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  • 1. Chapter 14 Using Magnetics in Petroleum Exploration by Edward A. Beaumont and S. Parker Gay, Jr.
  • 2. Edward A. Beaumont Edward 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 in geology from the University of Kansas. Currently, he is generating drilling prospects in Texas, Oklahoma, and the Rocky Mountains. His previous professional experience was as a sedimentologist in basin analysis with Cities Service Oil Company and as Science Director for AAPG. Ted is coeditor of the Treatise of Petroleum Geology. He has lectured on creative exploration techniques in the U.S., China, and Australia and has received the 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 Lake City, 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 in the U.S. Air Force; worked as a geophysicist and geologist for Utah Construction Co. and Mining and its subsidiary, Marcona Mining Co.; was a geophysicist for Asarco, Inc.; organized and managed the U.S. subsidiary of Scintrex Ltd. in Salt Lake City; and founded American 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 the International Basement Tectonics Assoc. He has written numerous papers and given many talks on basement control of geological structure and stratigraphy. He has published 16 papers on the interpretation of magnetic anomalies and their geological causes.
  • 3. Overview Introduction Basement fault blocks often correlate with structural and stratigraphic features in the sedimentary section that control trap location. Magnetic technology senses the earth’s magnetic field. This technology—and aeromagnetics, in particular—effectively delineates basement fault blocks through the use and interpretation of magnetic residual maps and profiles. Basement lithologic changes and the resulting magnetic susceptibility changes from block to block allow us to map the basement fault block pattern and to use this information in important new ways for finding oil and gas. This chapter discusses concepts of magnetics and how to apply them to petroleum exploration. In this chapter This chapter contains the following sections. Sections Topic Page A Magnetic Basics 14–4 B Interpreting Magnetic Data 14–8 C References 14–20 Overview • 14-3
  • 4. Section A Magnetic Basics Introduction The lithology of basement rocks controls regional and local magnetic variations in the earth’s field. Seeing local variations is critical to applying magnetic technology to petroleum exploration. This section discusses the basics of magnetic theory and mapping. In this section This section contains the following topics. Topic Page Basics of Magnetics Total Intensity and Residual Magnetic Maps 14-4 14–5 14–6 • Using Magnetics in Petroleum Exploration
  • 5. Basics of Magnetics Theory Magnetic disturbances caused by rocks are localized effects superimposed on the normal magnetic field of the earth. The distribution of magnetite in rocks is the primary cause of the local variations in the magnetic field observed in magnetic surveys. Magnetite is not the 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 greater than the magnetite content of sedimentary rocks. Consequently, variations in the magnetic field result mainly from basement rocks underlying the sedimentary section. The earth’s magnetic field is measured in nanoTeslas (nT; formerly known as “gammas”). Effect of magnetic latitude Magnetic anomalies for an object of the same size, composition, and depth have different signatures at different magnetic latitudes because the magnetic inclination—the angle at which 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 object at different latitudes in the northern hemisphere. In the southern hemisphere the profiles would be the opposite [north and south would reverse south of the equator and the inclination angles (i) would be negative]. Location: Magnetic North Pole Sphere Location: Magnetic Equator Total Intensity Magnetic Anomaly for Various Inclinations Figure 14–1. After Nettleton, 1962; courtesy Society of Exploration Geophysicists. Gravity vs. magnetic anomalies 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 and shape of structures, and determining offset of basement faults Defining basement blocks, locating intrusive bodies; generalized depth to basement Magnetic Basics • 14-5
