Architecture des Bassins & Géomatique

Transcription

1 Architecture des Bassins & Géomatique 1- Origine des Bassins Sédimentaires Déformation lithosphérique: forçages internes Sédimentation : forçages externes Importance des Bassins Sédimentaires 2- Cadre géodynamique des Bassins Sédimentaires Analyse de la subsidence Bassins liés à la divergence - rifts - marges passives Bassins liés à la convergence - bassins foreland Autre types de bassins 3- Évolution post-dépôt des Bassins Sédimentaires Compaction - Diagenèse Circulation des fluides Cas de la matière organique - systèmes pétroliers 1 1- Origin of Sedimentary Basins 1.1 Lithospheric deformation: Internal forçings 1.2 Sedimentation: External forçings 1.3 Sedimentary basins & societal issues 2 NASA What is a sedimentary basin? - it s a depression filled with sediments => Study of sedimentary basins = analysis of processes responsible for the origin of: 1- the depression (mostly controlled by internal forcings «Earth machine») 2- the sedimentary-fill (controlled by interaction of internal and external forcing) Distribution of sedimentary basins (sediment accumulation > 1km) Lithospheric deformation: Internal forçings Struture and reology of the Earth envelopes Basin forming driving mechanisms are related to processes within the rigid, cooled thermal boundary layer of the Earth known as the Lithosphere. atmosphere Solid crust => The Earth s interior is composed of number of compositional and rheological zones. The main compositional zones are the crust (low density rocks + sedim. cover), mantle (olivine) and core (metallic : iron & nickel). Molten outer.c. Solid inner C. Lithosphere mantle Outer core Rigid Lithosphere mantle 4 1

2 Basics about the Lithosphere Definitions Sedimentary basins are formed between solid and fluid envelopes of the Earth. Continental & oceanic crusts are compositionally different from the underlying mantle The outer mantle and the crust makes the lithosphere (rheological unit) The outer mantle has the same compostion as the underlying convective mantle (asthenosphere) Definition of the outer envelopes of the Earth that interact to form sedimentary basins 5 Basics about the Lithosphere parameters controling lihosphere rheology Characterization of the different layers of the lithosphere. Deformation of the lithosphere induces the formation of sedimetary basin. 6 Basics about the Lithosphere Principle of isostacy Total mass of each column must be equal. The Airy hypothesis: Depth of equal pressure Ocean Mantle (high density) mountain Continent (low density) Mountain root Blocks of the same density (material), but different thickness, floating about an equilibrium surface => uneven Moho (roots beneath mountains and rises beneath basins. Pratt model blocks of differing density (lighter beneath mountains and denser beneath basins) => flat Moho. basin «anti-root» Not verified by data 7 Basics about the Lithosphere Principle of isostacy weight of a column of lithosphere before basin formation = weight of a column after basin formation Only if locally Compensated! 8 2

3 Earth internal dynamics as driving mechanisms for basin formation (Courtillot, 2003) (Don Anderson 2004) => The Earth System can be regarded as a thermo-mechanical machine which consumes, transforms and releases energy in order to maintain equilibrium conditions. => Sedimentary basins rocks are recording devices (and sediments the tapes or CD s) which record (somewhat discontinuously) the equilibrium quest of the Earth system. 9 Internal driving forces of the Earth machine The internal heat is continuously dissipated outwards from the centre of the Earth in 3 ways : Conduction : Thermal energy transmitted between atoms => Internal solid Earth Advection : Movement of hot material to surface => Volcanoes and hot spot ; Convection : Movement of material in the mantle and outer core by density differentiation of the plastic material => Plate tectonics Global Heat Flow Map 10 Earth machine: mantle convection subduction Upper mantle Lower mantle Mantle one layer Mantle convection with phase transition (olivine -> spinel -> peroskivite subduction plume Mantle convection no phase transition avalanche Sam Butler, plume 11 Plate tectonics: lithosphere movements Pacific Ocean Convection cells Subduction zone America Relative movement Acretionary ridge Earth internal energy = Energy of accretion at the time of its formation + Energy related to formation of iron-rich core + Energy from decay of radioactive elements => dissipated at surface = heat flux. The heat flux propagated by convection in the plastic upper mantle is converted into mechanical energy (and localized partial melting) at the base of the Lithosphere and dissipated by plate motion and deformation. Lithospheric plates movements (1000 s km laterally, 1000 s m vertically). Atlantic Ocean W. Europe Lithosphere Asthenosphere 12 3

