True Stratigraphic Thickness

True stratigraphic thickness (TST) is the actual thickness of a rock layer measured perpendicular to its bedding planes. The number is what a geologist would get if they could lay the rock layer flat and measure across it from top to bottom. The reason this matters is that wells rarely intersect rock layers at right angles. A vertical well penetrating a tilted reservoir at 45 degrees of dip records a measured-depth thickness much greater than the rock's true stratigraphic thickness. A horizontal well drilled along bedding can record hundreds of metres of measured length while penetrating only a few metres of rock vertically. Converting measured well thickness to true stratigraphic thickness requires correcting for both the formation's dip and the well's own deviation. The corrected number is what reservoir engineers, geologists, and reserves auditors actually need.

Key Takeaways

True stratigraphic thickness is the thickness of a rock layer measured at right angles to its bedding. It is the number the rock would have if it were laid out flat. TST is the meaningful thickness for reservoir volume calculations, net pay accounting, and reserves estimates.
Three other thickness numbers also appear in well reports and have to be carefully distinguished from TST: measured depth thickness (the length of wellbore through the rock), true vertical thickness (the vertical distance between top and base, measured straight up), and apparent thickness (the thickness as it appears on a tilted seismic section before depth correction).
TST is calculated from measured-depth thickness by correcting for two angles: the formation dip and the well deviation. The basic formula is TST = MD-thickness times the cosine of the angle between the wellbore and the formation normal direction. The math is straightforward but easy to get wrong without careful attention to vector geometry.
The correction matters most in horizontal wells. A 2,500-metre horizontal well drilled exactly along the bedding plane through a 6-metre-thick reservoir has a TST contribution of zero from the lateral itself. The reservoir thickness is determined by the much shorter section where the well crossed the bedding (typically the curve and the toe of the lateral).
TST is the standard thickness used in volumetric reserves calculations: hydrocarbon-in-place equals net rock volume times average porosity times hydrocarbon saturation, where net rock volume uses TST as the thickness term. Using a different thickness (measured depth, true vertical, or apparent) produces overestimates that fail reserves audit.

Fast Facts

The Steno's Laws of Stratigraphy, formulated by Nicolas Steno in 1669, established the principle that sedimentary rock layers were originally horizontal at deposition and that the present-day dip reflects later deformation. True stratigraphic thickness restores the layer mathematically to its undeformed state, making it directly comparable across different parts of a basin where the dip varies. The concept is over 350 years old. Its day-to-day application in reservoir engineering, reserves estimation, and unconventional development has only grown in importance as horizontal drilling has made measured-depth thickness less and less informative on its own.

Why TST Differs From What the Driller Measures

Hold a deck of playing cards flat in front of you. Stack them slightly. The deck is about 4 centimetres thick measured straight up and down. Now tilt the deck 45 degrees and stick a wooden skewer straight down through it. The skewer enters the top card, passes through the deck, and exits the bottom card. The length of skewer inside the deck is much longer than 4 centimetres because the skewer is crossing the cards at an angle. If you wanted to know how many cards thick the deck is, you cannot just measure the skewer length; you have to correct for the tilt.

Wells through tilted rocks have the same problem. The driller measures how much wellbore is inside a particular rock layer by tracking the bit position. That measurement is the length of well inside the rock, not the thickness of the rock. To get the thickness of the rock (which is what tells you how much oil it could hold per unit area), you have to correct for the angle between the wellbore and the bedding planes.

The correction is most dramatic in horizontal wells. A horizontal well drilled deliberately along the bedding plane is staying inside one specific rock layer. The well might run 2,500 metres laterally inside a 6-metre-thick layer. The measured depth through that rock is 2,500 metres. The true stratigraphic thickness is 6 metres at most, possibly much less depending on exactly how parallel the well stays to the bedding.

How TST Is Calculated and Used

The standard formula for TST takes three inputs: the measured-depth thickness through the formation, the formation dip and azimuth, and the well's deviation (inclination and direction at the depth in question). The output is a vector calculation that gives the thickness perpendicular to the bedding plane. Modern petrophysics software (Techlog, Geolog, IP) computes TST automatically from logs and survey data. Geologists working on paper sometimes use simple cosine corrections that work when the well is vertical and the dip is the only correction needed.

The TST goes into reserves calculations. The Society of Petroleum Engineers reserves classification framework, the Canadian Oil and Gas Evaluation Handbook (COGEH) used by Canadian reserves evaluators, the SEC's Modernization of Oil and Gas Reporting rules, and the equivalent regulatory frameworks in Norway, Australia, and the Middle East all expect reserves calculations to use TST rather than measured-depth thickness for volumetric estimates. Auditors regularly find errors where reserves engineers have used the wrong thickness in their calculations, and the corrections sometimes shift booked reserves by tens of millions of barrels.

TST also drives well planning. When designing a horizontal well, the engineering team plans the lateral to maximize the time the well stays inside the productive layer's TST. A well that wanders out of the TST band and into adjacent (less productive) rock loses production for every metre spent outside the target.

True stratigraphic thickness is abbreviated TST. The contrasting terms are measured depth (MD) thickness, true vertical thickness (TVT, sometimes called true vertical depth thickness), and apparent thickness. Related terms include dip (the angle of inclination of a rock layer relative to horizontal; the parameter that drives the difference between TST and TVT or between TST and measured-depth thickness), measured depth (MD, the length of wellbore from surface to a given point along the well path; the raw measurement that must be corrected for deviation and formation dip to produce TST), true vertical depth (TVD, the vertical distance between two points in a wellbore, calculated from the directional survey; one of the inputs needed to correct measured depth to TST), net pay (the cumulative TST of rock that meets cutoff criteria for porosity, permeability, water saturation, and shale volume; the thickness used directly in volumetric reserves calculations), and reserves (the volumetric estimate of recoverable hydrocarbons in a reservoir; calculated using TST as the thickness term in the standard volumetric formula).

Why a Two-Hundred-Metre Difference Hides In a Wrong Number

A reserves engineer is auditing the booked reserves on a Bakken horizontal well in southern Saskatchewan. The well drilled 1,650 metres of lateral inside the Middle Bakken target. The original reserves report uses 1,650 metres as the "thickness" for the volumetric calculation. The calculated original-oil-in-place is 6.2 million barrels.

The auditor recalculates using true stratigraphic thickness. The Middle Bakken at this location has a TST of about 5.4 metres based on logs from the vertical pilot hole. The correct volumetric calculation should multiply the lateral length by the formation TST and the relevant porosity and saturation values, not the lateral length by itself. The corrected original-oil-in-place comes to about 380,000 barrels, not 6.2 million. The original calculation overstated reserves by a factor of roughly 16.

The error is not unusual on first attempts at unconventional reservoir reserves estimates. The fix is straightforward when caught: rebuild the calculation using TST as the thickness term, separate the lateral length factor (which determines how much TST-area the well drains, not how thick the rock is), and present the volumetric in a form that satisfies COGEH or SEC reporting rules. The well in question still has a sensible reserves estimate after the correction. It is just much smaller than the original number suggested. The lesson is that thickness is not just a length measurement: it is a specific geometric quantity, and using the wrong version of that quantity in a reserves calculation produces consequences that can run into the hundreds of millions of dollars when summed across a portfolio.