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