Subtle traps in sedimentary basins and their importance to hydrocarbon exploration

This thesis uses high-quality 3D seismic data from the Broad Fourteens Basin (Southern North Sea), Espirito Santo Basin (SE Brazil), and Taranaki Basin (New Zealand) to characterise the evolution of geological structures related to differential compaction and subsidence; also known as subtle hydrocarbon traps. Each chapter tackles deformation over a different geological feature, spanning from salt-withdrawal basins, to submarine channel complexes and associated mass-transport deposits. These chapters subsequently discuss the impact the results have on the hydrocarbon industry. Included in these discussions are the importance of subtle traps on carbon capture and storage, local sealing potential, and reservoir distribution. The Broad Fourteens Basin dataset was used to investigate concentric faults associated with salt withdrawal from below Triassic units. Throw-depth and throw-distance plots helped to understand the growth histories of the concentric faults. It was shown that these faults formed as a result of the bending of strata due to differential subsidence during salt withdrawal. Slip tendency analyses assessed the likelihood for faults to reactivate and transmit fluids whenever pore fluid pressure is increased. This approach simulated a typical profile during carbon capture and storage. It was shown that concentric faults will reactivate if pore fluid pressures are increased above 30 MPa at the relevant sub-surface depths, leaking fluids (including stored CO2) past regional seal intervals in the basin. Data from the Espirito Santo Basin were first used to assess the timing and magnitude of differential compaction over a submarine channel complex. Thickness-relief models helped quantify both the variations in thickness in overburden strata. Smaller channels associated with downslope knickpoints were located within the channel complex. Differential compaction over channels produced four-way dip closures, as coarse-grained sediments were deposited at the knickpoint base. These provide adequate structural traps after early burial. The Espirito Santo Basin 3D survey was used in a third chapter to assess how differential compaction affected sediment distribution over a mass-transport deposit. As large remnant and rafted blocks entrained within the MTD were buried, differential compaction produced anticlines over them. This created a rugged seafloor and the topographic highs confined sediment moving downslope, allowing it to pond in discrete depocentres. Results from the data analysis chapters were compared with compaction-related structures documented in the published literature. A novel classification for subtle structural traps associated with differential compaction was produced, separating each feature into one of four types; Type A: folds over tectonic structures >2 km wide; Type B: folds over sedimentary packages, typically elongate, ~500 m to 5 km wide; Type C: folds over topographic features that are 20 m to 2 km wide; Type D: folds over sub-seismic/outcrop features no larger than 20 m. The results of the classification can be used as a first assessment when recognising a compaction-related fold and to rapidly assess its evolution and effectiveness as a subtle hydrocarbon trap.