B. Milkereit 1, E. Adam 2, Z. Li 3, Wei Qian 1, T. Bohlen 4, D. Banerjee 1, and D.R. Schmitt 5

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1 Multi-offset vertical seismic profiling: an experiment to assess petrophysical-scale parameters at the JAPEX/JNOC/GSC et al. Mallik 5L-38 gas hydrate production research well B. Milkereit 1, E. Adam 2, Z. Li 3, Wei Qian 1, T. Bohlen 4, D. Banerjee 1, and D.R. Schmitt 5 Milkereit, B., Adam, E., Li, Z., Qian, W., Bohlen, T., Banerjee, D., and Schmitt, D.R., 4: Multi-offset vertical seismic profiling: an experiment to assess petrophysical-scale parameters at the JAPEX/JNOC/GSC et al. Mallik 5L-38 gas hydrate production research well; in Scientific Results from Mallik 2 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore and T.S. Collett; Geological Survey of Canada, Bulletin 585, 13 p. Abstract: Multi-offset vertical seismic profiling (VSP) techniques were employed in the JAPEX/JNOC/GSC et al. Mallik 3L-38 observation well to image the gas hydrate zones in the immediate vicinity of the Mallik 5L-38 gas hydrate production research well. Conventional vertical seismic profiling common depth point (VSP-CDP) transforms of the reflected wave field and resonance-scattering analysis from three-component VSP data provide information about lateral distribution of the gas hydrate and the internal structure of the gas hydrate zone. Compressional-wave velocities beneath the permafrost and within the gas hydrate zone averaged 24 m/s and 247 m/s, respectively. Within the gas hydrate zone, shear-wave velocities averaged 11 m/s. At the Mallik well site, the gas hydrate zone is characterized by strong perturbations in compressional-wave velocities. The resonance-scattering analysis and full wave-form sonic logs from the JAPEX/JNOC/GSC Mallik 2L-38 and JAPEX/JNOC/GSC et al. Mallik 5L-38 wells indicate that important lateral variations in gas hydrate distribution must exist near the top of the gas hydrate zone, at about 9 m depth. Résumé : Department of Physics, University of Toronto, Toronto, Ontario, Canada M5S 1A7 Oil and Gas Division, Hydro-Québec, Québec, Quebec, Canada G1V 4P1 P.O. Box 468, Yaohuamen Qixia District, Nanjing City, Jiangsu Province, Sinopec, P. R. China 2146 Department of Geosciences, Kiel University, Kiel, Germany Department of Physics, University of Alberta, Edmonton, Alberta, Canada T6G 2J1 1

2 GSC Bulletin 585 INTRODUCTION AND SURVEY DESIGN A multi-offset vertical seismic profiling (VSP) survey was conducted as part of the Mallik 2 Gas Hydrate Production Research Well Program (Dallimore et al., 4) to study the vertical and lateral variations of gas hydrate distribution. In layered and heterogeneous media, the angular- and frequency-dependent seismic responses are affected by the statistical distributions of physical properties. Investigations of the statistical nature 45 UTM Northing (m + 775) L eismic p olution s 36 5L-38 High-res of velocity and density perturbations provide insights into mechanisms governing wave propagation, as there may exist a strong correlation between the spatial properties of the velocity field of a reflective target and the lateral correlation length of the resulting seismic-wave field. At the Mallik well site, the gas hydrate zones are characterized by strong variations in compressional-wave velocities (Mi et al., 1999; Walia et al., 1999; Winters et al., 1999; Collett and Dallimore, 2). The synthetic seismograms derived from log and petrophysical data predicted bright seismic reflections. The regional seismic data imaged the top of the gas hydrate zone, but reflection strength and lateral 257 continuity were modest (see example in Collett and Dallimore, 2). High-resolution seismic 227 data (Hunter et al., ; Miller et al., 3) failed to image a prominent seismic response from the gas hydrate. Common techniques to 2L-38 assess lateral continuity of subsurface formations are two-dimensional and three-dimensional surface-seismic methods, offset VSP techniques, and cross-well seismic methods (Bauer et al., 4; Pratt et al., 4). rofile ) ee Fig. 2 (s 3L L UTM Easting (m + 51) 35 Figure 1. Detailed location map for the offset vertical-seismic-profiling survey, showing vibroseis source locations (circles), wells from the present Mallik program (filled circles) and the previous program (stars), and the high-resolution seismic profile shown in Figure 2. NE Mallik 5L-38 SW T B 1 5 m Figure 2. High-resolution seismic profile through the JAPEX/JNOC/GSC et al. Mallik 5L-38 gas hydrate production research well, with strong reflectivity from the permafrost section and patchy reflectivity from the gas hydrate zone (Schmitt et al., 4). Top of gas hydrate zone (T) and bottom of gas hydrate zone (B) are based on two-way travel times from the Mallik 2L-38 (Walia et al., 1999) and 3L-38 (this study) wells. 