Gravity Support for Hydrocarbon Exploration at the

Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 2 (1): 1-6 (ISSN: 2141-7016)

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Gravity Support for Hydrocarbon Exploration at the Prospect Level

1B. S. Badmus,

1�. K. Sotona and

2Krieger, M

1Department of Physics, University of Agriculture, Abeokuta, Nigeria

2Terrays Geophysics, GmbH and Co., KG Hamburg, Germany.

Corresponding Author: B. S. Badmus

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Abstract

Exploration sites with complex geophysical structures like salt bodies, significant to hydrocarbon exploration

are difficult in seismic interpretation to delineate. An integrated approach using land gravity information with

integrated seismic horizons in building a model for these complex structures via a 3-D gravity forward

modelling was used. The acquired Bouguer gravity data was filtered using low, high and band pass filter,

removing regional trends and high frequency anomalies. First horizontal derivatives and second vertical

derivative maps were obtained from the Bouguer gravity, revealing the pattern of faulting and enhance near-

surface features. A 3-D body was created and modelled with seismic horizons as constraint until the calculated

gravity effects of the model match the observed gravity or are deemed close enough.

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Keywords: bouguer gravity, salt domes, hydrocarbon traps, contour maps and seismic horizons

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I�TRODUCTIO�

The study of the earth’s gravity is a modern

application of classical Newtonian physics. The

gravity method measures small spatial differences in

the gravitational pull of the earth. Traditionally,

seismic reflection method is the most effective

method for detailed oil and gas exploration but can

only image flat layered geometry. In the case of

complex geometrical geologic area where there are

complex features like salt domes, it will be difficult

for seismic method to image and interpret around the

feature as well as predicting the geologic features

below the salt. The crystalline structure of salt makes

the reflection of seismic waves to be irregular and

inconsistence in this area. It is difficult and

sometimes impossible to interpret these seismic

reflections. The salt body is always a problem in

seismic method for hydrocarbon exploration.

Shadow zone are usually formed by seismic method

below the salt and this poor illumination makes it

sometimes difficult to image the potential

hydrocarbon traps. In such cases, a potential field

method for joint interpretation is required. The

integrated geophysical interpretation approach is the

use of several geophysical techniques in the same

area. This is important because the exploration

geophysicist selecting suitable different methods

obtain much more information. Gravity method

which is more preferable was used for integrated

interpretation coupled with the seismic data, since

magnetic data interpretation is theoretically more

complex because of the dipolar nature, latitude and

longitude dependent nature of the induced magnetic

response for a given body as well as rapid magnetic

field changes in space. Gravity measurements are

simple and moderate source of information about the

subsurface of an exploration target.

Gravity information has for several decades, been

successfully used in the Gulf of Mexico to address

the problem of defining the salt/sediment boundary,

where the best quality 3-D seismic data task cannot

meet the challenges (Nafe and Drake 1957, Bain et al

1993). Gravity fields at the Earth’s surface contain

anomalies from sources of various size and depth. To

interpret these fields, it is desirable to separate

anomalies caused by certain features from anomalies

caused by others. Salt diapirs play an important role

in hydrocarbon development and are significant for

petroleum exploration in highly matured areas. Salt

domes are emplaced when buried salt layer, because

of its low density and ability to flow, rises through

over laying denser strata in a series of approximately

cylindrical bodies. Locating the base of a salt body is

difficult with seismic reflection data. Gravity data in

combination with seismic reflection data can be used

to give joint interpretation. The relatively low

density of salt with respect to its surrounding renders

the salt dome a zone of anomalously low mass.

Gravity surveys provide a powerful method for the

location of features of this type because it shows

strong regional effect and regional gradient because

of the low gravity effect of the salt compare to the

surrounding sedimentary rocks. Analysis and

interpretation of this kind of geological structure

generally requires a 3-D structural model.

There are numerous contributions in the literature in

which the gravity method has been used to support

hydrocarbon exploration. Wallace, 1970 addresses

the difficulties of determining the shape and storage

capacity of basins by combining gravimetric and

seismic refraction interpretations to avoid drilling,

which can be expensive and difficult because of the

depth of alluvium and the large areas involved. The

Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 2 (1): 1-6

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result showed that the gravity-seismic method of

basin analysis provides useful numerical data in

arriving at a ground water storage capacity estimate.

