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The Value of 3D Seismic

Exploration for oil and gas involves the evaluation of a variety of information. 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. The seismic tool provides a cost efficient alternative to additional drilling.

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.

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 mother’s 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.

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

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