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Crestal fault reactivation on rising salt diapirs: An integrated analysis from large to small scales of observation

Tao, Ze 2018. Crestal fault reactivation on rising salt diapirs: An integrated analysis from large to small scales of observation. PhD Thesis, Cardiff University.
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The modes in which faults can propagate and grow through subsurface rocks and strata are key to the establishment of fluid paths in sedimentary basins; faults are potential conduits for fluid in some regions, at the same time they are associated with fault-related traps in others. The classical fault propagation models addressed in the published literature have so-far considered isolated, linkage (lateral-tip linkage and dip-linkage), constant-length, and coherent models. However, the propagation histories of faults in regions dominated by salt tectonics are scarcely documented; rather, the existing fault propagation models lack critical thinking when applied to crestal faults, particularly due to the limited resolution of imaged strata in most publications, and the relatively small size of crestal faults (length < 2.3 km, maximum throw < 50 m). With the increasing use of high-resolution seismic data in recent decades, it is now possible to undertake research into the evolution of both crestal faults and fluid flow paths in regions dominated by salt tectonics. In parallel, the uniqueness of crestal faults in terms of their scales has brought up important questions about how data resolution and scale variance influence many a fault analysis, and the current fault propagation models, when based on seismic and outcrop information. This research uses high resolution seismic data from the Espírito Santo Basin, offshore SE Brazil, to investigate the growth histories of crestal faults, fluid flow in an area of significant salt tectonics, and how crestal faults are associated with traps in supra- salt successions. To answer the question, in a second stage, of how scale variance can influence the analysis of faults’ propagation histories, data from Somerset (Bristol Channel) and the Ierapetra Basin (Crete) were collected in the field to broaden the database in this thesis from the larger, rift-basin scale to the seismic and sub-seismic scales. Segment linkage is predominant in areas where crestal faults grow. Interpreted crestal faults in SE Brazil propagated vertically and horizontally. Horizontal propagation was often hindered by natural barriers such as an accommodation zone (Chapter 4), or oblique transfer zones (Chapter 5), onto which faults terminate. Vertical propagation stopped when the fault meets the sea floor or when vertical propagation was accommodated by blind faults or larger (adjacent) faults showing relatively large displacements. Hence, this thesis shows that the propagation of crestal faults does not follow a ‘coherent growth model’. Rather, the geometry and history of propagation of discrete faults segments are not comparable. In SE Brazil, large fault segments propagated to link with non-reactivated small fault segments on the crest of the salt ridge, and can show later ‘blind’ propagation towards the surface. In terms of how scale variance can potentially (negatively) influence fault growth models interpreted on seismic data and in the field, a new quantitative method and two new parameters (sampling interval and module error) are introduced in this thesis for faults of multiple scales - from a few meters to 10s of kilometers. Sampling interval has a significant influence on the interpretation of fault growth histories. By changing one’s sampling interval: 1) the interpretation of fault geometries is significantly changed; 2) maximum fault throw values are underestimated; 3) fault segments are underrepresented; 4) the geometry of fault linkage zones is changed; 5) the width of fault linkage zones is underestimated; 6) fault interaction zones are lost. Using the SE Brazil seismic data, the accuracy of Throw-Distance plots was shown to be quantitatively lost when sampling intervals were larger than 37.5 m (every 3 shot-points) for the ‘unique’ crestal fault families in this thesis. However, this thesis demonstrates that sampling intervals adopted by interpreters should differ depending on the resolution of seismic data used, and the total length of investigated structures. A practical sampling interval/fault length ratio is therefore proposed in this work to address the caveats behind using variable (and indiscriminate) sampling intervals when analysing faults. Supra-salt sequences capable of promoting episodic fluid flow in regions of salt tectonics are of vital economic importance. Following on the two latter themes (crestal faulting and fault scaling), the thesis addressed the episodic fluid flow documented in the Espírito Santo Basin in a third stage. The results of this section are proposed as a case study for supra-salt sequences. In detail, seal failure is systematically recorded in the study area, and is interpreted to have contributed to most of the supra-salt fluid flow events investigated in SE Brazil. Six types of traps are therefore widely identified in supra-salt successions of the Espírito Santo Basin – all forming examples of trapping geometries in sedimentary basins associated with salt tectonics. Regardless of a thermogenic or diagenetic origin for fluid off Espírito Santo, the results in this thesis demonstrate important (and focused) fluid flow above salt giants when, at least, two critical conditions are observed: 1) a certain thickness of overburden strata is deposited on top of the salt structures, 2) the generation of highly developed (i.e. large) crestal fault systems is observed over these same salt structures. It is therefore postulated that, if overburden strata is thinner than a certain value, or pressure imposed by growing salt increases significantly, active salt intrusion occurring together with fluid flow will replace more focused fluid flow features in salt giants.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Earth and Environmental Sciences
Subjects: Q Science > QE Geology
Funders: China Scholarship Council
Date of First Compliant Deposit: 30 March 2021
Last Modified: 30 Mar 2021 09:22

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