  • 6. Total Intensity and Residual Magnetic Maps Introduction Magnetic variation or susceptibility may be analyzed using either total intensity or residual maps. Magnetic residual maps reveal much more detailed geologic features—in particular, the geometry and configuration of individual basement blocks. They bring out the subtle magnetic anomalies that result from the changes in rock type across basement block boundaries. Total intensity maps show larger scale geologic features, such as basin shape or anomalous rock types deep within the basement. What is total intensity? Total intensity is the measurement from the magnetometer after a model of the earth’s normal magnetic field is removed. It is generally a reflection of the average magnetic susceptibility of broad, large-scale geologic features. What is residual? Residual is what remains after regional magnetic trends are removed from the total intensity. Residual maps show local magnetic variations, which may have exploration significance. The regional trend of the total intensity can be calculated using a number of techniques, including running averages, polynomials, low-pass filters, or upward continuation techniques. The figure below shows magnetic profiles of total intensity, regional trend, and residual. Residual Regional Total Intensity Residual Figure 14–2. After Nettleton, 1962; courtesy Society of Exploration Geophysicists. 14-6 • Using Magnetics in Petroleum Exploration
  • 7. Total Intensity and Residual Magnetic Maps, continued Total intensity and residual map example The maps below are examples of a residual map (A) that was calculated from the total intensity magnetic map (B). The grid and small circles on the total intensity map are the flight path lines [approximately 2 km (1.2 mi) apart] and location points for the flight lines. The total intensity map strikingly does not resemble the residual map and would be of limited value for delineating basement fault blocks. Total intensity magnetics responds to rock types over broad areas as well as those deep within the crust. We can see by a careful examination of map B, however, that many of the 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 basins because many enhancement techniques employing residuals, derivatives, polynomials, or downward continuation exist to reveal the subtle magnetic anomalies that result from the changes in rock type across basement block boundaries. A. mi. Contour interval 2 nT km. Contour interval 10 nT B. Figure 14–3. From Gay, 1995; courtesy International Basement Tectonics Assoc. Magnetic Basics • 14-7
  • 8. Section B Interpreting Magnetic Data Introduction This section documents a number of one-on-one correlations of the basement fault block pattern, as mapped by modern aeromagnetic techniques. Structural and stratigraphic features in the sedimentary section that are important to petroleum exploration are displayed. Several pitfalls in aeromagnetic interpretation are due to the failure to recognize the existence of the basement fault block pattern and its control on the lithology of basement. These basement lithologic changes, and the resulting magnetic susceptibility changes from block to block, allow us to map the basement fault block pattern and to use this information in important new ways for exploring for oil and gas. In this section This section contains the following topics. Topic Page Basement Fault Blocks and Fault Block Patterns Local Magnetic Field Variations 14–11 Interpreting Residual Maps 14–13 Applying Magnetics to Petroleum Exploration 14-8 14–9 14–16 • Using Magnetics in Petroleum Exploration
  • 9. Basement Fault Blocks and Fault Block Patterns Introduction The basement fault block pattern in sedimentary basins was formed in multiple tectonic and metamorphic episodes during the Archean and Proterozoic eras. Basement tends to control 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 basement, that we generally find the faults or other structures in the overlying sedimentary section. These zones of weakness are periodically reactivated by tectonic stresses or gravitational loading. Consequently, they have influenced depositional patterns and locations of structures throughout geologic time. Study of shield areas The following Landsat and SLAR images of exposed Precambrian crystalline crust show highly lineated terrains and demonstrate that the linears fall into multiple parallel or subparallel sets of varying strike directions. These overlapping fracture sets cut the basement into blocks of varying shapes and sizes. This collection of basement blocks is the basement fault block pattern. Arabian Shield Canadian Shield African Shield Gayana Shield Figure 14–4. From Gay, 1995; courtesy International Basement Tectonics Assoc. Interpreting Magnetic Data • 14-9
  • 10. Basement Fault Blocks and Fault Block Patterns, continued Precambrian surface topography The intensity of fracturing and mylonitization of the rocks in shear zones explains why these zones generally erode low and why they tend to control the topography of the Precambrian surface. This surface, in turn, controls much of the structure in the lower part of the sedimentary section through gravitational compaction of the sedimentary rocks. Canadian Shield example The figure below is a geologic map of an area of crystalline basement in central Wisconsin on the southern edge of the Canadian Shield. Here, outcrops and rock exposures in shallow excavations, roadcuts, etc., abound. It is possible to map the basement geology in considerable 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 communication) up to 2.5 km or more. 5. The shear zones and geology truncate abruptly and change style across line A–A′. Figure 14–5. From LaBerge, 1976; courtesy International Basement Tectonics Assoc. 14-10 • Using Magnetics in Petroleum Exploration
