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 contours around a low value, a bullseye; lows (basins, synclines) as closed contours around a high value.
• Isochron maps (or isochore maps if depth-converted): contour maps of the thickness between two horizons. Low values mark thin intervals (condensation, non-deposition or erosion) and high values thick ones (accommodation from subsidence, depocentres); tightly spaced contours mark a rapid change in thickness.
• Fault polygons: for a normal fault, the strip on each horizon's map between the footwall cutoff, where the horizon meets the fault on the upthrown side, and the hangingwall cutoff on the downthrown side. Inside it the horizon is missing. Its width is the heave, $H = T/\tan\delta$ for throw $T$ and fault dip $\delta$: 46 m for 80 m of throw on a 60° fault. Each horizon cuts a fault at a different place, so every horizon has its own polygons.

Together these three deliverables form a complete 3D structural model. Every downstream analysis, volumetrics, well planning, reservoir modelling, 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 centre has a lower TWT than the edges, it is a structural high (shallower than its surroundings), a candidate trap. If the centre has a higher TWT, it is a structural low.
• Contour spacing indicates slope. Tightly spaced contours mean steep dip; widely spaced contours mean gentle dip. Uniform spacing over a broad region is a flat regional tilt.
• Contours that stop at a line and resume on the other side with values that jump indicate a fault. The jump in value across the fault polygon is the throw, in time.
• Contour noses indicate plunging folds. An elongate set of partial contours pointing off the edge of the map is a plunging anticline or syncline that does not close within the mapped area.

Closure and spill point, the trap fundamentals

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

• Closure: a contour that closes around a structural high. The crest is the highest point inside it, where a vertical well would first enter the trap.
• Spill point: the place where the trap first opens. The deepest contour that still closes around the crest (the largest TWT) is the spill contour; just below it the contour opens, at a saddle, at the edge of the mapped area, or where the reservoir meets a fault that lets oil across, and that opening is the spill point. Oil added beyond it escapes there, so the spill point sets the tallest column the trap can hold.

The closure is the height between them, $\Delta t = t_{\mathrm{spill}} - t_{\mathrm{crest}}$ in time and $\Delta z = V\,\Delta t/2$ in depth: 100 ms of time closure is 150 m at 3000 m/s. A large closure means a trap that can hold a tall column; 10 ms is a small trap however wide the dome.

The figure below builds a framework from one horizon and one normal fault, and fills the dome beside the fault until it spills. Plate (a) is the time-structure map, (b) a section along the line A to A′, and (c) the fault plane seen from the east. Change the fault's throw and its fault rock, and the sand's thickness, and watch where the oil escapes.

Juxtaposition: what faces the sand

In its opening state the dome holds a 124 m column, 99 ms of time closure. Its closing contour reaches the fault at 1567 ms, where 80 m of throw is less than the 100 m sand, so part of the sand faces sand across the fault and the oil leaks into the footwall. With sealing fault rock the same dome would hold 166 m, about 43 m more: that is what the fault seal is worth.

Wherever a closing contour meets a fault, the question is what lies across the fault from the reservoir. If the throw is greater than the sand's thickness, the sand faces shale on the other side, and oil cannot cross even through permeable fault rock: a juxtaposition seal. If the throw is smaller, part of the sand faces sand, and oil crosses through that window unless the fault rock itself seals it, by clay smear or crushed grains, which is what the shale gouge ratio of Section 3.2 estimates. An Allan diagram, plate (c), draws both sands on the fault plane so the windows can be seen (Allan, 1989).

Raise the throw in the figure to 120 m and the column grows to 160 m with no help from the fault rock. Thicken the sand to 150 m at the same throw and it falls to 136 m, because the thicker sand faces itself across the fault again. Move the fault west to 1.5 km and the dome closes by dip alone: 170 m, sealing or leaking.

Three kinds of trap

Structural traps fall into three broad categories, by what closes them:

• Four-way dip closure: the trap is closed on all sides by the horizon's own shape, a dome or an anticline. Its column does not depend on any fault, which makes it the lowest-risk structural trap. Ghawar (Saudi Arabia) and Forties (North Sea) are anticlines of this kind.
• Three-way, fault-dependent closure: dip closes three sides and a fault the fourth. The trap holds only if the fault seals there, by juxtaposition or by its fault rock. Common in extensional basins, including many fault-block fields of the northern North Sea.
• Closure against two or more faults: where faults intersect, or a fault block is tilted, dip may close only one or two sides and faults the rest. Every bounding fault must seal, so the risks multiply. Where a stratigraphic pinch-out closes part of the trap instead, it is a combination trap.

In practice a single prospect often combines them, a dome with faults cutting across it, producing a mix of four-way and fault-dependent compartments. 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:

1. Where is the crest? (Where to drill the exploration well.)
2. Where is the spill point, and how does the oil escape there? (What column can the trap hold?)
3. What is the trap area? (What map area does the spill contour enclose?)
4. Which faults bound the trap? (Which must seal for the trap to work, and do they seal by juxtaposition or by their fault rock?)
5. How does the structure plunge? (Will the trap extend laterally into mapped or unmapped area?)
6. 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 structure. On a dome with 25 ms of time closure a 20 ms interval draws one or two closed contours, too few to see its shape; a 5 ms interval draws four or five. The interval also limits how precisely the spill is known: it lies somewhere between the last closed contour and the first open one.
• 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 on the map. When a fault cuts a horizon, the contour values must jump across the fault polygon by the throw. A map whose contours run smoothly across a fault trace has been gridded through the fault, turning an offset into a ramp.
• Missing plunging features. A dome that plunges off the map edge may look as if it has no closure within the mapped area. Widen the 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, and small errors in velocity propagate into the depth map, moving the spill point and changing the closure. For volumetrics, 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 shows an apparent fault dip shallower than the true dip. Choose section lines perpendicular to the mean fault strike for accurate dip measurement.