore (MLT) to true vertical thickness (TVT), then to true stratigraphic thickness (TST), accounting for both wellbore deviation and the azimuth offset between the wellbore's drift direction and the bed's true dip direction. This is the deviated-well counterpart to the vertical-well isopach/isochore relationship (TST = TVT × cos(dip)) used elsewhere in this discipline.
Variables
| Symbol | Unit | Description |
|---|---|---|
| TVT | ft | True Vertical Thickness |
| TST | ft | True Stratigraphic Thickness |
| MLT | ft | The interval thickness as measured along the wellbore (i.e., the raw logged interval), before any correction for wellbore deviation or bed dip. |
| φ | ° | The inclination of the wellbore from vertical at the interval of interest (0° = vertical). |
| α | ° | The horizontal angle between the wellbore's drift azimuth and the true dip direction of the bed. |
| θ | ° | The true dip of the bedding at the interval of interest. |
Assumptions
- The bed has a uniform, planar dip and the wellbore trajectory is effectively straight (constant φ and azimuth) across the logged interval.
- α is measured as the acute horizontal angle between the wellbore's drift azimuth and the bed's true dip direction, not an arbitrary reference azimuth.
- MLT is the raw along-hole logged interval, not already corrected to TVD.
Limitations
- Breaks down (returns a non-physical result) when the wellbore runs nearly parallel to bedding for the given deviation/azimuth combination — always sanity-check that TVT is smaller than MLT.
- Assumes constant wellbore deviation and azimuth across the interval — a build/turn section spanning the logged interval requires splitting it into shorter, more nearly-constant sub-intervals first.
- This formula is corroborated across multiple independent secondary directional-survey and structural-geology technical sources, but Tearpock & Bischke's *Applied Subsurface Geological Mapping*, 2nd ed. (2003) — the textbook it is attributed to — was not directly page-accessed this session (the book's isopach-vs-isochore relationship was independently confirmed in Session 1, but this specific MLT-to-TVT wellbore-deviation formula was not). Treat the formula as secondary-source-corroborated, not a page-checked citation from Tearpock & Bischke directly.
Use Cases
- → Correcting logged thickness in a deviated or horizontal well: Convert a raw logged interval thickness from a directionally drilled well into a true vertical thickness suitable for isopach mapping alongside vertical-well control points.
- → Reservoir volumetrics from deviated-well logs: Recover an accurate net/gross-interval vertical thickness from a deviated appraisal or development well before feeding it into a volumetric or isopach calculation.
Related Calculations
Region Notes
Global
The azimuth-offset term matters most in high-angle or horizontal wells drilled up-dip or down-dip; in near-vertical wells this correction is small and the result converges to the simple TST = TVT × cos(dip) relationship used for vertical wells.
References
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