USE OF MAGNETIC FIELD AIDS OIL SEARCH

Robert S. Foote
Geoscience & Technology Inc.
Hurst, Tex.

The desire of all oil and gas explorationists is to find a method for oil and gas exploration that will:

• Remove from further evaluation most nonproductive areas.

• Select those targets most favorable for exploration.

• Classify the selected favorable targets.

• Indicate where seismic lines should be placed to be most effective.

• Operate over land and water equally.

• Be highly cost effective.

Such a method is operational.

It requires the measurement of the earth's magnetic field with the ability to determine and remove with high accuracy that part of the signal caused by changes in the magnetic basement, and measurement of rock drill cuttings when possible, only to add confidence that the magnetic mineral body is authigenic in origin (created in place), and not detrital.

Results show that (1) anomalous areas developed from aeromagnetic data and (2) drill holes shown to have anomalous authigenic altered drill cuttings are both 80-85% probable of oil and natural gas discovery, and similarly those areas not anomalous in either are 10-15% probable of oil and gas discovery.

The method involves the gathering of low terrain clearance high resolution data of the earth's magnetic field using the cesium vapor magnetometer or equivalent.

The magnetic field of the earth at its surface represents a composite of the effects of:

1. Magnetic properties of basement Precambrian rocks and basement topography

2. Surface and near-surface cultural iron contamination

3. Near-surface volcanic and intrusive debris, and

4. Authigenic magnetic alterations in near-surface sedimentary formations, the magnetic "fossil."

The method requires the successful removal of the first three effects. This gives the fourth, the near-surface authigenic magnetic alterations as a residual, and provides the sedimentary residual magnetic (SRM) anomalies as an indicator of the existence of subsurface oil and gas.

No other cause for creation of the SRM anomaly has been found. The method is also highly effective over water, where cultural magnetic material is at a minimum.

CONFIRMATION BY CUTTINGS

Measurement of the magnetic properties of drill hole cuttings will validate the SRM anomalies developed.

The existence of an SRM anomaly must result from the more magnetic mineral body in the near-surface attracting and concentrating otherwise approximately uniform distribution of the lines of force of the earth's magnetic field.

Cuttings from drilled oil wells and from small bore drill test holes can be used to verify any discovered SRM anomalies and will confirm their authigenic origin.

Most authigenic magnetic mineral bodies are found in the depth range of 200-2,000 ft, but in the case of the Jurassic Smackover oil trend area, southwestern Alabama, magnetic alteration is in the upper part of the Selma chalk at depths to 3,500 ft.

COLORADO STUDY AREA

A cesium vapor magnetometer survey was made over the eastern part of Cheyenne County, Colo., in January 1986 and covered about 1,250 sq miles.

This survey was made before the discovery of the Pennsylvanian Morrow sand trend north and south of the existing Stockholm field in Southwest Kansas. A map shows the area of the newly developed Morrow sand trend before its discovery, with four dry holes and one oil well (Fig. 1).

The enclosed rectangular areas of Fig. 1 now contain at least 85 oil wells and 48 dry holes.

This Morrow sand trend area (Fig. 2, larger block of Fig. 1) is strongly supported by the sedimentary residual magnetic field data from data gathered before field discovery. The axis of the trend coincides with the center region of the large SRM anomaly. The trend has SRM flight line individual anomalies shown with each anomaly block number representing the maximum alteration in nT x 10.

The small squares in the upper right-hand corner of certain anomaly rectangles indicate those anomalies having ideal magnetic field anomaly distributions as would be predicted from a distributed magnetic mineral body at depth.

Oil well and dry holes are shown, and those that have had drill hole cuttings measured for their magnetic susceptibility (MS) are marked for having a large, medium, or no alteration.

Figs. 3 and 4 show the MS as a function of cuttings depth for a typical oil well and dry hole, their chi-logs. Note the presence of anomalous magnetic cuttings from the well and the lack of such from the dry hole.

An SRM anomaly cluster can be seen over the newly developed Second Wind oil field (Fig. 5; small block of Fig. 1). Only one dry hole existed in the area (see Fig. 1) at the survey time, which has the small MS anomaly shown in Fig. 5.

The Mossbauer technique has been used on several area anomalous cuttings to show maghemite to be the magnetic mineral.

A summary of the correlations of wells and dry holes within and outside the periphery of the computer sedimentary residual magnetic anomaly clusters for the Cheyenne County area is:

Cluster SRM Oil Dry Total

Yes 228 63 291

No 46 289 335

Sum 274 352 626

The density of oil and gas locations in SRM anomalies is 228 of 291 or 78%. The density of dry locations without SRM anomalies is 289 of 335 or 86%.

CEMENT FIELD MS DATA

During the past 9 years the correlations of results of analysis of aeromagnetic data and measurements of the magnetic properties of borehole rocks with the presence and absence at depth of oil and gas reservoirs have been made and studied.1 2 3 4

The results presented are developed from an empirical study of data gathered from the previously discussed Cheyenne County data plus data from the Cement oil field area of Oklahoma and from Southwest Alabama.

In 1975 a survey was sponsored by the U.S. Geological Survey and directed by T. Donovan to measure from a low flying aircraft the magnetic and radiometric data over Cement oil field, Caddo and Grady counties, Okla.

Donovan published the results of the aeromagnetic data together with measurements of the MS of borehole shallow rock cuttings from wells over Cement field in 1979.5

The proton precession magnetometer used did not have the sensitivity required to measure any anomalous conditions, and iron contamination of oil field material prevented measurement regardless of the type of airborne magnetometer used.

