Reading Contour Maps

One important skill that an exploration geologist must acquire is an ability to interpret contour maps representing the spatial distribution of various properties in the subsurface. Although many people are familiar with contour maps in the form of topographic maps representing land surface elevation, geologists use contour maps to represent both structural characteristics of the subsurface, such as the elevations and thicknesses of formations, and distributions of rock and fluid properties. For example, a geologist may estimate vertically averaged porosity of a certain formation at a number of wells and then prepare a contour map representing the geographic variation of the average porosity by interpolating from the well data. In the Small County exercise you will be using contour maps of formation top elevations and formation thicknesses to guide your selection of promising locations for drilling exploratory wells. Contour maps of formation thickness are often called isopach maps.

A contour map is a two-dimensional representation of a three-dimensional surface or function. The contours connect points of equal "elevation" or value on the surface. Here is a 3D representation of a surface together with its representation in a set of contours, at the bottom of the figure:

3D surface with contours

This could represent the elevation of the top of a formation in Small County, in feet above sea level. The contour map of this surface is simply a view of the bottom portion of this figure, from straight above:

Contours of 3D surface

Unfortunately, a geologist cannot see and measure the entire surface or property that he or she is mapping. Instead, subsurface geological maps are prepared from scattered measurements, typically measurements made in wells. These scattered measurements must be interpolated in order to prepare a contour map. Interpolation is the process of estimating values at unmeasured locations by averaging values measured at surrounding wells or observation locations. Usually, this is a weighted average, with values measured at nearby wells weighted more heavily than values at more distant wells. Although there are many ways to compute these averaging weights, most of these approaches give very similar interpolated estimates and have similar characteristics, namely:

The most important point to keep in mind is that an interpolated contour map represents an estimate, not the truth. As you add more wells in the Small County exercise, the various contour maps will be updated to reflect the observations in these wells, so that these maps will gradually provide more accurate estimates of the true elevations, thicknesses, and property values in the subsurface.