Imaging the internal structure of an active fluid escape structure in the North Sea: 3-D full waveform inversion of the Scanner Pockmark OBS seismic dataset

Abstract

Fluid escape structures, often referred to as chimneys, are high permeability geological features that traverse low-permeability layers, thereby establishing hydraulic connections between deep strata, the shallow subsurface, and the seabed. These structures are typically identified via seismic reflection imaging, where they manifest as sub-vertical columnar anomalies. They are widespread in fine-grained sedimentary basins globally, including the Quaternary shallow sediments of the North Sea. The presence of shallow chimney structures can compromise the integrity of overburden cap rocks. This poses a challenge to the safe and permanent disposal of CO2 in underlying geological reservoirs, which is a key mitigation measure for controlling the global climate change. Despite their significance, the internal architecture and fine details of these structures remains poorly understood.
The Scanner Pockmark chimney is an active methane-venting fluid escape structure within Quaternary sediments of the Witch Ground basin in the North Sea, and serves as an exemplar for fluid escape structures overlying potential CO2 storage reservoirs in the North Sea. The Scanner Pockmark subsurface has been investigated using various geophysical techniques, including multiple seismic surveys. These surveys involved deploying 25 ocean bottom seismometer (OBS) instruments and recording a set of broadband seismic dataset from various marine gun sources.
In this work, the fine-scale architecture of the Scanner Pockmark chimney is imaged using 3-D travel time tomography and acoustic full waveform inversion (FWI) of the wide-angle and full-azimuth OBS seismic dataset. The 3-D FWI velocity model offers novel insights into the complex nature of the Scanner Pockmark fluid escape structure with unprecedented fine detail. It distinguishes genuine fluid migration pathways from imaging artifacts, reveals the fine-scale geometry of these pathways, and demonstrates their correlation with distinct stratigraphic units. Additionally, the FWI model suggests multiple episodes of active fluid expulsion, as evidenced by the similar geometries of low- and high-velocity structures, which are interpreted as active and cemented gas migration pathways, respectively. The gas saturation of shallow gas-bearing sediments was mapped, allowing for the estimation of in-situ gas volume. A generation mechanism for the Scanner Pockmark chimney was also postulated based on the geometry of inferred gas migration pathways. Furthermore, it was shown that a robust FWI result can be achieved by applying the FWI scheme to the vertical geophone component of the OBS dataset, and an effective inversion strategy for similar geological settings and datasets was proposed.

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Identifiers

ORCID for Farid Jedari-Eyvazi: orcid.org/0000-0002-9045-9792

ORCID for Tim Minshull: orcid.org/0000-0002-8202-1379

ORCID for Jonathan Bull: orcid.org/0000-0003-3373-5807

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