This paper, on Understanding Pore Pressures in Undrilled Areas Using Analogues, appeared in The Leading Edge in April 2014.
In recent years, new deepwater seismic-based exploration work has resulted in the revision of existing basin boundaries and the identification of new, potentially oil-bearing basins in the deepwater Labrador region. The petroleum potential in this deepwater area has also been encouraged by the identification of slick and seepage locations using 2D seismic data and satellite imagery. The importance is that surface slicks possibly are related to subsurface hydrocarbon migration. Thus, all recent data collated together show strong evidence for an active petroleum system in deep water. Many of the wells in shallow water have been drilled with low mud weights, suggestive of low pore pressures. However, where thick shale packages are present, significant overpressure is observed by significant kicks. Clearly, there is a close association between thick (and deep) shale packages and high pore pressure. Thus, one of the key risks in developing the deepwater potential is to understand the pressure regime. The success of this approach has been highlighted recently by successful discoveries such as the presalt Lulu field onshore Brazil and associated discoveries in Gabon and Angola and postsalt discoveries that include Jubilee field offshore Ghana and the associated discovery of Zaedyus field in French Guiana. The deepwater Vøring Basin of the Mid-Norway North Sea and the Labrador slope and deepwater share a similar passive margin setting to each other, similar facies associations, and structural development.
In the absence of well penetrations, analogues can be a useful method for establishing exploration potential and risk. In a largely unexplored region, the main tool for exploration is high-resolution imaging of the structure and stratigraphy using combined seismic, gravity, and magnetic data. Tullow Oil made the Zaedyus discovery in offshore French Guiana in 2011 using these data and the analogy from equatorial African discoveries. Deepwater settings generally have a series of common features, which include being shale prone, having less faulting, and having less uplift. Evidence for additional mechanisms of overpressure generation rather than disequilibrium compaction is less. All these features impact the pressure regime; for instance, likely pore-pressure regimes in deep water are overburden parallel.
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