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    Chap15 Chap15 Document Transcript

    • Chapter 15 Applying Gravity in Petroleum Exploration by David A. Chapin and Mark E. Ander
    • David A. Chapin David A. Chapin received his B.S. degree (1979) from George Washington University and his M.S. degree (1981) from Lehigh University, both in geology. He was employed by Gulf Research from 1981 to 1984. In 1984 he joined Arco, specializing in the interpretation of gravity and magnetics data. He rose to senior principal geophysicist, responsible for research in new exploration technologies utilizing potential fields’ data. In May 1998, he left Arco and became executive vice-president of LaCoste & Romberg. As an active member of SEG, he was a founding member of the SEG Gravity and Magnetics Committee and served as a member of the Technical Program Committees for 1989, 1993, and 1997. He is also a member of the Houston Potential Fields Group, AAPG, SPWLA, and Sigma Xi. He holds three patents in gravity methods and has authored numerous scientific papers. Mark E. Ander Mark E. Ander specializes in potential field and electromagnetic exploration methods. He is presently CEO of LaCoste & Romberg. He was vice president of EDCON from 1994 to 1996. Prior to that he spent fifteen years as a research geophysicist at Los Alamos National Laboratory. He received international attention for his geophysical scale measurements of the Universal Gravitational Constant and investigations into possible violations of Newton’s inverse square law of gravity. In 1970 he received a B.S. in mathematics and physics from Jacksonville University. He received an MS in physics in 1974 and a Ph.D. in geology and geophysics in 1980 from the University of New Mexico. His professional affiliations include AGU, SEG, GSA, and SPE. He is an author of many scientific articles and abstracts.
    • Overview Introduction Gravity offers significant applications to petroleum exploration. Gravity measurements are affected by changes in rock density. Surface gravity surveys and subsurface surveys made with a borehole gravity meter are effective in locating faults and geologic structures with density contrasts to their surroundings. The borehole gravity meter has broad application, from locating porosity in wildcats or through casing in old wells to monitoring fluid changes in productive reservoirs. This chapter discusses the traditional application of gravity, from surface surveys to the application of borehole gravity measurements. In this chapter This chapter contains the following sections. Section Topic Page A Applying General Gravity Methods 15–4 B Applying Borehole Gravity Methods 15–15 C References 15–25 D Annotated References 15–26 Overview • 15-3
    • Section A Applying General Gravity Methods Introduction Surface gravity methods help constrain subsurface structural interpretations and are particularly good at inexpensive reconnaissance studies in areas where either seismic is too expensive or where seismic imaging is poor. Gravity data provide another independently measured geophysical constraint for interpretation problems: lateral density distributions. Location and configuration of subbasins, identification and extensions of structures, and location of major subsurface faulting are the major uses of this technology. This section discusses concepts related to gravity acquisition, processing, and interpretation and applies these techniques to petroleum exploration. In this section This section contains the following topics. Topic Page Gravity Basics Processing Gravity Data 15–7 Interpreting Gravity Measurements 15–8 Models of Gravity Anomalies 15–10 Examples of Gravity Applications 15-4 15–5 15–12 • Applying Gravity in Petroleum Exploration
    • Gravity Basics Introduction Gravity methods help us identify the size, shape, and depth of anomalous masses. Gravity effectively images lateral density contrasts within the subsurface. It is particularly good at locating geologic structures horizontally. However, it is not as good as magnetics in determining depth to source because density distributions tend to be diffuse in the subsurface. Uses of gravity Gravity has the following uses: • Determining the shape of salt masses • Locating structures under thrust plates or volcanics • Locating major faults and determining sense of motion on faults • Finding reefs • Locating intrusives • Defining overall basin configuration • Determining structural trend continuation between wells and seismic data • Mapping large tectonic features Advantages of gravity Gravity has advantages over other methods: • Fast, inexpensive tool for evaluating large areas • Can distinguish sources at exploration depths • Nondestructive; measures an existing field through a passive measurement • Can use old data today and easily integrate with new data • Lends itself to simple enhancements • Scalar measurement can yield a pseudostructure map Disadvantages of gravity Following are the disadvantages of using gravity vs. other methods: • Needs geological and geophysical constraints to interpret • Does not directly provide a structural cross section without additional geologic input • Overlapping anomalies may confuse the interpretation • Data quality may deteriorate in rougher terrain • Tends to image gross structures; finer structures are more difficult to image • Resolution deteriorates with depth Theory Gravity effects caused by subsurface geology are superimposed upon the earth's overall gravity field. These effects, called anomalies, are typically less than 100 ppm of the total field. Several corrections are made to remove the earth's field from the total measurement to image these anomalies. For petroleum exploration, gravity is measured in milligals (mGal). Typical exploration anomalies are generally < 25 mGal. Typical gravity sensors are capable of measurements < 0.5 mGal. Applying General Gravity Methods • 15-5
