true vertical depth, Oil & Gas Glossary

What Is True Vertical Depth?

True vertical depth (TVD) is the vertical distance from a reference datum — typically the kelly bushing (TVDKB) or mean sea level (TVDSS) — to a specific point in a wellbore, calculated from measured depth along the wellbore trajectory using directional survey inclination and azimuth data, providing the actual subsurface depth needed for geological correlations, fluid contact mapping, structural contouring, and comparison of formation depths between wells with different trajectories.

Key Takeaways

TVD is always less than or equal to measured depth (MD); for vertical wells TVD equals MD corrected for KB elevation.
TVDSS (true vertical depth subsea) uses mean sea level as zero, enabling correlation between wells with different KB elevations.
TVD is computed from MD and directional survey data using the minimum curvature algorithm.
Horizontal wells at 90° inclination have MD increasing while TVD remains constant in the lateral section.
All reservoir contacts (OWC, GWC, GOC) and formation tops are reported in TVDSS for structural interpretation.

How TVD Is Calculated from Measured Depth

Measured depth follows the actual physical path of the wellbore — every curve, dogleg, and deviation accumulates MD regardless of the vertical component. True vertical depth captures only the vertical component of the wellbore path. For a perfectly vertical well, MD and TVD are equal (apart from the KB elevation correction). For a deviated or horizontal well, TVD grows more slowly than MD because the well is traversing a non-vertical path; in a fully horizontal lateral section at 90° inclination, TVD does not increase at all as MD increases — the well is moving laterally without descending.

The minimum curvature method computes TVD from directional survey stations by treating the wellbore as a smooth circular arc between each pair of adjacent survey stations. At each station, inclination (angle from vertical, in degrees) and azimuth (compass direction, in degrees) are measured by a magnetic or gyroscopic survey tool. The vertical increment between survey stations is computed as: ΔTVD = (ΔMD / 2) × (cos(I1) + cos(I2)) × RF, where I1 and I2 are the inclination angles at the two stations and RF is the ratio factor that accounts for the curvature of the arc. Summing these increments from surface gives the cumulative TVD at each depth point. Commercial directional drilling software automates this calculation and provides TVD logs alongside the MD-referenced log data for use in formation evaluation and reservoir engineering.

True Vertical Depth Applications Across International Jurisdictions

In Canada, TVDSS formation top depths are required in all AER formation evaluation submissions for deviated and horizontal wells. The AER's well completion report (WCR) submission system requires formation tops, fluid contacts, and completion intervals to be reported in both MD and TVD. Pool fluid contacts defined at exploration and appraisal wells are mapped in TVDSS space across the pool to establish the oil-water contact (OWC) depth for volumetric calculations; if contacts were reported in MD rather than TVDSS, contact depths from wells with different deviations would not be geologically comparable. Montney formation horizontal wells with total depths of 6,000-8,000 m MD typically have TVDSS at total depth of 2,000-2,800 metres — the difference is the 4,000-5,000 metre horizontal lateral section drilled at approximately 90° inclination.

In the United States, SEC reserve reporting standards require that reservoir volume estimates be based on depth maps in TVD, not MD. The SEC's Staff Legal Bulletin 5 confirms that TVDSS contour maps are required for structural trap definitions in proved reserve determinations. BSEE OCS well completion reports require TVD depths for all reported formation penetrations. In Norway, Sodir's Diskos database accepts well data in both MD and TVDKB/TVDSS; formation tops in the Norwegian Petroleum Directorate's factmaps are stored in TVDSS. NCS field development plans (PDO/PUD) present reservoir fluid contact depths and formation tops in TVDSS for all wells. In the Middle East, Saudi Aramco's maximum reservoir contact wells with lateral lengths of 7-12 km have MD values that exceed TVD by several kilometres; TVDSS formation depths are the reference standard for structural correlation across the Ghawar field.

Fast Facts

The difference between MD and TVD can be dramatic in extended-reach drilling. The world's longest measured depth wells — including wells at the Al Shaheen field in Qatar (Maersk Oil) with MD exceeding 12,290 metres — may have TVD values of only 3,000-4,000 metres below the kelly bushing. For such a well, the MD-to-TVD ratio is nearly 4:1. Every log curve plotted against MD in such a well spans four times the depth range plotted in TVD; a sand layer that appears to be 400 metres thick on the MD-referenced log may have a TVT (true vertical thickness) of only 15-20 metres — a factor of 20 difference that transforms from nuisance to critical accuracy requirement in volumetric reserve calculations.

