KB (Kelly Bushing)

KB (Kelly Bushing) is the depth reference datum used in oil and gas drilling and well logging, defined as the elevation of the rotary kelly bushing — the square or hexagonal drive mechanism on the rig floor through which the kelly passes — above mean sea level (for offshore wells) or above ground level (for onshore wells), from which all measured depths in the wellbore are recorded and reported.

Key Takeaways

All depth measurements in a well — casing shoe depths, formation top depths, perforated intervals, cement tops, and wireline log depths — are referenced to KB elevation unless explicitly stated otherwise, making KB the universal datum for well data within a given wellbore.
KB elevation is measured in metres or feet above mean sea level (AMSL) for all wells and is recorded in the well header, well completion report, and wireline log header so that all users can convert KB-referenced depths to true subsurface elevations by subtracting the KB elevation.
On modern top-drive rigs that do not use a kelly, the equivalent reference point is called the drill floor elevation (DF) or rotary table (RT) elevation, but the convention of calling this reference "KB" or reporting it as "KB equivalent" persists widely in industry practice regardless of rig type.
Subsea depth (depth below sea floor) and true vertical depth subsea (TVDSS) are calculated from KB-referenced measured depths using the KB-to-sea-floor or KB-to-mean-sea-level relationship, critical for comparing formation depths between wells drilled from different platform or rig heights.
KB elevation is a fixed surveyed value for a given well and does not change during or after drilling; errors in KB elevation measurement propagate into all subsequently calculated subsurface depths and must be corrected if discovered.

Fast Facts

Typical KB elevations for land rigs are 5 to 15 metres above ground level, depending on substructure height. For jackup drilling rigs in the Gulf of Mexico, KB can be 30 to 60 metres above mean sea level, adding significant depth to reach a target at a given true vertical depth subsea. For drillships in deepwater, KB may be 20 to 30 metres above sea level but must also account for the water column depth below sea floor. Depth conversion errors arising from KB reference confusion are a common source of depth discrepancies between wells drilled in different eras or by different operators in the same field, requiring careful investigation before any structural interpretation that correlates tops across multiple wells.

What Is KB (Kelly Bushing)?

Every measurement made in an oil or gas well — from the depth at which a formation is first encountered to the placement of production perforations — must be referenced to a defined elevation datum so that it can be compared with measurements from other wells, converted to subsurface true vertical depth, and used in reservoir models and geological interpretations. The Kelly Bushing (KB) is the conventional datum for this purpose.

The physical Kelly Bushing is the mechanism on the rig floor that transmits rotary motion from the rotary table to the kelly — the square or hexagonal pipe that in turn drives the drill string. The top face of the KB, or the master bushing on which the KB sits, defines the zero reference point from which drill pipe is measured as it is run into the wellbore. When a pipe connection is made and a new joint is lowered, the total depth of the string is read off at the KB level — hence "depth measured from KB."

On modern rigs using top drives instead of a kelly system, the equivalent rotary table or drill floor elevation serves the same reference function, though the term "KB" is retained in convention. The specific KB or equivalent elevation for each well is surveyed and recorded as a fixed value in the well header. All wireline log depth tracks, casing records, completion reports, and geological formation tops reported for that well use this fixed KB datum.

How KB Depth Is Used in Practice

When a geologist reports that a formation top is at 3,200 metres KB, it means the formation was first encountered at 3,200 metres of measured depth from the Kelly Bushing. To find the true vertical depth of that formation below sea level, two conversions are needed: subtract the KB elevation above sea level to get true vertical depth subsea (TVDSS), and apply a measured depth to true vertical depth correction if the well is deviated. For a vertical well with KB at 10 metres AMSL, a formation at 3,200 m KB is at 3,190 m TVDSS.

In a deviated horizontal well, the measured depth (MD) from KB increases much faster than true vertical depth (TVD) because the wellbore travels along a curved path that covers more total distance than its vertical component. At the end of a 3,000-metre horizontal section drilled at true vertical depth of 3,500 metres, the measured depth might be 6,500 m KB while the TVDSS is still approximately 3,490 m. All well-to-well depth correlations use TVDSS to correctly place formation tops at their true subsurface elevation, independent of the well trajectory or rig height.

Reservoir engineers use KB-referenced depths to build structural maps by converting formation tops from multiple wells to TVDSS, gridding the subsurface structure, and calculating oil-water contact depths, net pay thicknesses, and reservoir volumes. An error in one well's KB elevation causes that well's formation tops to plot at systematically incorrect subsurface depths, potentially distorting the structural interpretation across the entire field.

