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The application of amplitude-preserved processing and migration for carbonate reservoir prediction in the Tarim Basin, China

The application of amplitude-preserved processing and migration for carbonate reservoir... 406 Pet.Sci.(2011)8:406-414 DOI 10.1007/s12182-011-0158-5 The application of amplitude-preserved processing and migration for carbonate reservoir prediction in the Tarim Basin, China 1 2 1 2 Sam Zandong Sun , Yang Haijun , Zhang Yuanyin , Han Jianfa , Wang 1 1 1 Dan , Sun Wenbo and Jiang Shan Laboratory for Integration of Geology & Geophysics, China University of Petroleum, Beijing 102249, China Research Institute of Exploration & Development, PetroChina Tarim Oilfi eld Company, Korla 841000, China © China University of Petroleum (Beijing) and Springer-Verlag Berlin Heidelberg 2011 Abstract: Conventional seismic exploration method based on post-stack data usually fails to identify the distribution of fractured and caved carbonate reservoirs in the Tarim Basin, so the rich pre-stack information should be applied to the prediction of carbonate reservoirs. Amplitude-preserved seismic data processing is the foundation. In this paper, according to the feature of desert seismic data (including weak refl ection, fast attenuation of high frequency components, strong coherent noises, low S/N and resolution), a set of amplitude-preserved processing techniques is applied and a reasonable processing fl ow is formed to obtain the high quality data. After implementing a set of pre-stack amplitude-preserved processing, we test and defi ne the kernel parameters of amplitude-preserved Kirchhoff PSTM (pre-stack time migration) and subsequent gathers processing, in order to obtain the amplitude-preserved gathers used to the isotropic pre-stack inversion for the identifi cation of caved reservoirs. The AVO characteristics of obtained gathers fi t well with the synthetic gathers from logging data, and it proves that the processing above is amplitude- preserved. The azimuthal processing techniques, including azimuth division and binning enlargement, are implemented for amplitude-preserved azimuthal gathers with the uniform fold. They can be used in the anisotropic inversion to detect effective fractures. The processing techniques and fl ows are applied to the fi eld seismic data, and are proved available for providing the amplitude-preserved gathers for carbonate reservoir prediction in the Tarim Basin. Key words: Amplitude-preserved processing, amplitude-preserved pre-stack time migration, azimuth, carbonate reservoir, Tarim Basin fractures effectively, its considerable expense makes the 1 Introduction method difficult to implement widely. Considering the With further development of exploration and the increase fact that the field data acquisition is azimuthally limited, a in successful wells in carbonate reservoirs, complicated method of obtaining the amplitude-preserved processing carbonate reservoirs which are deeply buried in the Tarim flow which is used for fracture detection would be very Basin are gradually becoming important exploration goals. important. The aim of early processing does not serve for The conventional method of dissolved cave identification the pre-stack analysis, the amplitude-preserved capability is based on bead-like reflections from seismic profiles fails unsatisfactory, and the processing does not take anisotropy to determine the filling features of the storage space, and it into consideration. Additionally, most of the techniques usually neglects other heterogeneous reservoirs. Therefore, could not solve the problem of multiples elimination, relative more rich pre-stack seismic information should be employed amplitude preservation, fractures and caves imaging, so that to do AVO analysis and elastic inversion. So a technique to they could not satisfy the need in reservoir prediction and oil obtain high quality and amplitude-preserved pre-stack data is & gas detection (Tao et al, 2010). the key to the pre-stack data analysis. Ordinary methods like Considering the surface conditions and raw data coherence cube could not predict effective fractures (open characteristics in the Tarim Basin, the processing diffi culties or filled by fluid), and small cracks are usually neglected. in deep layers are analyzed at first in this paper, and then Moreover, although the shear wave data could recognize a set of amplitude-preserved data processing workflow for reservoir characterization is summarized and explained, based on former research and practical application (Guo, 2009; * Corresponding author. email: samzdsun@yahoo.com Wang et al, 2008). Under the guidance of this workfl ow, the Received March 18, 2011 Iterative analysis Pet.Sci.(2011)8:406-414 407 resolution and amplitude-preservation are improved, which employed. (5) Since the development of carbonate reservoirs lays the foundation for reservoir prediction. is affected by many factors such as sedimentation, tectonic movement and diagenesis, the secondary fractures and 2 Processing challenges of desert data and dissolved caves with variable sizes and complicated shapes are well developed (Zhao et al, 2007; Wu et al, 2005; Zhou amplitude-preserved processing workfl ow et al, 2008). Therefore, anisotropic information should be considered. 