Reprocessing strategy for shallower prospects from the available 3D data set Case history of Cambay Basin

Transcription

1 P-378 Reprocessing strategy for shallower prospects from the available 3D data set Case history of Cambay Basin M.Singh*, Vikash Chandra, USD Pandey Summary The occurrence of non commercial hydrocarbons is established in the wells in Hazad and Cambay shale in the area of study. For the exploratory locations to evaluate hydrocarbon potential of the area, reprocessing of 3D data pertaining to Jambusar Sarod area was taken up with the objective to improve imaging over the earlier processed data up to Cambay shale level. The Jambusar-Sarod area is situated in the Broach sub block, North of Gandhar field and nearest fields are Jambusar in south, Dabka in the NE and Padra in the east. The area of study falls under the administrative control of Bharuch district of Gujarat. Reprocessing project was taken up with the aim to bring out events up to 2500 m depth. The reprocessing was done by limiting the offset up to 3000 m and record length up to 5.0 sec as the deepest zone of interest was only up to 2.5 sec. Major emphasis was given to increase signal to noise ratio by attenuating the noise and carrying out velocity analysis at closer interval of 500 m. Interaction at every stage of processing helped in improving the image up to 2.5 sec thus fulfilling the objective of reprocessing. Introduction 3D data was acquired by a Geophysical Party of Western Onshore Basin during field season to map sands within Dadhar, Hazad and Olpad formation with geophysical objective as: Zone of Interest (TWT): ms Depth: m Figure-1: Wells drilled in the area The Objective of reprocessing was to identify structural and Stratigraphic traps in Dadhar, Hazad sands (Ankleshwar formation) and Sands within Cambay shale by improving imaging and signal to noise ratio. Zone of interest for reprocessing was up to sec. RCC Vadodara * Mail : singh_m@ongc.co.in

2 respectively. This formation has been sub-divided into following four members: Telwa Shale Ardol Member Kanwa Shale d. Hazad member Dadhar formation is underlain by Ankleshwar formation and overlain by Tarkeshwar formation. The petroleum system in the study area consists of Cambay Shale and Olpad as source rock, Hazad member,ymarker and Olpad as reservoir rocks and Kanwa shale and shale intercalations in Cambay shale and Olpad as the cap rocks. Wells drilled in the area of study Figure-2: Tectonic map of the Cambay Basin Brief Geology of the area The area of study lies in the Jambusar-Broach sub block of Cambay Basin and forms a part of North eastern rising flank of Broach depression. The Cambay basin is an intracratonic basin, which came into existence at the close of Mesozoic period by the development of tensional faults along its margins. The area is characterized by thick sedimentary section of Tertiary Quarternary age, which is underlain by Deccan Trap, Olpad formation, the oldest sedimentary sequence, overlies the Deccan trap with an erosional unconformity. It is composed of volcanic conglomerate, sandstone, silts and clays exclusively derived from basaltic trap showing digenetic altercations. Cambay shale sequence unconformably overlies the Olpad formation. This formation is composed of dark grey shale rich in organic matter and serves as a good source rock. This sequence was deposited during first marine transgression in upper Paleocene period in comparatively deep waters. A number of wells are drilled in the study area. Some of them are tabulated below: Well NO. Drilled depth Well m Well m Well m Well m Well m Acquisition Parameters 3D was acquired during field season with following parameters: The succeeding Ankleshwar formation lies unconformably over Dadhar formation. Ankleshwar formation comprising of alternating shale and sand sequences is understood to be formed during high-stand and low-stand sea level 2

3 Field Parameters Shooting geometry END ON orthogonal No. of receiver lines 12 No. of channels 128 per line Total active channels 1536 Receiver line interval 240 m (RLI) Group interval 40 m Shot line interval (SLI) 320 m Shot point interval 40m Shots per salvo 36 Far trace offset -Inline 5120 m Diagon 5511 m Bin size 20m x 20m Foldage 8 x 6= 48 Swath roll South to North Energy source Explosive Charge size 2.0 kg m (as per near surface Charge depth model) 6 Geophones, Bunched Receiver array (SM-24 geophones) Instrument Parameters A broad band filter was applied to suppress some of the coherent/random noise but the improvement was not appreciable. Efforts were made to attenuate noise by using different module available with Geocluster 5000 software after optimizing the parameter from the shot gather itself before deconvolution. This resulted effective deconvolution, better semblance and more confidence in velocity pick. Different tests were performed for Surface consistent deconvolution to determine operator lengths and prediction gaps. The parameters for reprocessing must be able to bring out improvement in zone of interest which is shallow in the present study The best results in terms of resolution and continuity of events was selected and corresponding parameters were applied on the data during the processing. The processed out put has shown considerable improvement due to following strategy adopted during reprocessing. Better reconditioning of data in gather mode Restricting the offset up to 3000 m Tuning the parameter for shallower levels Close grid velocity analysis with well control Interaction with interpreter Seismograph Recording format Sampling interval Record length Low cut filter High cut filter Notch filter Gain Scorpion IO System IV SEG Y 2 ms 7 sec OUT Hz OUT G0 (0 db) Reprocessing of data The presence of surface/near surface generated noises and cultural noises were found to affect the data. The dominant noise types identified in the area are: i) Ground Roll ii) Cultural Noise. 3