And the basin configuration can be obtained from

profiles taken across the gravity contour map and also

the average depth to basement is noted from the

gravity profiles. The total volume of alluvium can

then be established from this depth and the surface

area. Wallace, 1970 concluded that the gravity-

seismic method of estimating storage capacity in

deep alluvium is best adapted to regional surveys.

Shin’ya Onizawa et al., 2002, formulated a method

for simultaneous velocity and density inversion using

travel times of local earthquakes and gravity data to

investigate the subsurface structure of Izu-Oshima

volcano. In order to constrain the velocity inversion

and increase the spatial resolution of shallow velocity

structures, additional gravity data was introduced.

Gravity data contributes to the P-wave and S-wave

velocity models by imposing constraints between

seismic velocities and density. Huston et al, 2004

used gravity data in conjunction with prestack depth

migration of the seismic data in an iterative way to

build a better velocity cube, thereby leading to clearer

images of the base of the salt. Henrick et al., 2005,

carried out a detailed high-resolution land gravity

survey over the southern part of Bolivia in South

America, at station intervals of 500m along survey

lines spaced 800m apart. The area covered by the

gravity data generally extended beyond that of the

seismic data sets, and offered the opportunity to

extend the structures interpreted solely from the

seismic data. With an indication that interesting

structures may exist outside the existing seismic data

coverage and also that there is sufficient density

contrast across the various stratigraphic sections and

that more detailed gravity data should add useful

structural information where the seismic method is at

a disadvantage. The result obtained showed that

gravity information generally supports the existing

geological model and concepts, but indicated the

possibility of prospective areas outside the available

seismic data. Helen and Donald, 2007 illustrated

how a gravity derived model can be used effectively

to assist the construction of a seismic velocity model

for depth migration of seismic data collected in a

difficult data area where carbonates outcrop at

subsurface. The results showed that integrated

analysis of the two data sets support a thin skinned

deformational model; for the Norman Range with a

décollement in Upper Cambrian salt strata of the

Saline River Formation. Seismic method usually

encounters difficulties in imaging and interpreting

complex structures like salt body in hydrocarbon

exploration process. As a result of crystalline nature

of the salt body, the reflection of seismic waves is

irregular and inconsistence. It is difficult and

sometimes impossible to interpret these seismic

reflections. Shadow zones are usually formed by

seismic method below the salt and this poor

illumination makes it sometimes difficult to image

the potential hydrocarbon traps. All these motivated

the idea of integrated approach by using a potential

field method with seismic horizon as constraint for

joint interpretation and 3-D structural model.

LOCATIO� OF THE STUDY AREA

This study was carried out within Giforn, Northern

Germany covering an area in the range of 10.22o

~10.48oE longitude and 52.21

o ~52.51

oW latitude.

METHODOLOGY

The objectives of geophysical data interpretation are

to locate anomalous material, its depth, dimensions,

and properties. Gravity and seismic data were used

to study the subsurface geology by developing an

integrated interpretation which includes updated

transformations of the potential fields, anomalies

filtering and 3-D forward gravity modelling with

seismic horizons as a constraint.

This research work started with 21609 gravity

measurements covering over an area approximately

29.2km by 22.9km. The gravity data was reduced to

complete Bouguer anomaly using a reduction density

of 1.90g/cm3, which is comparable to typical North

Germany average crustal density. The Bouguer was

gridded to form an evenly spaced data to be able to

make a contour map from it. This next step of

anomalies separation is very important for the

analysis and interpretation of the Bouguer gravity,

because the anomalies of interest were superposed on

a regional field caused by sources larger than the

scale of study or too deep to be of interest. The

regional effects correspond to low frequencies or

large wavelength while the residual corresponds to

high frequency or low wavelength. The separation is

easier done in the frequency or wavelength domain

rather than in spatial domain. Data from spatial

domain was transformed to wavelength domain by

fast Fourier transform computer algorithm and

Geosoft’s Oasis Montaj software. Low pass filter

was then used to remove high frequency and small

scale spatial detail, so as to smoothen data or

enhancing larger weak features. This filter passes

longer wavelength and cut out all wavelengths

shorter than the cut off wavelengths. High pass filter

was used to remove low frequency, large scale spatial

detail and also enhances shorter wavelengths and cut

out all wavelengths longer than the cut off

wavelengths. While band pass filter were created

from the low pass filters and high pass filters after

choosing the best from the low pass and high pass.

