Seismic Probe Beneath A Civilization

Transcription

1 5th Conference & Exposition on Petroleum Geophysics, Hyderabad-2004, India PP Seismic Probe Beneath A Civilization P.L.N.Sarma 1, S.K.Chandola 2, M.P.Rao 1, K.Ramakrishna 1, A.Saha 2 & V.Singh 2 1 RCC,ONGC, Chennai GEOPIC, ONGC, Dehradun ABSTRACT : Growing urbanization has made the task of generating seamless 3-D seismic datasets increasingly challenging. Each element in the chain of 3-D seismic exploration, i.e., survey design, data acquisition, processing and interpretation, has to be specially tuned for such conditions. We present a 3-D seismic case study from Tiruvarur town in Cauvery Basin of India to demonstrate how an integrated quality management approach comprising smart 3-D survey design, meticulous data acquisition, objective oriented processing and analysis has yielded seamless subsurface coverage with high quality data in a thickly populated township. Since the inception of exploration activities, continuous subsurface seismic coverage was not available inspite of proven hydrocarbon potential in this area. The 3-D seismic volume generated in this study could be crucial for accelerating the exploration and production of stratigraphic prospects in Cretaceous and Paleocene Formations. INTRODUCTION The increase in computing resources and software development has drastically changed the E & P scenario of petroleum industry. It has provided opportunity to practicing geoscientists to use the seismic reflection signal to interpret subsurface stratigraphy, lithology and pore fluid content in addition to the mapping of subsurface structures. This requires evaluation of each element of seismic system and ensuring that each part of the system contributes optimally to the success of this method. This starts from signal radiation and extends through data acquisition and processing where correct parameter selections and their effective implementation is required to improve the signal-to-noise ratio and provide a faithful representation of the earth (Stone, 1994, Cardsen etal., 2000, Vermeer, 2002, Chandola etal., 2002). Most often, it has been observed that the quality of seismic data is affected due to access constraints (thick population, pipelines, drill sites, many other man made structures, etc.) that prelude the placement of source and receivers. This results in either data gaps or irregular sampling of the recorded seismic wave-fields. If the objectives of preserving signals are lost during data acquisition process, there can not be likelihood of optimum interpretation results. This paper is an effort to demonstrate the effectiveness of an integrated quality management approach in a thickly populated township through extensive 3-D survey design and analysis during data acquisition and then during processing. Several candidate geometries were tested using state-of-the-art 3-D survey design and analysis software. A technically superior, operationally efficient and cost-effective 3-D survey design was chosen for execution in Tiruvarur Town of Cauvery basin, India. The data was acquired using orthogonal geometry and processed with an eye on the product. The impact of this integrated quality management approach on interpretation of 3-D seismic data for hydrocarbon exploration and production has been discussed briefly. STUDY AREA The study area spreading across the producing Vijayapuram, Adiyakamangalam and Kamalapuram fields is situated in Nagapattinam sub-basin of Cauvery Basin, India which is known for hydrocarbon accumulations in different plays ranging from fractured basement to Oligocene (Fig.1). The generalised stratigraphy of the study area is given in (Fig.2). The prospectivity of different units, namely, Andimadam, Bhuvanagiri, Nannilam and Kamalapuram Formations in this area has already been established. Out of these, Kamalpuram Formation of Paleocene-Middle Eocene age consisting of clastic sediments is an important producer. Late Cretaceous regression in this passive margin rift basin with horst-graben morphology has resulted in a number of deeply incised submarine canyons. Discrete sandstone reservoirs of Paleocene and younger Eocene fills in these canyons are established hydrocarbon plays of significant potential. Floating mudclasts and slump structures are some of the sedimentary features within these sediments, indicating the probable depositional mechanism as slumping and debris flow. Andimadam-Kamalapuram is a well established petroleum system in the study area. Source rich sediments in the Andimadam Formation are known to have generated substantial quantam of hydrocarbons and are found distributed within the reservoirs of Andimadam, Nannilam and 209