KB Across International Jurisdictions

Canada (AER / WCSB): Alberta Energy Regulator well licensing and completion reporting requirements mandate that the KB elevation be recorded in the well licence application and in the final well completion report (WCR). The AER's well data submission system (Petrinex) stores KB elevation as a required field for every well, and all formation tops, perforation intervals, and casing shoe depths in Petrinex are understood to be in measured depth from KB unless flagged as true vertical depth. WCSB operators converting depths from legacy wells on paper records must confirm the original KB datum before using historical well data in modern reservoir studies.

United States (BSEE / State Agencies): BSEE offshore well completion reports and the API well number system both require KB elevation documentation. State oil and gas commissions (Texas RRC, Oklahoma CC, Colorado COGCC) require KB elevation in all well permit and completion filings. In the Permian Basin, where thousands of wells from different eras are being correlated for stacked pay development, verifying KB elevations on legacy wells is a standard quality control step before any cross-well formation top comparison.

Norway (Sodir): The Norwegian Petroleum Directorate / Sodir requires KB elevation (expressed as metres above mean sea level) in all well completion reports submitted through Diskos (the Norwegian national oil and gas data repository). All NCS well data in Diskos is stored with KB reference, and operators converting between different depth conventions must use the Diskos KB value as the authoritative reference. Equinor's geological data standards require explicit KB documentation in all internal and regulatory well databases.

Middle East (Saudi Aramco): Saudi Aramco's Engineering Standards require KB elevation to be precisely surveyed and recorded for all wells and used as the sole depth datum for all well data unless an explicit conversion to TVDSS is applied. Given the scale of Aramco's operations — thousands of wells across multiple giant fields — maintaining consistent KB referencing across decades of data is a critical data management requirement for field development planning and reserve estimation.

KB is also written as K.B. or Kelly Bushing. Equivalent terms for the same or closely related depth datum include drill floor (DF), rotary table (RT), and ground level (GL) on onshore wells where the rotary table is essentially at ground. Related terms include measured depth (MD), true vertical depth (TVD), TVDSS (true vertical depth subsea), depth reference, and well header. The datum should always be explicitly stated when reporting depths — "3,200 m MD KB" versus "3,190 m TVDSS" are unambiguous; "3,200 m depth" is ambiguous and should be avoided.

Tip: When correlating formation tops between two wells in the same field, always confirm that both wells use the same depth datum before comparing depths. A top that appears 15 metres shallower in one well than another may simply reflect a 15-metre difference in KB elevation between the two rig setups, not a genuine structural dip. Pull both wells' KB elevations from the regulatory database (AER Petrinex, Sodir Diskos, BSEE TIMS, or similar) and subtract them from the reported MD tops to get TVDSS before drawing any conclusions about structural geometry or formation dip.

FAQ

What is the difference between KB and ground level as depth datums?
Ground level (GL) is the elevation of the ground surface at the wellsite above mean sea level. KB is the elevation of the Kelly Bushing (or equivalent rotary table) above mean sea level, which is always higher than ground level by the height of the rig substructure (typically 5 to 15 metres for land rigs). Because all drill string measurements are made from KB, all wellbore depths are referenced to KB, not to ground level. Ground level is recorded as a separate survey value and is used for some regulatory purposes (depth to groundwater, surface casing depth requirements) but is not used as the drilling depth datum. Confusing KB and GL by 10 metres would offset all formation top depths by 10 metres relative to sea level — significant for precise structural mapping.

What is the KB equivalent on a deepwater drillship?
On a drillship, the rig floor and rotary table (KB equivalent) are elevated above sea level by the height of the vessel's moon pool deck, typically 20 to 30 metres. All measured depths are referenced to this KB elevation above sea level. The water depth (sea floor depth below sea level) is a separate measurement. To calculate TVDSS for a deepwater well, the formation MD from KB must be corrected for: the KB elevation above sea level (adds a positive offset), and the inclined wellbore geometry (if deviated). The sea floor depth is not subtracted from MD KB — it is a separate value used to calculate depth below sea floor for regulatory drilling hazard assessments and for plotting the wellbore trajectory in cross-sections.

Why KB Matters

Consistent depth referencing from the Kelly Bushing is the foundation upon which all subsurface geological and engineering interpretations are built. Without a well-defined and documented depth datum, formation correlations between wells are unreliable, structural maps are distorted, and reserve calculations based on net pay and contact depths are incorrect. In mature basins where thousands of wells have been drilled over decades by many different operators, KB elevation documentation and its consistent application across all well databases is one of the most fundamental data quality requirements for any regional geological study, development planning exercise, or regulatory compliance program.