Depth Reference
A depth reference is the fixed surveyed datum from which all wellbore depth measurements are recorded, with common references including kelly bushing (KB) elevation, rotary table (RT) elevation, ground level (GL), mean sea level (MSL), and seabed, each expressed in meters or feet above or below a common geodetic datum such as MSL, and critical for correlating logs and surveys from different wells that may have used different reference points.
Key Takeaways
- The kelly bushing (KB) is the most common depth reference for onshore wells; all measured depth (MD) and true vertical depth (TVD) values on wireline logs and drilling reports are measured from the KB elevation above mean sea level.
- Offshore wells use either the rotary table (RT) on the rig floor or the mean sea level (MSL) as the primary reference, with seabed depth recorded separately to allow subsea true vertical depth (TVDSS) calculations.
- Failure to apply depth reference corrections when correlating wells across a field can introduce apparent stratigraphic shifts of tens of meters, leading to incorrect formation tops, reservoir mapping errors, and misaligned seismic-to-well ties.
- The API well header for every well drilled in the United States must record the KB elevation, ground elevation, and water depth (if offshore) to enable consistent depth referencing across the national well database.
- Seismic data uses its own datum (the seismic processing datum, usually MSL or a local flat datum), and converting seismic two-way time to depth requires tying the well log TVD referenced to MSL (TVDSS) to the seismic datum.
Fast Facts
Kelly bushing elevations on land rigs typically range from 1 to 10 meters above ground level, depending on rig substructure height. On jack-up rigs, the rotary table may be 20 to 40 meters above mean sea level. On drillships and semi-submersibles in deep water, the rotary table can be 25 to 35 meters above MSL, while the seabed is hundreds to thousands of meters below MSL. All these offsets accumulate if a well header is misread or the reference datum is incorrectly assumed during log loading.
Tip: When loading well logs into a subsurface software package, always verify the KB elevation stored in the LAS file header matches the surveyed KB elevation in the well completion report. A 1-meter error in KB entry propagates through every formation top pick, contact interpretation, and net pay calculation in that well, and silently corrupts any volumetric model built on it.
What Is Depth Reference
A depth reference is the physical starting point from which the length of a wellbore is measured. Because wells are drilled from elevated structures, and because subsurface targets are described relative to a common horizontal plane such as mean sea level, every depth measurement requires a clearly defined origin. The depth reference connects the engineering world of drill-pipe tally and wireline cable depth to the geological world of formation tops, fluid contacts, and seismic reflectors.
Common depth references include the kelly bushing (KB), which is the top of the square hole in the rotary table through which the drill string passes on land rigs; the rotary table (RT) elevation, which is functionally the same measurement; ground level (GL), used for surface casing depth references; mean sea level (MSL), the global geodetic datum; and seabed, used for offshore measurements of mudline depth. Each reference must be surveyed with a level and referenced to a known geodetic benchmark so that wells from different drilling campaigns can be compared on a common spatial framework.
How Depth Reference Works
Measured depth (MD) is the along-hole distance from the depth reference to any point in the wellbore, accumulated by counting drill-pipe joints, wireline cable length, or coiled tubing footage. Because deviated and horizontal wells follow curved trajectories, MD always equals or exceeds true vertical depth (TVD). True vertical depth is derived from MD using the directional survey (inclination and azimuth readings), corrected to the depth reference datum. Subsea true vertical depth (TVDSS) is TVD minus the KB elevation above MSL, converting the depth reference to mean sea level for compatibility with seismic and regional geological maps.
Converting between depth references requires simple arithmetic: TVDSS = TVD (from KB) minus KB elevation above MSL. For a well with a KB at 650 meters above MSL and a formation top at 2,400 meters TVD from KB, the TVDSS of that formation top is 1,750 meters below MSL. When correlating with a nearby well that has a KB at 625 meters above MSL, the same formation top at 1,750 m TVDSS would appear at 2,375 meters TVD in that well. Ignoring this 25-meter KB difference would create a false 25-meter apparent dip on a structural map.
For offshore wells, the water depth from MSL to seabed must also be recorded. This allows calculation of reservoir depth below seabed (useful for pore pressure gradient calculations) and permits consistent depth conversion when integrating wells with shallow-water and deepwater seismic surveys that use different processing datums.
