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Fault families around salt structures: Implications for fluid flow, storage and production

Zhang, Qiang 2023. Fault families around salt structures: Implications for fluid flow, storage and production. PhD Thesis, Cardiff University.
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This thesis uses high-quality 3D seismic reflection volumes and well data from offshore Espírito Santo Basin (SE Brazil) and the Cleaver Bank High (Southern North Sea) to investi- gate fault families around salt structures and their implications for fluid flow, storage and pro- duction. The main aims are to understand: a) the distribution, geometry, scale and type of fault families around salt structures; b) the growth and development of fault families around salt structures and their relationship with halokinesis; c) the stress states necessary for the formation of fault families around salt structures; d) the role of fault families around salt structures in controlling fluid flow; e) the influence of salt structures and faults on the geothermal potential of surrounding strata. Hence, analyses were completed for fault families recognised in the two study areas, and such analyses included the mapping of plane and profile geometry, the map- ping of faults’ spatial distribution, and the gathering of maximum displacement/throw-length (Dmax/Tmax-L) and displacement/throw-depth (D/T-Z) data for selected faults. Stress analyses were applied to the interpreted fault families, providing important insights into the palaeostress states around salt structures. Multiple bottom-hole temperatures (BHTs) around salt structures and faults were utilised to reveal the influence of the latter on geothermal potential. In addition, fluid flow features and geothermal gradient anomalies were studied to address the role of dis- tinct fault families in fluid flow and trapping around salt structures. In a first stage, the distribution, evolution history and significance of tectonic faulting were investigated in a salt minibasin from offshore Espírito Santo Basin (SE Brazil). Inter- preted faults include crestal, corridor, listric and keystone faults, all showing different geome- tries, orientations and distributions. Maximum displacement-length (Dmax-L) and displace- ment-depth (D-Z) plots for selected eight (8) faults from four (4) fault families were compiled - together with eight (8) key structural maps - to understand their growth history. The results show that listric faults in minibasins can point out the presence of ductile, highly likely organic- rich shales and marls, which have similar seismic, lithological and petrophysical characters to known Cenomanian to Turonian source rocks. Three distinct evolutionary stages can be iden- tified in the study area during the Cenozoic, and these reveal the control of halokinesis on the growth and development of faults around salt structures. Normalised leakage factor analyses for all fault families indicate that keystone faults are the most favourable paths for fluid migra- tion in salt minibasins. Conversely, listric faults are likely to form barriers and baffles to fluid in their lower parts. In a second stage, the palaeostress state around a rising salt diapir developing on the Cleaver Bank High, Southern North Sea, was investigated. Fault families around salt structures, including radial, polygonal and keystone faults, were separated into eight zones. Stress inver- sions were applied to a total of 10,401 interpreted faults to reveal the palaeostress state around the diapir of interest (diapir K09). The results show that diapir K09 experienced multiple phases of growth during the Cenozoic and two of these phases were associated with discrete episodes of tectonic inversion. Halokinesis controlled the formation and growth of fault families around diapir K09 following three structural stages. Palaeostresses in the flanking zones of the salt diapir show marked differences when compared with its corner zones, but the combination of minimum principal palaeostresses from all flanking and corner zones forms a triangular stress ring around the salt diapir. The width of this stress ring was not only associated with the growth of salt diapir but was also strongly influenced by adjacent salt structures. The influence of salt structures and faults on the geothermal potential were investigated, in a third stage, over the Cleaver Bank High, Southern North Sea. The results of this part show that strata deposited above the Zechstein Group record geothermal gradients increasing pro- portionally to the thickness of this evaporite unit. Conversely, strata buried below the Zechstein Group reveal a moderate decreasing trend in geothermal gradients with increasingly thick salt. In parallel, large supra-salt faults can act as fluid paths to deep and hot fluid, a factor that results in the presence of high geothermal gradients in shallow strata around these same faults. In detail, geothermal gradients on the footwall of supra-salt faults are higher than that in their corresponding hanging-wall, but decrease markedly with distance away from these faults. Sub- salt faults influence the geothermal gradient of supra-salt strata in parts of the study area where there is very thin, or even absent, salt (<100 ms; or ∼230 m), forming distinct low-amplitude trails of fluid above these same faults. They also indirectly influence geothermal gradient by controlling the position, geometry and distribution pattern of salt structures. As a corollary, three potential geothermal exploration targets are suggested in the study area; one located on the footwall of a large supra-salt fault, one above thick salt, and a third target above very thin Zechstein strata, where low-amplitude fluid chimneys are found in seismic data. In conclusion, fault families around salt structures are divided in this thesis into clus- tered salt-related faults and scattered sub-salt and listric faults based on their scales and geom- etries. They can act as effective paths to deep and hot fluid into shallow strata, as indicated by the fluid flow features and geothermal gradient anomalies recorded around them. The growth or reactivation of steep faults propagating into sources of fluid (hydrocarbon) or heat (hot water and gas) are the most favourable migration paths in the two study areas considered. Salt-related faults can also provide a relatively accurate record of stress states associated with halokinesis, a character that has important implications to hydrocarbon exploration, carbon and energy stor- age around salt structures. The results in this thesis can thus be extrapolated to fault families formed in salt-rich basins worldwide.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Earth and Environmental Sciences
Funders: China Scholarship Council
Date of First Compliant Deposit: 18 January 2024
Last Modified: 19 Jan 2024 10:17

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