2.1 Processing challenges of desert data For data processing in the Tarim Basin, many problems 2.2 Amplitude-preserved processing workfl ow usually exist and can be listed as the following aspects: (1) Precisely imaging structure, predicting lithological Although the conventional shooting method is replaced by reservoir and detecting hydrocarbon, and monitoring residual exploding below the water table, seismic data still have poor oil-gas distribution during the development of oil-gas fi elds quality resulted from the complex desert surface conditions. are the three basic targets for data processing. Amplitude- The seismic data are featured as weak reflection, fast preserved processing is the foundation of the techniques of attenuation of high frequency components, strong coherent isotropic AVO inversion, anisotropic inversion, reservoir noises, and low S/N and resolution. (2) Processing and feature analysis and the detection of oil and gas. interpretation are heavily infl uenced by multiples produced by For the amplitude-preserved processing which is used for the low velocity layer in the desert surface. (3) The refl ection the prediction of fractured and caved carbonate reservoirs, of secondary carbonate reservoirs is always accompanied the most important factor is to eliminate the diffraction with complicated scattering and diffraction which need to waves which appear around caves and fractures, image the be eliminated by a reasonable migration technique (Li et refl ection energy, and make sure the seismic data have correct al, 2007). (4) Due to the effects of tectonic movement and amplitude. Additionally, we should also do well in the static weathering erosion, spatial structure characteristics in the correction, amplitude-preserved noise attenuation, amplitude subsurface are complicated and variable, and it is hard to compensation, surface consistent deconvolution and precise identify their effective seismic refl ection energy. So, special velocity analysis and residual static correction. amplitude-preserved processing techniques should be Geometry building Static correction Noise attenuation with amplitude-preservation Spherical spreading and transmission absorbed amplitude compensation Surface consistent amplitude compensation Surface consistent deconvolution Azimuthal processing CMP separation AVOZ Velocity analysis Surface consistent residual statics Predictive deconvolution Amplitude-preserved PSTM Pre-stack RNA Multiple attenuation AVO Stack Resolution and S/N enhancement Inversion and attributes analysis Pure and conditioned data in post stack domain Fig. 1 The workfl ow of amplitude-preserved processing (RNA: random noise attenuation) 408 Pet.Sci.(2011)8:406-414 The high quality seismic data which have preserved data from the Tarim Basin (Fig. 2). In CRP (common the information of heterogeneous features could be used reflection point) gathers, multiples are more obvious than in the pre-stack AVO inversion to predict caved reservoirs. those in CMP (common middle point) gathers. In order For a fractured carbonate reservoir with strong anisotropy, to perform pre-stack analysis, these multiples should be considering the limited azimuth data acquisition and the effectively removed, and Radon transform method is often need in AVOZ inversion, we performed azimuthal processing used. For random noise, TDNFK (3D noise attenuation in before migration, and then amplitude-preserved migration F-K domain) noise elimination method is preferable. The and the subsequent processing with data of each azimuth results of multiples suppression are shown in Fig. 2. Fig. 2(a) were finished. Finally we obtained the azimuth data which is the CRP gather after migration, and Fig. 2(b) is CRP gather can be used in the anisotropy analysis. Focusing on carbonate after eliminating the multiples, and Fig. 2(c) is the multiples reservoir prediction, a series of processing techniques removed. Fig. 2(d), (e), and (f) are the corresponding velocity aiming at obtaining the amplitude-preserved seismic data is spectrums of Fig. 2(a), (b), and (c). It can be clearly observed introduced in this paper, and the fl ow is shown in Fig. 1. The that the multiples have been effectively suppressed. results from isotropic and anisotropic inversion using the data 3.2 The sequence of TDNFK and migration from this fl ow both refl ect the distribution characteristics of caved and fractured carbonate reservoirs which illustrates the The pre-stack migration is quite sensitive to noise, which effectiveness of this strategy. can be enlarged after migration. Therefore, pre-stack noise elimination is the foundation for the following data processing 3 Key amplitude-preserved techniques (Zhang, 2006). The real seismic data in the Tarim Basin are of poor quality. They show strong surface waves, linear waves, 3.1 Multiples suppression multi-refraction waves and random noises. Several noise Multiples and random noise are prominent in the seismic suppression techniques are integrated to enhance the S/N 8000. 5000. 5000. 100. 8000. 100. 8000. 100. 2000. 2000. 2000. 5000. (a) (b) (c) (d) (e) (f) 3.5 4.5 5.5 Offset, m V, m/s Comparison of CRP before and after multiples suppression Fig. 2 ratio, and at the same time, to maintain the effective perform attribute analysis and AVO/AVA inversion (Sun, reflection signal. The linear waves are removed according 2002). In 1993, the Kirchhoff amplitude-preserved migration to the velocity difference, and the anomalous amplitude method was put forward by Schleicher (Schleicher et al, is suppressed using a median filter. The TDNFK method 1993). Furthermore, in 1999 Graham A. Winbow (Graham et is applied for surface wave suppression in f-K K domain. al, 1999) provided an explicit 3D amplitude-preserved weight x y When this method is conducted before and after migration function formula for pre-stack time migration and utilized respectively, different imaging results can be obtained, as true amplitude weight function estimation to compensate shown in Fig. 3. It is better for TDNFK to be performed after amplitude. In this paper, the pre-stack Kirchhoff amplitude- migration. In the amplitude-preserved data processing, noise preserved migration is applied in the fi eld data of this area, is subtracted from the original data, which is beneficial for and we mainly analyzed and tested the main parameters of amplitude preservation. this amplitude-preserved migration method in order to obtain high quality seismic data for pre-stack inversion. 3.3 Amplitude-preserved Kirchhoff pre-stack time 3.3.1 The selection of key parameters migration (1) Regroup offset Amplitude-preserved seismic data are required to correctly Firstly, the maximum and the minimum offset and the Time, s Pet.Sci.