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Integrated seismic and structural interpretation of reactivated fault systems in sedimentary basins

Maunde, Abubakar 2021. Integrated seismic and structural interpretation of reactivated fault systems in sedimentary basins. PhD Thesis, Cardiff University.
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This thesis investigates the impact of tectonic shortening on the reactivation and growth histories of normal fault systems from the Espírito Santo Basin (SE Brazil), Broad Fourteens Basin (Southern North Sea) and Nankai accretionary prism (SE Japan). In the Espírito Santo Basin, 3D seismic data were used to assess the impact of tectonic rafting, and associated gravitational instability of the continental slope, on the geometry and reactivation histories of faults formed above rollover anticlines. The data showed that the complex fault geometries observed above rollover anticlines are primarily due to three distinct stages of downslope gravitational instability of tectonic rafts. Stage 1 (post-Albian to Coniacian) caused the supra-salt strata to fragment into discrete blocks of strata (rafts) separated by large listric (roller) faults and associated wide rollover anticlines in their hanging-walls. In stage 2 (Early Santonian), the continued evacuation of salt from upper-slope regions to the base of the continental slope promoted the progressive downslope translation of rafted strata and triggered the reactivation of rollover faults in the form of crestal fault systems associated with local extensional collapse. Stage 3 (Middle/Late Eocene) caused tectonic rafts to be translated downslope until salt welds were formed and post-salt strata became grounded over pre-salt successions. The grounding (welding) of tectonic rafts over the pre-salt successions was progressive and accompanied by moderate translation of blocks during the Cenozoic. Differences in the degree of downslope translation of un-welded rafts further enhanced the reactivation of rollover faults, a phenomenon that promoted the migration of hydrocarbons from welded sub-salt source units into supra-salt reservoirs. As a corollary, this Chapter shows that tectonically generated pulses of compression and uplift, commonly related to the Andean Orogeny, are not the only mechanisms deforming post-salt overburden units in proximal extensional dominated regions of the southeast Brazilian margin. In fact, the close control of tectonic rafts gravitational instability induced a continuum of overburden deformation and contributed significantly to shaping the supra-salt overburden in the Espírito Santo Basin. In the Broad Fourteens Basin, 3D seismic and borehole datasets were used to investigate the effect of lithological variations on fault-throw distribution and segmentation, as well as on the geometry and reactivation histories of faults developed due to tectonic inversion. It was shown that the Late Cretaceous to Paleogene tectonic inversion affecting the Broad Fourteens Basin induced a continuum of deformation and contributed significantly to the formation and subsequent reactivation of Late Mesozoic faults. Late Cretaceous inversion (i.e., the Sub-Hercynian inversion episode) led to the formation of broad anticlines associated with normal faults in Upper Mesozoic strata. The progressive bending and stretching of outer-arc Mesozoic strata during the Early Paleocene inversion episode (i.e., Laramide episode) reactivated the older Sub-Hercynian faults. In a last stage, the Pyrenean (Oligocene) and Savian (Miocene) inversion episodes reactivated some of these faults upward into Tertiary strata. Offshore Nankai (SE Japan), 3D pre-stack depth migrated seismic data were used to analyse the geometry and growth of shallow faults associated with tectonic shortening in prominent thrust anticlines. These thrust anticlines show a trenchward increase in horizontal shortening and deform the seafloor at present in response to plate subduction off Nankai. It was shown that the presence of closely spaced vertically segmented fault arrays at shallow stratigraphic levels - with relatively small local throw maxima - relate to the existence of more incompetent (soft) intervals blanketing the Nankai accretionary prism. These shallow fault geometries accommodate a significant part of the bending and stretching strain occurring during the development of thrust anticlines and subsequent local stress redistribution during seismic events. The anomalous vertical fault throw distributions documented in the three study areas in this thesis are inconsistent with models of fault growth by uniform slip distribution and radial tip-line propagation. It is concluded that fault reactivation and growth by segment dip-linkages characterised the evolution and growth of all the faults interpreted in the three study areas. Where two separate faults with local throw maxima propagate towards each other and linked in throw-minimum in the throw-depth (T-Z) profiles. Fault segments with local throw maxima in the throw-depth (T-Z) profiles are early-stage fault segments and represent regions where faults localise first in competent intervals. Each segment of these early-stage faults propagates outwards until they encounter other fault strands and link together. Linkage points are located where local throw minima are recorded in less competent intervals. Consequently, the propagation of slip from fault segments (with local throw maxima) in the competent intervals into the incompetent intervals resulted into vertically segmented fault arrays. Vertical fault segmentation increases the chances of strata compartmentalisation and localised fluid flow through fault linkages, at the same time presenting significant risks when injecting CO2 in subsurface traps.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Earth and Environmental Sciences
Date of First Compliant Deposit: 26 January 2022
Last Modified: 26 Jan 2022 15:55

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