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etter understanding of geology on regional and prospectlevels using potential field data continues to encouragethe melding of these fields to produce enhanced inter-pretations. Three case studies are presented in this arti-

cle, all with different geologic settings and targets. All of theexamples draw on databases from non-exclusive gravity andmagnetic surveys.

Two large databases and their interpretation in Gabonand Nigeria were the topics of recent poster papers at theAfrican Symposium, sponsored jointly by the HoustonGeophysical Society and the Petroleum Exploration Society ofGreat Britain (PESGB) in Houston in September, 2003. Theyare illustrative of the approach that utilizes regional data (inthis case satellite-derived gravity) to develop the tectonicframework and more detailed shipborne or airborne gravityand magnetic data to focus on specific targets. The continuedhigh level of activity off West Africa attests to the wide-rang-ing potential for further development in this region. Manyareas, both onshore and offshore, have been extensivelyexplored with 2D and 3D seismic surveys and many wells.

Gravity and magnetic data provide a low cost way toscreen large areas as well as construct important alternativemodels to delineate subsurface structures and reach a betterunderstanding of the geology. The density contrasts presentedby the juxtaposition of sediments with shales and salt makedetailed gravity modelling in this region a valuable exercise.The magnetic data provide insight into mapping basementsurfaces and delineating shallower volcanics and in somecases shale or salt diapirs.

In the Gulf of Mexico, the reconstruction of the paleo-tec-tonic history is also enhanced by incorporating the potentialfield data. New work is ongoing which blends the geologicinformation with the gravity and magnetic data using region-al scale modelling techniques. Gravity and magnetic datahave been traditionally thought of as regional screening toolscapable of providing basin edges or basement mapping. Inrecent years, the application of these data has been greatlyexpanded to include modelling of prospect-level targets.

Gabon: geologic alternatives addressed usinggravity and magneticsGabon is the subject of increasing interest for oil explorationthese days. Geologic challenges that are encountered inGabon are not limited to national boundaries but are foundin neighbouring areas, as well as analogous environmentsaround the world. Specific to Gabon, however, is a wealth ofmulti-client airborne magnetic data (see Figure 1) that hasbeen collected over the years both onshore and offshore.High-resolution marine gravity data have been collected on anumber of exclusive seismic surveys, and are not generallyavailable for publishing. A description of the gravity data’sutility in this environment will be discussed here.

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Improving geologic understanding with gravityand magnetic data: Examples from Gabon,Nigeria and the Gulf of Mexico

John M. Jacques1, Marianne E. Parsons2, Antony D. Price2 and David M. Schwartz2 drawupon recent work to provide evidence as to why gravity and magnetic survey data can stillprovide vital geological clues for oil and gas exploration.

B

1 Robertson Research International Ltd., Llandudno, North Wales, UK. www.robresint.co.uk2 Fugro-LCT Inc., Houston Texas USA. www.fugro-lct.com

Figure 1 Location of Non-exclusive Magnetic Data.

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The majority of the oil and gas targets in Gabon are asso-ciated with salt structures and basement highs.Unfortunately, the salt makes seismic imaging very difficultand non-seismic methods have been used to aid in the geo-logic understanding of the area. In the Southern basin ofGabon, the salt (Ezanga formation) was initially depositedabove a common reservoir rock (the Gamba sandstone) andas a result of the forces of sediment loading, periodic exten-sion and compression and the reactivation of basement faultshas formed pillows, diapirs and salt walls. Similarly, in theNorthern and Interior basins of Gabon, the Ezanga salt for-mation is present and has been deformed to create variouscomplex structures. As would be expected, below the saltlayer the seismic imaging is very difficult, and also causesproblems when attempting to define the flanks, and even thetop of these complex salt structures. There are therefore twoseismic imaging problems that can be aided with the inclu-sion of potential fields: delineation of a basement surface,and better definition of the geometry of complex salt bodies.

Magnetic data can be analyzed in a number of ways, withenhanced techniques and imaging making it an increasinglyvaluable tool. The basic geophysical concept behind this isthat different rock types have different magnetic responses.For this reason, there is often a marked difference betweenthe magnetic susceptibility of the basement and that of theoverlying sediments. Depth to magnetic basement surfacesare created by carefully analyzing a network of 2D magneticflight lines with multiple techniques (Naudy, SPI, ExtendedEuler, Werner, Peter’s Half-Slope, Solokov, etc.). A surface isthen created by contouring the results of the profile analysisusing geologic insight. Fault block highs and grabens becomeevident and the basement fabric is revealed.

