Regional Seabed Geology and Engineering Considerations for Hibernia and Surrounding Areas

Abstract

The discovery of oil in the Hibernia region in 1979, and subsequently at the Ben Nevis, Terra Nova, Whiterose and Hebron locations has heightened the demand for information on surficial and subsurface sediments for northeastern Grand Bank. Both gravity-based production facilities and subsea installations require a detailed understanding of the seabed foundation zone and any variations in sedimentary and physical properties of near-surface seabed materials which may affect seabed strength and stability. High resolution single-channel airgun (0.16 l) seismic reflection profiles and information from industry and government boreholes show the near-surface stratigraphy to be dominated by a sequence of shallow, seaward-dipping, parallel reflections which are interpreted to have resulted from progradation and aggradation on the continental shelf. At least locally at the Hibernia site, the upper parallel reflection sequence has complex, internal lenticular reflections and dense interbedded sands, silty-clayey sands and minor gravel and cobbles. Zones of clinoform reflections are interbedded within the sequence. The largest overlies the Hibernia oilfield and is composed of medium to coarse sand with silt and clay interbeds. It is interpreted as a shallow marine progradational sequence, which may have resulted from meltwater deposition of glacially-eroded sediment. Sediment composition and stability within the progradational sequence varies locally and regionally. A regional unconformity near the seabed truncates the progradational and the parallel reflection units and underlies thin surficial sands and gravels that are periodically transported and reworked by bottom currents and scouring icebergs. Four (4) potential seabed constraints to offshore development are described: 1) nearfield sediment transport around seabed installations, 2) liquefaction of seabed foundation sediments as a result of seismic loading, 3) seafloor iceberg scouring, and 4) site-specific and regional geological variability of foundation sediments and their physical properties. Constraints 1–3 can be addressed with appropriate design and proper siting, while 4) needs to be recognized and quantified as seabed strength and composition will vary and therefore impact design and installation of facilities such as wellheads, flowlines and pipelines.