Gehrmann, R. A. S., Provenzano, G., Böttner, Christoph , Marn-Moreno, H., Bayrakci, G., Tan, Y. Y., Yilo, N. K., Djanni, A. T., Weitemeyer, K. A., Minshull, T. A., Bull, J. M. and Berndt, Christian (2021) Porosity and free gas estimates from controlled source electromagnetic data at the Scanner Pockmark in the North Sea. International Journal of Greenhouse Gas Control, 109 . Art.Nr. 103343. DOI 10.1016/j.ijggc.2021.103343.

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Abstract

We present porosity and free gas estimations and their uncertainties at anactive methane venting site in the UK sector of the North Sea. In the Scan-ner Pockmark area in about 150m water depth, we performed a multi-disciplinary experiment to investigate the physical properties of fluid flowstructures within unconsolidated glaciomarine sediments. Here we focus onthe towed controlled source electromagnetic (CSEM) data analysis with con-straints from seismic reflection and core logging data. Inferred backgroundresistivity values vary between 0.6–1 Ωm at the surface and 1.9–2.4 Ωm at150 mbsf. We calibrate Archie’s parameters with measurements on cores, andestimate porosities of about 50±10% at the seafloor decreasing to 25±3% at 150 mbsf which matches variations expected for mechanical compaction ofclay rich sediments. High reflectivity in seismic reflection data is consistentwith the existence of a gas pocket. A synthetic study of varying gas contentin this gas pocket shows that at least 33±8% of free gas are required to causea distinct CSEM data anomaly. Real data inversions with seismic constraintssupport the presence of up to 34±14% free gas in a 30–40 m thick gas pocketunderneath the pockmark within the stratigraphic highs of a till layer abovethe glacial unconformity in the Aberdeen Ground Formation.

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