The Value of 3D Seismic

Contributed by
Cooper, N.M.
Mustagh Resources Ltd., Calgary, 400 604 -- 1st Street SW, Calgary, Alberta T2P 1M7, e:mail: ncooper@mustagh.com

OVERVIEW

Exploration for oil and gas involves the evaluation of a variety of information. Well logs provide detailed information at specific locations, usually an area less than one meter surrounding the well bore. We use our knowledge of formation signatures to recognize depositional environments and try to project this information between wells.

Reflection seismic methods can be used to create images of the geologic changes between wells. These images can help complete a picture of the subsurface that should enhance the ability of the explorationist to successfully select future well locations. Seismic is an effective tool as long as it provides meaningful and helpful images for the desired objectives. The seismic tool must provide a cost efficient alternative to additional drilling.

3D seismic has become a common exploration and production tool. In every country where the author has consulted (18 countries on 6 continents), 3D techniques are either being used or are under serious consideration. The following map indicates the locations of some of the more intense onshore 3D activity. It is by no means a complete record of activity levels.

map of 3d seismic areas of use

What is it about 3D that is making it such a popular tool? What determines the cost of 3D seismic and how does it compare to 2D?

REVIEW OF 2D SEISMIC

Reflection seismic is a method that allows us to image changes in the subsurface geology by inducing an acoustic wave from near the surface of the earth and listening for the echoes from deeper stratigraphic boundaries (much like ultra-sound is used to create pictures of unborn babies in their motherís wombs).

2D seismic is recorded using straight lines of receivers crossing the surface of the earth. Acoustic energy is usually provided by the detonation of explosive charges or by large vibroseis trucks. The sound spreads out through the subsurface as a spherical wave front. Interfaces between different types of rocks will both reflect and transmit this wave front. The reflected signals return to the surface where they are observed by sensitive microphones known as geophones. The signals detected by these devices are recorded on magnetic tape and sent to data processors where they are adjusted and corrected for known distortions. The final processed data is displayed in a form known as "stacked" data.

Typical Costs of 2D Seismic

Play

Offset

Fold

Source

CDP

Cost

Type

(depth)

%

Interval

Size

(per km)

High Res

500

50

10

5

$7,500

Shallow

680

20

34

8.5

$6,500

Paleo U/C

960

12

80

10

$5,500

D-3

1400

14

100

12.5

$5,000

Deep

2000

20

100

12.5

$5,000

Foothills

4000

40

100

12.5

$30,000

Typical Costs of 3D Seismic

Play

Offset

Fold

Line

Bin

Cost

Type

(depth)

%

Spacing

Size

(per sq km)

High Res

500

20

100

5

$700,000

Shallow

700

10

200

15

$40,000

Paleo U/C

1000

14

240

20

$24,000

D-3

1400

18

290

25

$18,000

Deep

2000

20

400

30

$12,000

Foothills

4000

10

1120

40 x 100

$8,000

The above costs are approximate averages for the Western Canada Basin and should be used as guidelines for relative comparisons only. The "High Res" parameters refer to detailed 4D work performed of certain shallow enhanced recovery projects where the intent was to map advancing steam or fire fronts.

In order to record data with sufficient density over large areas, we require a large number of recording channels. The operations of 3D are considerably more elaborate than 2D and the daily cost of crew is substantially increased. However, the rewards include fewer dry holes, more optimized well locations, guidance for horizontal drilling projects, more complete evaluation of mineral rights and better understanding of the nature of prospects.

The following comparisons of 2D and 3D activities in Western Canada for the year of 1997 are compiled by personal communication with a sampling of clients, loose interpretation of industry statistics (total wells drilled from ERCB, crew activity levels from CAGC), and personal involvement in approximately 20% of seismic recorded in Canada. These interpretations do not represent any formal study or extensive surveys of the industry. None the less, we believe the following numbers represent the approximate state of the industry in a fairly accurate relative comparison.

2D versus 3D Seismic Activity Levels (1997)

2D

3D

Program Recorded

30,000 km

24,000 sq km
(1200 programs)

Crew Months

200

350

Channels per Crew

200

1200

Average Cost

$5,000

$350,000

Total Expenditure

$150,000,000

$420,000,000

2D versus 3D Estimated Results (1997)

2D

3D

Wells Drilled on Seismic

3000

8000

Drill Density

1 per 10 km

1 per 3 sq km

Seismic Costs /Well

$50,000

$52,500

Est. Completion Rate

60%

80%

Quality of Production

Fair

Good

Although 3D does not remove all exploration risk, it generally improves success rates and productive wells will more often be on optimal locations and should deliver better production and exhibit slightly longer life. One client who recently recorded a 3D over a well developed pool stated that six to ten of the dry holes associated with pool development would obviously not have been drilled if the 3D data was available prior to drilling. The costs of a 3D program may seem high, but the above figures indicate that exploration and development efficiency can be considerably enhanced by knowledgeable application of the 3D method.

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