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Abstract
Infiltration of shallow soils by naturally occurring hydrocarbons has been
documented in several deepwater environments worldwide. The potential for
significant soil modification, such as the development of authigenic carbonates
and alteration of the generally expected background geotechnical properties may
provide constraints to flowline routing, foundation installation and
engineering lifetime performance. This paper presents a review of the current
state of knowledge of the authors with special reference to recent
investigations in deepwater offshore Angola, and a suggested method for
identification, characterization and prediction.
Interpretation of 3D exploration seismic, enhanced by AUV (Chirp) data
facilitates an initial identification of areas prone to hydrocarbon
infiltration and the vertical and spatial extent of potential soil
modification. A first pass geotechnical characterisation is developed by
targeted seabed CPTs and soil sampling. The generation of an integrated
predictive model requires a multidisciplinary advanced testing programme,
including geophysical, geotechnical, geochemical and geological analyses.
Once the extent, nature, and formative processes of hydrocarbon-related soil
modification are understood, an assessment can be made of the challenges posed
to a field development. This provides the necessary input to foundation and /or
routing feasibility and determines if there is requirement to mitigate, through
avoidance or design.
Introduction
Recent deepwater discoveries of natural hydrocarbon extrusions, such as seeps
and mounds, have been made in various offshore settings including: Escanaba
Trough, Northeast Pacific (Koski et al., 2002); Gulf of Mexico, USA (Canet et
al., 2006); Porcupine Trough, offshore western Ireland (Naeth et al., 2006);
offshore California, USA (Naehr et al., 2000; Lorenson et al., 2009);
Australian Shelf, Timor Sea (O’Brien et al., 2002). This list is by no means
exhaustive, but serves to illustrate the global distribution of such
occurrences and that their profusion may be more significant than previously
thought (Figure 1). In many instances, occurences of carbonate (or
carbonate-rich claystone) are found in association with hydrocarbon
infiltrations such as in the Nile Delta, Egypt (Wheeler and Satdnitskaia,
2011), offshore Trinidad (Deville, 2008), Sea of Marmara (Zitter et al., 2008),
Guaymas Basin, California (Paull et al., 2007) and offshore Oregon, USA
(Johnson et al, 2008). These are believed to be have been formed in situ (or
authigenic) by post-depositional, diagenetic processes that have utilised
hydrocarbon that has been introduced to the shallow subsurface by natural
seepage.
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