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Salt Tectonics, Sediments and Prospectivity: an Introduction

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Salt Tectonics, Sediments and Prospectivity: an Introduction Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021 Salt tectonics, sediments and prospectivity: an introduction STUART G. ARCHER1*, G. IAN ALSOP1, ADRIAN J. HARTLEY1, NEIL T. GRANT2 & RICHARD HODGKINSON3 1Geology and Petroleum Geology, School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK 2ConocoPhillips UK Ltd, Rubislaw House, North Anderson Drive, Aberdeen AB15 6FZ, UK 3Bowleven plc., 1 North St Andrew Lane, Edinburgh, EH2 1HX, UK *Corresponding author (e-mail: [email protected]) Salt: an introduction rafting. Salt influences syn- and post-kinematic sediment dispersal patterns and reservoir distri- Salt is a crystalline aggregate of the mineral halite, bution and can therefore be important for the cre- which forms in restricted environments where the ation of stratigraphic traps. It can also form top hydrodynamic balance is dominated by evaporation. and side seals to hydrocarbon accumulations and The term is used non-descriptively to incorporate all act as a seal to fluid migration and charge at a evaporitic deposits that are mobile in the subsurface. more regional scale. Salt may also dramatically It is the mobility of salt that makes it such an inter- affect the thermal evolution of sediments due to its esting and complex material to study. As a rock, salt high thermal conductivity. A thick layer of salt is almost unique in that it can deform rapidly under cools sediments that lie below it while heating geological conditions, reacting on slopes ≤0.58 dip sediments above it. This effect cannot be underesti- and behaving much like a viscous fluid. Salt has a mated as it helps provide the favourable conditions negligible yield strength and so is easy to deform, for source rock maturation in the deepwater Gulf of principally by differential sedimentary or tectonic Mexico and Santos basins, even though sedimentary loading. Significant differences in rheology and overburden may be 5 km or more in thickness. Salt behavioural characteristics exist between the indi- can also impact reservoir quality. The role of salt in vidual evaporitic deposits. Wet salt deforms the diagenetic history of reservoirs through its largely by diffusion creep, especially under low control on hydrothermal pore waters is a crucial strain rates and when differential stresses are low. element in the risking of the deepwater Palaeo- Basins that contain salt therefore evolve and gene play of the Gulf of Mexico, for example. Salt deform more complexly than basins where salt is continues to kinetically evolve through time, not absent. The addition of halokinetic processes to only by the classical roller-diapir-pedestal-canopy/ the geodynamic history of a basin can lead to a collapse progression but also with varying rates of plethora of architectures and geometries. The rich deformation, in response to changing sedimentation variety of resultant morphologies have considerable rates and patterns. The relative timing of salt move- economic as well as academic interest. ment and its impact on source, reservoir, trap, seal Historically, salt has played an important role in and timing often governs the prospectivity in salt- petroleum exploration since the Spindletop Dome related basins. Beyond the realm of petroleum, salt discovery in Beaumont, Texas in 1906. Today, is also used as a resource for potash, gypsum and much of the prime interest in salt tectonics still nitrates and has the potential to be employed as a derives from the petroleum industry because many repository for radioactive waste or a top seal to of the world’s largest hydrocarbon provinces sequestered CO2. reside in salt-related sedimentary basins (e.g. Gulf of Mexico, North Sea, Campos Basin, Lower Congo Basin, Santos Basin and Zagros). An under- Salt tectonics, sediments and prospectivity: standing of salt and how it influences tectonics and a conference summary sedimentation is therefore critical to effective and efficient petroleum exploration. Within rift basins In January 2010, The Petroleum Group of the Geo- in particular, salt is seen to orchestrate the petroleum logical Society in conjunction with the Society for system. Through halokinesis it creates structural Sedimentary Geology (SEPM), convened an inter- traps, counter-regional dips on continental margins, national conference at Burlington House in and it can carry or entrain adjacent lithologies via London entitled Salt Tectonics, Sediments and From:Alsop, G. I., Archer, S. G., Hartley, A. J., Grant,N.T.&Hodgkinson, R. (eds) 2012. Salt Tectonics, Sediments and Prospectivity. Geological Society, London, Special Publications, 363, 1–6. http:// dx.doi.org/10.1144/SP363.1 # The Geological Society of London 2012. