Chap14

Chapter 14

Using Magneticsin Petroleum Exploration

by

Edward A. Beaumont

and

S. Parker Gay, Jr.

Edward A. BeaumontEdward A. (Ted) Beaumont is an independent petroleum geologist from Tulsa,Oklahoma. He holds a BS in geology from the University of New Mexico and an MS ingeology from the University of Kansas. Currently, he is generating drilling prospects inTexas, Oklahoma, and the Rocky Mountains. His previous professional experience wasas a sedimentologist in basin analysis with Cities Service Oil Company and as ScienceDirector for AAPG. Ted is coeditor of the Treatise of Petroleum Geology. He has lecturedon creative exploration techniques in the U.S., China, and Australia and has receivedthe Distinguished Service Award and Award of Special Recognition from AAPG.

S. Parker Gay, Jr.Parker Gay is president and chief geophysicist of Applied Geophysics, Inc., Salt LakeCity, Utah, a company he co-founded in 1971. He received his B.S. degree at MIT (1952)and his M.S. degree at Stanford University (1961). Gay served as a photointerpreter inthe U.S. Air Force; worked as a geophysicist and geologist for Utah Construction Co. andMining and its subsidiary, Marcona Mining Co.; was a geophysicist for Asarco, Inc.; orga-nized and managed the U.S. subsidiary of Scintrex Ltd. in Salt Lake City; and foundedAmerican Stereo Map Co. in 1970 and Applied Geophysics in 1971, which he now heads.Applied Geophysics is a geophysical contracting organization. In 1974, Gay cofounded theInternational Basement Tectonics Assoc. He has written numerous papers and givenmany talks on basement control of geological structure and stratigraphy. He has pub-lished 16 papers on the interpretation of magnetic anomalies and their geological causes.

Overview • 14-3

Basement fault blocks often correlate with structural and stratigraphic features in thesedimentary section that control trap location. Magnetic technology senses the earth’smagnetic field. This technology—and aeromagnetics, in particular—effectively delineatesbasement fault blocks through the use and interpretation of magnetic residual maps andprofiles. Basement lithologic changes and the resulting magnetic susceptibility changesfrom block to block allow us to map the basement fault block pattern and to use this infor-mation in important new ways for finding oil and gas. This chapter discusses concepts ofmagnetics and how to apply them to petroleum exploration.

Introduction

Overview

This chapter contains the following sections.

Sections Topic Page

A Magnetic Basics 14–4

B Interpreting Magnetic Data 14–8

C References 14–20

In this chapter

This section contains the following topics.

Topic Page

Basics of Magnetics 14–5

Total Intensity and Residual Magnetic Maps 14–6

14-4 • Using Magnetics in Petroleum Exploration

The lithology of basement rocks controls regional and local magnetic variations in theearth’s field. Seeing local variations is critical to applying magnetic technology to petrole-um exploration. This section discusses the basics of magnetic theory and mapping.

Introduction

Section A

Magnetic Basics

In this section

Magnetic Basics • 14-5

Magnetic disturbances caused by rocks are localized effects superimposed on the normalmagnetic field of the earth. The distribution of magnetite in rocks is the primary cause ofthe local variations in the magnetic field observed in magnetic surveys. Magnetite is notthe only magnetic mineral, but it is the dominant cause of magnetic anomalies (Nettleton,1962). The magnetite content of basement rocks can be two orders of magnitude greaterthan the magnetite content of sedimentary rocks. Consequently, variations in the magnet-ic field result mainly from basement rocks underlying the sedimentary section.

The earth’s magnetic field is measured in nanoTeslas (nT; formerly known as “gammas”).

Theory

Basics of Magnetics

Magnetic anomalies for an object of the same size, composition, and depth have differentsignatures at different magnetic latitudes because the magnetic inclination—the angle atwhich the magnetic force field is oriented with the earth’s surface—changes with latitude.The figure below shows profiles of magnetic total intensity anomalies for the same objectat different latitudes in the northern hemisphere. In the southern hemisphere the profileswould be the opposite [north and south would reverse south of the equator and the incli-nation angles (i) would be negative].

Effect ofmagneticlatitude

Figure 14–1. After Nettleton, 1962; courtesy Society of Exploration Geophysicists.