TVDSS Reference System and KB Elevation

The kelly bushing (KB) is the physical reference point on the rig floor from which measured depth is counted downward. Its elevation above mean sea level varies between wells — an onshore well on flat prairie may have KB at +700 m elevation while an offshore drillship may have KB at +30 m above sea level. To compare formation depths between wells on the same structure that were drilled with different rigs at different KB elevations, TVDSS is used. TVDSS = TVD from KB - KB elevation above mean sea level. Formations at the same geological depth will have the same TVDSS regardless of where the surface is or where the rig floor is — making TVDSS the universal reference for interwell correlation, structural mapping, and field-wide geology. On offshore wells, the rotary table elevation (RT) or mean sea level (MSL) may be used as additional reference datums alongside KB; all reported depths should specify the reference datum used to avoid ambiguity.

Tip: When comparing formation tops between an onshore and an offshore well in the same basin, always confirm that both wells are using TVDSS before attempting structural correlation. An onshore well at 700 m KB elevation reporting a formation top at TVDKB 2,300 m is at TVDSS 1,600 m (2,300 - 700). An offshore well at 30 m KB elevation reporting the same top at TVDKB 1,630 m is at TVDSS 1,600 m — the same structural depth. If either well's depth is treated as TVDSS when it is actually TVDKB, the apparent structural discrepancy of 670 m will be mistaken for a structural offset when it is actually just a reference datum difference. Always check and document the depth reference system before performing any interwell correlation.

True vertical depth is also referenced as:

TVD — the universal abbreviation used on log headers, completion reports, and reservoir models; the most commonly seen form in all oil and gas documentation
TVDSS — true vertical depth subsea; TVD referenced to mean sea level; the standard reference for interwell correlation and structural mapping where different wells have different KB elevations
TVDKB — true vertical depth from kelly bushing; TVD referenced to the rig floor measurement point; the primary reporting datum in well completion reports before TVDSS conversion

Frequently Asked Questions

Why is TVD used for structural contour maps rather than MD?

Structural contour maps show the depth of a formation horizon as a function of geographic position, and depth means vertical depth below a reference datum — not distance along a curved wellbore path. If an oil-water contact is at TVDSS 2,000 m in one well and the adjacent well, drilled as a highly deviated producer, penetrates the same contact at TVDSS 2,000 m (but at MD 4,200 m from its KB), the two wells define the same OWC plane. If MD were used, the contact would appear to be at different "depths" (different MD values) in the two wells, creating false structural relief on a MD-based depth map. TVDSS eliminates this confusion by measuring only the vertical component, ensuring that all wells on the same structural map use the same depth convention regardless of wellbore trajectory, rig elevation, or measurement technique.

How accurate is TVD computed from directional surveys?

TVD accuracy depends on the quality and frequency of directional survey measurements. In vertical wells, inclination surveys are taken every 30-90 metres; if inclination varies by less than 1 degree between stations, TVD accuracy is typically within 1-2 metres per 1,000 metres of wellbore length, or 0.1-0.2% of total depth. In highly deviated wells with significant curvature, survey station frequency must be increased in build-up and turn sections to maintain positional accuracy. The primary source of TVD error in magnetic surveys is magnetic interference from the drill string, which can cause azimuth errors that propagate into lateral (Northing, Easting) position errors but have smaller effects on TVD because TVD depends primarily on inclination rather than azimuth. Gyroscopic surveys avoid the magnetic interference problem and are used in wells where magnetic reference is unreliable (near casing, within a steel jacket platform, or in high-latitude areas where magnetic declination is large and variable).

Why True Vertical Depth Matters in Oil and Gas

Every quantitative use of wellbore data in oil and gas — formation evaluation, structural mapping, volumetric reserve estimation, fluid contact determination, reservoir pressure gradient calculation, and interwell correlation — requires that depths be expressed in a consistent vertical reference frame. The move toward horizontal wells and extended-reach drilling, where MD-to-TVD ratios of 2:1 to 5:1 are common, has made TVD conversion from MD not a minor correction but a fundamental data processing step that changes formation thickness estimates, contact depths, and reservoir volumes by factors of 2-5 compared to naive MD-based calculations. Errors in TVD from poor directional survey quality or incorrect KB elevation handling can cause well placement failures, incorrect reserve estimates, and erroneous structural interpretations that cost millions of dollars to correct through redrilling or reinterpretation. Correct TVD calculation is therefore a fundamental data quality requirement for all directional well operations.