Different mapping types

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8
Mapping
Surface and subsurface maps are important tools that geologists use to
find gas and oil. All maps are oriented with north to the top, south to the
bottom, east to the right, and west to the left.
Topographic Maps
A topographic map shows the elevation of the earth’s surface (fig. 8–1).
To illustrate the third dimension (elevation) on a flat, two-dimensional
map, contour lines are used. A contour line is a line of equal value on a map,
and a contour line on a topographic map is a line of equal elevation. A
contour line is always labeled with an elevation that is above or below
sea level. All along that contour line, the elevation is exactly the same.
For example, anywhere along the +400 ft contour line on a topographic
map, the elevation is exactly 400 ft above sea level. The contour interval of
a topographic map is the difference in elevation between two adjacent
contour lines. The contour interval of the topographic map in figure 8–1
is 100 ft. If the elevations on contour lines increase in a direction, the slope
is rising (fig. 8–2). If the contours are spaced relatively close together, the
elevation is changing rapidly, and the slope is steep. If the contours are
relatively far apart, the slope is gentle.
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Fig. 8–1. Land and a topographic map of the land
Fig. 8–2. Contoured topographic map
showing steep and gentle slopes
There are some important characteristics of contours on a topographic
map. Contour lines never cross. Contour lines are single lines; they never
branch. Contour lines are continuous; they always close or run off the map
and never end on the map.
Elevations can be accurately estimated from a topographic map. If a
point is on the +300 ft contour, it must be, by definition, exactly 300 ft
above sea level. If the point is about halfway between the +300 and +400
ft contour, an elevation of +350 ft is a good estimate. The shape of the
contours is characteristic for many topographic features such as hills,
ridges, and canyons.
A topographic map (or any contoured map) cannot be drawn without
some accurately surveyed points. After the elevations or values are located
on a map (spotted), contours can be drawn between the points. Contouring
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of any map can be done either by hand or computer. The position of a
contour line between two data points can be accurately located by using
proportions. For example, the 400 contour line must run between data
points of 402 and 399 (fig. 8–3). A straight line is drawn between the two
data points. Because there is a difference of 3 between the data points (402
and 399), the line is divided into three equal segments. The 400 contour line
is located one segment from the 399 point and two segments from the 402
point. Anything that can be expressed by mathematics can be programmed
into a computer, and computer-generated contour maps can be made.
Fig. 8–3. Locating a contour using proportions
Geologic Maps
A geologic map (fig. 8–4) shows where each rock layer crops out on the
surface of the earth. Each rock layer is given a different pattern, color, and
symbol on the map. The basic sedimentary rock layer used for geologic
mapping is called a formation. A formation is a mappable rock layer
with a definite top and bottom. Geologists have divided all sedimentary
rocks into formations. Each formation has a two-part name. The first
part is a town where the layer crops out on the surface. The second part
is the dominant rock type, such as sandstone or limestone. San Andreas
Limestone, Bartlesville Sandstone, and Barnett Shale are formation names.
If the sedimentary rock layer is a mixture of rock types, such as alternating
thin sandstones and shales, the word formation is used, for example, the
Coffeyville Formation. Formations can be subdivided into smaller units
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called members. A member is a distinctive but local bed in a formation
(fig. 8–5). It is also given a formal, two-part name. For example, the
Layton Sandstone Member is part of the Coffeyville Formation. Adjacent
formations of similar rocks can be joined to form a group and given a
geographic name (i.e., the Chase Group). If a rock layer occurs deep in the
subsurface and does not appear to crop out on the surface or if it is located
offshore, it is given a letter and number designation such as the H5 sands.
Fig. 8–4. Geologic map
Fig. 8–5. Stratigraphic column showing formations
and members
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A geologic map is a flat, two-dimensional representation of the earth’s
surface. The orientation of rock layers, the third dimension, is shown with
a strike-and-dip symbol. Strike is the horizontal orientation of a plane (fig.
8–6a), such as a sedimentary rock layer or a fault. It is measured with a
compass orientation, such as north 30˚ east. Strike is shown as a short line
on the geological map (fig. 8–6b) that is oriented in the measured compass
direction. Dip is the direction and vertical angle of the plane. It is measured
perpendicular (90˚) to the strike (fig. 8–6a). The dip symbol on the map is
a small bar attached to the middle of the strike line (fig. 8–6b). It points in
the direction that the plane goes down into the earth. The angle in degrees
is often on the dip symbol. The dip of a rock layer is the angle and direction
it goes into the subsurface. Drilling updip means that the drillsite will be up
the angle (dip) of the rock layer from the last drillsite. Updip in a reservoir
is usually a favorable position from a dry hole (fig. 8–7). You may assume
that any reservoir rock is filled with water. Gas and oil are lighter than
water and will flow (migrate) updip in the reservoir rock to a high area.
