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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
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)
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
Linear trend of porosity - but only over a relatively deep reservoir interval
Identical lithology (shallow marine sands)
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
Porosity loss related to cementation: quartz cementation
Quartz cementation kicks in at 60oC (data from Norwegian continental margin, Statoil group)
Porosity loss related to cementation: diagenetic (authigenic) clay minerals, such as illite
Illite cementation kicks in at 120oC, reduces porosity and serious impact on permeability
Porosity-depth curves for sandstones, shales and carbonates
Individual datasets shown by black lines, and spread of data shown by grey shading
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
Porosity-depth parameters for common sedimentary lithologies
Lithology Surface porosity c Grain density
0 km-1 kg m-3
Shale 0.63 0.51 2720
Sandstone 0.49 0.27 2650
Chalk 0.70 0.71 2710
Slaey sandstone 0.56 0.39 2680
Based on North Sea data, in Sclater & Christie (1980)
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)
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
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
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
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
Eustatic corrections
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
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.
Isostatic effect of a change in the water depth of the ocean
Initial ocean of depth h1
Increase in water depth to h2 results in sea level change SL
Sea-level change due to deposition of sediment in the ocean
Initial ocean with water depth hw
Sea-level change of SL results from deposition of sediment thickness hs
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
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
€
Y = Sρ m − ρ sb( )ρ m − ρ w( )
⎧ ⎨ ⎩
⎫ ⎬ ⎭− ΔSL
ρ w
ρ m − ρ w
⎛
⎝ ⎜
⎞
⎠ ⎟+ Wd − ΔSL( )
where SL is the palaeo-sea level relative to the present, and Wd is the palaeowater depth
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
€
C =ρ m − ρ sb( )
ρ m − ρ sb +D
g
2π
λ
⎛
⎝ ⎜
⎞
⎠ ⎟4
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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