A recently developed 3D magnetic tool is the 3DExtended Euler technique that integrates results from 3DEuler with Werner to achieve a tighter control on the solu-tion surfaces. An example of this is shown in the 3D image(Figure 2). Notice that the solutions form in planes. Theseare interpreted as fault planes, both in the basement and inthe sedimentary column.

These depth-to-magnetic source solutions can be insertedinto and displayed in a seismic volume. Since many of thetraps form as a result of the draping of later formations overbasement highs, the optimum locations for further geologicexploration are revealed. Understanding the basement, bothas a surface and through the fault planes produced in 3D,provides information both on the tectonic history, the pres-ent day traps and the potential pathways of the hydrocar-bons.

Perhaps one of the greatest benefits of the magnetic datais that prior to committing to an expensive seismic programin a difficult exploration terrain, one can perform detailedmagnetic analysis on the readily available magnetic datausing the newer techniques and imaging methods to increasegeologic understanding of the area. This may affect theacquisition parameters and the orientation of the 3D seismicsurvey being considered. In addition, the relative cost differ-ence between the magnetic and seismic methods can allow amore detailed laterally extensive analysis of the magneticdata after the initial seismic is shot to extend any trends seenin the seismic.

Another type of potential field data useful in Gabon forbetter subsurface geology definition is high-resolution gravi-ty data. The gravity method depends on a high-density con-trast between the geologic bodies of interest (in this case salt)and that of the surrounding sediment. Gravity modelling hasbeen used for discerning between a number of geologic sce-narios possible in this environment, based on both the seis-mic and the literature.

Among the geologic possibilities (see Figures 3 to 5) are:salt wall with greater width at the top than at the bottom (i.e.a diapiric shape in cross-section), a salt body that has becomedetached from its root or the autochthonous salt layer, or a

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Figure 3 Salt wall as a cross-section.

Figure 2 3-D Euler Solutions with Topography Surface

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salt wall that has had the upper portion ‘dissolved’ and filledin with unconsolidated sediment (see, for example,Teisserenc and Villemin, 1989). These three scenarios canlook very similar in the seismic data, however in a gravitymodel they have marked and measurable differences inresponse. Using gravity modelling to improve the geologicmodel, the pre-stack depth migration (PSDM) process canachieve much better results.

Another gravity method, which we believe has yet to beapplied in Gabon, is the direct acquisition of gravity gradientdata. This can be acquired using boat or aircraft, and is par-ticularly useful for modelling of small, shallow features. Dueto the sensitivity of the gravity gradients to shallow features,it is imperative that the topography or bathymetry is meas-ured careful, such that the effect of the bathymetry/topogra-phy can be reduced.

As always, geophysical methods work best when inte-grated with each other and with the geologic knowledge inthe area. The physical attributes measured with the gravityand magnetic methods provide important information thatneeds to be included to create the most complete geologicsolution.

Akata mobile shale: Tectonism in the OffshoreNiger DeltaExpanding on the concept that salt can be modelled using acombination of gravity and seismic data, the Akata mobileshale structures in the Niger Delta were examined for possi-ble gravity signatures, with a view to modelling theirresponse. Salt structures produce a relative gravity low dueto their inherent low density compared with surroundingsediments. This density is fortunately fairly uniform, makingthe surrounding sediment the main variable, definable bywell logs. The case for shale, however, is somewhat less con-sistent. Shale densities mentioned in the literature vary,dependent on a number of factors, such as water content,depth of burial and pressure regime, making modelling ofshales by gravity rather difficult. The fact that shale behavessomewhat like salt, in that it forms mobile diapirs generallysuggests that it has a density less than that of surroundingsediments.