(2011)8:406-414 409 193 433 673 913 1153 2.0 3.0 4.0 (a) 5.0 CDP (20m) 193 433 673 913 1153 193 433 673 913 1153 2.0 3.0 4.0 A A B B (c) (b) 5.0 CDP (20m) CDP (20m) Fig. 3 The infl uence of noise elimination sequence on imaging (a) Pre-stack time migration section (b) Noise elimination after migration (c) Noise elimination before migration offset increment are achieved by investigating and analyzing the corresponding parameter testing carried out for different the offset distribution of CMP gathers. Particularly, missing structural regions in the Tarim Basin, the dip of layer is traces in the same offset should be less than 30%, and the usually small. Therefore, in order to ensure the recognition of offset distribution of every CMP gather should be uniform local small structures, the protected dip value is chosen to be and as complete as possible. 45 degrees. (2) Migration aperture (4) Ray path The migration aperture refers to the seismic data The calculation of reflection travel time can be distribution for the migration. In general, the migration approximately simplified as a straight line with a lower aperture is controlled by three factors: the maximum dip calculation precision. The ray bending method calculates and depth of reflection interface, input data length and ray travel time according to the interval velocity model which path aperture. It was said that a small aperture may only is based on the Snell’s Law ray tracing method with high ensure structure imaging effects for small dip data and accuracy for complex and complicated areas. According to the characteristics of the high S/N, but not for steep angle data geological characteristics of the Tarim Basin and computing which at the same time would have structural distortion and ability, the bending travel time calculation method should be give rise to the “fl attened” phenomenon. A large aperture can selected. ensure structural imaging for steep dip, but the continuity of (5) Anti-alias fi ltering the refl ection section would become worse, and the S/N and The frequency of the migration section and imaging resolution would fall (Chen et al, 2002). quality can be directly influenced by anti-frequency (3) The angle of migration parameters. If frequency is too high, high-frequency noise The maximum migration angle refers to the maximum will be introduced and false frequency phenomenon will angle to be preserved during the migration process. On one be caused, and if frequency is too low, high frequency hand, it limits the scope of the migration operator extensions components will be lost. Aliasing always appears at the in the space. On the other hand, it has an influence on the shallow section, so an anti-alias filter is needed. We limit migration arithmetic as well as computing times. The larger the highest frequency to the scope of the effective wave and the migration angle, the more the computing times. Through adopt anti-alias fi ltering. Time, ms Time ms 410 Pet.Sci.(2011)8:406-414 3.3.2 Analysis of migration and processing results reasonable, and are advantageous for inversion. In order to test the amplitude preservation after data In the course of amplitude-preserved PSTM, signals processing and migration, the CRP gather after noise can be imaged appropriately through choosing a proper elimination and migration is compared with the theoretical directional factor, spherical divergence factor and phase synthetics, which is shown in Fig. 6. The reasonability of adjustment factor. For imaging of carbonate reservoirs in amplitude-preserved processing is affirmed by the similar the Tarim Basin, amplitude-preserved PSTM is the most AVO characteristics between synthetics and CRP gathers, important. The different CRP gathers by different Kirchhoff especially in the target area marked by the red frame. Pre- migration algorithms are selected and shown in Fig. 4. The stack inversion identifying carbonate reservoirs using the pre- amplitudes at 5.0 ms of different CRP gathers are extracted, stack gathers from this processing flow shows reasonable and are shown in Fig. 5. We can notice that the amplitudes results. marked by red and green lines are much more similar and 0.0 0.0 8000. 8000. 0.0 8000. 0.0 (a) (b) (c) (d) 3.0 3.0 3.0 3.0 4.0 4.0 4.0 4.0 5.0 5.0 5.0 5.0 Offset, m CRP gathers by different migration algorithms Fig. 4 0.0 (a) 3.0 ms (b) 4.0 ms (c) 5.0 ms 1.0 Offset, m Fig. 5 Amplitudes extracted from different CRP gathers in Fig. 4 is featured by frequency loss, and changes of amplitude, 4 Azimuth amplitude-preserved Kirchhoff phase, velocity and fault imaging. These features are in favor PSTM of extracting the information from anisotropic reservoir (Williams and Jenner, 2002). Although the scale of terranes or The P-wave is sensitive to anisotropic media, and it Amplitude Time, s Pet.Sci.(2011)8:406-414 411 Well logs Synthetics CRP gathers Time PI SI 3.55 3.6s 3.6 3.65 3.7s 3.7 3.75 3.8 3.8s 3.85 3.9 3.9s 3.95 4 4s 4.05 4.1 4.1s 1000 6800 1.0 1.8 1000 6800 Amplitude Offset, m Offset, m Fig. 6 Comparison between synthetics and PSTM CRP gathers fi ssures is less than seismic wavelength, the macro anisotropic 15° (Fig. 8(a)). The offsets and azimuths of bins can not be effects still can be observed (Jenner, 2002). Actually, there is guaranteed to uniform in the acquisition in this area, which a defi nite relationship between the amplitude changes versus will lead to AVO error. To avoid this and improve S/N, the bin azimuth and anisotropy. Specifi cally, if fractures in a certain size is enlarged. The data bin in the Tarim Basin can usually azimuth exist in the subsurface, the AVO characteristics be enlarged from 20m×20m to 40m×40m (Fig. 8(b)). of CRP gathers in the azimuths parallel with and normal 4.2 Azimuth velocity analysis and residual static to fractures are obviously different. The distribution and correction development densities of fractures can be reflected by the size and distribution of that difference (Zhang et al, 2004). Since the velocity and static measurement are different in Based on this, combined with the data in the Tarim Basin, the different azimuth data, velocity analysis and residual static azimuth range is chosen through analyzing the azimuth, offset correction should be performed respectively in different and fold times of CMP gathers at fi rst, and then the bins are azimuth data to protect the anisotropic characteristics enlarged to improve S/N and fold times. Finally, amplitude- before migration. Then amplitude-preserved PSTM preserved PSTM and other processing are applied to the method discussed above is applied to all these azimuth data azimuthal gathers. respectively. 