Depth Reference Across International Jurisdictions
In Canada and under the Alberta Energy Regulator (AER), all well records submitted through the Petrinex and OneStop systems require KB elevation in meters above sea level surveyed to a geodetic benchmark. The AER's Directive 059 and associated well data submission requirements specify that KB elevation, ground elevation, and total depth must be reported in the well licence and well completion report. The WCSB public well database (maintained by AER and BC Oil and Gas Commission) stores TVDSS formation tops for all wells, enabling province-wide stratigraphic correlation without manual KB correction.
In the United States, the American Petroleum Institute (API) well number system and associated header requirements mandate recording of KB elevation, ground elevation, and water depth for all wells. The USGS National Oil and Gas Assessment and state surveys such as the Texas Railroad Commission (RRC) and Oklahoma Corporation Commission (OCC) use these headers to populate public well databases. The Society of Professional Well Log Analysts (SPWLA) publishes LAS (Log ASCII Standard) format specifications that include mandatory header fields for KB elevation, ensuring wireline log files carry the necessary reference information for automated loading into subsurface software.
In Norway, the Norwegian Offshore Directorate (formerly Sodir) requires all wells on the Norwegian Continental Shelf to report water depth, air gap (from MSL to rotary table), and KB elevation in the well completion report submitted to the DISKOS national data repository. Norwegian wells consistently use MSL as the seismic and log datum, which simplifies regional seismic-to-well correlation across the North Sea and Norwegian Sea. All formation tops in DISKOS are stored as TVDSS values, enabling direct comparison across the highly variable water depths of the NCS.
In the Middle East, Saudi Aramco's internal well data standards align with Society of Petroleum Engineers (SPE) and API conventions, with KB elevation surveyed relative to the Saudi national geodetic datum. Onshore fields in the Arabian Peninsula involve wells with relatively consistent KB elevations because flat desert terrain limits variation in rig floor height above ground. Offshore wells in the Arabian Gulf record water depth and air gap. Saudi Aramco's GeoFrame and Petrel-based subsurface workflows automatically apply TVDSS corrections during well loading, enforcing consistent depth referencing across the thousands of wells in the Ghawar, Safaniyah, and Shaybah fields.
Synonyms and Related Terminology
Depth reference is closely related to the concepts of measured depth (MD) and true vertical depth (TVD). The term datum is used interchangeably with depth reference in some contexts, though datum more specifically refers to the geodetic reference plane (MSL) rather than the physical reference point on the rig. Kelly bushing, rotary table, and drill floor elevation are synonymous terms for the rig-floor depth reference. Subsea true vertical depth (TVDSS) and depth below MSL (DBMSL) refer to the same converted measurement. Log datum and survey datum are terms used in wireline and directional surveying to specify which reference was used for a specific dataset.
FAQ
What happens when depth reference information is missing from a log file?
If KB elevation is absent from a wireline log header, the log can still be loaded at its MD values but cannot be correctly placed in TVDSS space without external reference. This causes formation tops from that well to appear displaced relative to neighbouring wells, distorting structural maps and cross-sections. Industry practice is to flag such wells and retrieve the KB elevation from the original well completion report or drilling prognosis before incorporating the well into any structural or volumetric model.
How does depth reference affect pore pressure prediction?
Pore pressure prediction models (Eaton, Bowers) use depth below a reference surface, typically TVDSS, as input. If the wrong KB elevation is applied, the predicted pore pressure gradient shifts, potentially causing an engineer to select an incorrect mud weight for the next hole section. In deepwater drilling, the air gap between MSL and the rotary table is particularly important: failing to add the air gap when converting from MSL to RT reference can underestimate the hydrostatic head and result in an incorrect equivalent circulating density (ECD) calculation.
Why Depth Reference Matters
Every subsurface dataset, from wireline logs and core descriptions to seismic horizons and production data, must share a common depth framework before it can be integrated. Depth reference is the single conversion that ties rig-floor measurements to the global geodetic system, enabling geologists to correlate formation tops across dozens of wells, reservoir engineers to map fluid contacts consistently, and drilling engineers to plan wellbore trajectories that intersect the intended targets. Errors in depth reference are particularly insidious because they are silent: a 10-meter KB error appears as a 10-meter stratigraphic shift on every map and cross-section built from that well, propagating into reserve estimates, perforation intervals, and completion designs without triggering any obvious warning.