2 Figure 1 : Tectonic map of Cauvery basin showing the study area. Kamalapuram Formations. The trapping style of reservoirs within Kamalapuram and Nannilam Formation are primarily stratigraphic in nature. Three way closures and the associated lateral facies variation provide the updip seal for entrapment. Deep rooted faults and unconformity surfaces together act as conduits for migration from deeper source rocks of Andimadam formation. Limited areal extent, stratigraphic nature and internal heterogeneities of these thin clastic reservoirs pose a major challenge in exploration and development. Another major challenge to seismic data acquisition in this area is the thickly populated township of Tiruvarur (approximately 12 sq. km.) which has resulted in either huge sub-surface data gaps or significant loss of fold. Therefore, the analysis of earlier acquired 2-D seismic data could not facilitate the reliable mapping of these reservoirs due to data gap, irregular fold and significant variation in seismic signatures in and around the Tiruvarur Town. 3-D SEISMIC SURVEY DESIGN The inputs for carrying out 3-D survey design consisted of available 2-D seismic data, well data, topographic Thickness (m) N S Deposition Quaternary Cuddalore Shallow 20 Burdigalian inner shelf MIOCENE Madanam Lst. Shallow 25 Aquitanian inner reritic Shiyali C.st. Upper Inner shelf Chattian 30 Kovila- Shelf Edge Narimanam Lower Rupelian kkalapal Niravi S.st. Upper 36 Bathyal Upper Priabonlian Thirupundi Inner to 39 Bartonian middle shelf Karaikal Lutetian Shale 54 Lower Ypresian Outer shelf Kamalpuram Kamalapuram & Upper Thanetian To Adiyaka- 66 Maastrichtian mangalam Portonovo Upp. bathyal Shale 74 Companian Nannilam / Outer shelf Komarakshi Nannilam shale & To Kizhvalur 88 Santonian Upper bathyal 89 Lithology Env. Of Era Age Formation Oil/Gas Fields ma System Stages Group Conacian survey and geologic information. Thorough reconnaissance and DGPS surveys were carried out in and around Tiruvarur town for realistic assessment of obstacles and logistics. Around 200 stations were established to provide positional accuracy in real time. Based on the results of these surveys and other inputs, a 3-D survey design exercise was carried out using MESA CORE software to arrive at the optimum 3-D geometry for data acquisition. Following factors were considered while designing the 3-D survey: Survey Objectives CENOZOIC MESOZOIC Azoic Std. Chronostratigrpahy NEOGENE TERTIARY PALEOCENE EOCENE OLIGOCENE Middle Paleocene CRETACEOUS LOWER UPPER Turonian Cenomanian UTTATUR ARIYALUR NAGORE NARIMANAM Kudavasal shale Bhuvanagiri shale Sattapadi shale Lithostratigraphy Middle shelf To Outer shelf Albien Andimadam PreAlbien Shallow 108 Marine High energy Archean Basement Basement regime Figure 2 : Stratigraphic framework of the study area showing disposition of Kamalapuram, Nannilam and Andimadam formations within Paleocene and Cretaceous sequences. Primary objectives: The primary zone of interest falls in the depth range of m, with a deeper target at 3200m approximately. Therefore, the 3-D geometry should provide high trace density in the primary zone of interest, with adequate trace density for deeper target as well. Bin size: Considering the spatial sampling requirement and prestack merging of this 3-D dataset with surrounding 3-D vintages already available, a bin size of 20m x 40m was considered optimum. 210

3 3-D fold: Based upon the study of existing seismic data in the area, a 3-D fold of 36 to 42 was considered optimum. Logistics: Thickly populated town, road network, highrise buildings, etc., dominate a significant part of the survey area. Therefore, the selected geometry needed to be tolerant to these obstacles. This meant that the geometry should provide flexibility of shot-receiver movement around obstructions, without significantly affecting the offset-azimuth distribution and other survey attributes. Operational Aspects: Factors like available source (dynamite only due to non-availability of vibrators), receivers layout, available ground electronics, HSE aspects, etc. were also considered while selecting the acquisition geometry. Candidate Geometries: A large number of candidate geometries were designed and analysed based on 3-D attributes. The geometry given in table-1 was finally selected for data acquisition due to good offset-azimuth distribution, high trace density at the target levels, greater operational flexibility and better tolerance to obstructions. The unit template for the short-listed geometry is shown in Fig. (3). Table-1: 3-D survey parameters. Recording Instrument SN-388 Layout Orthogonal-Brick (Full swath roll) Bin size 20 m x 40 m No. of Channels 840 (140x6) No. of receiver lines 6 Receiver line interval 240 m No. of shots/ salvo 36 Shot line interval 400 m 3-D fold 42 (7x6) Record length 5.0 s Sampling Interval 2.0 ms DATA ACQUISITION Based on the survey design and analysis, 3-D seismic data was acquired in Tiruvarur town using the above geometry after optimizing the field and instrument parameters. A total number of 6300 shots were taken throughout the survey area to achieve a full fold migrated image area of 48 sq. km. In Order to minimize the cultural noise within township area, operations were scheduled early in the morning. Utmost care was taken for source placement to have proper energy Figure 3 : Unit template of the orthogonal- brick geometry selected for 3D data acquisition. 211

