Seafloor analogues: Identifying conventionally sub-seismic structures in polygonal fault systems

Abstract

Structural characterisation of sedimentary basins has never been more important with the continuing development of the Enhanced Oil Recovery (EOR) and Carbon Capture and Storage (CCS) industries. In particular, the identification of potential fluid flow pathways such as faults, is essential for accurately establishing basin/reservoir integrity. However, identifying these pathways is subject to the limitations of conventional 3D seismic imaging; only capable of resolving features on a decametre scale horizontally and metre scale vertically. While outcrops and cores can be used to identify smaller features, they are limited by the extent of the exposures. The disparity between these techniques highlights a resolution gap, suggesting many small structural heterogeneities remain unidentified. Structures that could suffer from this are polygonal fault systems (PFS), these are networks of non-tectonic faults, typically associated with the seals of hydrocarbon reservoir seals and are capable of acting as fluid pathways. PFS were first identified in the Kortijk Clay, the stratigraphic equivalent of the London Clay which reaches the seafloor within the Southern Bight of the North Sea. In an attempt to fully characterise fault networks within the London Clay and other formations containing these PFS, a combination of conventional and ultra-high resolution techniques were acquired, investigating both deep and coastal examples. For the first time, polygonal faults across a wide range of spatial scales are identified using an ultra-high-frequency 3D seismic volume from the 3D Chirp, with additional information provided by 2D boomer seismic lines and 1m-resolution bathymetry. The smallest faults measure 8 m and the largest at 926 m in length, with displacements between 30 cm & 4.44m. A direct comparison with conventional 3D seismic data of the same formation from the offshore Dutch sector reveals faults at a scale of 100s metres, with throws of tens of metres. Although this large dataset exhibits characteristics typical of PFS, these are far larger than those identified in the Southern Bight, indicating that PFS structures contain conventionally sub-seismic faults. High-resolution 3D seismic data can be used to identify these faults that could otherwise create additional unseen pathways, potentially influencing fluid migration within hydrocarbon basins.

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Identifiers

ORCID for Callum Richard Fry: orcid.org/0000-0002-6747-8792

ORCID for Justin Dix: orcid.org/0000-0003-2905-5403

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