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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

2

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

3

10m contour spacing

Problem: How to interpolate between the known points?

100

125150

175

15

Triangulation

16

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.

4

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

7

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

14

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

9

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