4 PROCESSING PARAMETERS Format conversion & From SEGY to CGG 1 Merging geometry format Geometrical Spherical Divergence 2 spreading correction (T**1.6) 3 Pre filter 06,10,80,90 Hz Restricting the offset up to 3000 m. 4 Reconditioning of data using different noise module and amplitude based auto editing. 5 Surface consistent amplitude balancing 6 Dephasing filter with the instrument impulse window : ms : ms Surface consistent Prediction distance : 8ms 7 Deconvolution :12m before stack s Operator length : 260 ms white noise (%) : Velocity analysis-i in a grid of 500m x 500m 9 3D residual statics - i Window ms 10 Velocity analysis-ii in a grid of 500m x 500m 3D 11 residual Window ms 12 Velocity analysis-iii in a grid of 500m x 500m 13 RMS velocity analysis in a grid of 500m x 500m 14 PSTM Kirchhoff Migration Aperture: 2000m x 2000m 15 Random noise attenuation 16 Decon after stack 17 Time Varying Filter Window : ms PD : 24 ms Op. length : 240 ms White noise % : 0.1 Up to 1500 ms Hz 2000 ms Hz 5000 ms Hz 18 Final PSTM stack In SEGY FORMAT Figure-3: PSTM Stack of Inline IL-1 (Earlier Reprocessed) Figure-4: PSTM Stack of Inline IL-1 (Reprocessed) Figure-5: PSTM Stack of Inline IL-2 (Earlier Reprocessed) 4

5 were made to remove the migration generated smiles and noises in the edges from the final data volume. The reprocessed out put is having improved imaging over earlier processed data. Similar attempts were made for the shallower prospect in West Patan area by restricting the far offset 2000 m and record length 3.0 sec. The processing parameters were tuned keeping in view the shallower objective. This strategy resulted remarkable improvement in zone of interest. Figure-6: PSTM Stack along Inline IL-2 (Reprocessed) Fig-9: Study area of West Patan Figure-7: PSTM Stack along Inline IL-3 (Earlier Processed) Fig-10: PSTM Stack along Inline PL-1 (Reprocessed) Figure-8: PSTM Stack along Inline IL-3 (Reprocessed) Extreme care was taken to preserve the frequency i.e. signal band width in the zone of interest to bring out the best sub-surface image. Close grid velocity analysis was carried out to generate reliable 3D velocity volume. Efforts 5

6 Acknowledgement The authors express their sincere gratitude to Director (Exploration), ONGC for his kind permission to publish this work. The authors also express their sincere gratitude to Basin Manager, Western Onshore Basin for assigning this project. Authors express there sincere gratitude to Head Geophysical Services for his guidance and keen interest in timely completion of project. Fig-11: PSTM Stack along Inline PL-2 (Earlier Processed) Thanks are due to Block Manager and concerned members of Block I of Western Onshore Basin for fruitful discussion and suggestions for improvement of processed out put. Authors express their sincere thanks to Geophysical Party and its members for acquiring data in logistically difficult area. Authors express sincere thanks to all the members of RCC for their support and suggestions. References Operation report on 3D Reflection Survey (Unpublished ONGC report) Processing report of Jambusar area (Unpublished ONGC report) Fig-12: PSTM Stack along Inline PL-1 (Reprocessed) The strategy adopted for the improved imaging during reprocessing for shallower prospects is very encouraging and needs to be adopted in future projects also. The interpreter is having more confidence in studying different attributes at shallower level with reprocessed out put. Conclusion Bahorich,M.S. and Farmer,S, D Seismic coherency for faults and stratigraphic features, The Leading Edge, Crowo and Alhilali, 1974 Amplitude of Seismic events and their dependence on the absorption- dispersion pairs of media. SEG Dallas, Also AAPG, Dallas 1975 Views expressed in this study are solely of the authors only and do not necessarily reflect the views of ONGC Data acquired in the area where both shallow and deeper prospects are promising, should be processed separately for both shallow and deeper prospect. For shallower prospect, offset should be restricted to get the reliable velocity model and to avoid contamination of data (both amplitude and frequency) due to far offsets. Improvement at shallower level is remarkable because of the strategy adopted during reprocessing. 6