Adding Value with Broadband Seismic and Inversion in the Central North Sea Seagull Area

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1 H2-2-1 Adding Value with Broadband Seismic and Inversion in the Central North Sea Seagull Area Marnix Vermaas, Andy Lind Apache North Sea, Aberdeen, UK Introduction The merits of modern broadband seismic data have been documented extensively. The main advantages are; reduced sidelobe interference, better penetration of deeper layers and the potential of a broadband seismic inversion without the need for prior well information. During the appraisal of the Seagull discovery in 2014/2015, an opportunity arose to extend nearby broadband seismic acquisition over the license area. A conventional Pre-Stack Depth Migration (Pre-SDM) seismic data set has been available over the area since The addition of the perpendicular shot broadband data set was aimed at enhancing the seismic image in terms of resolution, fault definition and suitability for quantitative interpretation. This paper briefly discusses the geology, seismic processing and interpretation and then focusses on the merits and challenges of using the broadband data in quantitative interpretation and reservoir modelling in the Seagull area. Geology and History The Seagull field is defined by a faulted, four way dip closure at the Base Cretaceous Unconformity (BCU) level with Triassic sandstones forming the primary reservoir and the Upper/Middle Jurassic sandstones providing a secondary reservoir. Two key maps defining the structural configuration are generated at the BCU and Triassic Lower Judy B levels. These provide an understanding of the potential reservoir extent and compartmentalization. The Triassic map is shown in Figure 1. The Triassic reservoir facies were deposited in an arid environment and are associated with stacked and single fluvial channels, splay deposits and playa lake deposits resulting in varying lateral and vertical flow properties. The field was discovered in 1992 by the 22/29-2 & 2S1 wells. The well discovered 38 meters of gross oil bearing Middle Jurassic Pentland sands and 134 meters of gross oil bearing Triassic sands. The discovery was appraised in 1992 by the 22/29-3 well. The Seagull acreage was relinquished by Shell in 2008 and subsequently acquired by Talisman Sinopec in 2009 in the 25 th Licensing Round. Apache farmed into the license in 2011 and JAPEX UK E&P LIMITED entered the license in In 2014, Talisman Sinopec and partners drilled the 22/29c-8 well to appraise the Triassic reservoir sands in a crestal location on the field. Apache has reported these well results as finding 672ft of net oil over a 1092ft column. In 2015, a DST was completed with reported results of a facility constrained flow of 8,700 Bo/d and 16MMcfp/d. In April 2016 Apache took over operatorship of the Seagull license.

2 Figure 1. Seagull area Lower Judy B depth structure with appraisal wells 22/29-2z, 22/29-3, 22/29c-8z and 22/29c-8y. The faulted nature of the depth structure provides an understanding of the potential reservoir compartmentalization. Seismic Acquisition, Processing and Interpretation In August 2014 Apache pre-committed to licensing some 600 sq. km of broadband and reprocessed Cornerstone seismic from CGG. The Cornerstone data was re-processed with Ghost Wavefield Elimination (GWE) and the Tomographic Multi-Layer (TOMO ML) derived velocity model for Pre-SDM. In order to maximise illumination, the North-South shot Cornerstone data was combined with the East-West shot broadband data. The surveys were migrated separately but with the same TOMO ML derived velocity model. The migrated data sets were phase matched, aligned through 3D warping and stacked using data weighted scalars resulting in the dual azimuth broadband data (DAZ). This DAZ is currently being used for interpretation. Only the single azimuth processed broadband data (SAZ) set is used for seismic inversion purposes. Figure 2 shows a comparison of the amplitude spectra of the Cornerstone and SAZ data sets highlighting the additional low frequency content.

3 Figure 2. Spectral comparison of the 2011 Cornerstone Pre-SDM with the SAZ broadband data in ~0.6sq. km. area around 22/29c-8 over a 750ms zone centered on the Triassic reservoir interval. The SAZ data clearly has additional low frequency content. Seismic Inversion Using constrained sparse spike inversion and a suitable low frequency model, the seismic amplitudes of the SAZ seismic are converted to acoustic impedance. The acoustic impedance volume is used as a conditioning volume for the 3D distribution of reservoir facies or properties away from the well locations. The resulting reservoir model will then be used for both volume in place calculations and dynamic simulations to test field development options and ultimate recoverable reserve ranges. The main objective of the inversion study is to use the acoustic impedance to gain additional confidence in the interpretation of the facies in the undrilled fault blocks of the Seagull area and provide guidance on the reservoir distribution away from the wells. A key element of the inversion is the broadband wavelet estimation; long wavelets are used to capture the lowest frequencies and constraints are used to better estimate the wavelet phase. Another key element is the construction of a low frequency model. Ideally the low frequency model is derived from seismic velocities without prior information from well data and interpreted surfaces. However, the available seismic velocities were derived from the North-South shot Cornerstone data prior to the drilling of the latest appraisal wells and as such are not always suitable for a low frequency model. Therefore, to assess the uncertainty several low frequency models were derived including those with well data and surfaces. Acoustic and elastic well log data of the reservoir and non-reservoir facies of the Triassic and Jurassic are analysed at well log and seismic frequencies. Seismic models are created to asses tuning frequencies and the degree to which facies can be distinguished from one another.

4 Inversion of the broadband seismic results in improved seismic to well ties compared to the older conventional data (Figure 3), giving additional confidence in the inverted impedance away from the wells. Figure 3. Relative Acoustic Impedance (RAI) well tie comparison over a Seagull well. The bandpass filtered (4-60Hz) Relative Impedance well log is shown for both sections. The broadband RAI has less sidelobe energy and matches the well data better with higher correlation. References DECC Relinquishment Reports, April 2008, P. 012, Part relinquishment of Blocks 22/29,223a and 29/7 Firth et al., 2014, Experiencing the full bandwidth of energy from exploration to production with the art of Broadseis, First Break, Volume 32, No 6 JafarGandomi et al., 2015, Assessing the Value of Low Frequencies in Seismic Inversion, 77 th EAGE Conference and Exhibition, Extended Abstract Hollingworth et al., 2015, Setting new standards for regional understanding mega-scale broadband PSDM in the North Sea, First Break, Volume 33, No 9

5 ten Kroode et al., 2013, Broadband seismic data The importance of low frequencies, Geophysics, Vol. 78(2), WA3-WA14. Reiser et al., 2015, Reservoir property estimation using only dual-sensor seismic data a case study from the West of Shetlands, UKCS, First Break, Volume 33, No 10 Schakel, M.D. and Mesdag, P.R., 2014, Fully data driven quantitative reservoir characterization by broadband data, SEG Technical Program Expanded Abstracts, pp Acknowledgements Thanks to: Apache North Sea and partners Talisman Sinopec Energy UK Limited and JAPEX UK E&P LIMITED for allowing us to present this work. CGG for allowing the use of their data. Tim Berg, Ron Roberts, Christa Steekelenburg and Dennis Yanchak for their valuable input.