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Geol 493Subsurface Mapping
Representing a 3D surface on a map
Movie
The arrow points at …
A. A ridge.
B. A valley.
What is a contour line?
Describe this
surface
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Simple Contouring Rules
• A contour line separates points that are higher from points that are lower
• Contour lines cannot cross• Contour lines cannot merge• Contour lines must close on themselves or
extend to the edge of the map• Contour interval must remain constant for the
entire map
How do you make a contour map?
• A contour lines separate points that are higher from points that are lower
• Space the contours between all the know points
• Honor all the data
Contour this data using 25m intervalDatum = 0 m
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10m contour spacing
Problem: How to interpolate between the known points?
100
125150
175
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Triangulation
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Gridding
Methods of interpolation
• Least squares
• Tangential
• Spline
• Weighted average
• Minimum curvature
• Polynomial
• Hyperbolic
• Kriging
• Trend surface
• Etc., etc., etc.!
a. Inverse distance
b. Kriging
c. Minimum curvature
d. Triangulation
e. Polynomial regressiona.
c.
b.
e.d.
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Advantages of Computer Contouring
• Fast
• Objective and unbiased
• Easily updated
• Ready for later processing
• Only practical way if you have 1000s of data points
Advantages of Hand Contouring
• Allows you to impart your geologic knowledge to the interpretation
Depth Structure Maps
Sea level
132 (132)
511 (502) -362
530 (525) -385
362(356)-211
Land surface
Cross Sectional View
Well 1Well 2 Well 4
Well 3Well 5
•Log Depth•(True Vertical Depth)•Subsea Depth
Dealing with multiple layers•Isopach maps•Plot the elevation difference between two maps
Mississippian Top PennsylvanianTop
1800
1800
1700
5
Pennsylvanian-Mississippian Isopach
500
600
300
400
800
700
Pennsylvanian plus Isopach
Which well would encounter the thickest sand?
Isopach Map
What about Unconformities?
Cross section
-8000
-8200
-7800
NESW
A
Unconformity Surface Map Top of Layer A
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Base of Layer A Isopach of Layer A
25
25
50
What about mapping other parameters?
• Extensive Surfaces:– Tops and Bottoms of formations
– Isopachs
– Fault surfaces
– Fluid contacts, i.e. Gas-Oil, Water-Oil
• Intensive Surfaces:– Porosity
– Fluid Saturation
– Production
• Black contours are structure
• Colors are porosity• Circles are production
Michigan
Michigan Tech
Porosity map from reservoir model
Gaspar et al. 2013
Mapping faults
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Contouring Faulted Surfaces• Faults break the continuity of a surface
Describe the Structure
Faulted surfaces
Normal Fault- North Sea
Virtual Seismic Atlas
Making a map of a faulted structure
1. Contour the surface without faults
2. Contour the fault plane
3. Find the intersection of the two maps
4. Correct the contours for the vertical separation across the fault
5. Clean up
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Faulted structure contour map
3
2
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1
Faults from well data• A normal
fault can remove a piece of the stratigraphy in a well
• Missing section is a indicates faulting
Cross SectionNW SE
Reverse Faults- cross section
Reverse Faulting Example UNION OOLITE THRUST MODEL WITH FRACTURE SWARM
(after Nelson, 2001)no scale
UNION OOLITE
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DEPI #5639 (047-055-00226) Gamma Ray log
UNION OOLITE(Hanging wall)
UNION OOLITE(Foot wall)
100’
PILOT KNOB THRUST FAULT
228’ vertical displacement
Thrust faults cause repetitions in the stratigraphy
Cross-section A - A’
A A’
0
-500
-1000
-1500
PILOT KNOB
ARISTA
NW SE
Scale:500’
500’
UNION
PICKAWAY
DENMAR
LITTLELIME
PRICE
Where is the highest point in the
structure in this
depth map?
A.
B.
C.
D.
E.
AD
C
B
E