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on September 24, 2021 2 S. G. ARCHER ET AL. Prospectivity. The aim was to bring together indus- 24 km long weld and is thought to be controlled trialists and academics to present a contemporary by the original thickness of the salt wall together view of salt at a global scale and to examine with the amount and direction of subsequent short- its influence on syn-kinematic sedimentation, on ening. Where contraction was normal to the salt basin evolution and ultimately on hydrocarbon wall, then the diapir was locally closed with little prospectivity. further deformation. However, where shortening The conference was one of the best attended in was oblique to the wall then significant post- recent years at the Geological Society and this weld dextral shearing and fracturing took place, volume incorporates 29 papers stemming from the which may affect the sealing capacity of such welds. conference. Salt tectonics obviously continues to Further detailed studies along the La Popa be an important theme within both academic and weld by Andrie et al. demonstrate that fluvial sedi- industrial geoscience communities alike. This col- mentation in a shortening-induced salt-withdrawal lection of papers creates a broad thematic set that basin displays distinct up-section changes in encompasses much of the recent research into salt fluvial facies distribution and the geometry of halo- and sediment dynamics. It is hoped that this kinetic folding. Fluvial channels in the lower part of volume will act as a valuable modern reference the sequence are typically thin, broad and display and a springboard for future studies. This overview variable palaeocurrents, while the upper parts of outlines the key findings and summarizes the key the sequence are marked by thick, stacked channels concepts that are presented more fully in the with weld-parallel palaeocurrents. In addition, papers in the main body of this special publication. halokinetic folding of sediments also intensifies This is the first Geological Society Special Publi- and becomes narrower up the sequence, reflecting cation (GSSP) dedicated to salt tectonics and associ- reduction in sedimentation rates compared to salt ated sedimentation since Alsop et al. (1996) more rise rates. than 15 years ago; it therefore represents a timely Kernen et al. provide a stratigraphic analysis of addition to this expanding field. Neoproterozoic sequences adjacent to an inferred The volume is separated into five main themes allochthonous salt sheet in the Central Flinders covering a variety of geographical and process- Ranges of South Australia. Sediments adjacent to linked topics relating to salt tectonics, sediments the salt represent a progradational sequence from and prospectivity. These include halokinetic- wave-dominated shelf deposits through to coastal sequence stratigraphy, salt in passive margin set- plain sediments. Halokinetic-sequence boundaries tings, Central European salt basins, deformation may be pronounced and marked by up to 508 of within and adjacent to salt and salt in contractional angular truncation. settings and salt glaciers. Smith et al. consider the fracture-controlled palaeohydrology of a secondary salt weld in the Halokinetic-sequence stratigraphy La Popa Basin. Isotopic and fluid inclusion analyses suggest that veins were formed after salt was evac- Halokinetic-sequence stratigraphy involves the uated and are more abundant near a bend in the application of sequence stratigraphic principals weld. In addition, such analyses also indicate that directly to salt-influenced sedimentary strata. This the weld acted as a vertical fluid conduit and a section opens with a paper by Giles & Rowan horizontal baffle. This has clear implications for that investigates concepts in halokinetic-sequence hydrocarbon sealing potential, which may also be stratigraphy and deformation. Hook geometry influenced by factors such as bends in welds and halokinetic sequences are narrow unconformity- amounts of shortening across such structures. bounded zones of deformation with pronounced angular (.708) discordances and abrupt facies Salt in passive margin settings changes. Wedge geometry halokinetic sequences display broader zones of folding, low-angle trun- Mohriak et al. discuss salt structures in Brazil, cations and gradual facies changes. Composite ranging from intracratonic Palaeozoic basins to halokinetic sequences may also form and reflect divergent margin basins formed during the Mesozoic variations in the ratio of diapiric rise to sediment break-up of Gondwana. Seismic reflection data accumulation rates. together with magnetic and gravity data across Rowan et al. describe the anatomy of an exposed Mesozoic rifts of the South Atlantic suggest a thick vertical salt weld
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