Geological Subsurface Maps Types of Geological Subsurface Maps Isochore and Isopach maps Types of subsurface maps

1. Geological Subsurface Maps
SubsurfaceMap can be defined as a geological map depicting geologic data or
features below the Earth's surface, especially a plan of mine workings, or a
structure-contour map of a petroleum reservoir or an underground oredeposit,
coal seam, key bed, faults, joints, or thickness etc.
Types of geological subsurface maps:
There are many types of subsurfacemaps, mostof these maps are prepared
by contouring or drawing lines at regular intervals representing depth or
thickness.
1. Thickness Maps:
Thickness maps representthickness variations and thickness trends of a given
unit. Isochoreand isopach maps are two different types of thickness map.
(i) Isopach Map:
Itis a contour map of equal values of true stratigraphic thickness. An isopach map
illustrates thickness variations within a tabular unit, layer or stratum. Isopach are
contour lines of equal thickness over an area.
Or we can simple say, an isopach is a line that connects points of equal True
thickness (i.emeasured perpendicular to bedding).
Isopach maps arealso referred to as True Stratigraphic Thickness (TST) maps.
Uses: Isopach maps areutilized in hydrographic survey, stratigraphy,
sedimentology, structuralgeology, petroleum geology and volcanology.
(ii). Isochore map:
An isochoreis a line that connects points of equal vertical thickness. Itdisplays
lines of equal thickness in a layer where the thicknesses are measured vertically.
They are also called as True Vertical Thickness (TVT) maps.

2. Note:the thicknesses displayed in an isochoremap of the layer will be
greater than the thicknesses displayed in an isopach map of the samelayer
2. Structural Maps and Sections:
A contour map that shows thesubsurfacefeatures of a reservoir or formation.
These maps clearly show faults, folds, and other geologic structures.
Itis a type of subsurfacemap whosecontours representthe elevation of a
formation, reservoir or geologic marker in space, such that folds, faults and other
geologic structures areclearly displayed. Its appearanceis similar that of a
topographic map, but topographic map displays elevations of the Earth's surface
Uses: Structuralmaps and sections are also examined for clues to crustal
movement and potential hazards, such as earthquakes, landslides, and volcanic
activity. Identification of fault lines can facilitate land useplanning by limiting
construction over potentially dangerous zones of seismic activity.

3. 3. GEOLOGICAL CROSS-SECTIONS:
A cross section allows us to take look at a portion of the crust. These maps show
both structuraland stratigraphic features of the Earth allowing us to predict
possibletraps and reservoir formations.
They representthe geologic data as maps but in the vertical view. There are
severaltypes of cross-sections, fromwhich somearementioned below,
(i). Correlation Cross-Sections:
They are the first figures to be drawn in the firstphase of exploratory drilling and
they enable the geologist to decide stratigraphic equivalences between the wells.
(ii). Structural Cross-Sections:
They show the present structuralaltitudes of rocks in relation to sea level as a
horizontaldatum.

4. (iii). Stratigraphic Cross Section:
They show the correlation of strata with respectto one of them selected as a
horizontaldatum.
4. Facies Map
Facies arebodies of sediment that are recognizably distinct fromadjacent
sediments that resulted fromdifferent depositional environments. Itis the total
characteristics of a rock including its chemical, physical, and biological features
that distinguishes it fromadjacent rock. Generally, geologists distinguish facies by
the aspectof the rock or sediment being studied rock.
Therefore, a map showing the distribution of differenttypes of rock attributes or
facies occurring within a designated geologic unit is called FACIES MAP.
GeologicCrossSectionsAcross the Mahomet Bedrock Valley

5. In other words, itcan also be defined as stratigraphic map indicating distribution
of sedimentary facies within a specific geologic unit.
5. Paleo-geologic Maps: Paleo-geologic maps were introduced by Levorsen in
1933
A map that shows the areal (relating to area) geology of an ancient surfaceat
some time in the geologic past, specially. such a map of the surfaceimmediately
below an unconformity (surfaceof contactbetween two groups of
unconformablestrata), showing thegeology as it existed at the time the surface
of unconformity was completed but beforethe overlapping strata were
deposited.

6. 6. Sub-crop Maps:
If rocks exposed at the present-day erosion surfacearereferred to as outcrops,
then now-buried rocks that wereexposed at ancient erosion surfaces arereferred
to as sub-crops.
Sub-crop maps show whatan area would look like if the topmostdeposits would
be removed. It is therefore very usefulfor people such as well drillers, who are
curious to know which rocks arepresent at a certain depth, instead of at the
surface.
7. Internal Property Maps:
Maps detect the characteristics of a single unit and its shape.
-Itincludes many types:
1. Iso-porosity maps.
2. Iso-volumeor iso-vol. maps.
3. Iso-concentration maps.
4. Iso-hydrocarbon maps.
5. Iso-bar maps
6. Iso-potentialmaps.
7. Water- encroachmentmap.
8. Geophysical Maps:
These maps depend on geophysicalanomaly (such as local variations or
irregularity in the normal pattern) which after correction may be attributed to
some geologic phenomena. –
Geophysicalsurveys arean important part of mapping bedrock, soil and
groundwater. Geophysics is also used for specific problemsettings such as,
bedrock conditions before an underground construction, mapping of natural

7. resources, risk analysis of landslides, detection of groundwater magazines and
mapping of contaminated sites.
9. Geochemical Maps:
These maps are used for mapping various kinds of chemical analysis of rocks and
their fluid contents. Itmay show the surfacedistribution of hydrocarbons where
those hydrocarbons arefound atthe surfacein large amounts than normal
indicating that there is a seepage of oil or gas.
An example of Geochemical Map

8. 10. Some Other Types of Maps Include:
Litho-facies maps: Contour map that shows thecharacter of the rock itself and
how it varies horizontally in the formation. This may determine whether a rock is
sandstone, shale, or some other type of rock.
Bio-facies maps: Similar to lithofacies but show variations in the occurrenceof
fossiltypes.
Iso porosity maps: The maps which show the lines of equal porosity in the
potential reservoir rock.
Iso volume maps: The maps which show the contours of equal porosity-meters
or porosity-ft(netthickness x porosity).
Iso potential maps: The maps which show the initial or calculated daily rate
production of wells in a pool.
Iso baric maps: The maps which show by contours the reservoir pressurein a
pool.
Iso concentration maps: The maps which show the concentration of salts in
oil-field water by contours.
Water encroachment maps: The maps which show the position of wells from
which water is produced along with the oil.