    • Gravity Basics, continued Gravity vs. magnetics The figure below is a schematic of the earth, showing its gravity field (left) and magnetic field (right). The gravity field always points downward; thus, the measurements can be scalar. In contrast, the magnetic field can point in any direction; therefore, vector information is more important in interpreting magnetics. Gravity Magnetics Figure 15–1. Acquisition and instrumentation Gravity can be collected on land, at sea, in the air, and by satellite. On land, sea, and air, most sensors consist of a mass at the end of a spring (see figure below). On land, the instrument is leveled and the mass is set to a null position for reading the spring tension. In dynamic gravity measurements at sea or in the air, the position of the mass and the spring tension is recorded continuously. The instrument is on a stabilized platform during the measurement to maintain a vertical position. In a dynamic gravity measurement, extra care must be taken to keep track of the platform's position. The resulting gravity is usually not as accurate as land data. Satellite gravity is derived from satellite radar altimetry of the sea surface. The sea conforms to the gravity field of the earth; the first derivative of the sea surface height is gravity at the sea surface level. Since the satellite can only measure gravity over water, only the marine areas and large lakes have such data. The data quality is somewhat comparable to surface-acquired data, although the wavelength resolution is usually worse for the satellite data. Sp rin g Te ns io n Mass Position Figure 15–2. 15-6 • Applying Gravity in Petroleum Exploration
    • Processing Gravity Data Routine processing techniques Gravity data as measured must be corrected for the earth's field (Chapin, 1996a). There are five categories of corrections. Correction Definition Key Input Parameter(s) Latitude Whole-earth effect at sea level related to the shape and the spin of the earth Latitude Free air Correction because the observation is not at sea level Elevation Bouguer Free air correction to add back the rock between the observation and sea level Elevation & surface density Terrain Simplified assumptions of Bouguer correction in high-relief areas Detailed topography & surface density Eötvös Gravity collected on moving platforms has different angular velocities than the earth’s for dynamic gravity Platform's velocity & heading One of the most critical parameters in typical surveys is the high accuracy needed in elevation control. Submeter accuracies are usually necessary, except for marine surveys which are, by definition, collected at sea level. Interpretive processing techniques After obtaining either free air gravity or Bouguer gravity, subsequent processing may be needed to enhance or suppress various geologic effects. For example, Bouguer gravity naturally has lower values over higher elevations and higher values over deep ocean basins because of variations in the crustal thickness, or isostasy. An isostatic correction to suppress this deep effect can be made if the data set is large enough (Chapin, 1996b). In smaller data sets, typically a long-wavelength surface can be removed from the data to suppress this effect. Other interpretive processing to enhance certain anomalies includes the following: • Band-pass filtering—selecting a range of wavelengths to display • Derivatives—edge-enhancing processes that tend to emphasize the shorter wavelength anomalies • Upward/downward continuation—a process that attenuates or deattenuates data to simulate what might be observed at different vertical datums Collectively, these are all termed regional/residual operators. Many different types are useful for different purposes. Applying General Gravity Methods • 15-7
    • Interpreting Gravity Measurements Procedure The table below outlines a suggested procedure for interpreting gravity. Step Action 1 Use well logs and outcrop data to make structural cross sections through critical areas containing gravity data to be interpreted. 2 Plot gravity profiles above structural cross sections and seismic sections. Add magnetic profiles if available. 3 From geology interpreted from data, build gravity model sections. Divide the section into intervals of approximately the same density. 