2 The horizontal resolution of surface-seismic methods is governed by the Fresnel zone, the portion of a horizontal reflector at depth from which reflected energy can reach a sensor within one-half wavelength of the first reflected energy (Sheriff, 2). The width of the Fresnel zone increases with increasing depth of the reflector. This limits the lateral resolution of surface-seismic methods. In this paper, three-component offset VSP data have been used to test the lateral continuity of seismic reflection at the Mallik well site. The locations of vibroseis sources and boreholes are shown in Figure 1. The location of a two-dimensional, high-resolution reflection-seismic profile across the Mallik site (Schmitt et al., 4) is included for reference. The lateral variations of compressional-wave (P-wave) and shear-wave (S-wave) velocities are best determined in the forward scattering (transmission) direction provided by three-component VSP data-acquisition geometry. Analysis of the transmitted seismic wave field helps to assess how well log data relate to the surrounding gas hydrate zone. In addition, the multi-offset VSP acquisition geometry provides information about the vertical distribution of gas hydrate (layering) within the reservoir zone and, from amplitude-versus-offset (AVO) response, information about the bulk elastic properties of the gas hydrate zone. In the vicinity of the Mallik well sites, most of the seismic energy is trapped in the permafrost layer, with only a fraction of the energy reaching the gas hydrate. Superimposed on the high-resolution section (Fig. 2) are the predicted two-way reflection times for the top and bottom of the gas

3 B. Milkereit et al. hydrate zone. Reflectivity for the gas hydrate appears patchy, with high amplitudes near the projected location of the Mallik well sites. In addition, flat high-amplitude reflection marks the base of the gas hydrate zone in the area southwest of the Mallik wells. This seismic section raises the possibility of lateral seismic-amplitude variations within the gas hydrate zone: are the amplitude variations due to difficult imaging conditions caused by the permafrost layer, or do lateral amplitude variations correlate with variations in gas hydrate concentration? SEISMIC MODELING AND MACRO VELOCITY MODEL Another way of looking at the side of a borehole is to use offset VSP and walkaway VSP techniques (using sensors in the borehole and surface-seismic sources). Lateral resolution is limited by the width of the Fresnel zone and the limited azimuth coverage by the surface-seismic sources. Conventional VSP techniques rely on the separation of downgoing and upgoing wave fields recorded in the borehole, whereas cross-well seismic methods investigate the region between two boreholes (sources are located in one borehole and receivers are located in the other). This method overcomes limitations imposed by surface-seismic sources, but information on the lateral continuity of a formation is restricted to the plane between the boreholes. Multi-offset VSP techniques can be employed to image a target in a complex geological setting. The integration of borehole geophysical logs, offset VSPs, and three-dimensional elastic seismic modelling studies provide new insights into the scale-dependent petrophysical parameters and the internal structure of the target. Results from the three-dimensional modelling study (Milkereit et al., 2) were used to refine survey design and evaluate processing strategies for the VSP experiments. It is important to note that only the JAPEX/JNOC/GSC Mallik 2L-38 well provides critical sonic and density logs for the complete sedimentary section (including permafrost). The raw P-wave velocity (Vp), S-wave velocity (Vs), and density logs have been kindly provided by Tim Collett. The Density 3 (g/cm ) P-wave (km/s) S-wave (km/s) Vp/Vs ratio S G S P Figure 3. Reprocessed logs from the JAPEX/JNOC/GSC Mallik 2L-38 well are the basis for the macro velocity model used in this study. Abbreviations: P, permafrost; S, sediments; G, gas hydrate. raw data required processing in order to compute the synthetic seismic responses, because portions of the logs were acquired through casing and Poisson s ratios below.25 had been computed from the velocity values. Before the application of a 27-point running median filter, Vp and Vs outliers were edited out. The logs were then resampled to a 4 m interval. For depth ranges where Poisson s ratio was below.25, the Vp/Vs ratio was fixed at 2.2 and Vs was recomputed from the corresponding Vp. This approach assumes that the initial S-wave velocity measurement was in error and the P-wave velocity was correct. Figure 3 shows logs of processed density, Vp, Vs, and the corresponding Vp/Vs ratio. A full suite of multicomponent, multi-offset VSP data was computed to analyze travel-time fluctuation, amplitude fluctuations, and frequency wave-number spectra of the