  • 11. Local Magnetic Field Variations Introduction Variations in the local magnetic field are due mainly to the following: • Lithologic changes of basement rocks with corresponding differences in magnetite content • Elevation changes on the top of basement where basement is of uniform magnetic susceptibility (k) However, lithologic changes tend to overwhelm the magnetic response of elevation changes 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 to show in the magnetic pattern. Most of this is due to detrital magnetite in sandstones. Elevation change due to a fault 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 of basement, is the cause of the anomaly. It also assumes uniform lithology and uniform magnetic susceptibility of basement across a fault. Given this (usually incorrect) assumption, we can calculate the depth of the fault and its throw from the shape and amplitude of an observed magnetic curve. If we do not know the exact susceptibility, we can calculate a series of curves to establish a range of probable values of the throw. In all cases, the magnetic high necessarily appears on the upthrown side of the fault. In the hypothetical cross section below, basement rock has the same susceptibility across the fault. Figure 14–6 Lithologic changes due to a fault Figure 14–7 shows a fault separating basement blocks of different lithologies and magnetic susceptibilities. If the average magnetic susceptibilities (k1 and k2) of the basement blocks are unknown, then we cannot determine the amount of throw of the fault—we cannot even determine the direction of throw if the signal resulting from susceptibility overrides that due to throw. Since susceptibilities of basement rocks commonly vary by hundreds, even thousands, of percent (Heiland, 1946; Jakosky, 1950; Dobrin, 1960) and the ratio of throw to depth of a fault can be, at most, 100%, then it follows that in most cases the magnetic response 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. Interpreting Magnetic Data • 14-11
  • 12. Local Magnetic Field Variations, continued Lithologic changes due to a fault (continued) Detecting basement hills The hypothetical cross section shows a fault juxtaposing basement blocks of different lithologies and susceptibilities. The curves above the cross section are the magnetic profiles where the magnetic field is vertical for k1 > k2 and k1 < k2. It assumes no throw on the fault (d = 0). The dashed curves show the magnetic response if the fault has a finite throw (d). Note how little impact the fault throw has on either profile. Figure 14–7. From Gay, 1995; courtesy International Basement Tectonics Assoc. The basement hill and obvious magnetic anomaly shown on the left side of the figure below assumes a uniform magnetic susceptibility for basement. However, given that basement is usually block faulted, is this type of feature detectable? If we are looking at a topographic prominence centered on a basement block, the detection problem becomes that shown on the right side of the figure. A series of adjacent basement blocks having different magnetic susceptibilities results in a residual magnetic pattern of alternating highs and lows (solid lines). When the basement block on which the hill is carved is more magnetic than surrounding blocks, the hill contributes slightly to the magnetic high over the block as shown. The slight increase in anomaly amplitude due to the hill (top dashed line) generally is not distinguishable from a similar increase due to a slightly higher magnetic susceptibility for the whole block; hence, the hill is not generally detectable. If the block on which the hill is carved is less magnetic than the adjacent blocks, then the hill results in a lesser amplitude of the magnetic low over that block, but the low is still present (bottom dashed line). The hill generally is not detected. Figure 14–8. 14-12 • Using Magnetics in Petroleum Exploration