4 penetration and ensuring effective ground coupling of geophones. The subsurface data gap in and around the town was minimized by adopting a series of innovative measures like: (i) Placement of geophones in a regular pattern within the town using claybags for plantation. Additional receivers were placed along regular receiver lines to compensate for fold loss while executing the recovery plan. (ii) Near real time mega recovery plan using MESA CORE software to arrive at the optimal source-receiver configuration for minimising subsurface data gaps and acquisition footprints. (iii) In field QC processing of the acquired data for midcourse correction, if needed. Consistent offset distribution from bin to bin in inline as well as in cross line direction helped in minimizing the acquisition footprints. Attempts were also made to allow minimum deviation from regular geometry by ascertaining the accurate positioning, and selecting azimuthally unbiased receiver pattern (bunched geophones) which provides identical response in inline as well as in cross line directions to reduce the acquisition footprints. Consequently, the fold loss near the town could be effectively compensated to a large extent as is evident from Figs. 4 (a, b, c & d). The real worth of the meticulous recovery plan and its execution can be realised in terms of the filling of useful offsets at the bin-level (Figs.5a, b, c & d). The analytical approach adopted during data acquisition has helped in filling the data gaps through useful offset and in minimising acquisition footprints caused due to source-receiver coupling and fold variations. Rescheduling of operations in the early morning have significantly reduced the cultural noise within the town. The raw field record in the heart of the town is shown in Fig.6 which clearly demonstrates the data quality and good S/N ratio. DATA PROCESSING The acquired seismic data was processed using standard time domain processing sequence given in Fig.7 (Yilmaz, 2001). The three main issues addressed during processing of this data were: (i) Merging of regular data with the recovery data shot around the town with different templates. Extensive QC (a) (b) (c) FOLD OFFSET m DATA GAP 1.50 SQ KM WITH SKIPS (d) FOLD OFFSET m DATA HOLE 0.85 SQ KM (RECOVER) Figure 4 : (a) 3D Full fold coverage for the study area before and (b) after executing mega recovery plan. (c) Near offset fold coverage ( m) before and (d) after executing mega recovery plan. The effective bridging of data gap and fold compensation is clearly evident from the comparison. 212

5 (a) (b) (c) BIN OFFSET DISTRIBUTION WITH SKIPS (d) SINGLE BIN OFFSET DISTBN WITH RECOVERY SHOTS Figure 5 : Offset histograms at bin- level (a) before and (b) after executing recovery plan. (c & d) zoomed versions of (a & b) above. The bins are filled with useful offsets as a result of the recovery shooting. Figure 6 : Typical raw field records of a swath from the survey area within Tiruvarur town. The records display good S/N ratio in spite of the cultural noise due to special field measures adopted in placement of receivers and scheduling of field operations during lean traffic hours. 213

6 Figure 7 : Generalized processing flow adopted for data processing. (ii) PROCESSING SEQUENCE 1. Raw data merging 2. Field statics application 3. Spherical divergence correction and exponential gain 4. Surface consistent amplitude correction 5. Auto editing 6. Surface consistent deconvolution 7. Velocity analysis I (1 km x 1km) 8. Residual statics correction 9. Dip moveout correction 10. Velocity analysis-ii (500m x 500m) 11. DMO stack D migration 13. Time variant filtering checking was adopted during conditioning of data while merging of the two types of datasets Normalising the variations in ground coupling conditions for shots and receivers caused by variable surface conditions within and around the town through surface consistent amplitude corrections; and (iii) Enhancing the signal-to-noise spectral bandwidth through extensive testing and selection of surface consistent deconvolution algorithms. Close grid velocity analysis and 3-D post stack migration were carried out for enhancing the seismic imaging. In order to reduce the noise level, time variant filtering was applied on finally processed volume. The objective oriented processing of the complex 3-D data set has brought out perceptible improvement in data quality through different stages as shown in Figs. 8(a,b) and (9a,b). Care was taken during processing in attenuating coherent noises, removing long and short wavelength statics, normal move out corrections, far trace muting, 3-D DMO and migration for enhancing the signal bandwidth to minimize the foot prints. The value added by the present campaign can be easily demonstrated by the comparison of existing seismic line. Additional features could be identified on migrated 3-D inline (Fig.10) passing through the heart of the town, which were not mappable earlier. The seismic line displayed in Fig.11a Figure 8 : NMO stack of 3D inline (a) before and (b) after incorporating recovery shots. Improvement in data quality through recovery shooting is clearly evident. 214

