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.
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.