contour interval, Oil & Gas Glossary

What Is a Contour Interval?

Contour interval (also called map contour spacing or contour step) is the constant vertical distance — measured in feet or meters of depth or elevation — between successive contour lines on a subsurface structure map, isopach map, or other geological contour map. Selected by the interpreting geoscientist before drafting begins, the contour interval governs the resolution and apparent precision of the interpretation: a small interval reveals fine structural detail while a large interval simplifies regional patterns and avoids over-interpreting sparse well control data.

Key Takeaways

Contour interval is the fixed vertical spacing between adjacent lines on a subsurface map — every line represents one interval of depth or elevation change.
Exploration-scale structure maps commonly use 50 to 200 ft contour intervals; field development maps use 10 to 25 ft to support precise wellbore placement.
A contour interval must be smaller than the structural closure being mapped — a 100 ft interval cannot resolve a 75 ft four-way closure.
Index contours (every fifth line, drawn bold) allow quick visual counting of structural relief without reading every label.
Contour interval selection depends on three factors: density of well and seismic control, total structural relief of the feature, and the operational purpose of the map.

How Contour Interval Is Selected in Subsurface Mapping

The geoscientist chooses a contour interval before the first line is drawn, because once the interval is set it determines what features the map can and cannot display. The primary constraint is data density. When well control is sparse — one well per township in a frontier basin, for instance — drawing 10 ft contours implies a level of precision the data cannot support. Most explorationists follow the rule that contour interval should not be smaller than the combined uncertainty of the depth picks and the structural elevation differences between control points. In practice this means basin-scale reconnaissance maps use 200 to 500 ft intervals, regional play maps use 50 to 200 ft, and single-field maps use 10 to 50 ft.

Structural relief governs the upper bound on contour interval. If a prospect has 300 ft of four-way dip closure, a 50 ft interval generates six contour lines enclosing the trap — enough to define shape, crestal position, and spill point with confidence. The same prospect mapped at a 100 ft interval produces only three enclosing lines, making crest location uncertain by half a contour interval (50 ft) and spill point definition problematic. The general guideline is that at least four to six contour lines should enclose any structural feature the map is intended to define. Features smaller than two contour intervals in relief are unresolvable and should not be drawn.

The operational purpose of the map shifts the decision toward finer or coarser intervals. A wildcat exploration map identifying drillable prospects on a regional seismic grid can tolerate a 100 ft interval because the question being answered is whether a closure exists, not where exactly to perforate. A horizontal well placement map for a tight oil play needs a 10 to 25 ft contour interval because a lateral placed 20 ft up-dip from the optimal landing zone may never encounter the best reservoir facies. Development drilling decisions — infill well locations, lateral landing targets, waterflooding pattern geometry — routinely justify the time and cost of tight-interval remapping as more wells provide additional depth control.

Fast Facts: Contour Interval

Typical exploration interval: 50–200 ft on structure maps; 500 ft on basin reconnaissance maps
Typical field development interval: 10–25 ft for horizontal well placement
Index contour frequency: Every fifth contour line drawn bold for easy visual counting
Minimum closure rule: Structural closure must exceed the contour interval to be mappable as a closed feature
Isopach maps: Contour interval measured in net pay feet or gross thickness feet, not elevation
Metric equivalent: 25 ft ≈ 8 m; 100 ft ≈ 30 m; 200 ft ≈ 60 m
Seismic vertical resolution limit: Typically 25–50 ft (one-quarter wavelength) — finer contour intervals than this exceed seismic data resolution
Software default: Most mapping packages (Petrel, Kingdom, IHS Petra) default to auto-calculated intervals — always override with a geologically reasoned value

Mapping Tip:

Before finalizing a contour interval, calculate the total structural relief of the primary target and divide by six. That result is the coarsest interval that will still enclose the feature with enough lines for meaningful interpretation. If the data density does not support that fine an interval, the prospect is under-controlled and additional well data or reprocessed seismic may be needed before committing to a drill location.

Contour interval is also referred to as:

Contour spacing — informal term used interchangeably in geological and engineering reports, emphasizing the visual distance between lines rather than the vertical value.
Contour step — common in European and international usage; refers to the same fixed vertical increment between successive lines.
Map interval — shortened form used in internal well planning documents and memoranda when the context is clear.
Isoline interval — used when discussing any type of contour map (isopach, isochron, pressure, temperature) rather than structure maps specifically.

Frequently Asked Questions About Contour Intervals

Why can't I just use the smallest possible contour interval for maximum detail?

Using an interval finer than what the data supports creates a false impression of precision. If your depth control points are accurate to plus or minus 30 ft, drawing 10 ft contours implies the surface is known to that resolution — it is not. Interpolated contours between sparse control points are interpretive, not measured. Overly fine intervals can mislead engineers into placing wellbores based on apparent structural features that exist only in the interpolation algorithm, not in the subsurface. Always match the interval to the actual uncertainty of your control data.

What is the difference between a contour interval on a structure map versus an isopach map?

On a structure map the contour interval represents vertical depth or elevation — each line connects points of equal subsea depth (for a depth-domain map) or equal elevation above sea level (for a shallow formation map). On an isopach map the contour interval represents thickness — each line connects points where the mapped interval is equally thick. Both use the same visual convention, but the geological meaning is different: structure maps define trap geometry while isopach maps define reservoir volume distribution. A single well map often shows both, with structure map contours in one color and isopach contours in another.

How does a coarse contour interval affect volumetric estimates?

Coarse contour intervals introduce uncertainty in spill point depth, closure area, and bulk rock volume calculations. When the spill point depth is uncertain by one contour interval (say, 100 ft), the hydrocarbon column height is uncertain by up to 100 ft, which can translate to tens of millions of barrels of uncertainty in the gross rock volume. This is why proved reserve booking typically requires tight-interval structure maps built from multiple wells with confirmed fluid contacts, not just exploration-phase interpretations with 200 ft intervals. Reserve auditors commonly request the contour interval and control point locations as part of their documentation review.

Why Contour Intervals Matter in Oil and Gas

The contour interval is one of the most consequential choices a geoscientist makes when building a subsurface map, because every downstream decision — well location, wellbore trajectory, completion interval, reserve estimate, and development plan — rests on the map's fidelity to the actual subsurface. An interval that is too coarse misses real structural features and may cause a well to miss the crest of a structure. An interval that is too fine relative to data density creates fictitious detail that misleads drillers and reservoir engineers. Getting the contour interval right is not a cosmetic decision; it is a fundamental part of the interpretation workflow and directly affects the quality of the geological model that guides capital allocation in every oil and gas basin worldwide.