transmitted and reflected wave fields. In the present modelling study, the gas hydrate reservoir is characterized by strong vertical variations (first-order discontinuities and gradient zones) in elastic parameters. From the acoustic impedance model of the study area (Collett and Dallimore, 2), the base of the high-velocity permafrost is at 65 m. Gas-hydrate-bearing rock types are located at depths of approximately 9 to 11 m. Figure 4 shows the vertical component, zero-offset data for the three synthetic reservoir models. Systematic travel-time and amplitude fluctuations occur and are most pronounced for large offset recordings. Travel-time fluctuations of large offset recordings reveal bulk anisotropy in the target zone if a large lateral correlation length is assumed. Amplitude fluctuations at large offsets correlate well with the vertical scale length of the media. Amplitude fluctuations are best explained by transmission losses (caused by layering) and converted S-waves (both in reflection and transmission). In practice, amplitude and travel-time fluctuations must be obtained from raw VSP data before any amplitude scaling, deconvolution, or alignment of direct (downgoing) energy. Figure 4a shows the results of a synthetic VSP experiment to evaluate the velocity and reflectivity structure of the reservoir. In order to obtain information about the angular reflection response of the target, the VSPs were calculated for offsets between and m (Fig. 4b). The thick, high-velocity, ice-bearing permafrost layer produces prominent multiple events. The true-amplitude, vertical-component recordings of the target zone are shown in Figure 4b. The prominent AVO response of the reservoir zone is evident. The high-velocity permafrost zone, however, limits the usefulness of the large offsets because much of the seismic energy gets trapped in a near-surface, high-velocity zone. In addition, lateral variations of true amplitudes point toward complex geometrical spreading corrections for large offset recordings. The modelling study indicates that offset-vsp data-acquisition geometry will yield new information about the angular-dependent reflective response from the gas hydrate zone. Figure 5a shows the background velocity model, based on the Mallik 2L-38 well, with ray-path geometry from the offset-vsp data acquisition. Trapping of seismic energy in the permafrost layer may explain the poor signal/noise ratio or reduced lateral continuity of seismic reflection images below the permafrost. 3

4 GSC Bulletin 585 The seismic elastic modelling study helped to optimize the acquisition parameters for the offset and walkaway VSP surveys that targeted a depth of 9 m. Based on the modelling results, acquisition of a walkaway VSP was selected to better evaluate the AVO reflection and transmission response from the reservoir zone. Based on the reflection amplitudes, it was decided to limit offset for the VSP recordings to less than m, the same offset range used in the VSP survey of the Mallik 2L-38 well by Walia et al. (1999). The predicted travel times for offset recordings are shown in Figure 5b. The predicted travel-time model was used as a reference for the observed travel times from the VSP surveys in the Mallik 3L-38 well. Time (S) a) Permafrost b) Depth: m T 1 Figure 4. a) Synthetic vertical seismic profiling (vertical component) based on the logs shown in Figure 3; note energy trapped within the high-velocity permafrost zone; box indicates target zone for reflections from gas hydrate zone. b) Synthetic vertical-seismic-profiling response for offsets up to m; note decrease of first-break arrivals and reflection amplitudes with increasing offsets. FIELD OPERATIONS AND ACQUISITION OF VERTICAL-SEISMIC-PROFILING DATA Dallimore et al. (4) have given a general overview of field operations at the Mallik drill site. A 24-hour period was reserved to complete an offset VSP through the gas hydrate layers encountered by the Mallik 3L-38 well. The VSP survey was completed by Schlumberger Ltd. in just 8 hours on February 22, 2, while cementing was occurring in the Mallik 5L-38 well (Fig. 6). During cementing, the ambient noise at the research well was at its lowest level and conditions were ideal for collecting high-quality VSP data. Since there was no headframe at the Mallik 3L-38 well, a 2 m crane was mobilized from Tuktoyaktuk to allow the deployment of Schlumberger s five-receiver Array Seismic Imager (ASI) VSP tool. The ASI downhole receiver array uses an innovative clamping technology (a permanent magnet), and each module is equipped with a 25 Hz natural-frequency triaxial-geophone accelerometer sensor that provides a linear response between 3 and Hz. The source testing was conducted with the vibroseis at 22 m from the well collar (the offset location). The single-vibrator Mertz model 18 produced usable frequencies as high as 18 Hz for the