  • 13. Interpreting Residual Maps Introduction A major pitfall in interpreting magnetic residual maps is assuming that magnetic highs and lows are caused by elevation changes on basement rocks in a sedimentary basin. To the contrary, most magnetic anomalies are caused by lithologic changes and the corresponding changes in susceptibility. As shown in Figure 14–5, the basement is of complex lithology and is highly fractured. The fractures divide the basement into blocks and are zones of weakness along which faults occur. The most important and most reliable information obtainable from aeromagnetic maps is the configuration (in plan view) of the underlying basement fault block pattern. Interpreting depth to basement It is futile to attempt to define accurately the vertical dimension, Z, of adjacent source bodies with magnetics because of the inherent ambiguity of potential field methods in determining Z (see, e.g. Skeels, 1947). Furthermore, seismic and subsurface methods measure depth so much more accurately than magnetics that it is unwise to try to compete with these excellent techniques. This is not to say, however, that we should not use magnetics to estimate the approximate thickness of the sedimentary section in a new basin, i.e., in determining whether it is 2, 5, or 10 km thick, for example, to a usual accuracy of about ±15% under favorable conditions. Interpreting fault throw There is a fairly reliable way to determine the direction of throw of certain basement faults from magnetic maps. Faults that vertically offset basement or other magnetic sources generally show abrupt amplitude changes of magnetic anomalies, both the highs and lows. In the figure below, a series of four northeast-trending magnetic anomalies on the 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 on both sides of this obvious down-to-the-east fault. The four anomalies disappear altogether along 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 downdropped the four anomalies beneath the level of detection—the preferred interpretation. Figure 14–9. Interpreting Magnetic Data • 14-13
  • 14. Interpreting Residual Maps, continued Interpreting shear zones The figure below shows a residual aeromagnetic map of an area on the north shelf of the Anadarko 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. The residual magnetic contours (a) are shown at a 2-nT interval. The interpreted shear zones are traced along the linear gradients separating the residual magnetic highs and lows and along truncation lines of anomalies. On the right figure (b), two faults located from subsurface mapping are shown, labeled U/D. The evidence for their existence is seen in subsurface mapping. Both are located exactly along the interpreted basement shear zones, or block boundaries, as represented by gradients on the magnetic map. Note, however, that most of the interpreted basement shear zones in this area have no corresponding overlying faults. These zones were never reactivated—at least not sufficiently enough to be detected by the existing subsurface data. Figure 14–10. Oil field example 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 block boundaries. Block boundaries, i.e., shear zones, generally erode low, so it follows that the interiors of blocks must, in many cases, correspond to basement topographic highs. West Campbell field appears to be a case in point and is most likely underlain by such a basement topographic prominence, although there are no wells to basement here to document it. The culmination of structural closure nearer the north end of the block rather than at its center is probably due to the south dip of basement in this area. 14-14 • Using Magnetics in Petroleum Exploration
  • 15. Interpreting Residual Maps, continued Interpreting fault The figure below is a residual magnetic map of another area in northern Oklahoma with faults superimposed. Here the sedimentary section is approximately 2 km (6500 ft) thick, location and the flight level was about 2.3 km (7500 ft) above basement. The faults shown were interpreted from a detailed subsurface study by Geomap Inc. The faults were mapped about 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 gradients corresponding to the interpreted basement shear zones. Some 64% of the total length of faults, in fact, lies on the predicted shear zones following magnetic gradients. Note also that many magnetic gradients in Figure 14–11 show no faults cutting the section. These may not have been reactivated. Figure 14–11. Details from Gatewood (1983). Interpreting Magnetic Data • 14-15
  • 16. Applying Magnetics to Petroleum Exploration Introduction Magnetics can be an extremely effective and economical exploration tool when properly employed. 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 Applications 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 Interpreting fault location in seismic sections Magnetics can be very valuable in interpreting seismic data by plotting residual magnetic profiles along seismic sections. This technique is valuable in looking for (1) subtle stratigraphic changes that can occur along basement block boundaries and (2) subtle fault offsets or other structural and stratigraphic features. The locations of the basement weakness 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 section from Logan County, Arkansas. The dark band corresponds to Cambrian through Mississippian sedimentary rocks. Note correlation between the location of the four normal faults interpreted in the seismic section and the location of faults in the magnetic profile (marked by diamonds). Figure 14–12. 14-16 • Using Magnetics in Petroleum Exploration
