geology · structural
Structural Closure (Trap Height)
Closure = Spill Point Depth − Crest Depth
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Inputs
ft
ft
Description
Computes structural closure — the vertical distance from a trap's structural crest (apex) down to its spill point, i.e. the maximum possible hydrocarbon column height the trap geometry alone can support. Both the crest depth and spill point depth are read from a structure contour map or cross-section; this calculator makes the resulting closure relationship explicit rather than requiring it be read off a map by eye.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Hc | ft | Structural Closure (Trap Height) |
| zc | ft | The subsea (or other consistent datum) depth of the structural apex — the shallowest point on the mapped horizon, i.e. the crest of the trap. |
| zs | ft | The depth of the trap's spill point — the elevation of the lowest structural contour that still closes around the crest (the saddle where the structure opens into the regional dip or an adjacent structure), read directly from a structure contour map. |
Assumptions
- Crest depth and spill point depth were both picked from the same horizon, on the same depth datum (e.g. both subsea true vertical depth).
- The spill point used is the trap's actual lowest closing contour, already determined from a structure contour map or 3-D interpretation — this calculator does not derive the spill point itself, only the resulting closure once both depths are known.
- Closure represents the maximum possible column height from structural geometry alone; it does not account for seal capacity, charge availability, or fluid contacts that may leave the trap only partially filled.
Limitations
- Purely a subtraction of two independently determined depths — the accuracy of the result is entirely dependent on the accuracy of the crest and spill point picks, which this calculator cannot itself validate.
- Does not compute the spill point or the areal closure map (the polygon within the lowest closing contour) — both require a full structure contour map or 3-D surface and are not reducible to a single scalar formula; use dedicated mapping/GIS software for those steps.
- A trap can be structurally closed (closure > 0) yet still be dry or only partially filled if seal capacity, charge, or fluid contacts limit the actual column — closure is a geometric ceiling, not a guarantee of fill.
Use Cases
- → Screening a prospect's maximum column height: Before running detailed volumetrics, quickly establish the structural ceiling on hydrocarbon column height once crest and spill depths are picked from a contour map.
- → Comparing multiple fault-bounded compartments: Rank several fault blocks or structural compartments in a field by their available closure to prioritize which are most likely to hold a significant column.
Related Calculations
Region Notes
Global
Always confirm the spill point depth comes from the correct saddle for the horizon and fault configuration being evaluated — a structure can have more than one candidate spill point (e.g. one down-dip, one across an adjacent fault), and the shallowest of these actually controls the trap's closure.
References
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