first part of the survey. Some mechanical problems that could not be solved at the Mallik site, however, required that the upper frequency limit be dropped to 12 Hz halfway through the survey. The surveying of the vibration point was done using a Trimble differential GPS provided by the University of Alberta. Broadband vibroseis source signal (8 18 Hz, linear sweep) for offset up to m was recorded using a three-component, five-level tool with sensor separation of 15 m. Ongoing cementing operations in the Mallik 5L-38 well caused noise problems for the VSP data acquisition; however, vertical stacking (seven-fold) overcame this noise problem and the final VSP recordings are of excellent quality. Figure 7 shows the vertical-component seismograph for zero-offset recording used for first-break picking. True-amplitude processing of the multi-offset VSP data focused on first-break travel-time picks, velocity analysis, and wave-field separation. The multi-offset VSP acquisition geometry provides valuable information about the vertical distribution (layering) within the reservoir zone, as well as information about the bulk elastic properties from the reflected P-wave and converted S-wave AVO response. Consistent wave-form data and minimal noise contamination made it possible to pick first-break arrivals for all offsets. The travel-time data for all offset recordings are provided in Appendix A. Due to operational constraints, the permafrost section of the well was not used for the VSP surveys; the minimum depth was therefore 56 m. The average P-wave velocities for the sedimentary section (24 m/s) and the gas hydrate zone (247 m/s) are consistent with results obtained by Walia et al. (1999) and Mi et al. (1999). The vertical surface source did not generate prominent downgoing S waves. The modelling study (Fig. 4) identified the optimum depth interval for downhole receivers to record a reflection response from the gas hydrate zone. The direct downgoing wave field exhibits clear first-break energy, no noise contamination 4

5 B. Milkereit et al. a) V (m/s) b) 1 Figure 6. Vertical seismic profiling data acquisition at the JAPEX/JNOC/GSC et al. Mallik 3L-38 well. Locations of surface sources relative to the Mallik 3L-38 well and existing surface-seismic data are shown in Figure 1. (m/s) Figure 5. a) Compressional-wave interval velocity model for the Mallik site, with ray paths for offset-vsp acquisition geometry. b) One-way travel times between source and receiver for offset-vsp geometry Figure 7. Vertical-component zero-offset recording in the JAPEX/JNOC/GSC et al. Mallik 3L-38 well, showing clear direct wave energy in the sediments (average compressional-wave velocity of 24 m/s) and the gas hydrate zone (average compressional-wave velocity of 247 m/s). 5

6 GSC Bulletin 585 Depth: m Figure 8. Offset vertical seismic profiling recordings (vertical component) from the gas hydrate zone in the JAPEX/JNOC/GSC et al. Mallik 3L-38 well: the five panels on the left show the downgoing wave field, and the five panels on the right show the reflected, upgoing wave field and stable wave forms (Fig. 8, left-hand set of five panels for source offsets ranging from 83 to 316 m). After wave-field separation and amplitude scaling, prominent reflected energy from the gas hydrate zone is shown in Figure 8 (right-hand set of five panels). For all offsets and depths, first-break travel times were determined (Appendix A) and compared with the regional-velocity model for the Mallik site (Fig. 5). The difference between observed and model travel times does not exceed 3 ms (Fig. 9). The differential travel times show no evidence for lateral-velocity variations near the source, such as thickness variations of the permafrost layer (lateral-velocity variations within the permafrost layer would be identified as vertical stripes in Fig. 9). There is evidence, however, for minor systematic travel-time variations at depth, as indicated by horizontal stripes in the differential travel times in Figure 9. These minor travel-time differences have not been modelled or inverted in order to improve the background velocity model. PROCESSING AND INTERPRETATION OF VERTICAL SEISMIC PROFILING DATA Based on first-break travel-time picks, the reflected (upgoing) wave field was shifted to align the reflections, to identify the origin (depth) of reflections, and to enable comparison with existing surface-seismic data. In addition, upgoing wave fields from the offset VSPs were processed to provide a commonmidpoint-reflection section in the plane defined by the borehole receivers and the surface-source locations. The VSP-CDP transformed sections should be used for comparison with the cross-well seismic data acquired between the Mallik 3L-38 and 4L-38 wells (Bauer et al., 4; Pratt et al., 4) The basic processing sequence for VSP data consisted of rotation of horizontal