4.1 Azimuth and offset analysis, and azimuth division 4.3 Common azimuth amplitude-preserved PSTM Considering the high cost, seismic data acquisition in the Based on the azimuth data divided in Fig. 8, the Tarim Basin is generally not applied to the whole azimuth. corresponding four common azimuth gathers are conducted Therefore, the distribution of azimuth-offset is not uniform migration. The four CRP gathers after PSTM and noise and usually lacks far offset components in some azimuths (Fig. attenuation are shown in Fig. 9, which locate at the red 7). There are two basic messages: (1) The mean aspect ratio dashed lines in the corresponding stack sections in Fig. 10. in this area is about 0.45 (Fig. 7(a)). (2) Through analyzing Obviously, the azimuthal processing is successful since the fold distribution versus azimuth (Fig. 7(b)), it is exhibited there are more distinct AVO characteristics in CRP gathers, as a shape of two peaks. About 66% data are distributed in better wave features and more continuous events in the stack the range between positive 30° and negative 30° azimuth section. In addition, the anisotropic characteristics of different (including corresponding opposite direction 150°-210°, the common azimuth data are outstanding, which are suitable east is regarded as 0°, increasing anti-clockwise). Finally, to for anisotropic inversion. The pre-stack anisotropic inversion meet the demand of anisotropic inversion, the azimuth angles using azimuthal data from this processing fl ow shows good from -30° to 30° are divided into four groups of common- results in effective fracture detection. azimuth gathers for processing, and each group differentiates Time, s 414 Pet.Sci.(2011)8:406-414 Petroleum Geophysics. 2009. 7(1): 1-3 preserved PSTM and the following noise attenuation should Jen ner E. Fractured reservoir characterization using P-wave AVOA be applied consistently. Attention should be paid to the analysis of 3D OBC data. The Leading Edge. 2002. 21(8): 777-781 preservation of effective reflection amplitude and quality Li G H, Gao H L, Zhang Z Q, et al. Reservoir prediction for the east of control in each step. Lun-Gu area. In: Thesis Collections of the Integration of Exploration (2) The amplitude-preserved pre-stack time migration and Development for Lun-Gu Oilfield in the Tarim Basin. Beijing: technique is the key to the accurate imaging of carbonate Petroleum Industry Press. 2007 (in Chinese) reservoirs in the Tarim Basin. This affects the later pre-stack Sch leicher J, Tygel M and Hubral P. 3-D true-amplitude finite-offset inversion and reservoir prediction. In order to obtain an image migration. Geophysics. 1993. 58(8): 1112-1126 of carbonate reservoirs in the Tarim Basin, the migration Sun J G. Kirchhoff-type true-amplitude migration and demigration. aperture was tested and finally defined as 6,000 m, and the Progress in Exploration Geophysics. 2002. 25(6): 1-5 (in Chinese) Tao Y G, Wang X W, Lü L, et al. Seismic data pre-stack processing effective imaged dip angle reached 45 degrees. techniques for carbonate reservoirs in the central Tarim Basin. Oil (3) For fracture prediction through anisotropic inversion Geophysical Prospecting. 2010. 45(2): 230-236 (in Chinese) based on P-wave data, it is an economic and effective Wan g X W, Lü L, Liu W F, et al. Seismic data processing techniques of method to realize processing common azimuth gathers, carbonate rocks in Tarim Basin. Lithologic Reservoirs. 2008. 20(4): which is a new direction of fracture prediction. To avoid fold 109-112 (in Chinese) nonuniform and improve S/N, the bin size, used to azimuth Wil liams M and Jenner E. Interpreting seismic data in the presence of migration, needs to be enlarged when seismic data are divided azimuthal anisotropy or azimuthal anisotropy in the presence of by different azimuths. seismic interpretation. The Leading Edge. 2002. 21(8): 771-774 Wu G H, Li Q M, Zhang B S, et al. Structural characteristics and Acknowledgements exploration fi elds of No.1 faulted slope break in the Tazhong area. Acta Petrolei Sinica. 2005. 26(1): 27-30 (in Chinese) This research is financially supported by National Basic Zha ng G S, Ma G G, Song Y L, et al. Crack detection by utilizing full Research Program of China (No.2011CB201100). The data 3D P-wave data. Oil Geophysical Prospecting. 2004. 39(1): 41-44 (in were provided by Exploration and Development Research Chinese) Institution of Tarim Oil Field Company. The authors would Zha ng Y. Application and prospects of the 3D seismic pre-stack like to thank Wang Di and Yang Pei for much work of time migration processing technique. Petroleum Exploration and proofreading. Development. 2006. 33(5): 536-541 (in Chinese) Zha o Z J, Wang Z M, Wu X N, et al. Genetic types and distribution forecast of available carbonate reservoirs in Ordovician in the central References area of the Tarim Basin. Petroleum Geology and Experiment. 2007. Che n Z D, Liu Z K and Li C B. 3D pre-stack depth migration velocity 29(1): 40-46 (in Chinese) analysis and automatic Monte Carlo velocity picking in depth. Zho u J G, Deng H Y, Fan G Z, et al. Geological-geophysical model and Chinese Journal of Geophysics. 2002. 45(2): 246-254 (in Chinese) prediction application of upper Ordovician Lianglitage reef-shoal Gra ham A W, William A and Schneider J. Weights for 3-D controlled reservoir in the Tazhong area, Tarim Basin. Marine Origin Petroleum amplitude prestack time migration. SEG Expanded Abstracts 18. Geology. 2008. 13(3): 17-23 (in Chinese) 1999. 1110-1113 Guo S X. Discussion on preserved amplitude processing of seismic data. (Edited by Hao Jie) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Petroleum Science Springer Journals

The application of amplitude-preserved processing and migration for carbonate reservoir prediction in the Tarim Basin, China

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Springer Journals
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Copyright © 2011 by China University of Petroleum (Beijing) and Springer-Verlag Berlin Heidelberg
