True Vertical Thickness: TVT vs True Stratigraphic Thickness, Isochore Maps, and Deviation Corrections in WCSB Mapping

True vertical thickness, abbreviated TVT, is the thickness of a bed or rock body measured straight down along a vertical line at a single point in the subsurface. It is the thickness you would record if you dropped a plumb line through the unit, regardless of how the bed is tilted or how the wellbore that penetrated it was oriented. TVT sits inside a family of thickness measurements that geologists must keep carefully distinct, because confusing them produces wrong volumes and wrong maps. Its most important companion is true stratigraphic thickness, TST, which is the thickness measured perpendicular to the bedding planes, after correcting for both the dip of the formation and the deviation of the well. When a bed is horizontal and the well is vertical, TVT, TST, and the raw measured thickness along the borehole are all identical. The moment the bed dips or the well deviates, they diverge: for a dipping bed penetrated vertically, TVT is always greater than or equal to TST, and the difference grows with the cosine of the dip angle, since TST equals TVT multiplied by the cosine of the formation dip. This geometry is why TVT is the natural quantity to contour for reservoir volume work. A map of true vertical thickness is called an isochore map, and it is the correct input for calculating gross rock volume and hydrocarbon pore volume because reservoir fluids are distributed against a vertical datum and fluid contacts are horizontal surfaces. A map of true stratigraphic thickness, by contrast, is called an isopach map, and it is the right tool for understanding depositional history because it shows how much sediment actually accumulated perpendicular to the layering. The two terms are frequently used loosely as synonyms in casual conversation, but in rigorous subsurface mapping they are different products built for different purposes. Deriving an accurate TVT from a logged well requires correcting the apparent thickness measured along the borehole for wellbore deviation, which comes from the directional survey, and for formation dip, which comes from a dipmeter or borehole image log. In the Western Canadian Sedimentary Basin, where structural dip in the foothills can exceed 40 degrees and horizontal and highly deviated wells are routine across the Montney, Duvernay, and Cardium, these corrections are not optional refinements; they are the difference between a defensible reserve volume and a badly inflated or deflated one, which is why TVT and its corrections are embedded in every WCSB mapping and volumetric workflow.

The Cosine Relationship in Practice

The link between the thickness types is pure trigonometry. For a unit dipping at 30 degrees penetrated by a vertical well, the apparent thickness logged along the borehole equals the TVT, but the true stratigraphic thickness is only TVT times cosine 30, about 87 percent of it. At 45 degrees that factor falls to 71 percent, and in steep foothills dips near 60 degrees the stratigraphic thickness is just half the vertical thickness. For a deviated or horizontal well the correction also folds in the borehole angle relative to bedding, which is why a horizontal well drilled along a flat bed can log hundreds of metres of apparent section through a unit only a few metres thick stratigraphically.

Building the Isochore for Volumetrics

To calculate gross rock volume a geologist contours TVT across the reservoir, multiplies by the mapped area, and applies net-to-gross, porosity, and hydrocarbon saturation to reach hydrocarbon pore volume. Because the isochore is referenced to vertical, it integrates cleanly against horizontal fluid contacts and structural depth maps. Modern workflows in SLB Petrel and similar packages compute TVT cell by cell from a structural framework and a dip model, so the isochore is generated automatically, but the underlying geometry and its sensitivity to dip remain exactly what a geologist contouring by hand would respect.

Related Terms

True vertical thickness is the quantity contoured on an isochore map for reservoir volume, in contrast to true stratigraphic thickness shown on an isopach. Its calculation depends on formation dip from a dipmeter or image log, combined with the well path. TVT feeds directly into gross rock volume, the foundation of volumetric reserve estimates, and it is measured against the vertical reference frame shared by true vertical depth, which positions the unit in absolute subsurface elevation.

Real-World WCSB Scenario: A Foothills Cardium Volume Check

A geologist mapping a Cardium sandstone target in the Alberta foothills west of Caroline had eight wells, several drilled deviated to reach beneath a thrust sheet where structural dip averaged 38 degrees. The raw log-derived thicknesses, uncorrected, suggested a gross rock volume that supported a CAD 14 million single-well development case, and an internal review flagged that no deviation or dip correction had been applied.

Reprocessing each well to true vertical thickness, correcting for borehole deviation and the 38 degree formation dip, trimmed the contoured isochore and cut estimated gross rock volume by about 22 percent. The revised volume moved the project below the economic threshold at prevailing prices, and the operator deferred the well, avoiding a marginal commitment that the uncorrected map had made look robust.