components into in-line and cross-line directions, deconvolution and band-pass filter test, wave-field separation in the frequency wave-number domain, and amplitude scaling. Front-end mute and travel-time shifts to align reflected energy were based on first-break travel times. The P-wave reflectivity for the zero-offset VSP recording is shown in Figure 1a. Prominent reflections are observed from the top of the gas hydrate zone (marked A ) and the base of the gas hydrate zone (marked C ). Most high-frequency VSP sections show a distinct reflection from a coal Time difference (ms) Figure 9. Comparison of observed and predicted travel times for offset vertical seismic profiling experiment in the JAPEX/ JNOC/GSC et al. Mallik 3L-38 well (see Fig. 5a, b); residual travel times do not exceed 3 ms; there is no evidence for severe lateral-velocity/thickness variations in the permafrost layer. 6

7 B. Milkereit et al. a) 1 ms NE b) ms C Figure 1. a) Compressional-wave reflectivity from zero-offset vertical seismic profiling recording. Abbreviations: A, reflection from top of gas hydrate zone; B, reflection from within gas hydrate zone (?coal seam); C, reflection from base of gas hydrate zone. b) Shear-wave reflectivity from offset vertical-seismic-profiling recording (horizontal component). Abbreviation: C, converted shear wave from base of gas hydrate zone (estimated shear-wave velocity in gas hydrate zone is 11 m/s). 1 m Figure 11. Vertical seismic profiling common depth point (VSP-CDP) transform for offset recordings in the JAPEX/ JNOC/GSC et al. Mallik 3L-38 well. Top (A) and bottom (C) of the gas hydrate zone are imaged between the well and the northeast-trending seismic-source locations. Note the prominent deep reflection marked R (see regional seismic data in Collett and Dallimore, 2). seam located within the gas hydrate zone (marked B ). For the zero-offset recordings, some numerical noise was caused by a high-frequency downgoing tube wave at late reflection times (the tube wave is aliased due to coarse spatial sampling). There is no evidence for prominent, direct S waves in the vertical-force vibroseis recordings. At large offsets, however, converted S waves (P to S conversion) were identified in the horizontal-component recordings. The reflected S wave from the base of the gas hydrate zone (marked C in Fig. 1b) aligns well for an average S-wave velocity of 11 m/s. The observed average S-wave velocity and the Vp/Vs and Poisson ratios for the gas hydrate zone agree well with earlier observations based on horizontal-force vibroseis recordings by Walia et al. (1999) in the JAPEX/JNOC/GSC Mallik 2L-38 well, and with petrophysical models for the Mallik gas hydrate (Lee, 2). 7

8 GSC Bulletin 585 The offset-vsp recordings are the basis for a VSP-CDP transform section, the purpose of which is to demonstrate lateral continuity of the reflection response in the plane between the borehole and the surface-seismic sources (Sheriff, 2). The VSP to CDP transform for the 316 m offset recording is shown in Figure 11. Continuity of horizontal reflections from the top and base of the gas hydrate zone (marked A and C, respectively, in Fig. 11) close to the Mallik 3L-38 well are well established. The reflection R from below the base of the gas hydrate zone should be tied to prominent reflections from regional seismic-exploration data in the Mallik area. The VSP to CDP transform provides an important link between logs and petrophysical data from the Mallik 3L-38, 4L-38, and 5L-38 wells, conventional VSP recordings in the Mallik 3L-38 well, and cross-well sections between the Mallik 3L-38 and 4L-38 wells (Bauer et al., 4; Pratt et al., 4). Close inspection of full wave-form sonic logs from the Mallik wells (Fig. 12) gives valuable hints for addressing the weak reflectivity from the top of the gas hydrate zone in the surface-seismic data. Both logs exhibit similar trends but differ in detail. The base of the gas hydrate stability zone (marked C in Fig. 12) is well aligned in the Mallik 2L-38 and 5L-38 wells. A prominent low-velocity zone (coal seam, A B C Mallik 5L-38 Mallik 2L Figure 12. Full wave-form sonic data from the JAPEX/JNOC/GSC Mallik 2L-38 and JAPEX/JNOC/GSC et al. Mallik 5L-38 wells. Note the high compressional-wave velocities (up to 3 m/s) associated with the gas hydrate. Abbreviations: A, top of gas hydrate zone; C, base of gas hydrate zone; B, low-velocity coal;?, local high-velocity (?gas hydrate) zone above A. Note the excellent agreement for C and the difference of at least 3 m for A. marked B in Fig. 12) correlates with reflections from within the gas hydrate zone. The top of the gas hydrate zone (marked A in Fig. 12), as indicated by a thick zone of high P-wave velocities (of more than 3 m/s), occurs at different depths in the Mallik 5L-38 and 2L-38 wells. In addition, the full wave-form sonic log from the Mallik 5L-38 