Subject
Earth Sciences; Mineral Resources; Industrial Chemistry/Chemical Engineering; Industrial and Production Engineering; Energy Economics
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1672-5107
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1995-8226
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10.1007/s12182-011-0158-5
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Abstract

406 Pet.Sci.(2011)8:406-414 DOI 10.1007/s12182-011-0158-5 The application of amplitude-preserved processing and migration for carbonate reservoir prediction in the Tarim Basin, China 1 2 1 2 Sam Zandong Sun , Yang Haijun , Zhang Yuanyin , Han Jianfa , Wang 1 1 1 Dan , Sun Wenbo and Jiang Shan Laboratory for Integration of Geology & Geophysics, China University of Petroleum, Beijing 102249, China Research Institute of Exploration & Development, PetroChina Tarim Oilfi eld Company, Korla 841000, China © China University of Petroleum (Beijing) and Springer-Verlag Berlin Heidelberg 2011 Abstract: Conventional seismic exploration method based on post-stack data usually fails to identify the distribution of fractured and caved carbonate reservoirs in the Tarim Basin, so the rich pre-stack information should be applied to the prediction of carbonate reservoirs. Amplitude-preserved seismic data processing is the foundation. In this paper, according to the feature of desert seismic data (including weak refl ection, fast attenuation of high frequency components, strong coherent noises, low S/N and resolution), a set of amplitude-preserved processing techniques is applied and a reasonable processing fl ow is formed to obtain the high quality data. After implementing a set of pre-stack amplitude-preserved processing, we test and defi ne the kernel parameters of amplitude-preserved Kirchhoff PSTM (pre-stack time migration) and subsequent gathers processing, in order to obtain the amplitude-preserved gathers used to the isotropic pre-stack inversion for the identifi cation of caved reservoirs. The AVO characteristics of obtained gathers fi t well with the synthetic gathers from logging data, and it proves that the processing above is amplitude- preserved. The azimuthal processing techniques, including azimuth division and binning enlargement, are implemented for amplitude-preserved azimuthal gathers with the uniform fold. They can be used in the anisotropic inversion to detect effective fractures. The processing techniques and fl ows are applied to the fi eld seismic data, and are proved available for providing the amplitude-preserved gathers for carbonate reservoir prediction in the Tarim Basin. Key words: Amplitude-preserved processing, amplitude-preserved pre-stack time migration, azimuth, carbonate reservoir, Tarim Basin fractures effectively, its considerable expense makes the 1 Introduction method difficult to implement widely. Considering the With further development of exploration and the increase fact that the field data acquisition is azimuthally limited, a in successful wells in carbonate reservoirs, complicated method of obtaining the amplitude-preserved processing carbonate reservoirs which are deeply buried in the Tarim flow which is used for fracture detection would be very Basin are gradually becoming important exploration goals. important. The aim of early processing does not serve for The conventional method of dissolved cave identification the pre-stack analysis, the amplitude-preserved capability is based on bead-like reflections from seismic profiles fails unsatisfactory, and the processing does not take anisotropy to determine the filling features of the storage space, and it into consideration. Additionally, most of the techniques usually neglects other heterogeneous reservoirs. Therefore, could not solve the problem of multiples elimination, relative more rich pre-stack seismic information should be employed amplitude preservation, fractures and caves imaging, so that to do AVO analysis and elastic inversion. So a technique to they could not satisfy the need in reservoir prediction and oil obtain high quality and amplitude-preserved pre-stack data is & gas detection (Tao et al, 2010). the key to the pre-stack data analysis. Ordinary methods like Considering the surface conditions and raw data coherence cube could not predict effective fractures (open characteristics in the Tarim Basin, the processing diffi culties or filled by fluid), and small cracks are usually neglected. in deep layers are analyzed at first in this paper, and then Moreover, although the shear wave data could recognize a set of amplitude-preserved data processing workflow for reservoir characterization is summarized and explained, based on former research and practical application (Guo, 2009; * Corresponding author. email: samzdsun@yahoo.com Wang et al, 2008). Under the guidance of this workfl ow, the Received March 18, 2011 Iterative analysis Pet.Sci.(2011)8:406-414 407 resolution and amplitude-preservation are improved, which employed. (5) Since the development of carbonate reservoirs lays the foundation for reservoir prediction. is affected by many factors such as sedimentation, tectonic movement and diagenesis, the secondary fractures and 2 Processing challenges of desert data and dissolved caves with variable sizes and complicated shapes are well developed (Zhao et al, 2007; Wu et al, 2005; Zhou amplitude-preserved processing workfl ow et al, 2008). Therefore, anisotropic information should be considered. 