These filters are applied to keep or pass only a

portion of the wavelength (residual) and remove the

rest (regional). The horizontal derivatives of the

Bouguer anomaly emphasize changes in the

horizontal gradient. This is an alternative way of

removing the regional trends in the data and


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providing a view of the overall pattern of faulting.

The second vertical derivatives (SVD) of gravity data

was applied to attenuate low-frequency signals,

enhances high frequency signals usually caused by

near-surface sources and separates anomalies

horizontally.

Gravity model was created to determine the density,

depth and geometry of the subsurface bodies.

Bouguer gravity, filtered data, and seismic horizons

were used as constraint to determine the density,

depth and geometry of the subsurface anomaly.

Forward modelling technique using 3-D irregularly

shaped bodies and seismic horizons to constrain the

geometry of the model was performed. A model of

the density structure up to depth of 4.9km, produced

through a 3-D forward modelling of the Bouguer

anomaly was produced. The gravitational field due

to the model was calculated and compared to the

observed gravity anomalies. The model parameters

were changed and re-calculated until the calculated

gravity effect of the model match the observed

gravity or are deemed close enough. The

geomaster’s software suite was used to combine the

gravity data and the seismic horizons for joint

integrated solution for these geological structures.

This software put emphasis on integrating various

types of information with high diligence on seismic

processing. For all application levels of potential

field data, the software has the right tools for joint

modelling and reliable geological interpretation.

Gravity anomaly data, filtered data and seismic

horizons were loaded into the geomaster’s software,

created an initial body and assigned density values to

the horizons and the newly formed body. Gravity

effects of the horizons and the initial body was

calculated and compare to the observed gravity. The

shape of this body and the density value of the

seismic horizons and that of the body were adjusted

until the observed and calculated gravity anomaly are

deemed close enough. This structure was positioned

at various depths and stationed to the depth that

makes the best fit between the calculated and

observed anomalies. The entire process was carried

out repeatedly until we obtain a model almost having

the same gravity effect as the observed gravity and

with the lowest standard deviation.

RESULTS A�D CO�CLUSIO�S

The Bouguer gravity anomalous map comprises of

both the regional and residual anomalies from both

deep and shallow sources. In this research work, the

gravity data in good coverage shows values between -

9.9 and 16.5 mGal. The Bouguer gravity anomaly

map displays three main positive anomaly and four

main negative anomaly trends. The main negative

areas are in the northwest, central, and south-eastern

part of the study area. Also the main positive

anomaly zones are in the south western part and

south southern part of the study area (Fig. 1.0). The

gravity values decrease from southern part to the

northern direction. The lowest gravity value is at the

centre and this shows the presence of a very low

density anomaly, which may be due to the presence

of a low density sediments probably salt dome. The

horizontal derivative map showed that the NW,

central, SE and SW parts of the study area have

strong horizontal gradient anomalies. The area of the

strong horizontal gradient can seen in the North

western part area showing short anomalies of NW-SE

orientation, and South western part area showing

short anomalies of E-W trends. The central part area

shows long anomalies of NW-SE strong orientation.

The south southern and south eastern parts show

short anomalies of NW-SE orientations as shown in

figure 3.0.

The SVD map emphasizes the expressions of local

features and removes the effects of large anomalies or

regional influences. The principal usefulness of this

enhancement is that the zero value contour lines on

the map follows sub-vertical edges of intra-basement

blocks or the edges of supra-basement disturbances

or faults. The centre of the SVD map indicated an

anomaly of very low density compared to the

surrounding regional geology and the 0mGal/km2

contour lines around this feature signifies the

boundary between this anomaly of lower density and

the surrounding geology (Fig 4.0). The band pass

filter showed anomalies at the centre with very low

density compared to the surrounding geological

trends. These anomalies may be a salt body because

the density is relatively much lower than the density

of the surrounding area (Fig 2.0). The proposed 3-D

model has an internal geometrical consistency; it is

compatible with available geophysical data as shown

in figures 5.0a-d. The gravity model revealed the

occurrence of a relatively low density body at the

central part of the site. This deep low density body

accounts for intermediate wavelength-negative

gravity anomaly observed at the central. The

modelled feature reveals a fair cylindricity of a deep

structure which exhibits a broad negative anomaly of

about -9mGal. The best fit between the calculated

and the observed is obtained assuming an uplift of the

crystalline body of density 2.15g/cm3. It can be

interpreted as a result of relative uplift of salt dome at

the centre of the study area because the density value

lies in the range of pure salt density value. The

model generated consists of 37 parallel NE-SE planes

(Figs. 6.0 & 7.0). The final 3-Ddensity structure

shows a very good fit between measured and

modelled gravity field (Figs. 6.0 – 7.0), and the

standard deviation difference of 900µGal (Figs. 8.0 &

9.0). The top of the low density body is at 0.06km

depth and the bottom is 3.392km deep. The width of

the central uplift at the top is about ~4km, at a depth

of ~0.56km; the width is about ~7km at depth of

about ~1.56km, while the width is about ~9km and at

the bottom is about ~13km.