4 Calculate a predicted gravity profile. Check the observed profile against the calculated profile. Where differences exist, adjust the gravity model and recalculate the gravity profile until a suitable match between observed and calculated is made. 5 Check the interpretation of the gravity map, i.e., location of faults, against the model profiles and all other available data. Interpreting structure In many places in the world, structural highs have higher density and are expressed in the data as gravity highs because dense basement rocks are closer to the surface. Therefore, a gravity map often is used directly as a pseudostructure map. However, there is not a one-to-one relationship between milligals and depth; therefore, the map must be viewed in a qualitative sense as a formline map. There are exceptions, as well, where structural highs are gravity lows because dense basement rocks are not closer to the surface. These structures may be of lower density than the surrounding rocks. Rock densities The range of densities for all rock types (igneous, metamorphic, and sedimentary) is typically 1.60–3.20 g/cm3. The density values of sedimentary rocks typically range from 1.80 to 2.80 g/cm3. Thus, small variations of density in sedimentary rocks may be invisible to the method. A 5–10% error in estimating subsurface densities from gravity is quite common. This is in contrast to magnetics, where typically there are orders of magnitude variations in susceptibilities. Horizontal layers Horizontal layers have no anomalous gravity response. Thus, it is impossible to determine the subsurface density distribution if there are no lateral changes. Layer-cake geology yields no anomalous gravity signal. A bed is considered infinite and horizontal if it is about five times wider in all directions than it is thick, with no dip. 15-8 • Applying Gravity in Petroleum Exploration
    • Interpreting Gravity Measurements, continued Interpretation ranges Gravity interpretation can produce a range of answers. The better the geologic and geophysical constraints, the better the interpretation. A completely unconstrained interpretation produces several acceptable answers that can all produce the identical anomaly. While it is often easy to rule out certain classes of interpretations as geologically unreasonable, it is best to start with good constraints or to test reasonable geologic questions. Depth-tobasement determination Gravity is not as good at depth-to-basement or depth-to-density anomaly estimations as other geophysical methods. Though possible to do, it is often difficult to determine the appropriate depth to geologic source unless other constraints exist. Positions of geologic bodies Gravity is particularly good at locating horizontal positions of geologic bodies that have a different density than the surrounding rock—ore bodies or salt-cored bodies, for example. Interpreting fault location Faults usually can be identified through either steep gradients or truncation of trends. The figure below contains two Bouguer gravity maps of Southern California, showing the expression of faults. In the left map, locations a and b show strong gradients, which indicate faults. A series of truncated trends (dashed) near location c also indicate faults. The right map shows the actual location of the major faults over the same gravity map. Figure 15–3. Applying General Gravity Methods • 15-9
    • Models of Gravity Anomalies Introduction Three key parameters of the source body affect the size and shape of the gravity response: • Density • Depth • Size In the following discussion, 2-D cross sections demonstrate each of these parameters on theoretical gravity profiles. While the models may not be geologically reasonable, the concepts they demonstrate provide important building blocks for more complex geometric modeling, which is often performed to solve real exploration problems. When modeling gravity effects, it is much more important to constrain the size (shape) and depth of the geologic body than it is to constrain the density. Effect of density The amplitude of a gravity anomaly has a linear relationship to density. Positive density contrasts produce gravity highs; negative density contrasts produce gravity lows. The wavelength of the anomaly is unaffected by differences in the density. The figure below shows the different gravity responses to a body with different positive density contrasts. In the upper half of the diagram are the gravity responses. The lower half of the diagram is a cross section. Values for the different densities are written next to the gravity response in the upper part of the figure. Figure 15–4. 15-10 • Applying Gravity in Petroleum Exploration
    • Models of Gravity Anomalies, continued Effect of depth The amplitude of the gravity signal varies as a function of 1/depth2 to the source. The figure below shows the gravity responses to a body of positive density contrast buried at different depths. The upper half of the diagram shows the gravity responses (labeled A, B, and C) to a body buried to depths A, B, and C, shown in the cross section in the lower half. Figure 15–5. Effect of size The gravity response is related directly to the amount of anomalous mass. Size differences in three dimensions are X3 functions. The figure below shows the gravity responses to bodies of the same density, at approximately the same depth, that are of different sizes. Figure 15–6. Applying General Gravity Methods • 15-11