4 Plate tectonics Lithospheric surface of the Earth showing plate tectonics (plate boundaries, earthquakes and volcanoes). 13 Sedimentary basins and Plate tectonics Distribution of sedimentary basins (sediment accumulation > 1km) Continental passive margins, subduction zones, foreland of present or ancient mountain belts, centre of cratons. 14 How to create a depression? -> 3 lithospheric processes account for subsidence 1- Cooling 2- Stretching/ thinning 3- Loading 20 C 1300 C 20 C 1300 C Ma 5km 2km Cooling: example Age of the oceanic Lithosphere (oceanic floor) Lithosphere emplaced at midoceanic ridges (accretion) and then moves apart symetrically (seafloor spreading) Bathymetry of the oceanic Lithosphere (oceanic floor) increases away from oceanic ridges 16 4

5 Thermal contraction of the Oceanic Lithosphere Sea level Accretion Very high geotherm -2km -5km z age Cooling of oceanic lithosphere Increasing bathymetry of oceanic floor Increasing thickness of oc. lithosph. 0 C 1300 C geotherms 17 Stretching/Thinning La lithosphère étirée s amincit - failles dans la croûte supérieure, - remontée manteau sous la zone amincie Sand-box Analog modeling Reflexion seismic - North Sea rift Michon, 2000 Copyright 2008 Virtual Seismic Atlas Marsden et al.'s (1990) interpretation of BIRPS' NSDP84-1 deep seismic line. MARSDEN, G, YIELDING, G, ROBERTS, A & KUSZNIR, N Application of the flexural cantilever simple-shear/pure-shear model to the north sea. In: Blundell, D & Gibbs, A (eds). Tectonic Evolution of the North Sea Rifts. Oxford University Press, Oxford. 18 Loading of the lithosphere => flexure Atlantic accretionary ridge Niger fan Congo fan Sedimentary load of the Congo deep-se-fan ( > 5km thick) => Deflexion by flexure of the oceanic lithosphere (subsidence) Congo drainage area Modifié d après Uchupi, Different types of basins according to plate tectonic setting: spatial and temporal evolution from one type to another 20 5

6 1.2. Sedimentation: External forcings Burdigalian (Digne foreland Basins) Tidal sediments = Sediment deposition controled by the tides (cyclic phenomenon). Tides results from combined attraction of the Moon and the Sun on the oceans (& on the crust). Sedimentation records variations of parameters external to the Earth 21 Baie du Mont Saint Michel Energy External forcings Periodic changes in the Earth s orbital parameters affect the amount of radiation from the Sun. The energy dissipated by the Sun varies with time => variation in radiation received by the Earth. => The total amount of solar radiation received on the Earth s surface governs long-term (100 s of millions of years) or on short-term ( s years) temperature of the atmosphere and hydrosphere. Through complex feedback loops, this has direct and indirect consequences on Climate and associated exogenic transfer processes. => Climate forcing affect the way the sedimentary basins are filled 22 Insolation : sun s energy Sun s energy Sun s energy tilt m 2 ->242W/m 2 1m 2 ->342W/m 2 High latitudes receives less energy than inter-tropical areas Insolation seasonal variation NO tilt No seasonal variation of insolation Increased yearly average temperature 23 Milankovitch cycles T= tilt or obliquity E = eccentricity P = precession Orbital parameters of the Earth have been acting over the whole history of the planet (albeit changes in periodicity and amplitude). Milankovitch cycles have been recorded in sediments with different intensity through time. During Quaternary Milankovitch cycles are particularly well expressed (Glaciations stages) 24 6

