Abstract
Evidence of hydrocarbon leakage has been well documented across the SW Barents Sea and is commonly associated with exhumation in the Cenozoic. While fault leakage is thought to be the most likely cause, other mechanisms are possible and should be considered. Further study is required to understand what specific mechanism(s) facilitate such leakage, and why this occurs in some locations and not others. In a case study of the Snøhvit Field, we use seismic and well data to quantify fault- and top-seal strength based on mechanical and capillary threshold pressure properties of fault and cap rocks. Magnitude and timing of fault slip are measured to acknowledge the role that faults play in controlling fluid flow over time. Results based on theoretical and in situ hydrocarbon column heights strongly indicate that across-fault and top-seal breach by capillary threshold pressure, and top-seal breach by mechanical failure are highly unlikely to have caused hydrocarbon leakage. Instead, top-seal breach caused by tectonic reactivation of identified faults is likely to have facilitated hydrocarbon leakage from structural traps. The results of this case study acknowledge the different mechanisms by which hydrocarbons can leak from a structural trap. Employing both a holistic and quantitative approach to assessing different seal capacities reduces the likelihood that a particular cause of hydrocarbon leakage is overlooked. This is particularly relevant for the Snøhvit Field in its dual capacity as a producing gas field and as a carbon sequestration site since both systems rely on a thorough understanding of seal capacity and leakage potential.
Highlights
Accumulation and retention of hydrocarbons within a structural trap depends upon access to charge and the presence of a lateral-seal and top-seal (Fig. 1) (Dolson 2016)
Since fault behaviour and its effect on fluid flow is inherently dynamic rather than static, analysis of how fault activity changes over time must be considered. 67 In order to comprehensively assess the role of fault-seal and top-seal breach in causing hydrocarbon leakage, we investigate seal and retention processes in an area where there is good control on present hydrocarbon column heights as well as paleo-hydrocarbon column heights, namely the Snøhvit gas field in Hammerfest Basin
The recurring presence of deep paleo-oil shows, seismic imaging of gas chimneys, seabed pockmarks and the high number of discoveries confirm that hydrocarbon charge is abundant (Doré & Jensen 1996; Chand et al 2012; Ostanin et al 2012, 2013; Duran et al 2013). Most traps in this area are underfilled due to partial leakage; this makes the Hammerfest Basin an excellent area to study the key controls on seal integrity and breaching. 77 The paleo-columns show that all traps in the study area have undergone partial leakage, and 78 through this study, we aim to investigate the mechanisms by which leakage occurred
Summary
Accumulation and retention of hydrocarbons within a structural trap depends upon access to charge and the presence of a lateral-seal and top-seal (Fig. 1) (Dolson 2016). Given these are in place, it is the interplay between retention and charge that determines the height of the hydrocarbon column (Zieglar 1992). Studies over the same basin have tended to focus on single processes to explain hydrocarbon leakage, such as mechanical seal failure (Makurat et al 1992, Gabrielsen et al 1997), fault reactivation (Hermanrud et al 2014; Mohammedyasin et al 2016), differential uplift and tilting (Doré & Jensen 1996) and isostatic adjustment in response to ice retreat (Ostanin et al 2013)
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