TVD: Definition, True Vertical Depth in Well Logging, and Depth Reference Conventions

What Is TVD?

TVD is the abbreviation for true vertical depth, the vertical distance measured from a surface reference datum (typically the kelly bushing or mean sea level) to a specific point in a wellbore, computed from measured depth and directional survey data using trigonometric methods, and used universally in well log presentations, completion reports, and reservoir models to reference formation depths in a consistent vertical coordinate system independent of wellbore trajectory.

TVD in Well Log Presentations and Formation Evaluation

In well log presentations for deviated wells, depth tracks are typically provided in both measured depth (MD) and true vertical depth (TVD). The MD track follows the physical length of the wellbore and is the reference for tool position during logging; the TVD track converts each depth increment to its vertical equivalent using the directional survey data. When a log is plotted on a TVD depth scale, the vertical spacing between formation features is proportional to the actual vertical distance in the earth — not to the path length along the borehole.

This distinction is critical for several formation evaluation calculations. Net pay thickness, which directly enters hydrocarbon volume calculations, must be computed in TVT (true vertical thickness) or TST (true stratigraphic thickness) rather than in MD. A horizontal well that traverses a 10-metre thick sand at 85° inclination will show the sand as 114 metres of MD log (10 / sin(5°)) while the true vertical thickness is only 10 metres. Using the MD-measured 114 metres as the pay thickness in a volumetric calculation would overestimate in-place volumes by more than a factor of ten. Log curves plotted on TVD depth tracks show formation features at their correct relative vertical positions, making TVD the required depth reference for all petrophysical calculations and geological correlations in deviated wells.

TVD Applications Across International Jurisdictions

In Canada, TVD log presentations are required in AER well completion report submissions for all deviated and horizontal wells. The AER's electronic well data submission system WellWiki and DLIS file format specifications require TVD columns alongside MD in all formation evaluation data uploads. WCSB horizontal well programmes for Montney, Duvernay, and Cardium plays produce log data that is standard-displayed in both MD and TVDSS to support structural correlation and completion interval selection. AER Directive 059 specifies TVD reporting requirements for formation tops and fluid contacts in well completion reports.

In the United States, SEC reserve reporting guidance requires that proved reserve volumes be calculated using geological maps in TVDSS depth; TVD log tracks are therefore required for all deviated producer wells where reserves are being booked. BSEE well completion reports for OCS wells require TVD and TVDSS depth references for all perforated and tested intervals. In Norway, Sodir's factpages and the national Diskos wellbore database store all formation data referenced to TVDKB and TVDSS; the Norwegian Petroleum Directorate publishes national TVD-based depth maps for all NCS formations. In the Middle East, Saudi Aramco's extensive deviated and horizontal well programmes require high-quality TVD calculations from multi-station directional surveys for well placement in thin oil columns above aquifer contacts.

TVDSS and Structural Geology Integration

The most important use of TVDSS (TVD referenced to mean sea level) is enabling geologically consistent comparisons between wells with different surface elevations and different rig floor heights. In a field with onshore and offshore wells, wells drilled from platforms at different elevations, or wells in mountainous terrain where surface elevation varies by hundreds of metres, using TVDKB (TVD from kelly bushing) for interwell correlation would introduce false structural variations equal to the difference in KB elevations. TVDSS removes the surface elevation effect, giving all wells a common vertical reference that reflects the true subsurface depth below the same geological datum. Isopach maps (formation thickness maps), isochore maps (vertical thickness maps), structure contour maps, and fluid contact correlation maps are all constructed in TVDSS to ensure geologically consistent spatial representations of the subsurface.

TVD Synonyms and Related Terminology

TVD is also referenced as:

• True vertical depth — the spelled-out form used in technical reports, regulatory submissions, and log headers; TVD is the standard abbreviation used in daily operations
• TVDSS — true vertical depth subsea; the TVDSS variant that references to mean sea level; the standard for structural mapping and interwell correlation; TVDKB is the KB-referenced variant used in single-well completion reports
• Vertical depth — colloquial shorthand; "at 2,000 m vertical depth" means TVDKB 2,000 m; used in operational drilling discussions to distinguish from MD; less precise but commonly understood in context

Related terms: true vertical depth, measured depth, depth-derived, directional survey, minimum curvature

Frequently Asked Questions

What is the difference between TVDKB and TVDSS?

Both TVDKB and TVDSS measure vertical depth from a datum, but they use different zero-reference points. TVDKB measures vertical depth from the kelly bushing — the physical reference point on the rig floor where measured depth counting begins. TVDSS measures vertical depth from mean sea level, using the relationship TVDSS = TVDKB - KB elevation above MSL. For an onshore well with KB at 650 metres above sea level, a formation at TVDKB 3,000 m is at TVDSS 2,350 m. For an offshore drillship with KB 30 metres above sea level, a formation at TVDKB 3,000 m is at TVDSS 2,970 m. When comparing formation tops between these two wells, only the TVDSS values (2,350 m and 2,970 m) are directly geologically comparable; the TVDKB values appear 620 m apart in "depth" due purely to the surface elevation difference.

How does TVD scaling affect log interpretation in horizontal wells?

In a horizontal well section drilled at 90° inclination, MD increases continuously along the lateral while TVD remains essentially constant (within fractions of a metre per 100 metres of lateral for nearly horizontal trajectories). If a petrophysicist works with the log plotted on a MD depth scale, the formation appears to have enormous vertical thickness — every 100 metres of lateral becomes 100 metres of apparent log. On a TVD depth scale, the same lateral section would appear as a nearly flat horizontal line with only minor vertical variation. For any vertical thickness or volume calculation, TVD-referenced log data must be used. In practice, horizontal well log analysis typically uses TVD for formation top picks and pay height determination, while MD is used for perforation placement and stimulation stage spacing because operations are physically performed along the MD wellbore path.

Why TVD Matters in Oil and Gas

TVD is not merely a technical nicety — it is the depth reference that makes all interwell geological comparisons valid. Every barrel of proved reserves, every structural map presented to investors, every hydraulic fracture model, and every reservoir simulation grid is fundamentally built on TVD-referenced data. As horizontal drilling has become the standard in unconventional resource plays and complex offshore developments, the gap between MD and TVD has grown from a minor correction to a fundamental factor that can change formation thickness estimates, contact depths, and reserve volumes by factors of 2-10. The entire data infrastructure of the modern oil and gas industry — from wellbore databases to 3D reservoir models to SEC filings — depends on correct TVD computation and consistent use of the TVDSS reference datum. Getting TVD right is the foundational quality requirement that everything else in subsurface characterisation is built upon.