    • Examples of Gravity Applications Satellite gravity, The figure below shows a satellite-derived free air gravity color image of the Andaman Sea area of Southeast Asia. Cooler colors are gravity lows; warmer colors are gravity Andaman Sea highs. A number of important tectonic elements can be interpreted easily from the image. The trenches are seen as deep blue, and the ridges are seen as bright red. The offshore extension of the Sumatra wrench fault system can be seen, as well as the active spreading center within the Andaman Sea (between the Alcock and Sewell seamounts). This type of data is useful for broad tectonic interpretations and to extend known geologic elements from onshore to offshore areas. Figure 15–7. Courtesy ARCO Exploration and Production Technology. 15-12 • Applying Gravity in Petroleum Exploration
    • Examples of Gravity Applications, continued Land gravity from Sudan In the figure below, the upper panel shows a map of a land gravity survey taken near Khartoum, Sudan. The stippled contours are gravity values of –50 mGal and lower. The exploration objective was to identify new Mesozoic subbasins along the Central African Rift system. These subbasins cannot be seen at the surface because they are covered by a thin veneer of alluvium, sand, and river deposits. However, the subbasins, because they are lower density than cratonic basement, can be identified in the gravity data as large gravity minima, shown in the stipled pattern. Nearby in a similar geologic setting, Chevron made a series of discoveries in the Muglad area using gravity as a primary exploration tool (Giedt, 1990). The gravity lows in this map indicate hitherto undiscovered subbasins, identified along gravity profile A–A' in the bottom panel. Figure 15–8. Modified from Jorgensen and Bosworth (1989) and Millegan (1990). Applying General Gravity Methods • 15-13
    • Examples of Gravity Applications, continued Marine gravity from U.S. East Coast The figure below shows an integrated constrained interpretation of gravity and seismic data along USGS survey line 25. Line 25 is a dip line from offshore southern New Jersey across the continental margin (over the Baltimore Canyon Trough). The free air gravity was modeled using a Talwani-type 2-D algorithm (Talwani et al., 1959) and was constrained by a reflection seismic interpretation, wells, and seismic refraction data. A complex and detailed geologic cross section is the result of this study. The relationship between the synrift and postrift sediments was resolved in part by gravity modeling. In combination with the magnetic data, the type and configuration of the basement can also be resolved. Gravity interpretation had significant exploration impact in determining depth of sediment burial, migration direction, and trap development for hydrocarbon exploration within the Baltimore Canyon Trough. Figure 15–9. Modified from Grow et al. (1988) and Sheridan et al. (1988). Gravity model by D.R. Hutchinson and J.A. Grow (1980), courtesy USGS. 15-14 • Applying Gravity in Petroleum Exploration
    • Section B Applying Borehole Gravity Methods Introduction Borehole gravity is a density logging technology. It is the only logging method that can directly measure density at a significant distance away from a well. It is also the only logging method that can reliably obtain density through well casing. This information is often vital for accurate porosity measurements and in determining the presence and amounts of oil, gas, and water in a hydrocarbon reservoir. This section discusses concepts of borehole gravity theory, acquisition, and applications to petroleum exploration. In this section This section contains the following topics. Topic Page Basics of Borehole Gravity 15–16 The Borehole Gravity Tool 15–17 Examples of Borehole Gravity Applications 15–19 Applying Borehole Gravity Methods • 15-15
    • Basics of Borehole Gravity Borehole gravity Borehole gravity is especially effective for the following exploration and production purposes: uses Exploration purposes • Locating nearby salt structures • Locating distance to nearby structures (e.g., reefs) for step-outs and sidetracks • Better synthetic seismograms Production purposes • Measuring bulk density when radioactive tools are too risky • Logging cased holes for lithologic changes • Calculating overburden for hydrofracture jobs • Monitoring injection fluids • Monitoring reservoirs during fluid withdrawal • Exploring for bypassed, behind-casing gas zones • Evaluating reservoir porosity, especially in carbonate reservoirs where other tools are not as reliable Borehole gravity advantages The following characteristics give borehole gravity surveys advantages in certain situations: • Directly measures bulk density • Is a deep imaging tool • Is effective in both cased and uncased wells • Is unaffected by washouts, hole rugosity, or mud invasion effects • Can help determine seismic wavelet scale density • Is a passive measurement, e.g., does not have active radioactive sources Borehole gravity disadvantages The following characteristics give borehole gravity surveys disadvantages in certain situations: • Direction away from the well to distant source cannot be determined without other information • Engineering limitations of the tool restricts use to certain candidate wells (hole size, low deviation, slow reading) • Only a few tools presently available for use • Expensive to operate Theory Density effects caused by downhole geology can be detected by very sensitive instrumentation and by knowing precisely where the sensor is located in the borehole. For petroleum exploration, gravity typically is measured in microgals (µGal). Typical exploration anomalies are on the order of < 50 µGal. The present borehole gravity tool is capable of a 3-µGal repeatability. 15-16 • Applying Gravity in Petroleum Exploration