Sphere

Location:Magnetic NorthPole

Location:MagneticEquator Various Inclinations

Total Intensity Magnetic Anomaly for

The table below summarizes the differences between gravity and magnetics.

Characteristic Gravity Magnetics

Anomaly cause Horizontal density variations Magnetite content variations

Best use Defining large-scale geologic features Defining basement blocks, locating intru-and shape of structures, and determining sive bodies; generalized depth to basementoffset of basement faults

Gravity vs.magneticanomalies

14-6 • Using Magnetics in Petroleum Exploration

Magnetic variation or susceptibility may be analyzed using either total intensity or resid-ual maps. Magnetic residual maps reveal much more detailed geologic features—in par-ticular, the geometry and configuration of individual basement blocks. They bring out thesubtle magnetic anomalies that result from the changes in rock type across basementblock boundaries. Total intensity maps show larger scale geologic features, such as basinshape or anomalous rock types deep within the basement.

Introduction

Total Intensity and Residual Magnetic Maps

Total intensity is the measurement from the magnetometer after a model of the earth’snormal magnetic field is removed. It is generally a reflection of the average magnetic sus-ceptibility of broad, large-scale geologic features.

What is totalintensity?

Residual is what remains after regional magnetic trends are removed from the totalintensity. Residual maps show local magnetic variations, which may have exploration sig-nificance. The regional trend of the total intensity can be calculated using a number oftechniques, including running averages, polynomials, low-pass filters, or upward continu-ation techniques. The figure below shows magnetic profiles of total intensity, regionaltrend, and residual.

What isresidual?

Figure 14–2. After Nettleton, 1962; courtesy Society of Exploration Geophysicists.

Total Intensity

Residual

Residual

Regional

Magnetic Basics • 14-7

The maps below are examples of a residual map (A) that was calculated from the totalintensity magnetic map (B). The grid and small circles on the total intensity map are theflight path lines [approximately 2 km (1.2 mi) apart] and location points for the flightlines. The total intensity map strikingly does not resemble the residual map and would beof limited value for delineating basement fault blocks.

Total intensity magnetics responds to rock types over broad areas as well as those deepwithin the crust. We can see by a careful examination of map B, however, that many ofthe features shown by the residual map are vaguely apparent in the total intensity data.Fortunately, we no longer have to interpret such total intensity maps in petroleum basinsbecause many enhancement techniques employing residuals, derivatives, polynomials, ordownward continuation exist to reveal the subtle magnetic anomalies that result from thechanges in rock type across basement block boundaries.

Total intensityand residualmap example

Figure 14–3. From Gay, 1995; courtesy International Basement Tectonics Assoc.

Total Intensity and Residual Magnetic Maps, continued

Contour interval 2 nTContour interval 10 nT

mi.km.

B.

A.

14-8 • Using Magnetics in Petroleum Exploration

This section documents a number of one-on-one correlations of the basement fault blockpattern, as mapped by modern aeromagnetic techniques. Structural and stratigraphicfeatures in the sedimentary section that are important to petroleum exploration are dis-played. Several pitfalls in aeromagnetic interpretation are due to the failure to recognizethe existence of the basement fault block pattern and its control on the lithology of base-ment. These basement lithologic changes, and the resulting magnetic susceptibilitychanges from block to block, allow us to map the basement fault block pattern and to usethis information in important new ways for exploring for oil and gas.

Introduction

Section B

Interpreting Magnetic Data

This section contains the following topics.

Topic Page

Basement Fault Blocks and Fault Block Patterns 14–9

Local Magnetic Field Variations 14–11

Interpreting Residual Maps 14–13

Applying Magnetics to Petroleum Exploration 14–16

In this section

Interpreting Magnetic Data • 14-9

The basement fault block pattern in sedimentary basins was formed in multiple tectonicand metamorphic episodes during the Archean and Proterozoic eras. Basement tends tocontrol most of the local structure and much of the stratigraphy within the overlying,younger sedimentary section. It is along the shear zones, or block boundaries of the base-ment, that we generally find the faults or other structures in the overlying sedimentarysection. These zones of weakness are periodically reactivated by tectonic stresses or gravi-tational loading. Consequently, they have influenced depositional patterns and locationsof structures throughout geologic time.

