isopach map

An isopach map illustrates thickness variations within a tabular unit, layer or stratum.

An isopach map is similar to an isochore map, but these terms actually describe different methods of displaying thickness variations within a layer.

An isopach map displays lines of equal thickness in a layer where the thicknesses are measured perpendicular to the layer boundaries.

These lines look a bit like the isobars on a weather map, but the lines are isopachs, and the map an isopach map.

Isopach maps are utilized in hydrographic survey, stratigraphy, sedimentology, structural geology, petroleum geology and volcanology.

In cases where patch reefs are expected, isopach maps would be less useful and the reefs would have to be identified where possible on seismic.

It is also possible to construct facies and isopach maps from logs and using these maps, predictions can be made about the distribution of potential reservoir rocks.

To the north of the Uinta/Cottonwood arch during the Sevier orogeny there was a basement high area gently dipping to the north identified by isopach maps.

Prediction of porosity in the Z3 Carbonate can be done in the same way as for the Z2 Carbonate, using either facies or isopach maps (Figs. 4c).

Screenshot of an Isopach map generated by reservoir modeling software for an 8500 ft deep oil reservoir 28 ft thick located in the Erath field, Vermilion Parish, Erath, Louisiana.

(To digress for a moment, it's a curious fact that most isopach maps for pumice deposits reveal winds blowing generally in an easterly direction; the ash deposits are all concentrated to the east of the volcano.

Thus, an isochore and isopach map are the same only when both the top and bottom surfaces of the layer shown are horizontal (i.e., the true stratigraphic and true vertical thicknesses are the same).

Unfortunately the terms isopach and isochore are widely confused, and many times maps of True Vertical Thickness (TVT), which by definition are isochore maps, are incorrectly labeled isopach maps.

When the layer shown is inclined, as is usually the case, the thicknesses displayed in an isochore map of the layer will be greater than the thicknesses displayed in an isopach map of the same layer.

Using a knowledge of facies distribution, it is possible to predict where these potential reservoirs might occur and as facies distribution can be related to the overall thickness of each carbonate unit, it is also possible to make tentative predictions of porosity distribution from isopach maps.

Nevertheless, in areas where a carbonate platform is developed, as in N.E. England and the southern North Sea, a similar model to that applied to Z2 and Z3 Carbonates presumably can be used, and it should be possible to make tentative predictions of porosity distribution from isopach maps (Fig. 4a).