However, recent measurement of the MS of many wells in Cement field indicates the depths and magnetic alteration intensities of authigenic magnetic cuttings. Fig. 7 shows the field outline and the results of MS measurement of 52 wells and dry holes.

The percentage of the 34 oil or gas wells having anomalous magnetic cuttings is 82%, and the percentage of the 18 dry holes not having anomalous drill cuttings is 83%.

A typical oil well chi-log (Fig. 8) and a dry hole chi-log (Fig. 9) are also shown. Most oil and gas wells show the prominent authigenic alteration zone to be between 800-1,000 ft (Fig. 10).

SOUTHWEST ALABAMA DATA

A large cesium vapor aeromagnetic survey area, including parts of Escambia, Monroe, Conecuh, Clarke, and Washington counties, Ala., has been processed to develop the sedimentary residual magnetic field.

Drill hole cuttings samples for a total of 65 oil or gas wells and dry holes have been measured for their MS at the Geological Survey of Alabama sample library in Tuscaloosa to a depth of about 5,000 ft.

The SRM data for the area of Vocation oil field in Monroe County is shown (Fig. 11). Several drill hole cuttings are available for correlation of MS data within the field area.

The chi-log of a Vocation field oil well indicates the Selma chalk to be the only region of strong authigenic magnetic alterations (Fig. 12). The chi-log for an area dry hole indicates no magnetic alteration of the Selma chalk at about 3,300 ft (Fig. 13).

The Getty Oil Co. 1 Quimby shows a maximum MS change of X70 in the top of the Selma chalk. Mossbauer technique measurement of the anomalous chalk from this well gives maghemite as the magnetic mineral.

Does agreement exist between the SRM cluster anomalies and anomalous magnetic drill cuttings? Of 46 oil exploration holes where comparison can be made, SRM/MS (Yes) = 41, and SRM/MS (No) = five, or the percent correlation for agreement = 89%.

The 2,000 sq mile Southwest Alabama aeromagnetic survey area has 54 oil or gas wells and 346 dry holes in regions where contamination from the area iron does not prevent accurate measurement.

SRM cluster anomalies agree with exploration drilling as:

Cluster

Cluster SRM

SRM (no)

(yes) fields Single Total

OH 40 9 5 54

Dry 5 - 341 346

Sum 45 --- 355 --- 400

considering wells having SRM (no), single well, as noneconomic; SRM (yes) with oil = 82%, and SRM (no) with dry = 98.6%.

MS measurement of cuttings from 65 drill holes in Southwest Alabama is made.

Selma chalk having an anomalous magnetic interval < 100 ft is considered not to be anomalous, giving:

Anoma-

Anoma- lous MS

lous MS 100' thick

Oil 3 14

Dry 47 1

giving MS (oil) = 82% and MS (dry) = 98%.

RESULT COMBINED

All areas covered by an aeromagnetic survey include a total of 1,026 oil or gas wells and dry holes. The results:

Cluster SRM Oil Dry Total

Yes 268 68 336

No 60 630 690

giving oil or gas in SRM = 80% and dry out of SRM = 91%.

MS for all areas includes measurement of 331 exploration drill hole cuttings:

Anomalous

MS Oil Dry Total

Yes 103 28 131

No 33 167 200

giving oil or gas with MS (yes) = 79% and dry with MS (no) = 84%.

Many of the 33 MS (no) oil and gas wells are included even though initial several hundred feet of sample are missing.

Because of these missing samples, some anomalous MS wells may not be identified, thus lowering the probability for encountering anomalous MS wells.

Many other areas have been previously measured for the SRM and comparative MS, with the percentage of oil or gas wells with an MS anomaly averaging 80-85% and for dry holes without the MS anomaly 85-90%.

The SRM results give the probability for oil or gas discovery between 80-85% within the SRM anomaly and a probability of about 90% for no production outside the SRM anomaly.

Several of the 60 wells not included within the SRM anomaly are single wells with no subsequent offset and may be noneconomic.

SUMMARY, CONCLUSIONS

New computer software provides accurate removal of the magnetic basement part from the composite earth's total magnetic field.

Identification can be made of the contribution to the magnetic field of effects of small near-surface magnetic mineral bodies to about 0.3 nT in areas of low geologic magnetic noise.

REFERENCES

1. Foote, R. S., integrating airborne and subsurface magnetic data, Fifth Thematic Conference, Reno, Nev., 1986, pp. 233-2417.

2. Foote, R.S., Aeromagnetic field data analysis/rock magnetic susceptibility, a method for locating oil and gas reservoirs, Institute for the Study of Earth and Man/Southern Methodist University Research Conference on Vertical Hydrocarbon Migration, Fort Burgwin, N.M., October 1987.

3. Foote, R.S., Correlations of borehole rock magnetic properties with oil and gas producing areas, APGE Bull., Vol. 3, No. 1, 1987, pp. 114-134.

4. Foote, R.S., and Long, G.J., Correlations of oil and gas producing areas with magnetic properties of the upper rock column, eastern Colorado, APGE Bull., Vol. 4, No. 1. 1988, pp. 47-61.

5. Donovan, T.J., Forgey, R.J., and Roberts, A.A., Aeromagnetic detection of diagenetic magnetite over oil fields, AAPG Bull., Vol. 63, 1979, pp. 245-248.

Copyright 1992 Oil & Gas Journal. All Rights Reserved.