depth map, Oil & Gas Glossary

A depth map is a two-dimensional representation of subsurface structure whose contours are expressed in true vertical depth rather than seismic traveltime, produced by applying a velocity model to a time-domain interpretation. In seismic reflection work, the raw product of processing is a picked horizon posted in two-way traveltime, measured in milliseconds, because sound waves are what the survey actually records. That time structure map is geophysically honest but commercially useless on its own, because a drilling engineer cannot set casing points, calculate hydrostatic pressure, or estimate reservoir volume in units of time. The conversion from time to depth multiplies the time structure grid by a velocity field, turning milliseconds into metres and feet, and the result is the depth map that anchors well planning, prospect ranking, and reserve booking. The velocity field is never a single number; it is built by integrating well tops from offset control wells, sonic and check-shot surveys, vertical seismic profiles, and stacking or migration velocities from the processing stream, then reconciling these sources into a geologically sensible model. Because velocity varies laterally with lithology, compaction, and overpressure, a flat time anomaly can hide a real structural closure, and an apparent time high can vanish once a faster overburden is honoured, so depth conversion is where geophysics and geology must agree. In the Western Canadian Sedimentary Basin, depth maps drive targeting on Cardium, Viking, Nisku, and Leduc horizons where a few metres of structural relief separate a productive well from a dry hole, and where Devonian reef pinnacles such as those in the Leduc and Nisku plays demand accurate velocity pull-up corrections beneath the high-velocity carbonate. Operators typically report structural depths in metres subsea (mSS) or metres true vertical depth subsea (TVDSS), and a competent depth map carries an explicit statement of its velocity model, its datum, and its residual error at control points. The map is the deliverable that a geologist hands to the drilling group, that a reservoir engineer gridsfor volumetrics, and that a landman references when defining a drilling spacing unit, which is why depth conversion error is one of the most closely audited numbers in any prospect package. A depth map without a documented velocity basis is treated with suspicion, because the same time interpretation can yield materially different closures depending on which conversion method is chosen.

Key Takeaways

Time Is Recorded, Depth Is Derived: Seismic surveys measure two-way traveltime in milliseconds, not depth. A depth map is produced only after a velocity model multiplies the time structure grid, converting milliseconds into metres and feet. The quality of the depth map is limited entirely by the quality of that velocity field, so a poorly constrained velocity model produces a confidently drawn but wrong map.
Velocity Models Integrate Many Sources: A defensible velocity field blends well tops, sonic logs, check-shot and vertical seismic profile surveys, and processing velocities. In the WCSB, offset well control from decades of drilling gives dense calibration in mature fairways such as Pembina and the Viking trend, letting geophysicists tie the seismic to real formation depths within a few metres.
Lateral Velocity Change Creates False Structure: Because velocity varies with lithology, compaction, and overpressure, a time high can disappear on depth conversion and a time flat can reveal a real closure. Velocity pull-up beneath fast Devonian carbonates such as Leduc and Nisku reefs is a classic WCSB pitfall that has produced both phantom prospects and missed pay.
Datum and Units Must Be Explicit: WCSB depth maps are posted in metres subsea (mSS) or true vertical depth subsea (TVDSS) referenced to a stated seismic datum and replacement velocity. Mixing kelly bushing depths with subsea contours, or metres with feet, is a common and expensive error that misplaces casing points and corrupts volumetric grids.
The Map Drives Money Decisions: Depth maps set drilling targets, define spacing units under AER Directive 065 reservoir management, and feed the gross rock volume used in reserve booking. A conversion error of even five to ten metres near a structural spill point can move a prospect from economic to sub-economic, which is why depth error is audited in every prospect review.

Time-to-Depth Conversion Methods and Their Trade-offs

Depth conversion methods range from simple to rigorous. A single vertical function multiplies time by one average velocity, adequate only for gentle, layer-cake geology. Layer-cake conversion assigns an interval velocity to each mapped layer and stacks them, honouring vertical velocity change but not lateral variation. A gridded velocity model interpolates velocity laterally between wells, capturing compaction trends and facies change. The most rigorous approach, prestack depth migration, images the data directly in depth using a full velocity model, and is standard for structurally complex Foothills thrust belts in Alberta where dips are steep and velocities change abruptly. Each step upward in rigour costs more processing time and demands more velocity control, so the method is matched to the structural risk and the value at stake.

Reconciling Seismic Depth Against Drilled Well Tops

The truth test of any depth map is the mistie at a well: the difference between the mapped seismic depth and the actual formation top drilled. Geophysicists post these residuals at every control well and grid them into a correction surface, effectively bending the seismic depth to honour hard data. In a mature WCSB pool with dozens of penetrations, misties are typically driven below a few metres, giving high confidence for infill targeting. On an exploration prospect with a single distant control well, misties can exceed twenty metres, and the depth map carries a stated uncertainty band that a prospect review team weighs against the size of the mapped closure before committing capital.

Fast Facts

The velocity pull-up beneath a Leduc reef in central Alberta can exceed thirty metres. Sound travels roughly twice as fast through the tight Devonian carbonate as through the shale that flanks it, so a flat-lying formation below the reef appears as a false structural high on an uncorrected time map. Early explorers who drilled these apparent highs without depth conversion sometimes found the target flat or downdip, a lesson that made velocity modelling standard practice across the basin decades ago.

A depth map is the end product of a workflow that touches several related concepts. It begins from a two-way traveltime interpretation, the raw time-domain measurement that must be converted. The conversion depends on a velocity model built partly from a check-shot survey, which ties seismic time to depth at a wellbore. The finished map is often used to compute gross rock volume, the structural container that, combined with porosity and saturation, yields the volumetric reserve estimate that justifies drilling.

WCSB Field Scenario: Depth Conversion on a Nisku Pinnacle

A junior operator maps a Nisku pinnacle reef prospect near Bashaw, Alberta on a 3D survey, picking a bright time closure of about forty milliseconds of relief. The initial single-velocity depth conversion suggests forty metres of vertical closure and a gross rock volume supporting roughly 1.2 million barrels of recoverable oil, and the prospect is budgeted at a CAD 4.8 million drill-and-complete. Before spudding, the geophysics team rebuilds the velocity model using sonic control from two offset Nisku wells and applies a layer-cake correction for the fast carbonate cap.

The rigorous conversion collapses the closure to twenty-two metres and shifts the crest 180 metres northeast of the original pick. The team moves the surface location and bottomhole target to honour the corrected crest, drills a successful well encountering 14 metres of Nisku pay, and books reserves against the smaller but validated closure. Had they drilled the uncorrected time high, the wellbore would have landed downdip near the reef flank, risking a marginal or wet completion and a written-off CAD 4.8 million.