    • The Borehole Gravity Tool How the tool measures gravity The figure below illustrates the fundamentals of measuring density using a borehole gravity sensor. Two gravity measurements, g1 and g2, are made downhole, separated in depth by ∆z. The value G is the universal gravity constant. Thus, the gravity gradient, ∆g/∆z, is related directly to the density of the intervening layer. The result is a direct computation of the bulk density of that layer. Figure 15–10. Depth of investigation The figure below shows how the depth of investigation is tunable by means of varying the separation between two gravity measurements, ∆z. The rule of thumb is that 90% of the gravity effect can be imaged at a distance away from the borehole within five times ∆z. Figure 15–11. After McCulloh et al., 1968; courtesy SPWLA. Applying Borehole Gravity Methods • 15-17
    • The Borehole Gravity Tool, continued Depth of investigation of various logging methods The following table, based on data from Beyer (1991), shows conservative estimates of the depth of investigation using various density logging tools. Borehole gravity can sample the farthest distance and investigate the most formation. Logging Method Radial Distance for 90% Effect Formation Volume Investigated in. cm ft3 m3 2.6 6.6 1.5 0.04 Gamma-gamma log 8 20 17 0.5 Neutron log 14 36 40 1.1 Sonic log 18 46 59 1.7 Borehole gravity log 600 1,500 78,532 2,224 Conventional 5.25-in. (13-cm) core Logging procedure In a typical logging operation, several downhole station locations are planned ahead of time. The tool is fitted to a conventional wireline and lowered to each station. Once the tool is stopped (in some wells it must be clamped to the side of the well), the measurement begins. The gravity values are telemetered to an operator at the surface. Because of vibrations, seismic activity, or residual tool movement, the measurements may take some time to settle to an acceptable noise level. Tool limitations There are three major problems with the present tool. 1. It is large. The smallest typical configuration is 41⁄8" OD, but more widely used configurations are up to 51⁄4". 2. Because the sensor must be set to vertical to make a reading, it can only measure in wells of 14° deviation or less. 3. It takes a long time to make measurements, and it is not a continuous logging tool. The wireline must stop at each individual station, and the tool takes an average of 5–15 min per station for a reading. This means a typical borehole gravity logging operation can run from 24 to 48 hrs. For the most part, these problems are engineering limitations of the present tool that could be overcome with new and modern sensor development. There is an active project underway to redesign the present tool to make it more useful in modern wells. 15-18 • Applying Gravity in Petroleum Exploration
    • Examples of Borehole Gravity Applications Distant reef exploration The broad departure between the BHGM and gamma-gamma density logs in this Michigan reef example reveals the edge of the reef complex is within a few hundred feet of the well. The overlying low-density zone near the top of the log is salt. The sharp difference in density at the arrow is caused by a remote higher porosity zone not detected by the gamma-gamma density log. The broader difference anomaly observed over the length of the interval is explained by the influence of the entire reef complex. The figure below shows three logs. The log under the scale on the left is the difference between the BHGM density measurements and the gamma-gamma log density measurements. The logs under the scale on the right are the density values measured by the BHGM (left line) and the gamma-gamma tools (right line). Figure 15–12. From Rasmussen, 1975; courtesy The Log Analyst. Applying Borehole Gravity Methods • 15-19
    • Examples of Borehole Gravity Applications, continued Salt body geometry In many Gulf of Mexico prospects, salt plays a key role in acting as a structural trap. Overhanging salt often forms seals, and sediments on salt flanks can have structural and stratigraphic pinch-outs against the salt. The exact shape of the salt is critical in understanding these traps. Unfortunately, seismic imaging often tends to be poor in these prospects. In this synthetic model (taken from a real structure), if a borehole gravity log were run, it would be able to tell conclusively which of the two seismic interpretations shown below in the figure was valid. Either interpretation would have a significant impact on the completion and economics of the exploration play. Figure (A) is predicted BHGM logs through a salt body in the Gulf of Mexico, (B) is a model of the salt body, and (C) is a seismic section through the salt body shown in the model (B). (C) Seismic Figure 15–13. Courtesy ARCO Exploration and Production Technology, 1997. 15-20 • Applying Gravity in Petroleum Exploration