This concept was initially tested using interpreted sec-tions made available by the AAPG in Course Manual andAtlas of Structural Styles on Reflection Profiles from theNiger Delta by Ajakaiye, D. E., and Bally A. W., from which2D gravity and magnetic models were constructed. Thesefairly simple models appear in Figure 6 and 7, with a fixeddensity of 2.35 g/cm3 for the Akata mobile shale, and someestimated densities for the overlying sediments. Despite thesebeing highly simplified models, areas that are difficult toimage seismically and are known shale structures, are bestmodeled gravitationally as shale at a lower density than sur-rounding material. These structures appear in the Tzz

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Figure 5 Dissolved top of salt wall, in-filled by unconsolidatedsediments with full salt wall response in green for comparison.

Figure 4 Detached salt body with full-salt wall response ingreen for comparison.

Figure 6 Model D3 based on Figure 1. Ajakaiye, D. E., andBally A. W., “Course Manual and Atlas of Structural Styleson Reflection Profiles from the Niger Delta”. AAPG © 2002Reprinted by permission of the AAPG whose permission isrequired for further use.

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enhancement of the Bouguer gravity (a vertical derivative ofthe gravimetry data, scaled to units of Eötvös) as anomalyminima, with the observed data in blue, and the calculatedresponse from the model in red in the middle panel.

The initial models for this investigation are full-earthmodels, encompassing gravitational elements as deep as theMantle surface. This is done to account as much as possiblefor the longer and regional wavelengths, as opposed to usingarbitrary filters to isolate the signal of interest. The Tzzenhancement is then applied to the observed and calculatedgravity profiles, which effectively accentuates the responsesfrom shallower structures.

Exact fit between calculated and observed has not beencompletely achieved, as these shale structures are threedimensional in nature. The 2D models shown here, with their

assumption of infinite extent in and out of the plane of themodel, do not completely characterize the true response seenin observed data. If detailed, perhaps prospect-level quantifi-cation of a shale structure were required, a 3D model wouldbe most appropriate.

The calculated and observed magnetic responses appear inthe uppermost panel, and the fit is achieved for this parame-ter by varying susceptibility in the basement compartments,where the majority of the magnetic response is expected to befound. Depth to magnetic source estimations using a numberof techniques such as Euler, Werner and Bean Ratio A, werealso made using the magnetic profile from Figure 6 (whitesymbols, different symbol for each technique), profile D3,largely confirming that the basement in this locality has littletopographic expression, and is of the approximate depth seenin the model. This basement character is in keeping with theconcept that the crust near these profiles is oceanic in nature.

Figure 8 displays the relative line location of Figures 6 and7, overlain on the Tzz enhancement of Bouguer gravity men-tioned previously. Major Akata shale structures are indicatedas Tzz gravity lows, along with the published locations ofmajor oil fields. This figure demonstrates that almost allmajor shale structures, from diapirs (strong, localized lows)to toe-thrusts (broad shallow lows) are clearly seen in thegravity enhancement. Moreover, trends of shale structurequickly become apparent, such as the linear trend northwestof the Bonga field, allowing rapid qualitative mapping of theAkata shale structures.

The magnetic data can also find utility (as mentioned pre-viously) in estimating depth to magnetic basement, whichwould put an upper limit on the thickness of source rocks, thebase of which may not be well imaged by seismic methods.Major crustal features are also apparent in the magnetic data,such as transform faults, aiding the regional tectonic under-standing, and perhaps defining zones of weakness such asshears or faults in basement, that are exploited by the mobileAkata shale in the formation of diapirs and toe-thrusts. Therealso seems to be some association between the inferred sedi-ment pathways as interpreted from satellite altimeter derivedgravity (courtesy of Dick Gibson Consulting), seen in Figure9, and the 5.7 km match filter (Cowan and Cowan 1993) ofthe reduced to pole magnetics. This could be due to magneticmaterial enrichment in the channel systems, or by control ofbasement topography on the drainage pattern. Either case isworthy of further investigation as channel systems form reser-voir sections for the major fields in this area (Tuttle et. al.1999).

Gravity and magnetic data traditionally found use inregional, large-scale tectonic understanding of a basin. Thesedata, in conjunction with public domain satellite altimeterderived gravity, are useful for such purposes, but are also ofsufficient resolution and quality for more detailed work. Asdetailed above, the Akata shale structures of interest can be

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Figure 7 Model D7 based on Figure 24. Ajakaiye, D. E., andBally A. W., “Course Manual and Atlas of Structural Styleson Reflection Profiles from the Niger Delta”. Limited mag-netic data were available for this line. AAPG © 2002Reprinted by permission of the AAPG whose permission isrequired for further use.