7 Figure 9 : NMO stack of 3D inline with surface consistent amplitude corrections (a) before and (b) after incorporating recovery shots. Amplitude changes due to variations in source-receiver conditions are minimized by adopting the above process. Figure 10 : Post stack time migrated section of 3D inline from the present campaign. Basement high observed on this data was not discernible earlier due to data gap. 215

8 shows huge data gap beneath the town whereas the seismic line extracted from present campaign (Fig.11b) shows interpretable event continuity, structural and stratigraphic features beneath township. DATA INTERPRETATION The processed 3-D seismic volume was taken up for interpretation. After analyzing the well log data, seismic to well log calibration was carried out to establish the correlation between seismic events and geologic interfaces. Synthetic seismograms were generated using sonic, density logs and VSP time depth curve available in different wells of study area. Synthetic data then was tied to the seismic data, performing a constrained minor stretching and/ or squeezing to fit the major events. This approach has resulted in high quality match between synthetic and real seismic data at different well locations (Fig. 12). After well-to-seismic tie at different locations, the seismic events corresponding to the tops of different horizons, namely, Kamalapuram, Paleocene, Cretaceous and G1 marker were tracked throughout the 3-D seismic data volume. A display of seismic section from the 3-D data volume showing different correlated events is shown in Fig.13. The time structure map corresponding to Paleocene top generated from the present 3-D volume is shown in Fig.14. Preliminary analysis of 3-D seismic data volume indicates that the seismic expression of producing Early Eocene reservoirs of Kamalapuram Formation confined to the lows on Paleocene top level are characterized by the discontinuous reflections with complex internal configuration. Data quality as seen in migrated volume suggests that window-based seismic attributes and acoustic impedance derived from 3-D seismic volume may be useful in delineating the stratigraphic reservoirs. TIRUVARUR TOWN AREA Figure 11 : Comparison of (a) existing 2D seismic line with (b) 3D inline from present 3D volume. True worth of the present campaign is realized in effective bridging of huge data gap seen on 2D line with interpretable events in the zone of interest. 216

9 s Figure 12 : Synthetic seismic response along with the well-toseismic tie from the study area. A good correlation is observed between the synthetic and well data. Figure 14 : Time-structure map at the top of Paleocene level derived from the 3D data. The structural lows are favorable locales for deposition of channel sands within Kamlapuram formation. ACKNOWLEDGEMENT The authors thank Oil and Natural Gas Corporation Ltd. for granting permission to publish this work. The extraordinary efforts put in by the data acquisition and processing teams in making this project successful are also gratefully acknowledged. Figure 13 : Interpreted inline from the present 3D volume showing good quality data with sequence boundaries and probable stratigraphic features in the Paleocene and Cretaceous sequences. CONCLUSION The integrated quality management approach adopted in the study from seismic survey design through data processing has resulted in generating high quality 3-D seismic data in a logistically difficult area. The campaign has yielded continuous sub-surface coverage, which was hitherto not available in the earlier 2D/3D campaigns. The preliminary interpretation of the 3-D seismic volume shows interpretable stratigraphic features in Cretaceous and Paleocene Formations which could be interesting from hydrocarbon exploration and production point of view. This innovative seismic probe beneath a flourishing civilization could be a trendsetter for future 3-D seismic investigations under similar logistics. The views expressed in this paper are solely of authors, and do not necessarily reflect the views of the organization. REFERENCES Chandola, S.K., Sharma, A.K., Ramakrishna, K., Saha, A. and Singh, V., 2002, Effective 3-D survey designs in limited channel environment: Can we achieve them?, Proceeding of 4 th conference & Exposition on Petroleum Geophysics, Mumbai-2002 pp Cordsen, A., Galbraith, M. and Done W.J., 2000, Planning land 3- D seismic surveys, Geophysical developments, Series No.9, SEG publication. Stone, D.G., 1994, Designing seismic surveys in two and three dimensions, Geophysical references, Series No.5, SEG publication. Vermeer, Gijs J.O., 2002, 3-D seismic survey design, Geophysical references, Series No.12, SEG publication. Yilmaz, Oz, 2001, Seismic data analysis: processing, inversion and interpretation of seismic data, Investigation in Geophysics, Series No.10, SEG publication. 217