Gravity Support for Hydrocarbon Exploration at the Prospect Level

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1 Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 2 (1): 1-6 Scholarlink Research Institute Journals, 2011 (ISSN: ) jeteas.scholarlinkresearch.org Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 2 (1): 1-6 (ISSN: ) Gravity Support for Hydrocarbon Exploration at the Prospect Level 1 B. S. Badmus, 1. K. Sotona and 2 Krieger, M 1 Department of Physics, University of Agriculture, Abeokuta, Nigeria 2 Terrays Geophysics, GmbH and Co., KG Hamburg, Germany. Corresponding Author: B. S. Badmus 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 nearsurface 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. Keywords: bouguer gravity, salt domes, hydrocarbon traps, contour maps and seismic horizons 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. 1 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

2 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 gravityseismic 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 o ~10.48 o E longitude and o ~52.51 o W 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 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/cm 3, 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 2

3 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 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/km 2 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/cm 3. 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 ), 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. 3

4 Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 2 (1): 1-6 (ISSN: ) 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, , 1957 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. 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, , 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 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. Figure 1.0: Buguer Gravity Anomaly Map 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. Figure 2.0: Band pass filter 1-60km 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,

5 Figure 3.0: First Horizontal Derivative Figure 5.0b: Body and Observe Gravity Figure 4.0: Second Vertical Derivative Figure 5.0c: Body and Observe Gravity Figure 5.0a: Body and Observe Gravity Figure 5.0d: Body and Observe Gravity 5

6 Figure 6.0:Calculated Gravity Effect Figure 9.0: The Geomasters window showing the std Dev of the modeled body Figure 7.0: Observed Gravity Effect Figure 8.0: The Geomasters window showing the std Dev of Cubic body 6