394
Chapter 8 Subsidence
b
SEDIMENT DECOMPACTION
a
POROSITY (n) OF UNIT A
o
0.2
0. 4
0.6
O~~~--~~~~~
SHALE
4
5
POROSITY (n) OF UNIT B
o
0.2
0.4
o +-......... ~-,-L..r...l....-...,.
1
1-. , , ,
"- .... ,
TIME
........
5
E
4~
(/)
(/)
w
3~
~
I
21Z
w
::2
1 Ci w
(/)
~ ........... ·_ ... o
o
E
~
"
UNCORRECTED
.... "..( FOR DECOMPACTION
E
~
Fig. 8.4. Graphical
method for decompacting presen t
sediment thicknesses of units Ap,
Bp, Cp to their
original thicknesses (Aj , Bj , Cj ).
a Growth of sedimentary column
with time, t, and
progressive compaction of buried
units. b Two examples of porosity-depth curves
for different sediments demonstrating decompaction
of units A and B
(Ap to Aj or A z,
respectively, and
Bp to Bj ) using Eq.
(8.12) in text. c
Resulting corrected subsidence
curve in comparison to uncorrected
curve; no changes
in paleo-water
depth and eustatic
sea level
~2
SANDSTONE
, ,
a..
w
o
~3
C
",
2w
u
z w
Cl
3Ui
CD
::J
(/)
,
a::
::l
CD
CORRECTED
" "
4
SUBSIDENCE HISTORY
4
5
8.2.2 Sediment Decompaction
The first step in baekstripping is to reeonstruet the
original sediment thieknesses, h s1 ' of the growing
sedimentary fill from the basin floor up to dated
stratigraphie boundaries in partieular exposures or
weil logs. Provided a sedimentary eolurnn with the
present thiekness, h.p, and mean porosity, 1\" had an
original mean porosity, n1, then its original thiekness,
h s1 , was
I-n
5
(cf. Seet. 13.2). A graphical method approximating
deeompaetion in eombination with Eq. (8.12) for
seetions eonsisting of different lithologie units is
shown in Fig. 8.4. This proeedure has to be repeated
for aU time steps and units eonsidered. The resulting
eurve eorreeted for deeompaetion demonstrates that
without this eorreetion subsidenee is underestimated
for the first phase and overestimated for the last
phase of basin development.
h - _ _ Ph
si -
sp
I-ni
(8.12) 8.2.3 Tectonic Subsidence from Decompaction
The initial mean porosities for partieular lithologie
units and depths below the sedimentary surfaee ean
be estimated by using known porosity-depth eurves
and Isostatic Adjustment
Having determined deeompaetion of the different
units of a sediment seetion, the next step in
baekstripping is isostatie adjustment. This adjustrnent
Chapter 8 Subsidence
b
SEDIMENT DECOMPACTION
a
POROSITY (n) OF UNIT A
o
0.2
0. 4
0.6
O~~~--~~~~~
SHALE
4
5
POROSITY (n) OF UNIT B
o
0.2
0.4
o +-......... ~-,-L..r...l....-...,.
1
1-. , , ,
"- .... ,
TIME
........
5
E
4~
(/)
(/)
w
3~
~
I
21Z
w
::2
1 Ci w
(/)
~ ........... ·_ ... o
o
E
~
"
UNCORRECTED
.... "..( FOR DECOMPACTION
E
~
Fig. 8.4. Graphical
method for decompacting presen t
sediment thicknesses of units Ap,
Bp, Cp to their
original thicknesses (Aj , Bj , Cj ).
a Growth of sedimentary column
with time, t, and
progressive compaction of buried
units. b Two examples of porosity-depth curves
for different sediments demonstrating decompaction
of units A and B
(Ap to Aj or A z,
respectively, and
Bp to Bj ) using Eq.
(8.12) in text. c
Resulting corrected subsidence
curve in comparison to uncorrected
curve; no changes
in paleo-water
depth and eustatic
sea level
~2
SANDSTONE
, ,
a..
w
o
~3
C
",
2w
u
z w
Cl
3Ui
CD
::J
(/)
,
a::
::l
CD
CORRECTED
" "
4
SUBSIDENCE HISTORY
4
5
8.2.2 Sediment Decompaction
The first step in baekstripping is to reeonstruet the
original sediment thieknesses, h s1 ' of the growing
sedimentary fill from the basin floor up to dated
stratigraphie boundaries in partieular exposures or
weil logs. Provided a sedimentary eolurnn with the
present thiekness, h.p, and mean porosity, 1\" had an
original mean porosity, n1, then its original thiekness,
h s1 , was
I-n
5
(cf. Seet. 13.2). A graphical method approximating
deeompaetion in eombination with Eq. (8.12) for
seetions eonsisting of different lithologie units is
shown in Fig. 8.4. This proeedure has to be repeated
for aU time steps and units eonsidered. The resulting
eurve eorreeted for deeompaetion demonstrates that
without this eorreetion subsidenee is underestimated
for the first phase and overestimated for the last
phase of basin development.
h - _ _ Ph
si -
sp
I-ni
(8.12) 8.2.3 Tectonic Subsidence from Decompaction
The initial mean porosities for partieular lithologie
units and depths below the sedimentary surfaee ean
be estimated by using known porosity-depth eurves
and Isostatic Adjustment
Having determined deeompaetion of the different
units of a sediment seetion, the next step in
baekstripping is isostatie adjustment. This adjustrnent