Building a structural framework: from picks to 3D model

The three core deliverables

A structural framework, as produced by a typical interpretation team, consists of three products:

• Time-structure maps: contour maps of the two-way time to each picked horizon. The contours show the shape of the horizon surface as if you were standing above it looking down. Structural highs (domes, anticlines) appear as closed contour "bullseyes"; lows (basins, synclines) appear as inverted bullseyes.
• Isochron maps (or isochore if depth-converted): contour maps of the THICKNESS between two horizons. Thin areas (tight contours) indicate condensation, non-deposition, or erosion; thick areas (wide contours) indicate accommodation space from subsidence or basin depocenters.
• Fault polygons: the map-view traces of each fault at each horizon level, often displayed as filled polygons showing the fault's horizontal projection. Faults are 3D surfaces and each horizon intersects the fault at a different map position; the polygon shows this intersection.

Together these three deliverables form a complete 3D structural model. Every downstream analysis — volumetrics, well planning, reservoir modeling — starts from this framework.

Reading a time-structure map

A time-structure contour map shows isolines of TWT on the horizon surface. Reading it:

• Closed contours (roughly circular or elliptical) indicate a dome or a basin. If the center has LOWER TWT than the edges, it is a structural HIGH (shallower than surroundings) — a candidate trap. If the center has HIGHER TWT, it is a structural LOW (deeper).
• Contour spacing indicates slope. Tightly-spaced contours = steep dip; widely-spaced contours = gentle dip. Uniform contour spacing over a broad region = flat regional tilt.
• Linear contour disruptions indicate faults. Contours abruptly offset across a line means the horizon is cut by a fault at that location, with the magnitude of offset equal to the fault throw.
• Contour "noses" indicate plunging folds. An elongate set of partial-closure contours pointing off the edge of the map is a plunging anticline or syncline that hasn't fully closed within the mapped area.

Closure and spill point — the trap fundamentals

Every structural trap is defined by two key features on the contour map:

• Closure: a closed contour encircles a region that traps hydrocarbons. The highest point of the closed contour is the structure CREST — where a vertical well would first enter the trap.
• Spill point: the LOWEST TWT at which the closure is still closed. Below this depth, the contour "spills" (opens at a saddle or map boundary, letting hydrocarbons escape). The spill point sets the maximum vertical column the trap can hold.

Closure relief = spill TWT − crest TWT. This is the vertical height of the closure. A large closure relief (e.g., 100 ms ≈ 150 m at typical velocities) means a big trap capacity; a small closure relief (e.g., 10 ms) means a tiny trap even if the dome is laterally extensive.

The widget below lets you see closure and spill-point analysis on a synthetic dome with a single fault. Slide the parameters to see how closure changes.

Exercise — map a structural trap

• The widget starts with a 200 m amplitude dome east of a normal fault, seen in map view (top, with contour bullseyes) and cross-section along A-A' (bottom). The yellow contour highlights the CLOSURE and the dashed yellow line in section shows the SPILL POINT.
• Slide Dome amp down from 200 to 50 m. Watch what happens to the closure: at smaller amplitudes the dome barely rises above regional tilt, and the closure shrinks or disappears. The verdict will switch to "marginal" or "no closure" — a real prospect-killer in interpretation.
• Slide Dome amp back up to 300 m. The closure is now huge: deep trap relief, robust four-way closure. Crest is shallow, spill is deep, closure relief is large.
• Slide Fault throw from 0 m to 200 m. With zero throw, the fault has no structural expression — the dome produces four-way closure independently. At 200 m throw, the fault becomes significant; the closure pattern becomes a THREE-WAY fault-assisted closure (fault on one side, dome contours on the other three). Note that SGR analysis (§3.2) would also apply to the fault seal.
• Slide Fault pos to different x values. When the fault is east of the dome, it doesn't interact with the trap. When it cuts through the western flank, it creates fault-dependent closure. When it cuts through the crest, it can split the dome into two compartments.
• Slide Section Y to move the cross-section line up and down the map. The A-A' cross-section updates live to show the horizon profile + fault + trap fill at that Y position. This is exactly the workflow of real interpretation: you look at the map, pick a cross-section line across a feature, and see how it looks in profile.
• Look at the stats panel. "Closure relief" is the number that matters most for trap volumetrics. Multiply it by the trap area (estimated from map) and by the reservoir net-to-gross and porosity to get a hydrocarbon-in-place estimate. (Volumetrics is the topic of §3.6.)

Three kinds of trap

Structural traps fall into three broad categories based on what closes them:

• Four-way structural closure: the trap is closed on ALL sides by the horizon's own topography — a dome. No faults involved. Safest structural trap because it doesn't depend on fault sealing. Example: Ghawar (Saudi Arabia), Forties (North Sea), most Gulf of Mexico salt-related fields.
• Three-way closure (fault-assisted): the horizon closes on three sides by topography and on the fourth side by a fault. The fault must seal for the trap to hold — SGR analysis (§3.2) determines this. Common in extensional basins like the North Sea (many Brent-play fields).
• Fault-only closure: the horizon has a regional dip away from a high point, with NO structural dome. Closure depends entirely on the fault. A stratigraphic or lithological pinchout against the fault may be involved. Higher risk: two things must work (the fault must seal AND the reservoir must pinch out cleanly).

In practice, a single prospect often combines elements of all three — a dome with faults cutting across, producing a mix of four-way and fault-assisted closures. Evaluating each compartment separately is the interpreter's job.

From framework to prospect

A complete structural framework lets you answer the questions that drive prospect evaluation:

• Where is the crest? (Where to drill the appraisal well.)
• Where is the spill point? (What hydrocarbon column can the trap hold?)
• What is the trap area? (At what depth contour is the closure defined, and what lateral extent does that contour enclose?)
• What faults bound the trap? (Which faults must seal for the trap to work, and what is the SGR of each?)
• How does the structure plunge? (Will the trap extend laterally into mapped or unmapped area?)
• Are there sub-seismic faults that could compartmentalize the reservoir? (Based on fault population statistics from observable larger faults.)

Each of these questions is answered by inspecting the framework, not by measuring any single pick. The framework is the interpretive synthesis.

Common framework-building pitfalls

• Incorrect contour interval. Choose a contour interval that matches the structural amplitude. A 10 ms interval for a 30 m dome gives 3 contours — not enough to see the structure. A 2 ms interval for the same dome gives 15 contours — excellent. Adjust the interval until the structure is well-sampled.
• Extrapolating beyond the data. Contour maps are interpolated from discrete horizon picks; away from pick coverage, they are inferred. Always look at the pick-coverage map before interpreting a contour feature — a "closure" could be an extrapolation artefact.
• Ignoring fault displacement effects on the map. When a fault cuts a horizon, the map should show contours OFFSET across the fault trace (by the throw). Maps that show smooth contours across faults are wrong — they haven't accounted for the fault displacement.
• Missing plunging features. A dome that plunges off the map edge may look like it has no closure when plotted within the mapped area. Widen the area coverage if the structure may extend further.
• Depth conversion errors. Time-structure maps are in TWT, not depth. Converting TWT to depth requires a velocity model. Small errors in velocity propagate into the depth map, changing the spill point and closure relief. Critical for volumetric calculations — always note whether a framework is in time or depth.
• Cross-section artefacts from arbitrary lines. A cross-section along an oblique line through a fault will show APPARENT fault dip shallower than the true dip. Choose section lines perpendicular to the mean fault strike for accurate dip measurement.