Fluid flow paths in sedimentary basins : implications for exploration in challenging geological environments

Seismic interpretation and attribute analysis of three-dimensional seismic data often reveals focused fluid flow features such as pockmarks, mud volcanoes, hydrothermal vent complexes and amplitude anomalies. This thesis analyses three distinct data volumes from the Jæren High (Central North Sea), East Breaks area (northern Gulf of Mexico Basin), and Vøring Basin (mid-Norwegian margin), where fluid flow features are observed. The three study areas reveal a variety of these features across a range of depths and associated structures within sedimentary basins, from salt walls and pods, to salt minibasins and magmatic sill complexes. Thus, this thesis aims to improve our understanding of fluid migration in sedimentary basins and discuss the impact of the results on the hydrocarbon industry. A comprehensive seismic and borehole dataset from the Jæren High is used to investigate the distribution and timing of buried pipes and pockmarks, which reveal seal breach across salt welds and Mesozoic strata. A semi-automated method in ArcGIS is used to map and characterise 196 depressions in three seismic surfaces. Depressions range in width from 225 m to 842 m and in vertical relief from 14 m to 178 m, scales that are comparable with the literature but that expand the limited evidence of large, 'mega' pockmarks, particularly in buried strata at the depth of prolific hydrocarbon reservoirs. It is shown that Upper Cretaceous depressions are likely to be drape features and pockmarks formed during the Upper Jurassic and Lower Cretaceous. Burial history modelling reveals gas maturity in Carboniferous strata, which may have sourced the observed pockmarks. High-amplitude anomalies within- and outside of pipes suggest further fluid seepage across salt welds, a character increasing the risk of seal breaching on the Jæren High if this structure is considered as a carbon capture and storage site. Data from the East Breaks area of the northern Gulf of Mexico Basin are used to analyse the morphology and distribution of 720 pockmarks and 62 mud volcanoes on the sea floor, correlating these with underlying structures and source intervals in a major salt province. Salt diapirs and associated crestal faults are the main fluid focusing pathways, whilst high-amplitude anomalies in salt minibasins may indicate gas pockets or hydrocarbons that have not leaked to the sea floor. The relationship between the depth of fluid source and size of pockmarks and mud volcanoes is deemed complex, in contrast to the expected correlation between increasing pockmark size and depth of source. In fact, the shallow plumbing system in East Breaks is dominated by pockmarks, whilst mud volcanoes are sourced from the deeper parts of the salt minibasins. The influence of magmatic sills, dykes and hydrothermal vent complexes on fluid migration is explored in the Modgunn Arch dataset from the southern Vøring Basin, Norwegian Sea. A total of 85 buried hydrothermal vent complexes are mapped, from which three examples of stacked vent complexes are identified, indicating important reactivation of existing conduits. Amplitude anomalies in younger strata suggest later-stage fluid seepage across hydrothermal vent complexes, highlighting the importance of magmatic structures in controlling fluid migration pathways. Larger vents tend to be fed by deeper sills, while a greater proportion of the larger vents are associated with amplitude anomalies. In conclusion, the presence of pipes, conduits and faults weaken or strengthen the host rock, depending on the in-situ stress states and cementation of these pathways. This will impact the degree of overpressure increase required for modern day reactivation of these pathways. As a result, this thesis explores the complexity of focused fluid migration pathways through sedimentary basins with the aim of understanding the importance of subtle features on seismic data as recurrent fluid flow paths. These are important considerations when exploring in challenging geological environments such as salt-rich and magmatic basins, as well as when identifying sites suitable for carbon capture and storage.