Figure 8 Location of models shown in Figure 7 and 8 overTzz enhancement of Bouguer gravity. Main Akata mobileshale structures and approximate location of Bonga fieldlabeled. Other published locations of oil fields in green.

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successfully interpreted both qualitatively using the gravityenhancements, and also quantitatively using 2D and /or 3Dmodelling in conjunction with seismic data. Modelling of thegravity response in 3D, in conjunction with interpreted 3Dseismic, allows reduction of ambiguities in the seismic inter-pretation by incorporating independent geophysical methods.This approach should find most use in areas where shalestructures seismically obscure targets of interest in a fashionsimilar to overhanging salt in such areas as the Gulf ofMexico and North Sea.

Source rock distribution and quality in the Gulf ofMexico: Inferences from plate tectonic and gravityand magnetic modellingUsing the Gulf of Mexico as a case example, it has recentlybeen demonstrated (Jacques & Clegg 2002a, b) that anunderstanding of the plate tectonic history of a region isessential in providing a tool to extend our knowledge ofsource rock and reservoir distribution into frontier areas. Asa means of understanding how tectonics through time haveplayed a key role in determining the present day petroleumsystems of a group of genetically-related basins, an advancedexploration programme has to be developed that is designedto focus on the unresolved problems and alternative modelsfor the tectonic evolution of that region, with the first aim ofidentifying what is known and evaluating what is presumedabout its infrastructural framework. To achieve this, it hasbeen shown (Jacques 2002) that we should integrate thediverse range of multiple geological datasets (structural, geo-physical, geochemical and sedimentological) available toidentify new play fairways and to extend existing play con-cepts into frontier areas, such as deepwater and sub-salt.Geographic information systems (GIS) provide this technolo-gy and the medium by which spatial and temporal relation-ships can be observed across numerous data layers, andqueries can be performed to evaluate data reliability and toperform multi-scenario analyses. This advanced query func-tionality provides a powerful way of assessing alternative geo-logical models.

Although GIS provides the explorationist with extremelypowerful state-of-the-art software for geospatial analyses, theusefulness of performing such analyses is totally dependent onthe quality and quantity of the information in the databaseand on the explorationist’s knowledge of the tectonostrati-graphic history of the region. It is therefore essential, that the‘basic building blocks’ of the region are recognized and,placed in a plate tectonic context, can be confidently used asa predictive, dynamic model. One of the ways to achieve thisis to use potential fields data (gravity and magnetics) to pro-duce detailed structural/geological coverage for the entireregion. As the first and most important phase of the explo-ration programme, the integration of potential field data withvarious geological data sets can be used to define:

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Figure 9 Match filter (5.7 km) of Reduced to Pole Magneticswith interpreted sediment pathways from Satellite AltimeterDerived Gravity (Dick Gibson Consulting).

Figure 10 Permo-Triassic reconstruction of Pangea (afterJacques & Clegg 2002a), showing pre-rift continental blockoutlines, and their configuration and relationships. Presentday basement, igneous and volcanics, and selected structuralelements rotated for reference.

Figure 11 Example mega-regional gravity and magnetic 2-Dprofile across the western Gulf of Mexico. Public domaindata displayed here.

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■ Predominant structural basement fabrics that characterizethe region

■ Distribution of different crustal types (continental, oceanicand heterogeneous) and their boundaries

■ Salt distribution and geometry■ Extent of igneous intrusives and volcanics

Once the present day structural/geological framework of theregion has been established, it is then necessary to define indi-vidual tectonic units (‘building blocks’) that will form the basiccomponents for developing a composite plate tectonic model.For this to happen, the ‘original’ continental block outlines(pre-tectonic block shapes and their positions), the distributionof principal crustal types, and the position of first order tecton-ic elements have to be identified and rigorously assessed, againusing potential fields data. Shaded relief images from mapenhancements of potential fields data are used to define saltgeometry and distribution, and distinguish between deep base-ment faults, crustal domain boundaries and sedimentarygrowth fault systems. This should ultimately result in thedevelopment of a paleotectonic-template that can be used tocreate the composite plate tectonic model.