2.1 Processing challenges of desert data For data processing in the Tarim Basin, many problems 2.2 Amplitude-preserved processing workfl ow usually exist and can be listed as the following aspects: (1) Precisely imaging structure, predicting lithological Although the conventional shooting method is replaced by reservoir and detecting hydrocarbon, and monitoring residual exploding below the water table, seismic data still have poor oil-gas distribution during the development of oil-gas fi elds quality resulted from the complex desert surface conditions. are the three basic targets for data processing. Amplitude- The seismic data are featured as weak reflection, fast preserved processing is the foundation of the techniques of attenuation of high frequency components, strong coherent isotropic AVO inversion, anisotropic inversion, reservoir noises, and low S/N and resolution. (2) Processing and feature analysis and the detection of oil and gas. interpretation are heavily infl uenced by multiples produced by For the amplitude-preserved processing which is used for the low velocity layer in the desert surface. (3) The refl ection the prediction of fractured and caved carbonate reservoirs, of secondary carbonate reservoirs is always accompanied the most important factor is to eliminate the diffraction with complicated scattering and diffraction which need to waves which appear around caves and fractures, image the be eliminated by a reasonable migration technique (Li et refl ection energy, and make sure the seismic data have correct al, 2007). (4) Due to the effects of tectonic movement and amplitude. Additionally, we should also do well in the static weathering erosion, spatial structure characteristics in the correction, amplitude-preserved noise attenuation, amplitude subsurface are complicated and variable, and it is hard to compensation, surface consistent deconvolution and precise identify their effective seismic refl ection energy. So, special velocity analysis and residual static correction. amplitude-preserved processing techniques should be Geometry building Static correction Noise attenuation with amplitude-preservation Spherical spreading and transmission absorbed amplitude compensation Surface consistent amplitude compensation Surface consistent deconvolution Azimuthal processing CMP separation AVOZ Velocity analysis Surface consistent residual statics Predictive deconvolution Amplitude-preserved PSTM Pre-stack RNA Multiple attenuation AVO Stack Resolution and S/N enhancement Inversion and attributes analysis Pure and conditioned data in post stack domain Fig. 1 The workfl ow of amplitude-preserved processing (RNA: random noise attenuation) 408 Pet.Sci.(2011)8:406-414 The high quality seismic data which have preserved data from the Tarim Basin (Fig. 2). In CRP (common the information of heterogeneous features could be used reflection point) gathers, multiples are more obvious than in the pre-stack AVO inversion to predict caved reservoirs. those in CMP (common middle point) gathers. In order For a fractured carbonate reservoir with strong anisotropy, to perform pre-stack analysis, these multiples should be considering the limited azimuth data acquisition and the effectively removed, and Radon transform method is often need in AVOZ inversion, we performed azimuthal processing used. For random noise, TDNFK (3D noise attenuation in before migration, and then amplitude-preserved migration F-K domain) noise elimination method is preferable. The and the subsequent processing with data of each azimuth results of multiples suppression are shown in Fig. 2. Fig. 2(a) were finished. Finally we obtained the azimuth data which is the CRP gather after migration, and Fig. 2(b) is CRP gather can be used in the anisotropy analysis. Focusing on carbonate after eliminating the multiples, and Fig. 2(c) is the multiples reservoir prediction, a series of processing techniques removed. Fig. 2(d), (e), and (f) are the corresponding velocity aiming at obtaining the amplitude-preserved seismic data is spectrums of Fig. 2(a), (b), and (c). It can be clearly observed introduced in this paper, and the fl ow is shown in Fig. 1. The that the multiples have been effectively suppressed. results from isotropic and anisotropic inversion using the data 3.2 The sequence of TDNFK and migration from this fl ow both refl ect the distribution characteristics of caved and fractured carbonate reservoirs which illustrates the The pre-stack migration is quite sensitive to noise, which effectiveness of this strategy. can be enlarged after migration. Therefore, pre-stack noise elimination is the foundation for the following data processing 3 Key amplitude-preserved techniques (Zhang, 2006). The real seismic data in the Tarim Basin are of poor quality. They show strong surface waves, linear waves, 3.1 Multiples suppression multi-refraction waves and random noises. Several noise Multiples and random noise are prominent in the seismic suppression techniques are integrated to enhance the S/N 8000. 5000. 5000. 100. 8000. 100. 8000. 100. 2000. 2000. 2000. 5000. (a) (b) (c) (d) (e) (f) 3.5 4.5 5.5 Offset, m V, m/s Comparison of CRP before and after multiples suppression Fig. 2 ratio, and at the same time, to maintain the effective perform attribute analysis and AVO/AVA inversion (Sun, reflection signal. The linear waves are removed according 2002). In 1993, the Kirchhoff amplitude-preserved migration to the velocity difference, and the anomalous amplitude method was put forward by Schleicher (Schleicher et al, is suppressed using a median filter. The TDNFK method 1993). Furthermore, in 1999 Graham A. Winbow (Graham et is applied for surface wave suppression in f-K K domain. al, 1999) provided an explicit 3D amplitude-preserved weight x y When this method is conducted before and after migration function formula for pre-stack time migration and utilized respectively, different imaging results can be obtained, as true amplitude weight function estimation to compensate shown in Fig. 3. It is better for TDNFK to be performed after amplitude. In this paper, the pre-stack Kirchhoff amplitude- migration. In the amplitude-preserved data processing, noise preserved migration is applied in the fi eld data of this area, is subtracted from the original data, which is beneficial for and we mainly analyzed and tested the main parameters of amplitude preservation. this amplitude-preserved migration method in order to obtain high quality seismic data for pre-stack inversion. 3.3 Amplitude-preserved Kirchhoff pre-stack time 3.3.1 The selection of key parameters migration (1) Regroup offset Amplitude-preserved seismic data are required to correctly Firstly, the maximum and the minimum offset and the Time, s Pet.Sci.