Introduction

Basement Fault Blocks and Fault Block Patterns

The following Landsat and SLAR images of exposed Precambrian crystalline crust showhighly lineated terrains and demonstrate that the linears fall into multiple parallel orsubparallel sets of varying strike directions. These overlapping fracture sets cut the base-ment into blocks of varying shapes and sizes. This collection of basement blocks is thebasement fault block pattern.

Study of shieldareas

Figure 14–4. From Gay, 1995; courtesy International Basement Tectonics Assoc.

Canadian Shield

Arabian Shield African Shield

Gayana Shield

14-10 • Using Magnetics in Petroleum Exploration

The intensity of fracturing and mylonitization of the rocks in shear zones explains whythese zones generally erode low and why they tend to control the topography of the Pre-cambrian surface. This surface, in turn, controls much of the structure in the lower partof the sedimentary section through gravitational compaction of the sedimentary rocks.

Precambriansurfacetopography

CanadianShield example

Figure 14–5. From LaBerge, 1976; courtesy International Basement Tectonics Assoc.

Basement Fault Blocks and Fault Block Patterns, continued

The figure below is a geologic map of an area of crystalline basement in central Wisconsinon the southern edge of the Canadian Shield. Here, outcrops and rock exposures in shal-low excavations, roadcuts, etc., abound. It is possible to map the basement geology in con-siderable detail. Five things stand out:1. A series of parallel to subparallel shear zones has been mapped.2. There is obvious periodicity to the shear zones, the spacing between them varying from

about 4–8 km (2.5–5 mi).3. There are rock type changes across these zones.4. The width of the shear zones varies from about 1 km (LaBerge, personal communica-

tion) up to 2.5 km or more.5. The shear zones and geology truncate abruptly and change style across line A–A′.

Interpreting Magnetic Data • 14-11

Variations in the local magnetic field are due mainly to the following:• Lithologic changes of basement rocks with corresponding differences in magnetite con-

tent• Elevation changes on the top of basement where basement is of uniform magnetic sus-

ceptibility (k)

However, lithologic changes tend to overwhelm the magnetic response of elevationchanges in basement caused by fault throws or basement highs unless basement is deep(> 5 km, approximately). In this case the slightly magnetic sedimentary rocks begin toshow in the magnetic pattern. Most of this is due to detrital magnetite in sandstones.

Introduction

Local Magnetic Field Variations

The presence of a fault is a common interpretation of a magnetic increase or decrease.This interpretation assumes the fault throw, which changes the elevation to the top ofbasement, is the cause of the anomaly. It also assumes uniform lithology and uniformmagnetic susceptibility of basement across a fault. Given this (usually incorrect) assump-tion, we can calculate the depth of the fault and its throw from the shape and amplitudeof an observed magnetic curve. If we do not know the exact susceptibility, we can calculatea series of curves to establish a range of probable values of the throw. In all cases, themagnetic high necessarily appears on the upthrown side of the fault.

In the hypothetical cross section below, basement rock has the same susceptibility acrossthe fault.

Elevationchange due to a fault

Figure 14–6

Figure 14–7 shows a fault separating basement blocks of different lithologies and magneticsusceptibilities. If the average magnetic susceptibilities (k1 and k2) of the basement blocksare unknown, then we cannot determine the amount of throw of the fault—we cannot evendetermine the direction of throw if the signal resulting from susceptibility overrides thatdue to throw. Since susceptibilities of basement rocks commonly vary by hundreds, eventhousands, of percent (Heiland, 1946; Jakosky, 1950; Dobrin, 1960) and the ratio of throwto depth of a fault can be, at most, 100%, then it follows that in most cases the magneticresponse due to susceptibility overrides that due to throw. The result is that many faults(perhaps as high as 40–50%) show a magnetic low on the upthrown side.

Lithologicchanges due toa fault

14-12 • Using Magnetics in Petroleum Exploration

The hypothetical cross sectionshows a fault juxtaposing base-ment blocks of different litholo-gies and susceptibilities. Thecurves above the cross sectionare the magnetic profiles wherethe magnetic field is vertical fork1 > k2 and k1 < k2. It assumes nothrow on the fault (d = 0). Thedashed curves show the magnet-ic response if the fault has afinite throw (d). Note how littleimpact the fault throw has oneither profile.

