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Subsidence history - the key to understanding basin evolution What we have: boreholes (sediment thickness and age) seismic reflection data (sediment thickness, stratigraphic patterns) micropaleontology (water depth at time of deposition) sedimentology (water depth?) What we want: flexural/isostatic and ‘true’ (thermal, tectonic) components of subsidence What we need: true (depositional) thickness of each unit porosity and bulk density of the sediment column over time

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An example Top panel is the data (thickness, age, lithology of each unit in situ) Middle panel is decompacted depth to base of each unit with time Lower panel is total postrift subsidence (blue, uncorrected) and ‘tectonic’ or ‘thermal’ subsidence (red, corrected for Airy isostasy)

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Porosity changes during subsidence/burial Some definitions: Mechanical compaction (gravitational load of overlying water-saturated sediments; compaction is a strain) causes porosity loss Physico-chemical compaction (pressure solution) is common in carbonates Cementation (porosity loss without strain), related to temperature rather than loading

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Linear trend of porosity - but only over a relatively deep reservoir interval Identical lithology (shallow marine sands)

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Tertiary sands, southern Louisiana: based on over 17,000 cores, averaged every 1000 feet (300 m) Linear trend in porosity with depth: thought to be due to the compaction of ductile rock fragments Data over depth range <1000 m to 6000 m

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Porosity loss related to cementation: quartz cementation Quartz cementation kicks in at 60 o C (data from Norwegian continental margin, Statoil group) Porosity loss related to cementation: diagenetic (authigenic) clay minerals, such as illite Illite cementation kicks in at 120 o C, reduces porosity and serious impact on permeability

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Porosity-depth curves for sandstones, shales and carbonates Individual datasets shown by black lines, and spread of data shown by grey shading

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Porosity-depth curves: the exponential model Porosity reduces to 1/e of its surface value at a depth of 1/c km Porosity at depth y ( y ) is equal to 0.exp(-cy) c is called the porosity-depth coefficient

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Porosity-depth parameters for common sedimentary lithologies LithologySurface porosityc Grain density 0 km -1 kg m -3 Shale0.630.51 2720 Sandstone0.490.27 2650 Chalk0.700.71 2710 Slaey sandstone0.560.39 2680 Based on North Sea data, in Sclater & Christie (1980)

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The effects of compaction can be removed iteratively by a process called decompaction This yields the thickness of each unit at each time from deposition (initial = true thickness) to present day (final = observed)

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Subsidence history and backstripping True sediment thicknesses and true sediment accumulation rates are calculated by decompaction Decompacted subsidence curves need to be corrected for (a) variations in palaeobathymetry and (b) variations in eustasy The isostatic effect of the sediment load is then removed to reveal the ‘driving’ tectonic/thermal subsidence by backstripping

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Palaeobathymetric corrections Decompacted depths are calculated relative to a stationary reference datum - sea level The sediment surface at a particular time, however, may have been below (or above) sea level Estimation of water depth variations as a function of time allow the decompacted curve to be corrected For the same ‘driving’ or ‘tectonic’ subsidence, water depth changes may cause major variations in sediment thicknesses

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Effects of initial water depth on sediment thickness for a given tectonic subsidence Tectonic subsidence is identical in: (a) (top) where there is a large initial water depth, and (b) (bottom) where there is no initial water depth

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Eustatic corrections Changes of absolute sea level over time change the position of the datum used to plot subsidence Changes of eustatic sea level are isostatically compensated. The new elevation of sea level after isostatic compensation is called freeboard Freeboard is about 70% of the change in the height of the water column

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Eustatic corrections

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Quaternary sea-level fluctuations, Gulf of Mexico Detailed glacio-eustatic Pleistocene record (dashed) and oxygen isotope record from deep sea benthonic foraminifera (solid) Glacio-eustatic changes in the Quaternary

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The global (eustatic) sea level curve The long-term ‘first-order’ curve probably relates to the balance between ocean ridge and subduction fluxes, which change the volumetric capacity of the ocean basins Shorter-term eustatic variations relate to the locking-up and release of ocean water in terrestrial ice caps during glaciation and deglaciation For subsidence analysis, initially use only the first order curve. The Haq ‘global cycle chart’ is unreliable.

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Isostatic effect of a change in the water depth of the ocean Initial ocean of depth h 1 Increase in water depth to h 2 results in sea level change SL

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Sea-level change due to deposition of sediment in the ocean Initial ocean with water depth h w Sea-level change of SL results from deposition of sediment thickness h s

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Backstripping the sediment load Assuming Airy isostasy, the effect of the sediment load can be removed simply using Y = S{( m - sb )/( m - w )} where Y is the tectonic (or ‘driving’) subsidence, S is the decompacted (total) subsidence, m and w are the mantle and water densities, and sb is the bulk sediment density, which varies with time/depth

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The corrected tectonic (or ‘driving’) subsidence The tectonic subsidence after corrections for changes in water depth (palaeobathymetry), and eustatic sea level, assuming Airy isostasy, is given by where SL is the palaeo-sea level relative to the present, and W d is the palaeowater depth

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Flexural isostasy The sedimentary fill of a basin acts as a load on the underlying lithosphere, which may therefore support it by flexure rather than by local (Airy) compensation The degree of compensation C of the load is dependent on the flexural rigidity and the wavelength of the load For a sinusoidal load of wavelength, the degree of compensation C is

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Subsidence history 3 classes: (1)Stretched basins; rapid synrift subsidence, passing into gradual concave-up postrift thermal subsidence, or basin inversion (2) Flexural basins; convex-up signature (3) Strike-slip basins; very rapid subsidence, short-lived, common inversion

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