well indicates thin layers of high P-wave velocities tens of metres above location A. The offset and walkaway VSP data-acquisition geometry provides the ideal framework to study angular-dependent reflection and transmission responses of a target zone at depth. For the new Mallik data set, however, the surface high-velocity permafrost layer has a major effect on the geometrical spreading factors. The spreading correction is larger than the amplitude-versus-offset (AVO) reflection response from the gas hydrate zone. The negative AVO trend for reflections from the gas hydrate zone is well established for the Mallik VSP site (reflection amplitudes decrease with increasing angle of incidence). Geometrical spreading, reflection, and transmission amplitudes are summarized in Appendix B. RESONANCE-SCATTERING ANALYSIS How can the lateral homogeneity of the gas hydrate zone be assessed? The gas hydrate zone is characterized by large P-wave velocity perturbations (ranging from 1 to 3 m/s). In addition, the differences between the sonic logs from the Mallik 2L-38 and 5L-38 wells (Fig. 12) indicate significant lateral variations. Based on the P-wave velocity perturbations in the gas hydrate zone, a strong correlation can be expected between the spatial properties of the target zone and the lateral correlation length of the resulting reflected or transmitted seismic-wave field. Information about short-wavelength, horizontal-scale parameters are contained in the transmitted (forward scattered) elastic-wave field. Most promising results have been obtained from full-wave-form resonance spectra (Milkereit et al., 3). The procedure for computing a resonance spectrum for three-component VSP data is as follows. At each depth interval, the three components are rotated into the ray co-ordinate system, so that one component (radial component) points into the direction of the incident P-wave. The first arrival is windowed, whereby the same time window must be applied to the three components. A resonance spectrum is obtained by dividing the amplitude spectra of the transverse component and the radial component at each depth interval. Zero amplitude in a resonance spectrum indicates definite polarization of the direct P-wave into the ray direction, which is expected for very weak lateral heterogeneity along the path of the direct wave. High amplitudes in a resonance spectrum, however, are observed if energy of the direct wave is observed on the horizontal components due to scattering at small-scale lateral heterogeneities near the receiver. The peak frequency may provide information on the composition and shape of the scattering structure. 8

9 B. Milkereit et al In-line direction Cross-line direction Sonic log Wavelength (m) Wavelength (m) Figure 13. Resonance-scattering response measured in the in-line and cross-line directions (horizontal component pointing toward surface source) for the JAPEX/JNOC/GSC et al. Mallik 3L-38 well; top of the gas hydrate zone in well 3L-38 shows evidence of pronounced lateral heterogeneity, particularly in the northwest-southeast cross-line direction. Mallik 3L-38 Mallik 5L-38 Mallik 4L-38 Mallik 2L-38 The resonance-scattering analysis (Fig. 13) reveals the presence of scale-dependent petrophysical parameters in the gas hydrate zone. Two zones of pronounced short-wavelength lateral heterogeneity have been mapped in the vicinity of the Mallik 3L-38 well: zone A coincides with the top of the gas hydrate zone, at a depth of about 9 m, and zone B, below m, coincides with the prominent low-velocity zone. The scale length of lateral variations ranges from 2 to 5 m. The three-component VSP data indicate that the heterogeneity at the top of the gas hydrate zone is more pronounced in the north-south direction than in the east-west direction. Surface-seismic methods are employed to image subsurface structures for gas hydrate exploration in two and three dimensions. Boreholes and well logs provide key information about the vertical distribution of geological and petrophysical data. As lateral resolution of surface-seismic data decreases with increasing depth of investigation, borehole-based seismic techniques such as offset VSP and cross-well surveys must be employed to assess the lateral continuity of formations, reservoirs, and target zones of interest. Resonance-scattering analysis of three-component VSP data offers an opportunity to detect heterogeneities close to the borehole, thereby closing the existing resolution gap between surface-seismic methods (hundred(s) of metres) and conventional well logs (decimetres). SUMMARY Permafrost Sediments? Fault Gas hydrate Sediments Figure 14. Schematic diagram of the main results from the offset-vsp survey at the Mallik site; the base of the permafrost and the gas hydrate zone show no evidence for lateral depth variations; the top of the gas hydrate zone lacks continuity