7 Sediment accumulate in basins if: 1- there is a gravity-driven flux of sediment (erosion/ transport/ deposition) => base level 2- there is space available to trap the sediment => accommodation space Sediment are generated if: Deformation of the topographic surface of the lithosphere induced by internal forcing (mountain-building, volcanism, thermal uplift ). Erosion of the topography, mobilization of detritals, transport, deposition. All processes governed by gravity. Processes strongly dependent on external forcing (climate ). Biological activity contributes to sediment flux. in-situ carbonate production in favourable environments («carbonate factory» in ocean, lakes) -> climate-dependent reworked carbonates behaving as detritals plants residues (coal) (Bio-) Chemical activity = weathering, alteration, evaporation, precipitation Base-level Base level (Wheeler, 1964) : is an abstract, non physical, surface ; is above the earth surface where deposition occurs, below where erosion occurs, and upon where there is an equilibrium (e.g., bypass) ; represents the surface where sediment flux would be constant (i.e., a balance would exist between sediment supply and removal) ; is a potentiometric surface (i.e., the surface along which the energy of sediment flux is minimized) ; is a dynamic surface (i.e., it vibrates with respect to the physical surface in time and space) ; exists in a system where space, energy and mass are conserved Available space => Accommodation Basin subsidence accommodation Eustacy Intraplate deformation Accommodation : it is the rate (measured in m/ma) at which space is being made available for sediments to be trapped in the basin. It is the result of the vertical movements of the basement (subsidence + lithoshere deformation) and of eustacy (World ocean level). Sediment flux may or may not fill the availlable space. This is determined by the balance of sediment rate and accommodation. Sed. Rate < Accomm => underfilled basin, water depth increases (starved basin, condensation surface) Sed Rate = Accomm => basin remains at the same water-depth => persistance of sedimentary facies through time Sed. Rate > Accomm => basin being filled, water-depth decreases, coarsening and shallowing up sequences. 27 «Eustacy» vs «Relative sea-level change» Eustacy = variation of the global World Ocean (all seas & oceans being connected) this is due to changes in the shapes of the ocean floor ( variable rates of sea-floor spreading, mantleconvection induced uplift, ) or of the volume of water in the World Ocean (growth or decay of polar icecaps, soil moisture, water thermal expansion ). Several Eustatic Curves have been compiled and progressively improved (Haq, Miller, Kominz, ). They can be applied everywhere. Haq Eustatic Curve Relative sea-level change = variation of water depth in one basin. It s the combination of eustacy, and local constraints: subsidence/uplift and sediment flux. Relative sea-level change in a basin can be approached by analysis of the stratal architecture combined with sedimentary facies. sediment flux Bst vertical mvt Relative sea-level Eustacy 28 7

8 Periodic changes in forcings => cycles Periodic (or not) changes in the controlling processes => record cycles Combination of stacking of several signals => complex stratigraphic record - Basin analysis aims at deciphering these signals - sedimentary basinfill contains these signals => Archives Signals of different time/space scale => record of stacked (nested) cycles - several nested sequences in the stratigraphic record 29 Canterbury Basin, New Zealand Aggradation: Sed. Rate Accomm Stratal geometry progradation aggradation 2 mains patterns: several possible causes f(subsidence, sediment flux, sea-level) Divergent: Differential subsidence Sed. Rate > Accomm Onlap Sed. Rate < Accomm Progradation : Sed Rate Accomm bathymetry Down-lap Condensed section 30 Sedimentation pattern of Neogene passive margins Maximum Flooding Surface Fluvial & delta Slope shales sequence boundary Reworked clastics Modifié d après Bartek et al, Eustacy Pliocene Miocene Oligocene 31 Orbital parameters of the Earth variable sun energy received outer envelopes temperatures climate sedimentation Valanginian, S. France Stratigraphic record 32 8

9 Lithofacies = Lithology Texture structure Sedimentology : lithofacies Lithofacies is the set of physical features of a sedimentary rock. Lithofacies provides info on depositional conditions. Mineralogy, granulo, morphometry Mode of association of constitutive elements Geometry of the sedimentary body Source, transport, duration, environment,bathymetry Mode of transport & deposition Hydrodynamics biochemicals, biological indicators 33 synthesis Sedimentary basins result from the complex interaction of internal and external forcings_ Reading the sedimentary record allows to decipher the controlling factors and their temporal evolution Sedimentary basins & societal issues Salt Geothermy Aquifers Stones Iron ore Gas storage sequestration Natural resources Fossil energy 35 Argiles imperméables Sedimentary basins World population (1994) Cropland (Jon Foley & al) Distribution of sedimentary basins (sediment accumulation > 1km) World distribution of population, mostly in sedimentary basins (favourable to agriculture, economic activity and easy communication) World distribution of cropland : Land use for agriculture is prevalent in sedimentary basins (West American foreland basin, Mississippi valley, Indian Foreland basins, continental margins of Asia and Australia, intra cratonic basins of Europe and Canada, ). Note that this map partly mirrors the population distribution 36 9

10 erosion & weathering P.J.Combes Natural Reactor = ore formation Dissolved metallic ions Sediments Sediment deposition & ions precipitation subsidence ores eosion & weathering 37 M. Séranne Biosphere (Carbon) sol Natural Reactor = hydrocarbons generation migration Maturation f(temperature, pressure, time): Organic matter -> kerogene -> Oil -> gas! Organic mater (anoxiclake) burial oil Biosphere soil 38 Consommation ressources naturelles /an / personne Ressources minérales eau Énergie fossile La vaste majorité des ressources naturelles provient des bassins sédimentaires