(2011)8:406-414 409 193 433 673 913 1153 2.0 3.0 4.0 (a) 5.0 CDP (20m) 193 433 673 913 1153 193 433 673 913 1153 2.0 3.0 4.0 A A B B (c) (b) 5.0 CDP (20m) CDP (20m) Fig. 3 The infl uence of noise elimination sequence on imaging (a) Pre-stack time migration section (b) Noise elimination after migration (c) Noise elimination before migration offset increment are achieved by investigating and analyzing the corresponding parameter testing carried out for different the offset distribution of CMP gathers. Particularly, missing structural regions in the Tarim Basin, the dip of layer is traces in the same offset should be less than 30%, and the usually small. Therefore, in order to ensure the recognition of offset distribution of every CMP gather should be uniform local small structures, the protected dip value is chosen to be and as complete as possible. 45 degrees. (2) Migration aperture (4) Ray path The migration aperture refers to the seismic data The calculation of reflection travel time can be distribution for the migration. In general, the migration approximately simplified as a straight line with a lower aperture is controlled by three factors: the maximum dip calculation precision. The ray bending method calculates and depth of reflection interface, input data length and ray travel time according to the interval velocity model which path aperture. It was said that a small aperture may only is based on the Snell’s Law ray tracing method with high ensure structure imaging effects for small dip data and accuracy for complex and complicated areas. According to the characteristics of the high S/N, but not for steep angle data geological characteristics of the Tarim Basin and computing which at the same time would have structural distortion and ability, the bending travel time calculation method should be give rise to the “fl attened” phenomenon. A large aperture can selected. ensure structural imaging for steep dip, but the continuity of (5) Anti-alias fi ltering the refl ection section would become worse, and the S/N and The frequency of the migration section and imaging resolution would fall (Chen et al, 2002). quality can be directly influenced by anti-frequency (3) The angle of migration parameters. If frequency is too high, high-frequency noise The maximum migration angle refers to the maximum will be introduced and false frequency phenomenon will angle to be preserved during the migration process. On one be caused, and if frequency is too low, high frequency hand, it limits the scope of the migration operator extensions components will be lost. Aliasing always appears at the in the space. On the other hand, it has an influence on the shallow section, so an anti-alias filter is needed. We limit migration arithmetic as well as computing times. The larger the highest frequency to the scope of the effective wave and the migration angle, the more the computing times. Through adopt anti-alias fi ltering. Time, ms Time ms 410 Pet.Sci.(2011)8:406-414 3.3.2 Analysis of migration and processing results reasonable, and are advantageous for inversion. In order to test the amplitude preservation after data In the course of amplitude-preserved PSTM, signals processing and migration, the CRP gather after noise can be imaged appropriately through choosing a proper elimination and migration is compared with the theoretical directional factor, spherical divergence factor and phase synthetics, which is shown in Fig. 6. The reasonability of adjustment factor. For imaging of carbonate reservoirs in amplitude-preserved processing is affirmed by the similar the Tarim Basin, amplitude-preserved PSTM is the most AVO characteristics between synthetics and CRP gathers, important. The different CRP gathers by different Kirchhoff especially in the target area marked by the red frame. Pre- migration algorithms are selected and shown in Fig. 4. The stack inversion identifying carbonate reservoirs using the pre- amplitudes at 5.0 ms of different CRP gathers are extracted, stack gathers from this processing flow shows reasonable and are shown in Fig. 5. We can notice that the amplitudes results. marked by red and green lines are much more similar and 0.0 0.0 8000. 8000. 0.0 8000. 0.0 (a) (b) (c) (d) 3.0 3.0 3.0 3.0 4.0 4.0 4.0 4.0 5.0 5.0 5.0 5.0 Offset, m CRP gathers by different migration algorithms Fig. 4 0.0 (a) 3.0 ms (b) 4.0 ms (c) 5.0 ms 1.0 Offset, m Fig. 5 Amplitudes extracted from different CRP gathers in Fig. 4 is featured by frequency loss, and changes of amplitude, 4 Azimuth amplitude-preserved Kirchhoff phase, velocity and fault imaging. These features are in favor PSTM of extracting the information from anisotropic reservoir (Williams and Jenner, 2002). Although the scale of terranes or The P-wave is sensitive to anisotropic media, and it Amplitude Time, s Pet.Sci.(2011)8:406-414 411 Well logs Synthetics CRP gathers Time PI SI 3.55 3.6s 3.6 3.65 3.7s 3.7 3.75 3.8 3.8s 3.85 3.9 3.9s 3.95 4 4s 4.05 4.1 4.1s 1000 6800 1.0 1.8 1000 6800 Amplitude Offset, m Offset, m Fig. 6 Comparison between synthetics and PSTM CRP gathers fi ssures is less than seismic wavelength, the macro anisotropic 15° (Fig. 8(a)). The offsets and azimuths of bins can not be effects still can be observed (Jenner, 2002). Actually, there is guaranteed to uniform in the acquisition in this area, which a defi nite relationship between the amplitude changes versus will lead to AVO error. To avoid this and improve S/N, the bin azimuth and anisotropy. Specifi cally, if fractures in a certain size is enlarged. The data bin in the Tarim Basin can usually azimuth exist in the subsurface, the AVO characteristics be enlarged from 20m×20m to 40m×40m (Fig. 8(b)). of CRP gathers in the azimuths parallel with and normal 4.2 Azimuth velocity analysis and residual static to fractures are obviously different. The distribution and correction development densities of fractures can be reflected by the size and distribution of that difference (Zhang et al, 2004). Since the velocity and static measurement are different in Based on this, combined with the data in the Tarim Basin, the different azimuth data, velocity analysis and residual static azimuth range is chosen through analyzing the azimuth, offset correction should be performed respectively in different and fold times of CMP gathers at fi rst, and then the bins are azimuth data to protect the anisotropic characteristics enlarged to improve S/N and fold times. Finally, amplitude- before migration. Then amplitude-preserved PSTM preserved PSTM and other processing are applied to the method discussed above is applied to all these azimuth data azimuthal gathers. respectively. 