Lithologicchanges due to a fault(continued)

Figure 14–7. From Gay, 1995; courtesy International BasementTectonics Assoc.

The basement hill and obvious magnetic anomaly shown on the left side of the figurebelow assumes a uniform magnetic susceptibility for basement. However, given that base-ment is usually block faulted, is this type of feature detectable? If we are looking at atopographic prominence centered on a basement block, the detection problem becomesthat shown on the right side of the figure. A series of adjacent basement blocks having dif-ferent magnetic susceptibilities results in a residual magnetic pattern of alternatinghighs and lows (solid lines).

When the basement block on which the hill is carved is more magnetic than surroundingblocks, the hill contributes slightly to the magnetic high over the block as shown. Theslight increase in anomaly amplitude due to the hill (top dashed line) generally is not dis-tinguishable from a similar increase due to a slightly higher magnetic susceptibility forthe whole block; hence, the hill is not generally detectable. If the block on which the hill iscarved is less magnetic than the adjacent blocks, then the hill results in a lesser ampli-tude of the magnetic low over that block, but the low is still present (bottom dashed line).The hill generally is not detected.

Detectingbasement hills

Figure 14–8.

Local Magnetic Field Variations, continued

Interpreting Magnetic Data • 14-13

A major pitfall in interpreting magnetic residual maps is assuming that magnetic highsand lows are caused by elevation changes on basement rocks in a sedimentary basin. Tothe contrary, most magnetic anomalies are caused by lithologic changes and the corre-sponding changes in susceptibility. As shown in Figure 14–5, the basement is of complexlithology and is highly fractured. The fractures divide the basement into blocks and arezones of weakness along which faults occur. The most important and most reliable infor-mation obtainable from aeromagnetic maps is the configuration (in plan view) of theunderlying basement fault block pattern.

Introduction

Interpreting Residual Maps

It is futile to attempt to define accurately the vertical dimension, Z, of adjacent sourcebodies with magnetics because of the inherent ambiguity of potential field methods indetermining Z (see, e.g. Skeels, 1947). Furthermore, seismic and subsurface methodsmeasure depth so much more accurately than magnetics that it is unwise to try to com-pete with these excellent techniques. This is not to say, however, that we should not usemagnetics to estimate the approximate thickness of the sedimentary section in a newbasin, i.e., in determining whether it is 2, 5, or 10 km thick, for example, to a usual accu-racy of about ±15% under favorable conditions.

Interpretingdepth tobasement

There is a fairly reliable way to determine the direction of throw of certain basementfaults from magnetic maps. Faults that vertically offset basement or other magneticsources generally show abrupt amplitude changes of magnetic anomalies, both the highsand lows. In the figure below, a series of four northeast-trending magnetic anomalies onthe west (two highs, two lows) abruptly loses amplitude along a northwest-trending line(A–A′) that crosscuts them. The high and low magnetic trends can be identified easily onboth sides of this obvious down-to-the-east fault. The four anomalies disappear altogetheralong another northwest-trending line farther east (B–B′). This may be a strike-slip fault,which is not common in this area, or another down-to-the-east fault that has down-dropped the four anomalies beneath the level of detection—the preferred interpretation.

Interpretingfault throw

Figure 14–9.

14-14 • Using Magnetics in Petroleum Exploration

The figure below shows a residual aeromagnetic map of an area on the north shelf of theAnadarko basin in Oklahoma where the sedimentary section is approximately 3.6 km(12,000 ft) thick and basement lies about 3.8 km (12,500 ft) beneath flight level. Theresidual magnetic contours (a) are shown at a 2-nT interval. The interpreted shear zonesare traced along the linear gradients separating the residual magnetic highs and lowsand along truncation lines of anomalies. On the right figure (b), two faults located fromsubsurface mapping are shown, labeled U/D. The evidence for their existence is seen insubsurface mapping. Both are located exactly along the interpreted basement shearzones, or block boundaries, as represented by gradients on the magnetic map. Note, how-ever, that most of the interpreted basement shear zones in this area have no correspond-ing overlying faults. These zones were never reactivated—at least not sufficiently enoughto be detected by the existing subsurface data.