One would almost never want to drill downdip from a dry hole; one would
want to drill updip.
a
b
Fig. 8–6. (a) Strike and dip of a sedimentary rock layer
and (b) strike-and-dip symbol on a geologic map
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Updip
Updip
Fig. 8–7. Updip from a dry hole
A stratigraphic column (fig. 8–5) is a convenient method for presenting
the vertical sequence of rocks on a geologic map or in a basin. Any
deformation of the rocks, such as faulting or tilting, has been removed.
The youngest formation is at the top of the column, and the oldest is
located at the bottom. The column is drawn as a cliff of weathered rocks
with the weaker rock types (e.g., shales) indented. Stronger rock types (e.g.,
sandstones) protrude outward as they would weather in nature.
Common geological symbols (fig. 8–8) are used for rocks, structures,
and wells on a geological map.
Base Maps
A basemap is a map that shows the location of all the wells that have been
drilled in an area. Spotting a well involves locating a wellsite and placing the
well symbol (fig. 8–8) on a base map. Base maps can also include seismic
lines and other data.
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Fig. 8–8. Common geological symbols
Global Positioning System
Accurate positioning is very important to geologists, geophysicists, and
petroleum engineers. They need to know the exact location of proposed
drillsites, existing wells, and seismic lines. These sites used to be located
with considerable time and expense using surveying tools. Since the 1980s,
accurate location in all weather and anywhere on the earth with no cost
has been determined by the Global Positioning System (GPS). GPS involves
the use of satellites and a receiver. There are 24 solar-powered satellites
very precisely orbiting the earth twice a day at an altitude of 12,550 miles
(20,200 km) in six planes with four satellites each. Each satellite transmits
extremely accurate time signals and the satellite’s orbital information. The
receiver at the location has an antenna tuned to each satellite’s frequency,
a processor, and a very stable clock. It compares the satellite time signal
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with the same time on the receiver to determine how much time it took
the satellite signal to reach the receiver. It then uses that information
to compute the distance from the receiver to the satellite. By using the
computed distance from three, or more accurately from four satellites, the
location, usually in latitude and longitude, and the altitude of the receiver
are calculated and displayed on the receiver. Very precise receivers can
calculate positions to less than 10 ft (3 m) on average.
Subsurface Maps
Three important types of subsurface maps are structural, isopach,
and percentage. All three maps use contour lines to describe a subsurface
rock layer.
Structural map
A structural map uses contour lines to show the elevation of the top of a
subsurface sedimentary rock layer (fig. 8–9). The contour lines are usually
in minus feet below sea level, as most rocks are located below sea level. An
important structural map would be one contoured on the top of a potential
reservoir rock or drilling target.
Fig. 8–9. Structural map
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Domes, anticlines, and faults can be identified on structural maps.
Both a hill on a topographic map and a dome on a structural map have a
bull’s eye pattern (fig. 8–10) with the highest elevation in the center. Both
a ridge on a topographic map and an anticline on a structural map have
a concentric but oblong pattern (fig. 8–11) with the highest elevation in
the center. Dip-slip faults are characterized by a rapid change in elevation
along a relatively straight line (fig. 8–12). A normal dip-slip fault that causes
a lost section in the rock layer being mapped (see fig. 5–18 in chapter 5) is
seen on the map as two lines separating the contour lines (fig. 8–13).
Fig. 8–10. Topographic map of a hill and structural map of a dome
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Fig. 8–11. Topographic map of a ridge and structural map of an anticline
Fig. 8–12. Fault on a structural map
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Fig. 8–13. Normal dip-slip fault on a structural map
Isopach map
An isopach map (fig. 8–14) uses contour lines to show the thickness of a
subsurface layer. If an oil or gas field has been drilled, an isopach map can
be made of the reservoir rock pay zone. The pay zone is the vertical distance
in a well that produces gas and/or oil. Grosspay contours the entire reservoir
thickness including nonproductive water-bearing and shaly zones. Net pay
contours only the productive thickness of the reservoir. A net pay isopach
map of a reservoir is used to calculate the oil and gas volume and reserves.
Fig. 8–14. Isopach map
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An isopach map can be used in exploration to delineate a sandstone
pinch-out (fig. 8–15a) where the isopach contour line becomes zero. The
aerial patterns of beach and river channel sandstones are seen on an
isopach map (fig. 8–15b).
Fig. 8–15. Isopach map of (a) a sandstone pinch-out and (b)
a beach or river channel sandstone
An isopach map of a limestone layer can also be used to locate a reef. A
reef is a mound and is shown by thick contour lines (fig. 8–16). Barrier reefs
that are long (fig. 8–16a) can be distinguished from pinnacle reefs that are
circular (fig. 8–16b).
Fig. 8–16. Isopach map of (a) a barrier reef and
(b) a pinnacle reef
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Percentage map
A percentage map (fig. 8–17) plots the percentage of a specific rock type
such as sandstone in a formation. Higher percentages of reservoir quality
rocks, such as sandstones and carbonates, imply a better reservoir quality.
Fig. 8–17. Sandstone percentage map of a formation composed
of sandstone and shale
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