4.1 Azimuth and offset analysis, and azimuth division 4.3 Common azimuth amplitude-preserved PSTM Considering the high cost, seismic data acquisition in the Based on the azimuth data divided in Fig. 8, the Tarim Basin is generally not applied to the whole azimuth. corresponding four common azimuth gathers are conducted Therefore, the distribution of azimuth-offset is not uniform migration. The four CRP gathers after PSTM and noise and usually lacks far offset components in some azimuths (Fig. attenuation are shown in Fig. 9, which locate at the red 7). There are two basic messages: (1) The mean aspect ratio dashed lines in the corresponding stack sections in Fig. 10. in this area is about 0.45 (Fig. 7(a)). (2) Through analyzing Obviously, the azimuthal processing is successful since the fold distribution versus azimuth (Fig. 7(b)), it is exhibited there are more distinct AVO characteristics in CRP gathers, as a shape of two peaks. About 66% data are distributed in better wave features and more continuous events in the stack the range between positive 30° and negative 30° azimuth section. In addition, the anisotropic characteristics of different (including corresponding opposite direction 150°-210°, the common azimuth data are outstanding, which are suitable east is regarded as 0°, increasing anti-clockwise). Finally, to for anisotropic inversion. The pre-stack anisotropic inversion meet the demand of anisotropic inversion, the azimuth angles using azimuthal data from this processing fl ow shows good from -30° to 30° are divided into four groups of common- results in effective fracture detection. azimuth gathers for processing, and each group differentiates Time, s 414 Pet.Sci.(2011)8:406-414 Petroleum Geophysics. 2009. 7(1): 1-3 preserved PSTM and the following noise attenuation should Jen ner E. Fractured reservoir characterization using P-wave AVOA be applied consistently. Attention should be paid to the analysis of 3D OBC data. The Leading Edge. 2002. 21(8): 777-781 preservation of effective reflection amplitude and quality Li G H, Gao H L, Zhang Z Q, et al. Reservoir prediction for the east of control in each step. Lun-Gu area. In: Thesis Collections of the Integration of Exploration (2) The amplitude-preserved pre-stack time migration and Development for Lun-Gu Oilfield in the Tarim Basin. Beijing: technique is the key to the accurate imaging of carbonate Petroleum Industry Press. 2007 (in Chinese) reservoirs in the Tarim Basin. This affects the later pre-stack Sch leicher J, Tygel M and Hubral P. 3-D true-amplitude finite-offset inversion and reservoir prediction. In order to obtain an image migration. Geophysics. 1993. 58(8): 1112-1126 of carbonate reservoirs in the Tarim Basin, the migration Sun J G. Kirchhoff-type true-amplitude migration and demigration. aperture was tested and finally defined as 6,000 m, and the Progress in Exploration Geophysics. 2002. 25(6): 1-5 (in Chinese) Tao Y G, Wang X W, Lü L, et al. Seismic data pre-stack processing effective imaged dip angle reached 45 degrees. techniques for carbonate reservoirs in the central Tarim Basin. Oil (3) For fracture prediction through anisotropic inversion Geophysical Prospecting. 2010. 45(2): 230-236 (in Chinese) based on P-wave data, it is an economic and effective Wan g X W, Lü L, Liu W F, et al. Seismic data processing techniques of method to realize processing common azimuth gathers, carbonate rocks in Tarim Basin. Lithologic Reservoirs. 2008. 20(4): which is a new direction of fracture prediction. To avoid fold 109-112 (in Chinese) nonuniform and improve S/N, the bin size, used to azimuth Wil liams M and Jenner E. Interpreting seismic data in the presence of migration, needs to be enlarged when seismic data are divided azimuthal anisotropy or azimuthal anisotropy in the presence of by different azimuths. seismic interpretation. The Leading Edge. 2002. 21(8): 771-774 Wu G H, Li Q M, Zhang B S, et al. Structural characteristics and Acknowledgements exploration fi elds of No.1 faulted slope break in the Tazhong area. Acta Petrolei Sinica. 2005. 26(1): 27-30 (in Chinese) This research is financially supported by National Basic Zha ng G S, Ma G G, Song Y L, et al. Crack detection by utilizing full Research Program of China (No.2011CB201100). The data 3D P-wave data. Oil Geophysical Prospecting. 2004. 39(1): 41-44 (in were provided by Exploration and Development Research Chinese) Institution of Tarim Oil Field Company. The authors would Zha ng Y. Application and prospects of the 3D seismic pre-stack like to thank Wang Di and Yang Pei for much work of time migration processing technique. Petroleum Exploration and proofreading. Development. 2006. 33(5): 536-541 (in Chinese) Zha o Z J, Wang Z M, Wu X N, et al. Genetic types and distribution forecast of available carbonate reservoirs in Ordovician in the central References area of the Tarim Basin. Petroleum Geology and Experiment. 2007. Che n Z D, Liu Z K and Li C B. 3D pre-stack depth migration velocity 29(1): 40-46 (in Chinese) analysis and automatic Monte Carlo velocity picking in depth. Zho u J G, Deng H Y, Fan G Z, et al. Geological-geophysical model and Chinese Journal of Geophysics. 2002. 45(2): 246-254 (in Chinese) prediction application of upper Ordovician Lianglitage reef-shoal Gra ham A W, William A and Schneider J. Weights for 3-D controlled reservoir in the Tazhong area, Tarim Basin. Marine Origin Petroleum amplitude prestack time migration. SEG Expanded Abstracts 18. Geology. 2008. 13(3): 17-23 (in Chinese) 1999. 1110-1113 Guo S X. Discussion on preserved amplitude processing of seismic data. (Edited by Hao Jie)

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Petroleum Science – Springer Journals

Published: Dec 8, 2011

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