Contour Map: Visualizing Subsurface Structure in Petroleum Exploration

What Is a Contour Map?

Contour map (also called a structural map or isoline map) is a two-dimensional representation in which lines of equal value connect points of identical elevation, depth, thickness, or reservoir property, allowing geoscientists and engineers to visualize the three-dimensional shape of subsurface formations from well log and seismic data. In petroleum exploration, contour maps reveal structural highs that may form hydrocarbon traps, delineate reservoir thickness, and quantify the spatial distribution of petrophysical properties such as porosity and net pay across a field.

Types of Contour Maps in Petroleum Geology

The most fundamental map in petroleum geology is the structure contour map, which plots lines of equal subsea depth to a picked formation top or seismic horizon. Each contour line represents points at which the formation occurs at the same depth below sea level. Closed contour loops at the shallowest depths define structural closures — potential hydrocarbon traps — and the area enclosed by the lowest closing contour (the spill point) defines the maximum trap area. Geoscientists construct these maps by hand-contouring well control combined with two-dimensional or three-dimensional seismic interpretation, with modern workflows performed entirely in subsurface interpretation platforms such as Petrel, Kingdom, or OpenWorks.

Isopach maps contour the true stratigraphic thickness of a formation — the perpendicular distance between the top and base surfaces measured normal to bedding. Isochore maps contour the true vertical thickness, which is the vertical distance between the same two surfaces. In flat-lying strata the two are identical, but in dipping formations the isochore is always thicker than the isopach by a factor of 1/cos(dip angle). Reservoir engineers and geologists use isopach maps for volumetric calculations because true stratigraphic thickness correctly represents the rock volume available for fluid storage. Net pay isopach maps apply additional log-derived cutoffs — typically minimum porosity, maximum water saturation, and minimum net-to-gross — to isolate only the intervals that will contribute to production.

Beyond structural and thickness maps, the discipline encompasses a broad suite of petrophysical property maps: average porosity maps, permeability maps derived from core-log transforms, water saturation maps, and net-to-gross ratio maps. These are contoured at the formation or flow-unit level and loaded into reservoir simulation models as input property grids. Regional maps covering basin-scale trends — such as source rock total organic carbon (TOC) maps, thermal maturity (Ro) maps, and pressure maps — guide exploration screening by identifying the most prospective areas before committing to seismic acquisition.

Contour Map Synonyms and Related Terminology

Contour map is also referred to as:

• Structure map — industry shorthand specifically for a structure contour map showing depth to a formation top
• Isoline map — the general cartographic term for any map contoured on a single variable; used in academic literature
• Depth map — common usage in seismic interpretation workflows referring to a time-to-depth converted structure contour map
• Grid map — refers to the digital representation of a contoured surface as a regular x-y grid of z-values, the standard format in subsurface software

Related terms: isopach, structure contour map, closure, spill point, gross rock volume, net pay, formation top

Frequently Asked Questions About Contour Maps

How is contour interval chosen on a petroleum structure map?

Contour interval is selected based on two factors: the density of control data and the vertical relief of the features to be mapped. On a regional exploration map with well spacing of tens of miles, a 200–500 ft interval is typical because well picks are not accurate enough to justify finer resolution. On a development-stage reservoir map with wells spaced at 40–160 acres, intervals of 20–50 ft are common and can be supported by the data. A general rule is to use an interval small enough that at least 4–6 contour lines appear across the main structural feature of interest. Using too fine an interval with sparse data produces spurious detail; too coarse an interval masks real structural complexity.

What is the difference between a time structure map and a depth structure map?

Seismic data is recorded in two-way travel time (milliseconds), so the first structural interpretation product is a time structure map showing the two-way time to a horizon. To convert this to a depth structure map — which is what drillers and reservoir engineers need — geoscientists apply a velocity model that transforms time values to depth. Velocity varies with lithology, compaction, and pore pressure, making the time-to-depth conversion one of the largest sources of uncertainty in pre-drill trap evaluation. Depth uncertainty of ±50–200 ft on a prospect at 10,000 ft TVD is common even with good seismic and nearby well control.

How do contour maps define a hydrocarbon trap?

A trap is defined on a structure contour map by a closed contour — a contour line that completely encircles a structural high without opening at the map edge. The highest closed contour defines the spill point, which is the deepest level at which hydrocarbons can be retained before migrating to an adjacent low or spilling out of the trap entirely. The vertical distance from the crest to the spill point is the closure relief, and the area enclosed by the spill-point contour multiplied by the net pay thickness gives the gross rock volume used in volumetric reserve estimates. Fault-bounded traps require the fault plane map to be combined with the structure map to identify the closing geometry.

Why Contour Maps Matter in Oil and Gas

Contour maps are the foundational communication tool of the subsurface team, translating complex three-dimensional geological and engineering data into a format that geoscientists, engineers, economists, and executives can interpret jointly. Every volumetric reserve estimate, every well location decision, and every field development plan rests on a set of contour maps describing the trap geometry, reservoir thickness, and property distribution. As the oil and gas industry has shifted toward more complex resource plays and deepwater prospects, the precision and uncertainty quantification of contour mapping have become increasingly critical — modern probabilistic reserve assessments generate hundreds of realizations of structure and isopach maps to capture the full range of closure and thickness uncertainty rather than relying on a single deterministic map.