    • Examples of Borehole Gravity Applications, continued Monitoring well drawdown One of the most attractive aspects of borehole gravity applications is its ability to detect gas, oil, and water contacts at large distances from the borehole. It can do this through multiple casing strings and formation damage—conditions where the neutron density tool performs poorly. In many hydrocarbon reservoirs, the oil has a gas cap. Frequently, these reservoirs have an underlying water zone. The shape of these interfaces over time is critical to production strategy. Methods can determine where those contacts exist in the wellbore, but only borehole gravity can determine their shape away from the well. Because the interfaces are mobile with time, their movement can be monitored with borehole gravity. The figure below shows a synthetic model of the configuration of a theoretical drawdown gas cone around a producing well, modeled after the Prudhoe Bay field, Alaska. Since so little is known about the shape of gas coning, present logging methods can severely underestimate the true gas–oil contact in the reservoir away from wells. Borehole gravity can determine the shape of the gas cone as well as locate the true gas–oil contact at a distance from the producing wells. Logs A, B, and C correspond to different gas cones shown in the model. ∆∆∆∆∆∆ ++++++ xxxxxx γ−γ BGHM density density density differences Figure 15–14. Courtesy ARCO Exploration and Production Technology, 1997. Applying Borehole Gravity Methods • 15-21
    • Examples of Borehole Gravity Applications, continued Bypassed pays Because the borehole gravity meter is the only tool that can measure bulk density away from the borehole, it is ideal to use for finding bypassed pay zones. In the figure below, Case #1 shows a model of laterally homogenous geology and no density anomalies. Case #2 shows a region of lower density, possibly signifying the presence of missed hydrocarbons 60 m from the well. The density difference detects the distant density contrast as a broad, anomalous low with its minimum centered at the correct depth. Such a zone may be within range of a borehole sidetrack. In Case #3 the low-density missed pay zone is within 15 m of the well, and a strong density difference exists. Such pay zones may be in the range of a possible well completion after hydrofracturing the reservoir. In Case #4 the missed pay is about 1 m from the well, and the density difference is very pronounced. Such pay zones are within the range of normal well completions but would still be undetected by any other logging method. These examples show that borehole gravity can indicate the presence of bypassed pay zones 1–60 m from the well. Once the well is cased, no other logging tool can do this. This is why the borehole gravity tool is currently the best technology available to search for bypassed hydrocarbons in existing wells. Figure 15–15. After Beyer, 1991; courtesy SEG. 15-22 • Applying Gravity in Petroleum Exploration
    • Examples of Borehole Gravity Applications, continued Combining BHGM with tomography Between-well imaging jointly uses borehole gravity with seismic tomography. Because of the unique distant resolution capabilities of borehole gravity, these data provide a useful integrating tool at the seismic wavelet scale. In the Gulf of Mexico example shown in the figure below, Amoco used its borehole gravity log to help interpret a detailed crossborehole seismic tomography image. The two wells were located less than 250 ft apart. Two faults, F1 and F2, are seen in both data sets, and excellent correlations are made of various sands labeled M5, M6, M8, M9, M10, and M10A. Note that the well on the left encountered more pay sands than the well on right. Also note that the M6 sand is missing in the well on the left. The between-well structural and stratigraphic changes in only 250 ft can be understood by combining the interpretations of the two comparable distant imaging tools: borehole gravity and seismic tomography. F2 M5 M5 F1 M8 M6 M9 M10 M10A M10A Figure 15–16. After Lines et al., 1991; courtesy CSEG Recorder. Applying Borehole Gravity Methods • 15-23
    • Examples of Borehole Gravity Applications, continued Monitoring gas production The figure below shows an example of a borehole gravity tool that succeeded where conventional open-hole and cased-hole logging methods had failed. In the upper part of the log, the gamma-gamma density log underestimates the gas saturation by about 15%. In the lower part of the log, wash-out zones are dominant, affecting the gamma-gamma log but not the BHGM log. Over these intervals, borehole gravity gives a more reliable and higher overall density measurement. The reservoir was fractured at 853 m, and a normally tight reservoir started to produce gas. The second BHGM logging run shows the lower density of the fractured, gas-filled producing interval. Shallower than 810 m, both gammagamma and BHGM logs agree. The borehole gravity tool was used to measure secondary gas saturation in a fractured limestone reservoir. Figure 15–17. After van Popta et al., 1990; courtesy SPE. 15-24 • Applying Gravity in Petroleum Exploration