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CO�CLUSIO� This study has demonstrated how 3-D gravity

modeling with integrating geophysical and geological

information can help to reveal the subsurface

structures that are difficult for seismic method in

hydrocarbon exploration within area of complex

geological setting. The study emphasizes the need of

3-D modeling and gravity integrated interpretation in

highly complex geological terrain. The needs of an

integrated geological and geophysical approach to

improve the understanding of the subsurface

structures were revealed. The integrated approach of

the gravity and seismic have high reliability in

resolution and accuracy, and the model is realistic in

the sense that the density, depth and geometry of the

subsurface causative body were suggested. The

calculated and the observed anomalies are deemed

sufficiently alike and a standard deviation of 900µGal

was obtained.

The model also revealed an uplift of a fair cylindrical,

deep, crystalline structure of density 2.15g/cm3;

which is an average density of salt, at depth between

0.06km and 3.392km. For a research of this kind,

where salt dome occurring at the site of hydrocarbon

exploration, the integration of gravity with seismic

makes a lot of sense as low density body displays a

negative anomaly effect best detected by gravity

analysis using seismic horizons as a constraint. The

integration procedure superflows as the density of the

body suggests and also predicts which type of body is

present beneath the subsurface via the density

information predicted. The 3-D modelling is very

useful in complex geologic setting and interpretations

of the geological data with a view to have a fore site

of what is most is likely to be present in the

exploration site, even before any analysis.

REFERE�CES

Bain, J.E., Weyand, J., Horscroft, T.R., Saad, A.H.,

and Bulling, D.N (1993). “Complex Salt Features

Resolved by Integrating Seismic, Gravity, and

Magnetics.” EAEG/EAPG 1993 Annual Conference

and Exhibition, expanded abstracts.

Henrik T. A and Timothy R. B (2005). AMG Mc

Phar Integration of seismic and non-seismic methods

for hydrocarbon Exploration: a Bolivian case history

GEOHORIZONS July 2005/27-29

Helen I. J. and Donald. C. L (2007). Benefit of

integrated seismic and gravity exploration. An

example from Norman wells NWT. Fold-Fault

Research Project, University of Calgary.

Huston, D. C., Huston D. E. and Johnson, E. (2004).

Geostatistical integration of velocity cube and log

data to constrain 3-D gravity modelling, deepwater

Gulf of Mexico: The Leading Edge, 23, 842-846.

Nafe, J. E., and C. L. Drake, (1957). Variation with

depth in shallow and deep water marine sediments of

porosity, density, and the velocity of compressional

and shear waves, Geophysics, 22, 523–552, 1957

Shin’ya, O., Hitoshi, M., Hidefumi, W and Shikou, S,

(2002) A method for simultaneous velocity and

density inversion and its application to exploration of

subsurface structure beneath Izu-Oshima volcano,

Japan. Earth Planets Space, 54,803-817, 2002.

Wallace D. E. (1970). Estimating storage capacity in

deep allunium by gravity-seismic methods. Bulletin

of the international association of scientific

hydrocology, XV, 2 6/1970

Figure 1.0: Buguer Gravity Anomaly Map

Figure 2.0: Band pass filter 1-60km


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Figure 3.0: First Horizontal Derivative

Figure 4.0: Second Vertical Derivative

Figure 5.0a: Body and Observe Gravity

Figure 5.0b: Body and Observe Gravity

Figure 5.0c: Body and Observe Gravity

Figure 5.0d: Body and Observe Gravity


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Figure 6.0:Calculated Gravity Effect

Figure 7.0: Observed Gravity Effect

Figure 8.0: The Geomasters window showing the std

Dev of Cubic body

Figure 9.0: The Geomasters window showing the std

Dev of the modeled body

Documents

Gravity Support for Hydrocarbon Exploration at the