  • 17. Applying Magnetics to Petroleum Exploration, continued Developing leads Magnetic basement mapping in petroleum exploration can be applied to the search for leads or prospects that can be quickly and economically developed by comparing known from analogs traps or structure (and/or stratigraphy) with the basement fault block pattern. Some areas have never been tested by the drill where the structure at basement level is analogous to that over nearby producing properties. Some of these leads become viable prospects when subjected to follow-up seismic profiling or other appropriate exploration techniques. A common type of structural or stratigraphic data used to correlate to the magnetic data is subsurface mapping, developed from well data. However, on overseas projects or in frontier areas, the best (or only) data available may be 2-D seismic surveying. In either case, the procedure is to search for “look-alikes” on the magnetic data that correspond to features over known producing fields. Since the magnetic data can be acquired in continuous fashion over large areas at a very economical price, many good leads can be developed in a short time. Laying out seismic programs 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 of faulting. This being the case, we can expect that many of the basement shear zones have been 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 of the probable poor seismic definition due to fracturing along these zones and the possibility of sideswipe. Lines B, C, and E, on the other hand, are good places to run seismic surveys 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 C would have found West Campbell field (WCF) but line A would not. Figure 14–13. Interpreting Magnetic Data • 14-17
  • 18. Applying Magnetics to Petroleum Exploration, continued Interpreting fault location in map view Magnetics can be quite useful in interpreting existing seismic programs after they have been shot. In the example below, two 2-D seismic lines have been placed purposely in the worst 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, does not connect straight across to fault pick G on line 2, nor even to H or I, which are some distance away. Instead, it hooks up to J, making this fault very oblique to the seismic lines. 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 not connect to the other seismic line at all; they terminate somewhere in between. Figure 14–14. Depth estimates Depth estimates from aeromagnetic data can determine values for broad areas, such as the approximate thickness of the sedimentary section in a basin or at a limited number of points within the basin. Using depth estimates to distinguish between the depths of adjacent magnetic anomalies invites trouble. 14-18 • Using Magnetics in Petroleum Exploration
  • 19. Applying Magnetics to Petroleum Exploration, continued We might question the strong emphasis on magnetics for mapping the basement fault Magnetics vs. other techniques block pattern. However, is there any other way to reliably map this pattern beneath the sedimentary section? Methods that depend on surface information—Landsat, SLAR, conventional photo geology, and surface geology—are of limited value. That leaves only gravity and seismic techniques. However, gravity techniques generally do not separate adjacent basement blocks because of the lack of density contrast between adjacent blocks and because of interference from density differences within the sedimentary section. On seismic data, the basement reflector is often difficult to recognize beneath complex structure and because of a lack of velocity contrast with the dense dolomites that overlie the basement in many areas. Furthermore, both seismic and gravity methods are expensive to apply over broad areas and cannot provide even a tiny percentage of the area coverage that can be obtained with magnetics 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 by magnetics and are best used for this purpose. Interpreting Magnetic Data • 14-19
  • 20. Section C References Dobrin, M.B., 1960, Introduction to Geophysical Prospecting, 2d ed.: New York, McGrawHill, 446 p. Gatewood, L., 1983, Viola–Bromide and Oil Creek Structure (map): Oklahoma City, privately sold and distributed. Gay, S.P., Jr., 1995, The basement fault block pattern: its importance in petroleum exploration, 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, PrenticeHall, 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 central Wisconsin: Proceedings of the First International Conference on the New Basement Tectonics, 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. 14-20 • Using Magnetics in Petroleum Exploration