Tectonostratigraphic evolution of a salt giant during passive margin development: Santos Basin, offshore Brazil
Creators: name: Rodriguez Rondon, Clara Rosa
Publisher: name: Imperial College London
Year: 2017

description: The stratigraphy of ancient salt giants provides constraints on syndepositional basin tectonics and physiography, and/or variations in climate or sea level. Moreover, subsequent deformation and dissolution of salt directly controls the distribution and architecture of the sedimentary overburden. Hence, salt directly influence the overall tectono-stratigraphic evolution and hydrocarbon potential of salt-dominated passive margins. This study focuses on the Santos Basin, a salt-dominated basin offshore eastern Brazil. To date, few studies have attempted to characterise the architecture of this stratified salt giant (i.e., Ariri Fm.) deposited during Late Aptian opening of the South Atlantic Ocean. In addition, no studies have characterised the deep-water salt tectonic structural style or salt dissolution and its control on deep-water sedimentation. In this study, 3D seismic reflection and borehole data are integrated to constrain the regional tectono-stratigraphic evolution of the Ariri Fm., from its deposition, to its subsequent deformation and partial dissolution. The integration of 3D seismic and borehole data shows that an up 2.5 km thick Ariri Fm. comprises four key evaporite-dominated intervals of distinct composition recording at least 12 cycles of basin filling and dessication in &lt; ca. 530 ka. Moreover, this study reveals that rifting-inherited basin physiography controlled salt thickness and composition leading to stratigraphic variations across structural domains. Overall, thinner salt and higher anhydrite net thickness occur towards structurally high domains, compared to, structurally lower domains where the salt is thicker and anhydrite net thickness is less. In addition, results of this work suggest that more anhydrite was deposited closer to the entrance of fresh seawater, whereas more bittern salts were deposited in more hydrologically restricted locations. Seismic stratigraphy and seismic attribute analysis illustrate how subsequent gravity-driven downdip salt flow directly controlled the Santos Basin deep-water slope morphology, influencing sediment dispersal and architecture. During the Turonian-middle Campanian, sand-rich channels and lobes were confined within proximal minibasins and to the hangingwalls of landward-dipping, salt-detached listric faults. During the middle Campanian-to-Paleocene, sand-rich channels and lobes eventually filled and bypassed proximal minibasins, with deposition then occurring further downslope. The work presented here illustrates how syn-depositional seabed deformation, driven by passive and active diapirism, and salt-detached thrusting and folding downslope directly controlled sediment distribution and architecture. During the Paleocene-to-middle Oligocene, continued rise of salt walls dissected previously deposited deep-water systems, with increased deposition of MTCs. In addition, during the Palaeocene, dissolution of the crest of salt walls led to the truncation of intrasalt stratigraphy and formation of a salt karst seascape, with subterranean dissolution driving overburden deformation. Widespread salt dissolution likely occurred due to downward migration of NaCl-undersaturated seawater through the thin unlithified roofs of salt walls. Dissolution of the crests led to local caprock formation and the subsequent disruption, collapse and rotation of the overburden. An alternative hypothesis is that salt dissolution occurred due to the updip lateral migration of formation fluids from deep-water channels and lobes deposited along the flanks of the crests of salt walls. This thesis helps to improve our understanding of ancient salt giants, and the control salt tectonics and salt dissolution has on deep-water reservoir deposition and architecture. In addition, findings of this work have direct implications for assessing the Santos Basin post-salt hydrocarbon potential of this previously underexplored interval.