geology · formation tops
Target Window Thickness
Thickness = Base of Target Depth − Top of Target Depth
click formula to derive ↑
Inputs
ft
ft
Description
Computes the vertical thickness of the target window available for landing and staying in a horizontal well — the interval bounded by the top and base structural/stratigraphic picks of the intended target zone. A simple, direct application of the same top-minus-base interval-thickness relationship used elsewhere in this discipline (e.g. isopach_isochore_conversion), applied here to the specific well-planning question of how much vertical room a landing target actually provides.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Hw | ft | Target Window Thickness |
| ztop | ft | The depth of the structural/stratigraphic pick bounding the top of the intended landing target zone, on a consistent datum (e.g. subsea true vertical depth). |
| zbase | ft | The depth of the structural/stratigraphic pick bounding the base of the intended landing target zone, on the same datum as the top pick. |
Assumptions
- Both the top and base picks were made on the same correlative structural/stratigraphic surfaces and the same depth datum.
- The target window is treated as a simple vertical interval at the point of interest — it does not itself account for how that interval's thickness or structural attitude may change along the planned lateral (use structure_gradient or target_window_thickness at multiple points along the planned path to track that).
- The top/base picks already reflect whatever operational tolerance (e.g. distance from a fluid contact) the well plan requires — this calculator does not itself impose or validate any operational safety margin.
Limitations
- Purely a subtraction of two independently determined depths, the same limitation structural_closure carries — the result is only as accurate as the two picks feeding it.
- Reports a single-point thickness only; real target windows commonly thin or thicken along a planned lateral (the same structural-thinning problem published geosteering case studies routinely encounter), so a single evaluation at the vertical/pilot-hole location does not guarantee the window stays open for the full lateral length.
- Does not itself account for wellbore inclination or the horizontal component of staying within the window while geosteering — it reports the available vertical thickness only, at a single vertical section.
Use Cases
- → Pre-drill landing-zone screening: Quickly check how much vertical room a proposed target zone offers before committing to a landing point, using offset-well or seismic-derived top/base picks.
- → Comparing candidate target zones: Rank multiple candidate benches or intervals in a stacked-pay setting by available window thickness to prioritize the most forgiving target for geosteering.
Related Calculations
Region Notes
Global
Published geosteering case studies from structurally complex plays commonly report target windows as tight as 15-25 ft (roughly 4.5-7.6 m) over laterals of 10,000 ft or more — a computed window near or below that range is worth flagging for closer structural-uncertainty review before finalizing a landing plan.
References
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