Placed in a time-referenced framework, the movements ofthe possible first order structures identified from the potentialfields data are evaluated on the basis of several basic con-straints: (1) if their kinematics are justifiable at local throughto regional scale, from both field-scale observations and platetectonic modelling; (2) if the timing of these movements arerealistically feasible and can be used to explain continentalblock overlap and underlap issues at different stages of theregion’s evolution; and (3) they provide a means of explainingpresent-day structural and/or geological characteristics andrelationships of the region. Only when compliance with theseconstraints is achieved for individual structures (e.g., continen-tal transfer and oceanic transform faults) are they consideredto represent an integral part of the region’s tectonic develop-ment. The resultant ‘paleo-template’ (an example appears inFigure 10 for reference) can be further evaluated and modifiedusing potential fields data, in particular, by creating a suite of2D mega-regional gravity/magnetic profiles that traverse theregion (example in Figure 11).

With a framework in place, different crustal types can bedefined with greater confidence, the kinematic history of majorcontinental blocks can be determined and the distribution andtiming of deformational activity can be explained across theregion. The end result is a thoroughly assessed set of palinspas-tic basemaps These can be used to create a series of palaeotec-tonic reconstructions onto which depositional environmentscan be compiled for key source rock and reservoir horizons(Figure 10). Enhanced with datasets of source rock and reser-voir characteristics, combined with a variety of other tech-niques, such as basin dynamic modelling, drainage net analy-ses and burial history modelling, this advanced exploration

programme can be successfully used to identify new andextend existing play fairways into frontier areas. Again, thepotential fields data is an invaluable set of data, which can beused in the subsequent stages of the exploration programme,particularly during the basin dynamic and burial history mod-elling stages, to predict paleo-heat flow gradients for determin-ing basin subsidence and source rock maturation histories.

ConclusionsGeologists and geophysicists have many tools at their disposalto aid in developing a complete understanding of their areas ofinterest. Incorporating all data available goes a long waytowards enhancing that understanding, especially when highquality data are readily available from literature or datalibraries.

AcknowledgementsWe gratefully acknowledge Fugro-LCT and Mabon(www.mabonltd.com) for permission to show the Niger Deltamerged shipborne gravity and magnetic data, and GibsonConsulting for the Satellite Gravity Atlas of Rifted Margins ofthe World, Fugro Airborne Surveys for the aeromagnetic dataoffshore Gabon, and the AAPG for display of models generat-ed from seismic interpretations in Course Manual and Atlas ofStructural Styles on Reflection Profiles from the Niger Delta”by Ajakaiye, D. E., and Bally A. W. (2002).

Selected ReferencesAjakaiye, D. E., and Bally A. W. (2002) Course Manual andAtlas of Structural Styles on Reflection Profiles from the NigerDelta. AAPG 2002 (Reprinted by permission of the AAPGwhose permission is required for further use).Cowan, D. and Cowan, S. (1993) Separation filteringapplied to aeromagnetic data. Exploration Geophysics, 24,24, 429-436.Jacques, J. M. 2002, Geographic information systems as anadvanced exploration tool. Offshore, 62,11, 58-59, 90.Jacques, J. M. and Clegg, H. (2002a) Late Jurassic source rockdistribution and quality in the Gulf of Mexico: inferences fromplate tectonic modeling. Gulf Coast Association of GeologicalSocieties, Transactions, 52, 429-440.Jacques, J. M. and Clegg, H. (2002b) Gulf of Mexico LateJurassic source rock prediction: Integrating tectonics andgeochemistry with GIS technology. Offshore, 62, 10, 106-107, 164.Teisserenc P. and J. Villemin (1989) Sedimentary Basin ofGabon – Geology and Oil Systems, in Edwards, J.D. andSantogrossi, P.A., Divergent/passive margin basins. AAPGMemoir #48, 117 – 199Tuttle, M. L. W., Charpentier, R. R., and Brownfield, M. E.(1999) The Niger Delta Petroleum System: Niger DeltaProvince, Nigeria, Cameroon, and Equatorial Guinea, Africa.USGS Open File Report 99-50-H.

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