Interpretingshear zones

Figure 14–10.

In Figure 14–10, also note the structural high apparent in Devonian strata about 800 m(2500 ft) above basement in the West Campbell field, conveniently nestled between blockboundaries. Block boundaries, i.e., shear zones, generally erode low, so it follows that theinteriors of blocks must, in many cases, correspond to basement topographic highs. WestCampbell field appears to be a case in point and is most likely underlain by such a base-ment topographic prominence, although there are no wells to basement here to documentit. The culmination of structural closure nearer the north end of the block rather than atits center is probably due to the south dip of basement in this area.

Oil fieldexample

Interpreting Residual Maps, continued

Interpreting Magnetic Data • 14-15

The figure below is a residual magnetic map of another area in northern Oklahoma withfaults superimposed. Here the sedimentary section is approximately 2 km (6500 ft) thick,and the flight level was about 2.3 km (7500 ft) above basement. The faults shown wereinterpreted from a detailed subsurface study by Geomap Inc. The faults were mappedabout 500 m (1600 ft) above basement and show 30–90 m (100–300 ft) of displacement.Note the high degree of correlation between these faults and the residual magnetic gradi-ents corresponding to the interpreted basement shear zones. Some 64% of the total lengthof faults, in fact, lies on the predicted shear zones following magnetic gradients. Note alsothat many magnetic gradients in Figure 14–11 show no faults cutting the section. Thesemay not have been reactivated.

Interpreting faultlocation

Figure 14–11. Details from Gatewood (1983).

Interpreting Residual Maps, continued

14-16 • Using Magnetics in Petroleum Exploration

Magnetics can be an extremely effective and economical exploration tool when properlyemployed. Its proper use, however, depends on the following: • Avoiding several pitfalls described herein• Integrating the magnetics with seismic, subsurface, and other data• Developing the basement fault block pattern from the magnetic data• Using concepts of basement control in working with all data sets

Introduction

Applying Magnetics to Petroleum Exploration

Magnetics can be applied to petroleum exploration for many reasons:• Aiding 2-D and 3-D seismic interpretations• Laying out new 2-D and 3-D seismic programs• Aiding in exploration programs based primarily on subsurface (well) data• Estimating depth to basement over broad areas

Applications

Magnetics can be very valuable in interpreting seismic data by plotting residual magneticprofiles along seismic sections. This technique is valuable in looking for (1) subtle strati-graphic changes that can occur along basement block boundaries and (2) subtle fault off-sets or other structural and stratigraphic features. The locations of the basement weak-ness zones provide focal points for examining the seismic data more closely.

The figure below shows an example of a magnetic profile on an interpreted seismic sectionfrom Logan County, Arkansas. The dark band corresponds to Cambrian throughMississippian sedimentary rocks. Note correlation between the location of the four normalfaults interpreted in the seismic section and the location of faults in the magnetic profile(marked by diamonds).

Interpretingfault location in seismicsections

Figure 14–12.

Interpreting Magnetic Data • 14-17

Magnetic basement mapping in petroleum exploration can be applied to the search forleads or prospects that can be quickly and economically developed by comparing knowntraps or structure (and/or stratigraphy) with the basement fault block pattern. Someareas have never been tested by the drill where the structure at basement level is analo-gous to that over nearby producing properties. Some of these leads become viableprospects when subjected to follow-up seismic profiling or other appropriate explorationtechniques. A common type of structural or stratigraphic data used to correlate to themagnetic data is subsurface mapping, developed from well data. However, on overseasprojects or in frontier areas, the best (or only) data available may be 2-D seismic survey-ing. In either case, the procedure is to search for “look-alikes” on the magnetic data thatcorrespond to features over known producing fields. Since the magnetic data can beacquired in continuous fashion over large areas at a very economical price, many goodleads can be developed in a short time.