    • References Beyer, L.A., 1991, Borehole Gravity Surveys: SEG Short Course notes, June, 350 p. Excellent source for general principles of borehole gravity. Very good figures and references. Chapin, D.A., 1996a, The theory of the Bouguer gravity anomaly: a tutorial: Leading Edge, May 1996, p. 361–363. A summary of modern techniques for processing raw gravity data. Chapin, D.A., 1996b, A deterministic approach towards computing isostatic gravity residuals: case history from South America: Geophysics, vol. 61, no. 4, p. 1022–1033. Details about new ways of computing isostatic residuals and computing the correct Bouguer reduction density. Giedt, N.R., 1990, Unity field, in E.A. Beaumont and N.H. Foster, eds., Treatise of Petroleum Geology Atlas of Oil and Gas Fields: AAPG Structural Traps 3, p. 177–197. Grow, J.A., K.D. Klitgord, and J.S. Schlee, 1988, Structure and evolution of Baltimore Canyon Trough, in R.E. Sheridan and J.A. Grow, eds., The Geology of North America, vol. I-2: GSA, p. 269–290. Jorgensen, G.J., and W. Bosworth, 1989, Gravity modeling in the Central African Rift system, Sudan: rift geometries and tectonic significance: J. Afri. Earth Sci., vol. 8, p. 283–306. Lines, L.R., H. Tan, and S. Treitel, 1991, Velocity and density imaging between boreholes: CSEG Recorder, vol. 16, no. 6, p. 9–14. A unique case study that integrates borehole gravity with the processing and interpretation of cross-well tomography. Millegan, P.S., 1990, Aspects of the interpretation of Mesozoic rift basins in northern Sudan using potential fields data: Expanded abstracts with biography, SEG 60th Annual Meeting, p. 605–607. McCulloh, T.H., J.R. Kandle, and J.E. Schoellhamer, 1968, Application of gravity measurements in wells to problems of reservoir evaluation: Transactions of the 9th Annual SPWLA Logging Symposium. Fundamental work describing the distance of sources seen by the borehole gravity meter. Rasmussen, N.F., 1975, The successful use of the borehole gravity meter in northern Michigan: The Log Analyst, September–October, p. 1–10. Sheridan, R.E., J.A. Grow, and K.D. Klitgord, 1988, Geophysical data, in R.E. Sheridan and J.A. Grow, eds., The Geology of North America, vol. I-2: GSA, p. 177–196. van Popta, J., J.M.T. Heywood, S.J. Adams, and D.R. Bostock, 1990, Use of borehole gravimetry for reservoir characterisation and fluid saturation monitoring: Expanded Abstracts, SPE Europec 90 Conference, p. 151–160. Use of time-lapsed borehole gravity logging to monitor fluid movement away from the borehole. References • 15-25
    • Annotated Bibliography General information Alixant, J.L., and E. Mann, 1995, In-situ residual oil saturation to gas from timelapse borehole gravity: Proceedings of the annual SPE Conference, p. 855–869. Use of reservoir monitoring technique in the Rabi-Kounga field, Gabon. Blakely, R.J., 1995, Potential Theory in Gravity & Magnetic Applications: Cambridge, Cambridge Univ. Press, 441 p. The state-of-the-art in modern potential field theory. Good sections on the various gravity corrections with models to show the effect and an excellent description of Fourier filtering. Brady, J.L., D.S. Wolcott, and C.L.V. Aiken, 1993, Gravity methods: useful techniques for reservoir surveillance: Proceedings of the SPE Western Regional Conference, p. 645–658. Predicted use of borehole gravity in reservoir monitoring of the Prudhoe Bay and Kuparuk fields, Alaska. Chapin, D.A., and M.E. Ander, 1999, New life for borehole gravity?: AAPG Explorer, vol. 20, no. 2, p. 24–29. A description of the borehole gravity method for the nonspecialist. Dehlinger, P., 1978, Marine Gravity: Amsterdam, Elsevier Scientific Publishing Co., 322 p. A thorough discussion of methods and interpretation of gravity with special emphasis on the stabilized platform meter and navigational methods. Dobrin, M.B., 1976, Introduction to Geophysical Prospecting: New York, McGraw-Hill Book Co., 630 p. Excellent source; complementary to Nettleton (1976). Edcon, 1977, Borehole Gravity Meter Operation and Interpretation Manual: Denver, Edcon, 110 p. Excellent source for general principles of borehole gravity. Has very good figures. Gournay, L.S., and W.D. Lyle, 1984, Determination of hydrocarbon saturation and porosity using a combination borehole gravimeter (BHGM) and deep investigating electric log: Proceedings of the 25th Annual Meeting of SPWLA, p. WW1–WW14. Methodology to detect bypassed hydrocarbons by comparing borehole gravity with electric logs. Heiskanen, W.A., and F.A. Vvening-Meinesz, 1958, The Earth and Its Gravity Field: New York, McGraw-Hill Book Co., 470 p. A classic. Contains theory and techniques for dealing with large-scale gravity data sets. Jageler, A.H., 1976, Improved hydrocarbon reservoir evaluation through use of borehole-gravimeter data: Journal of Petroleum Technology, vol. 28, no. 6, p. 709–718. A good review of the borehole gravity method. LaCoste, L.J.B., 1967, Measurement of gravity at sea and in the air: Reviews of Geophysics, vol. 5, no. 4, p. 477–526. A complete and extensive review of the development and use of the stabilized platform meter for shipborne and airborne acquisition. Also contains a useful discussion of the water-bottom gravity meter. LaFehr, T.R., 1983, Rock density from borehole gravity surveys: Geophysics, vol. 48, no. 3, p. 341–356. How to compute whole-rock apparent densities from borehole gravity data. 15-26 • Applying Gravity in Petroleum Exploration
