geology · formation tops
Formation Top Subsea Depth
TVDSS = TVD − KB Elevation
click formula to derive ↑
Inputs
ft
ft
Description
Converts a well's true vertical depth (TVD), referenced to the Kelly Bushing, into a subsea depth (TVDSS) referenced to mean sea level — the common datum needed to correlate formation tops between wells that have different KB elevations.
Variables
| Symbol | Unit | Description |
|---|---|---|
| SS | ft | Subsea Depth (TVDSS) |
| KB | ft | Elevation of the Kelly Bushing (or other depth-reference datum) above mean sea level — the well's zero-depth reference. |
| TVD | ft | True vertical depth of the formation top or pick, measured downward from the Kelly Bushing. |
Assumptions
- TVD is already the true vertical depth (deviation-corrected), not measured depth (MD) along a deviated wellbore.
- KB elevation is referenced to mean sea level, matching the same vertical datum used across the field or basin being correlated.
- A positive result represents depth below mean sea level, following the most common industry sign convention.
Limitations
- Different operators or basins sometimes use ground level or derrick floor rather than KB as the zero datum — always confirm which reference elevation a given well's TVD is measured from before combining data across wells.
- Does not itself perform an MD-to-TVD deviation correction — use the drilling discipline's directional-survey calculations first when working from measured depth in a deviated well.
- Assumes a single, static KB elevation for the well — workovers or rig changes that alter KB height require re-referencing older depth data.
Use Cases
- → Cross-well correlation: Put formation top picks from wells with different KB elevations onto a common subsea datum before contouring or cross-section construction.
- → Structure map construction: Subsea depths (not KB-referenced depths) are the standard input for structure-contour maps used in prospect and field development mapping.
Related Calculations
Region Notes
Global
Always double check the sign convention in use — some software and older paper maps report subsea depths as negative numbers (elevation-style, decreasing downward from zero at sea level) rather than positive depth-below-sea-level; this calculator reports positive-down.
References
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