Developing leadsfrom analogs

Suppose we have developed a basement fault block pattern as shown in the figure below.Also suppose this area has been tectonically active and is characterized by a fair degree offaulting. This being the case, we can expect that many of the basement shear zones havebeen reactivated and are now the locus of faults and fractures in the sedimentary section.Thus, A, D, and F in the figure are the wrong places to run 2-D seismic lines because ofthe probable poor seismic definition due to fracturing along these zones and the possibili-ty of sideswipe. Lines B, C, and E, on the other hand, are good places to run seismic sur-veys because of the probable lack of fracturing and faulting at these localities. In addition,gravitational compaction structures are generally found within blocks; thus, line B or Cwould have found West Campbell field (WCF) but line A would not.

Laying outseismicprograms

Figure 14–13.

Applying Magnetics to Petroleum Exploration, continued

14-18 • Using Magnetics in Petroleum Exploration

Magnetics can be quite useful in interpreting existing seismic programs after they havebeen shot. In the example below, two 2-D seismic lines have been placed purposely in theworst possible positions relative to the basement fault block pattern.

Assuming all the basement shear zones represent faults in the sedimentary section, then“hooking-up” the faults in this area is a problem. Fault pick C on line 1, for example, doesnot connect straight across to fault pick G on line 2, nor even to H or I, which are somedistance away. Instead, it hooks up to J, making this fault very oblique to the seismiclines. This is not a very common way of connecting faults on most seismic interpretations.The connection of B to H is straightforward but, again, is diagonal to the seismic lines,whereas F–K runs diagonally in the opposite direction. Fault picks D, G, E, and I do notconnect to the other seismic line at all; they terminate somewhere in between.

Figure 14–14.

Depth estimates from aeromagnetic data can determine values for broad areas, such asthe approximate thickness of the sedimentary section in a basin or at a limited number ofpoints within the basin. Using depth estimates to distinguish between the depths of adja-cent magnetic anomalies invites trouble.

Depthestimates

Applying Magnetics to Petroleum Exploration, continued

Interpretingfault location inmap view

Interpreting Magnetic Data • 14-19

We might question the strong emphasis on magnetics for mapping the basement faultblock pattern. However, is there any other way to reliably map this pattern beneath thesedimentary section? Methods that depend on surface information—Landsat, SLAR, con-ventional photo geology, and surface geology—are of limited value. That leaves only gravi-ty and seismic techniques.

However, gravity techniques generally do not separate adjacent basement blocks becauseof the lack of density contrast between adjacent blocks and because of interference fromdensity differences within the sedimentary section. On seismic data, the basement reflec-tor is often difficult to recognize beneath complex structure and because of a lack of veloci-ty contrast with the dense dolomites that overlie the basement in many areas.Furthermore, both seismic and gravity methods are expensive to apply over broad areasand cannot provide even a tiny percentage of the area coverage that can be obtained withmagnetics for the same price.

The conclusion is that both seismic and gravity are excellent follow-up tools for profiling,or “cross-sectioning,” specific leads developed from the basement fault block pattern bymagnetics and are best used for this purpose.

Magnetics vs.other techniques

Applying Magnetics to Petroleum Exploration, continued

14-20 • Using Magnetics in Petroleum Exploration

Dobrin, M.B., 1960, Introduction to Geophysical Prospecting, 2d ed.: New York, McGraw-Hill, 446 p.

Gatewood, L., 1983, Viola–Bromide and Oil Creek Structure (map): Oklahoma City, pri-vately sold and distributed.

Gay, S.P., Jr., 1995, The basement fault block pattern: its importance in petroleum explo-ration, and its delineation with residual aeromagnetic techniques, in R.W. Ojakangas, ed.,Proceedings of the 10th International Basement Tectonics Conference, p. 159–207.

Heiland, C.A., 1946, Geophysical Exploration: Englewood Cliffs, New Jersey, Prentice-Hall, 1013 p.

Jakosky, J.J., 1950, Exploration Geophysics: Los Angeles, Trija Publishing Co., 1195 p.

LaBerge, G.L., 1976, Major structural lineaments in the Precambrian of centralWisconsin: Proceedings of the First International Conference on the New BasementTectonics, Utah Geological Assoc., p. 508–518.

Nettleton, 1962, Elementary Gravity and Magnetics for Geologists and Seismologists:Society of Exploration Geophysicists Monograph Series 1, 121 p.

Skeels, D.C., 1947, Ambiguity in gravity interpretation: Geophysics, vol. 12, p. 43–56.

Section C

References