datum, Oil & Gas Glossary

A datum is a depth reference point, typically established at the time the well is completed, against which subsequent depth measurements are corrected or correlated. Every depth recorded in a well, whether a casing shoe, a perforation interval, a formation top, or a logging tool reading, is meaningless unless it is tied to an agreed zero point, and that zero point is the datum. In drilling operations the most common datum is the top of the kelly bushing, abbreviated KB or RKB for rotary kelly bushing, which is the steel assembly that sits in the rotary table on the rig floor and through which the drillstring passes. Because the kelly bushing is a fixed, repeatable point above ground level, drillers measure pipe and wireline depths from it, and those measurements are reported as depth below KB. Other reference points are used depending on the application: ground level (GL), drill floor (DF), rotary table (RT), and mean sea level (MSL) for regional and seismic work. The relationship between them is captured by the KB elevation, the vertical distance from the kelly bushing down to mean sea level, which lets any well depth be converted to a true subsea elevation. This conversion is what makes wells comparable to one another and to seismic data. A formation top logged at 1,840 m below KB in one well and 1,795 m below KB in a neighbouring well cannot be correlated directly if the two rigs had different KB elevations; only after both are referenced to a common datum such as MSL do the structural relationships, dip, and thickness become trustworthy. The problem grows more important in directional and horizontal wells, which dominate modern WCSB development in the Montney and Duvernay. In a deviated well there are two depths for every point in the hole: measured depth (MD) along the borehole and true vertical depth (TVD) straight down from the datum. Both are referenced to the same KB datum, but only TVD reflects real burial depth and pressure, so the directional survey converts MD to TVD against the datum before any geological or reservoir interpretation. In geophysics the equivalent concept is the seismic reference datum, a flat artificial surface, often set at or near sea level, to which all seismic traveltimes are corrected so that near-surface elevation and weathering-layer variations do not distort the image. Tying a well to a seismic section therefore requires reconciling the well's KB datum with the survey's seismic datum, a routine but critical step in WCSB exploration and development geophysics.

Key Takeaways

Top of kelly bushing is the standard well datum: Drilled and logged depths are measured from the top of the kelly bushing (KB or RKB), a fixed point on the rig floor. Every depth in the well report, from casing shoes to formation tops, is implicitly depth-below-KB unless another reference is stated, which is why KB elevation must accompany the data.
KB elevation converts to subsea datum: The vertical distance from the kelly bushing down to mean sea level lets any depth be restated as a true subsea elevation. Without this number, depths from wells drilled by rigs of different floor heights cannot be correlated, and formation tops will appear structurally offset purely from rig geometry.
MD and TVD share the datum but differ downhole: In deviated and horizontal WCSB wells, measured depth follows the borehole while true vertical depth drops straight from the same datum. Only TVD reflects real burial depth and pressure, so directional surveys convert MD to TVD against the datum before any reservoir interpretation.
Seismic reference datum is a separate flat surface: Seismic processing corrects all traveltimes to an artificial flat datum, often near sea level, to remove the effects of surface topography and the weathering layer. This datum is not the well's KB, so tying a log to a seismic line requires reconciling the two reference systems.
Errors cascade through correlation: A mistaken or undocumented datum propagates into every correlated horizon, isopach map, and volumetric estimate built on it. Datum discipline at the wellsite, recorded in the AER-filed survey and tour reports, is the foundation of trustworthy subsurface mapping.

Referencing Depths in Directional and Horizontal Wells

In a Montney horizontal, the well may reach a measured depth of 5,500 m while its true vertical depth is only about 2,400 m, because most of the length is the near-horizontal lateral. Both numbers reference the same KB datum, but they answer different questions. MD tells the driller how much pipe is in the hole and where to set tools; TVD tells the geologist the actual depth of the target and the reservoir pressure regime. The directional survey, filed with the AER, records inclination and azimuth at regular stations and integrates them to produce a TVD profile tied to the surface datum. Misreporting the KB elevation shifts every TVD value and corrupts the structural picture.

Reconciling Well and Seismic Datums for the Tie

A well-to-seismic tie checks that the reflectors in a processed seismic volume line up with the formation tops and acoustic boundaries seen in the well. To do it, the interpreter must place both on a common vertical reference. The well log is in depth below KB; the seismic is in two-way traveltime referenced to the seismic datum. A check-shot or vertical seismic profile provides the time-depth relationship, and the KB-to-seismic-datum offset is applied so the synthetic seismogram generated from the well aligns with the field data. A datum mismatch of even a few tens of metres can throw the tie off by a full seismic loop, leading to mispicked horizons.

Fast Facts

The kelly bushing became the universal depth datum largely by accident of rig design rather than by deliberate standardization. Because the kelly drove the drillstring through the rotary table, the top of that bushing was simply the most convenient fixed steel surface from which a driller could measure pipe going into the hole. The convention stuck so firmly that even on modern top-drive rigs that no longer use a kelly at all, depths are still reported as KB or RKB, preserving a reference point named for a piece of equipment many crews never physically use.

A datum is the zero point for both measured depth and true vertical depth, the two depth systems that diverge in any deviated well and must both be tied back to it. It is established and tracked relative to the kelly bushing, the rig-floor assembly whose top serves as the conventional reference. In geophysics the well datum must be reconciled with the seismic reference datum used in seismic reflection processing so that logs and seismic images share a common vertical frame.

Real-World WCSB Scenario: Duvernay Pad Correlation

An operator drilling a multi-well Duvernay pad near Fox Creek, Alberta, set each of four horizontals from a rig whose KB elevation was about 1,012 m above mean sea level. A fifth well, drilled later by a different rig with a KB elevation of 1,019 m, logged the Duvernay top 7 m deeper below KB than its neighbours, suggesting a structural low that did not exist. The geologist re-referenced all five wells to subsea elevation using the recorded KB elevations.

Once corrected to a common MSL datum, the Duvernay top fell on a smooth, gently dipping surface across all five wells, and the apparent 7 m offset vanished entirely. The lesson, reinforced on every WCSB pad, is that the datum is not a formality: a few metres of unrecorded rig-floor height can manufacture phantom structure and misdirect a landing-zone decision worth millions.