and may be responsible for the patchy seismic reflectivity in the nearby high-resolution seismic profile. Broadband vibroseis source signal (8 18 Hz, linear sweep) was recorded using a three-component, five-level receiver array in the JAPEX/JNOC/GSC et al. Mallik 3L-38 well. Processing of the VSP data focused on velocity analysis, wave-field separation, resonance-scattering analysis, and integration of full-wave-form sonic logs. Key results from the offset VSP and resonance-scattering data analysis are summarized in Figure 14. Beneath the permafrost, P-wave velocities average 24 m/s and, within the gas hydrate zone (from 87 to 11 m), they average 247 m/s. Four prominent P-wave reflections were observed: from the top of the gas hydrate zone (87 9 m depth), from a low-velocity zone within the gas hydrate zone (at approx. m), from the base of the gas hydrate zone (at 11 m), and from below the borehole. Large offset recordings image S-wave reflections from the base of the gas hydrate zone. Secondary-wave reflections from the base of the gas hydrate zone are converted waves produced by sources located at distances of more than m from the well. In addition, broadband seismic-source signals up to 12 Hz provide the basis for resonance-scattering analysis of three-component VSP data. The resonance-scattering analysis reveals the presence of pronounced heterogeneities near the top of the gas hydrate zone. The analysis of resonance spectra may become a new tool that helps to assess how well log data relate to the surrounding reservoir zone. 9

10 GSC Bulletin 585 ACKNOWLEDGMENTS The Mallik 2 Gas Hydrate Production Well Research Program participants included eight partners: Geological Survey of Canada (GSC), Japan National Oil Corporation (JNOC), United States Geological Survey (USGS), GeoForschungsZentrum Potsdam (GFZ), India Ministry of Petroleum and Gas (MOPNG), BP-ChevronTexaco-Burlington joint venture parties, and United States Department of Energy (USDOE). In addition, the program was supported by the International Continental Scientific Drilling Program (ICDP). The Geological Survey of Canada co-ordinated the science activities, and Japex Canada Limited acted as the designated operator for the fieldwork. Laboratory services in Inuvik were provided by the Inuvik Research Centre, part of the Aurora Research Institute. Funding for the VSP data acquisition was provided by a University of Toronto start-up grant to Bernd Milkereit. The authors acknowledge the excellent field operations conducted by Schlumberger Ltd. for the offset VSP survey. They also thank the two reviewers, Warren Wood and Ingo Pecher, for helpful comments. REFERENCES Bauer, K., Pratt, R.G., Weber, M.H., Haberland, C., Ryberg, T., and Shimizu, S. 4: Introduction and initial data analysis of the Mallik 2 crosshole seismic experiments; in Scientific Results from the Mallik 2 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore and T.S. Collett; Geological Survey of Canada, Bulletin 585. Collett, T.S. and Dallimore, S.R. 2: Integrated well log and reflection seismic analysis of gas hydrate accumulations on Richards Island in the Mackenzie Delta, N.W.T., Canada; Canadian Society of Exploration Geophysicists Recorder, v. 27, no. 8, p Dallimore, S.R. et al. 4: Overview of the science program for the Mallik 2 Gas Hydrate Production Research Well Program; in Scientific Results from the Mallik 2 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore and T.S. Collett; Geological Survey of Canada, Bulletin 585. Hunter,J.A.,Miller,R.D.,Doll,W.E.,Carr,B.J.,Burns,R.A.,Good,R.L., Laflen, D.R., and Douma, M. : Feasibility of high-resolution P- and S-wave seismic reflection to detect methane hydrates; Geological Survey of Canada, Open File 385, 42 p. Lee, M.W. 2: Biot-Gassmann theory for velocities in gas hydrate-bearing sediments; Geophysics, v. 67, p Mi, Y., Walia, R., Hyndman, R.D., and Sakai, A. 1999: Vertical seismic profile in the Mallik 2L-38 gas hydrate research well in the Canadian Arctic (expanded abstract); 69th SEG Conference and Exhibition, November 1 4, 1999, Houston, Texas; Society of Exploration Geophysicists, Tulsa, Oklahoma, Conference Expanded Abstracts, 1999, p (online; exabshist/, Session BHRP4) Milkereit, B., Bohlen, T., Adam, E., and Banerjee, D. 2: Reservoir imaging and monitoring a modelling study (abstract); 64th EAGE Conference and Exhibition, Amsterdam, The Netherlands; EuropeanAssociationofGeoscientistsandEngineers, Amsterdam, The Netherlands, Conference Abstracts, p Milkereit, B., Bohlen, T. And Qian, W. 3: Resonance scattering analysis of 3-component VSP data (expanded abstract); 73rd SEG Conference and Exhibition, Dallas, Texas; Society of Exploration Geophysicists, Tulsa, Oklahoma, Conference Expanded Abstracts, p (online; archive/exabshist/, 3, Session VSP3). Miller, R.D., Hunter, J.A., Doll, W.E., Carr, B.J., Burns, R.A., Good, R.L., Laflen, D.R., and