Exploration Insights February 2020 | Page 24

Exploration Handbook | 25
Subsurface Mapping: Understanding the
Subsurface Structural Disposition » Structure Contour Maps feature lines of equal
depth or elevation to a specific horizon or formation
in the subsurface. The common reference datum for
structure contour maps is mean sea level.
By: Georgina Wright » Topographic Maps are quantitative representations of
surface relief. They demonstrate contour lines of equal
elevation from mean sea level.
WHAT IS A SUBSURFACE MAP?
A subsurface map is a plane surface representation of geological features beneath the Earth’s surface,
typically in plan view or a horizontal projection. These maps detail the depth, thickness, distribution, and
structure of rock layers in the subsurface.
One of the most common types of subsurface map is contour maps. These can be constructed
in depth, where each contour line represents a point of equal elevation or depth, above or below a
reference datum. Alternatively, they may be represented in seismic two-way travel time, which can be
used as a proxy for depth. Other map types, such as isopachs and isochrons, are described in more
detail in the next section of this article.
An example of a highly simplified, subsurface depth contour map is shown in Figure 1.
» Isopach Maps comprise lines of equal thickness,
representing the true stratigraphic thickness of a
formation or rock layer.
» Isochore Maps feature lines of equal thickness,
representing the true vertical thickness of a formation
or rock layer.
» Isochron Maps are the seismic equivalent of isochore
maps, featuring lines of equal seismic travel-time to
transit a formation or rock layer.
APPLICATIONS OF SUBSURFACE CONTOUR
MAPS
-100m
-110m
-120m
-130m
-140m
-150m
Reservoir Unit
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Subsurface contour maps are critical for the evaluation of
complex subsurface features. They allow geoscientists to
create and share information about the subsurface, making
them some of the most important tools used to explore for
hydrocarbons, and develop proven reserves (Tearpock and
Bischke, 2002). They help geoscientists answer questions,
such as:
» Where are the major depocenters?
» Is a prospective source rock buried deeply enough to
be thermally mature?
» At what depths are the reservoir targets? Are porosity
and permeability likely to be preserved?
Contour maps, therefore, have a number of different uses:
Line of cross section
» To define subsurface structures and outline their
extents
-120m
-130m
-140m
-150m
Figure 1> Schematic cross section and structure contour map showing present-day subsurface stratigraphy. Contours represent
depths below mean seal level to the top of the reservoir unit.
CONTOUR MAP TYPES
Subsurface contour maps come in a variety of formats, and each map depicts something different
about the subsurface structure and stratigraphy. Examples include:
» To provide depth predictions of key geological horizons
or intervals of interest, such as top reservoir
» To screen the prospectivity of an area objectively,
through play fairway evaluation, by assessing,
for example, source rock maturity and reservoir
effectiveness
» To provide inputs to basin models to help understand
the subsurface thermal regime
» To assist with the calculation of gross rock volume
(GRV) for use in estimating stock tank oil initially in
place (STOIIP)
“A subsurface map
is a plane surface
representation of geological
features beneath the
Earth’s surface, typically in
plan view or a horizontal
projection.”