Comparison of Kirchhoff and Wave Equation PSDM : A Case Study from West Patan Area

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1 P-352 : A Case Study from West Patan Area B. K. Medhi*, C. B. Yadava, Kuldeep Prakash, Kunal Niyogi, S. K. Das GEOPIC, Oil & Natural Gas Corporation Ltd, Dehradun Summary West Patan area of north Cambay basin is geologically very complex. The area is characterized by strong lateral velocity variation as well as steep dips. Selection of proper imaging tool for such an area is essential to map structural closures, pinch out prospects and fault closures. Wave equation PSDM is advantageous in case of strong lateral velocity variations. More over it takes care of multi-arrival of the reflections, thus handles more complex structures. But in case of very steep dips, Kirchhoff PSDM is more successful, whereas Wave Equation PSDM has a dip limitation. For deciding the optimum imaging approach, image of a small portion of the area was generated by Wave Equation pre-stack depth migration and Kirchhoff pre-stack depth migration. It was found that Wave Equation PSDM had given a superior quality output, but has taken very large computing time. Introduction West Patan area of north Cambay basin falls at the northwestern margin of Ahmedabad-Mehsana Techtonic block of the basin. Exploration efforts have not yet yielded the desired result in Patan area as compared to its southern counterparts, where numerous oil fields are under production. Several wells drilled in and around the area have given encouraging trend to go for detailed exploratory efforts. Presence of oil influx in well Dharnoj-2, oil indication in well Patan-2, South Patan-4, West Patan-2, presence of commercial oil in South Patan-5 and South Patan-7 have established vast areal extent of oil and gas in Patan area in Olpad sediments, mostly in the flanks of Patan depression. Available 2D data is not sufficient enough for detailed understanding of the area, and 3D seismic data was acquired in the area (figure-01) for further detailing of the prospects identified in Olpad formation and to bring out subtle features in the area. The area is geologically very complex and characterized by presence of numerous parallel faults. Selection of proper imaging tool is essential to map structural closures and pinch out prospects at Kalol level and fault closures at Olpad level. For deciding the optimum imaging approach for this data set, image of a small portion of the area was generated by Wave Equation pre-stack depth migration and Kirchhoff pre-stack depth migration. Present work deals with the outcome of different imaging approaches on this data set. Figure-01 : Map of Cambay basin showing area of study rediffmail.com

2 Figure-02 : A part of representative shot gather Input Data 3D seismic data was acquired using 408 UL recording instrument and explosive as energy source. A detail of acquisition parameters is given in Table-01. In general quality of input data is fair to good. Figure-02 shows a part of representative shot gather. Data is infested by cultural noise, especially near village areas and strong ground roll. Pre-processing and Signal Conditioning After geometry updation, seismic data was subjected to band pass filter and automated noise suppression modules followed by spatial amplitude smoothening. Figure-03 shows the same shot gather as in figure-02, but after signal conditioning. It was observed that most of the ground roll energy as well as cultural noise were taken care by different noise handling algorithms. Table-02 shows a brief of signal processing steps involved in this study. QC stacks were generated at different stages of processing to assess the improvement in the data. Figure-04 is the stack of an inline passing through the central portion of the area. It has numerous diffraction events and bow tie at 2

3 deeper levels. It is observed that, in general, at shallower levels the geology is flat, but beyond 1000 ms two way time, it is very complex. Figure-03 : Shot gather after signal conditioning 3

4 Figure-04 : QC Stack of an inline passing though the central portion of the area after deconvolution Imaging Issues Processing team was equipped with two different PSDM algorithms; one based on Kirchhoff approach and other one Wave Equation depth migration. The two algorithms have distinct advantage and disadvantage over the other. Kirchhoff migration considers only one arrival whereas Wave Equation migration takes care of multi pathing and thus handles more complex structures and allows simpler amplitude treatment. Wave equation depth migration is advantageous wherever we have very strong lateral velocity variation but often struggles to image very steep dips, where Kirchhoff migration has generally been more successful. Although wave equation migration is expensive to pursue, but with large PC cluster, wave equation prestack depth migration of a small 3D data set is not a big issue nowadays. To evaluate the effect of two approaches of pre-stack depth migration on the present data set, PSDM was run on test portion of the data using Kirchhoff as well as Wave Equation algorithm. Figure-05 is the output of an inline passing through the central portion of the area with Kirchhoff PSDM whereas figure-06 is with Wave Equation PSDM. It is found that the overall quality of wave equation PSDM output is much superior to Kirchhoff PSDM. Pinch out prospect falling in the left portion of the section is well defined, which is not so in Kirchhoff PSDM output. Even the steep dipping boundary of the central syncline is better imaged in figure-06. Conclusion For deciding the appropriate imaging approach for the 3D data acquired in geologically complex West Patan area, pre-stack depth migration with Kirchhoff as well as Wave equation algorithm was attempted on the test portion of the data set. It was found that Wave Equation PSDM had given a superior quality output, but has taken very large computing time. 4

5 Figure-05: Output of Kirchhoff PSDM of an inline passing through the central portion of the area Figure-05: Output of Wave Equation PSDM of an inline passing through the central portion of the area 5

6 References Nanxun Dai et al, 2002, 3D wave equation PSDM optimizes and improves imaging of sub-salt prospects, The Leading Edge. Jianxiang Ren et al, 2005, Pre-stack wave equation depth migration in VTI media, The Leading Edge. Dimitri Beve et al, 2005, Which depth migration method should you use? A road map through the maze of possibilities, The Leading Edge. B. Duquet et al, 2003, Poster paper 3D multi arrival Kirchhoff vs wave equation migration: SEG/EAGE salt model case study, The Leading Edge Acknowledgements The authors place on record their sincere thanks to Director (Exploration), ONGC, for his kind permission to publish this work. Thanks are due to Shri T R Muralimohan, DGM(GP), GEOPIC, ONGC for valuable technical discussions from time to time. NB: The views, expressed here, are solely of the authors and do not necessarily reflect the views of ONGC. 6