Douma, M. 3: High resolution seismic imaging of the hydrate stability zone: Mallik, Canada; AAPG Annual Meeting, Salt Lake City, Utah; American Association of Petroleum Geologists, Tulsa, Oklahoma, Conference Abstracts (online; techprogram/session_1974.htm). Pratt, R.G., Hou, F., Bauer, K., and Weber, M.H. 4: Wave-form tomography images of velocity and inelastic attenuation from the Mallik 2 crosshole seismic surveys; in Scientific Results from the Mallik 2 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore and T.S. Collett; Geological Survey of Canada, Bulletin 585. Schmitt, D.R., Welz, M., and Rokosh, C.D. 4: High-resolution seismic imaging over thick permafrost at the 2 Mallik scientific wellbore site; in Scientific Results from the Mallik 2 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore and T.S. Collett; Geological Survey of Canada, Bulletin 585. Sheriff, R.E. 2: Encyclopedic Dictionary of Applied Geophysics, (Fourth Edition); Society of Exploration Geophysicists, Geophysical Reference Series 13, 429 p. Walia, R., Mi., Y, Hyndman, R.D., and Sakai, A. 1999: Vertical seismic profile (VSP) in the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate well; in Scientific Results from JAPEX/JNOC/GSC Mallik 2L-38 Gas Hydrate Research Well, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore, T. Uchida, and T.S. Collett; Geological Survey of Canada, Bulletin 544, p Winters, W.J., Pecher, L.A., Booth, J.S., Mason, D.H., Relle, M.K., and Dillon, W.P. 1999: Properties of samples containing natural gas hydrate from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate well, determined usinggas HydrateAndSedimentTestLaboratoryInstrument (GHASTLI); in Scientific Results from JAPEX/JNOC/GSC Mallik 2L-38 Gas Hydrate Research Well, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore, T. Uchida, T. and T.S. Collett; Geological Survey of Canada, Bulletin 544, p

11 B. Milkereit et al. APPENDIX A Offset vertical seismic profiling travel times Table A-1. Direct compressional-wave travel times (in ms) from vertical seismic profiling recordings (surface sources and downhole receivers). Depth (m)

12 GSC Bulletin 585 APPENDIX B Reflection and transmission amplitude trends Amplitude-versus-offset (AVO) response provides useful information about changes in petrophysical parameters in layered-earth models. Zero-offset vertical seismic profiling (VSP) provides important links between surface-seismic and borehole geophysical data. Multi-offset VSP data allow the extraction of AVO (or amplitude versus angle of incidence) information from three-component borehole seismic data. Single- or low-fold VSP data, however, are challenging for AVO analysis because multiples, converted waves, S waves, and tube waves may introduce a low signal-to-noise environment. Multi-offset VSP data can be utilized to study angular-dependent reflection and transmission responses for CMP or common receiver geometries. Figure B-1 shows reflection and transmission geometries suitable for amplitude versus angle of incidence studies. It is important to note that the VSP geometry offers unique opportunities to calibrate AVO trends by analyzing reflection and transmission response. Figure B-2 shows true-amplitude reflected and transmitted arrivals from the offset VSP survey. In practice, AVO analysis is based on plane-wave reflection co-efficients (Zoeppritz equations or small-angle approximations). For small and intermediate depths of investigation, however, plane-wave approximations do not suffice and geometrical spreading corrections must be applied. For multi-offset VSP experiments, the direct (transmitted) wave field can be used to compensate for offset-dependent geometrical spreading. Figure B-3 documents important geometrical spreading effects for the offset VSP survey. The geometrical spreading for seismic arrivals from above the gas hydrate zone differ from geometrical spreading estimates from the base of the gas hydrate zone. Due to these difficulties, the decision was made not to invert the AVO trend seen in Figure B-2. A Offset B Offset V + V+ C Offset D Offset V + V + Figure B-1. Offset vertical seismic profiling data-acquisition geometries for A) common receiver reflection; B) common receiver transmission; C) common midpoint reflection; and D) common midpoint transmission. 12

13 B. Milkereit et al. RMS amplitude Figure B-2. True-amplitude data from the JAPEX/JNOC/GSC et al. Mallik 3L-38 well; offset-vsp survey sorted for reflected and transmitted arrivals; note that reflection amplitude in common-receiver and common-midpoint geometry decreases with offset, whereas transmission amplitudes increase with offset RMS amplitude Figure B-3. True-amplitude recordings for receiver located at depths of 875 and 113 m in the JAPEX/JNOC/GSC et al. Mallik 3L-38 well; note variations in geometrical spreading. 13