    • Annotated Bibliography, continued General information (continued) Maute, R.E., and L.S. Gournay, 1985, Determination of residual oil saturation with the borehole gravity meter: SPE Middle East Oil Techology Conference, p. 185–188. How to compute oil saturation from borehole gravity logs. Nettleton, L.L., 1976, Gravity and Magnetics in Oil Prospecting: New York, McGrawHill Book Co., 464 p. Excellent source, particularly for acquisition and historical methods. Slightly out of date with current methods. Highly readable. Smith, N.J., 1950, The case for gravity data from boreholes: Geophysics, vol. 15, no. 4, p. 605–636. Groundbreaking paper that describes the modern concepts for use of a borehole gravimeter. Telford, W.M., L.P. Geldart, R.E. Sheriff, and D.A. Keys, 1976, Applied Geophysics: Cambridge, Cambridge Univ. Press, 860 p. Very complete. Contains a lot of theory. Interpretation— general concepts Al-Chalabi, M., 1971, Some studies relating to nonuniqueness in gravity and magnetic inverse problems: Geophysics, vol. 36, no. 5, p. 835-855. Potential field ambiguity as proven by model studies. Ervin, C.P., 1977, Short note: theory of the Bouguer anomaly: Geophysics, vol. 42, no. 7, p. 1468. An important paper reiterating basic concepts. Pawlowski, R.S., 1992, Tutorial: gravity anomalies for nonspecialists: Leading Edge, Sept. 1992, p. 41–43. A nice review of what gravity anomalies really are, based upon models. Romberg, F.E., 1958, Key variables of gravity: Geophysics, vol. 23, no. 4, p. 684–700. A highly readable article outlining many basic interpretation concepts. Skeels, D.C., 1947, Ambiguity in gravity interpretation: Geophysics, vol. 12, no. 1, p. 43–56. Proof that gravity cannot be interpreted uniquely. Interpretation— residuals Coons, R.L., J.W. Mack, and W. Strange, 1964, Least-square polynomial fitting of gravity data and case histories, in G.A. Parks, ed., Computers in the Mineral Industries: Stanford Univ. Publications, vol. 9, no. 2, p. 498–519. Discussion and use of the polynomial residual method. Elkins, T.A., 1951, The second derivative method of gravity interpretation: Geophysics, vol. 16, no. 1, p. 39–56. Development and use of the second derivative method using grid convolution operators. Fuller, B.D., 1967, Two-dimensional frequency analysis and design of grid operators, in Mining Geophysics, vol. II: Tulsa, SEG, p. 658–708. A landmark paper comparing frequency domain operations to space domain operations. Kanasewich, E.R., 1981, Time Sequence Analysis in Geophysics, 3rd ed.: Edmonton, University of Alberta, 480 p. An excellent, modern and complete source for frequency domain theory, written mainly for seismic applications. Annotated Bibliography • 15-27
    • Annotated Bibliography, continued Interpretation— residuals (continued) Interpretation— modeling Nettleton, L.L., 1954, Regionals, residuals, and structures: Geophysics, vol. 19, p. 1–22. A fairly complete discussion about the use of grid residuals. Skeels, D.C., 1967, What is residual gravity?: Geophysics, vol. 32, p. 872–876. A discussion of regional/residual separation. Bhattacharyya, B.K., 1978, Computer modeling in gravity and magnetic interpretation: Geophysics, vol. 43, p. 912–929. A review of the theory of various modeling schemes. Cady, J.W., 1980, Calculation of gravity and magnetic anomalies of finite-length right polygonal prisms: Geophysics, vol. 45, p. 1507–1512. A modified version of Talwani et. al. 1959 method of gravity modeling for a 2-D geometry. Caution: there are numerous errors in this paper. This is the method used in most current modeling programs. Parker, R.L., 1972, The rapid calculation of potential anomalies: Geophys. Jour. of the Royal Astronomical Society, vol. 31, p. 447–455. Development of the theory to calculate gravity or magnetic anomalies in the frequency domain, which can be applied to modeling or inversion techniques. Talwani, M., J.L. Worzel, and M. Landisman, 1959, Rapid gravity computations for two-dimensional bodies with application to the mendocino submarine fracture zone: Journal of Geophysical Research, vol. 64, no. 1, p. 40–59. Landmark paper for forward-type 2-D polygonal gravity modeling. Interpretation— miscellaneous concepts Chandler, V.W., J.S. Koski, W.J. Hinze, and L.W. Braile, 1981, Analysis of multisource gravity and magnetic anomaly data sets by moving-window application of Poisson’s theorem: Geophysics, vol. 46, no. 1, p. 30–39. Use of Poisson’s relationship to identify joint magnetic and gravity sources. Hammer, S., 1983, Airborne gravity is here!: Geophysics, vol. 48, no. 2, p. 213–223. A controversial presentation of the airborne gravity method with examples of recent usages. A sales pitch for Carson Geoscience, it is also worthwhile to read discussions in the March and April 1984 issues of Geophysics by N.C. Steenland, A.T. Herring, and W.C. Pearson and January 1985 by M.J. Hall. LeFehr, T.R., 1965, The estimation of the total amount of anomalous mass by Gauss’s theorem: Journal of Geophysical Research, vol. 70, p. 1911–1919. How to analyze the gravity anomaly to compute the amount of anomalous mass. 15-28 • Applying Gravity in Petroleum Exploration