286
The Stratigraphic Architecture of Fluvial Depositional Systems
REFLECTION
COEFFICIENTS
. �
r
'
'
WAVELET
ZERO PHASE
10·15·45·50 HZ
NORMAL POLARITY
'
'
·, ,,·,
0
I ::::: .5
I .....
1·:_:_:_:_:
lnm
II It I
1.0
Fig. 9.43. Synthetic seismogram for a well through the
Fort Union Formation and related deposits, Powder River
Basin, Wyoming. (Ray 1982, reprinted by permission)
Paleosols may be characterized by distinctive
petrophysical properties, and may therefore be
mappable using wireline logs (Sect. 9.5.2) and seismic methods. Paleosols are commonly distinctive
stratigraphic markers (Sect. 9.2) and in some cases
are used to define sequence boundaries (Sect.
13.3.1). They can therefore be of considerable stratigraphic importance. Hanneman et al. (!994)
mapped calcic paleosols in some Miocene beds in
Wyoming using seismic methods and wireline logs
and demonstrated that their petrophysical visibility
was caused by their high densities and interval
velocities.
The Paleocene Fort Union Formation of the Powder River Basin, Wyoming, provides an example of a
seismic reflection survey of a fluvial-lacustrine deposit (Ray 1982). A synthetic seismogram is shown in
Fig. 9.43 and Fig. 9.44 is a seismic cross section. Five
unconformities have been recognized in this succession, at 810, 1000, 1295, 1360, and 1670 ms (depths on
the right-hand side of the section, Fig. 9.44). These
unconformities separate distinct stratigraphic sequences. A particularly instructive example of the
lateral changes in seismic facies discussed by Anstey
(1980) is shown by the sequence between the 1000and 1295-ms unconformities (sequence C). Note the
east-dipping clinoform structure of this sequence.
Note also the change in reflection character from
low-amplitude, low-frequency, low-continuity reflectors in the upper part of the section, particularly
in the we�t, to a regipn of high-continuity, highamplitude, high-frequency reflectors in the middle
of the unit (the clinoform structure is particularly
clear here), to a zone oflow- to moderate-continuity,
low-frequency, low-amplitude reflectors in the
lower part of the sequence. These changes are interpreted as lateral facies changes from discontinuous
fluvial sandstones and shales in the west, to
interbedded sandstones and shales of the lake-margin delta clinoforms, to uniform lacustrine shales in
the east.
The ideas in the preceding paragraphs, including
those of Anstey (1980), Brown and Fisher (1977), and
Ray (1982), relate to the recognition of large-scale
fluvial depositional systems within other types of
stratigraphic units. Much more difficult is the mapping of individual sandstone bodies within a fl uvial
system. As pointed out by Neidell and Beard (!985),
careful attention to processing and presentation of
seismic data can lead to considerable refinements in
our ability to map subtle stratigraphic objectives. A
knowledge of the structure of the propagating waveform and the use of color to display amplitude variations are two vital aspects of this work. Fulthorpe
(1991) demonstrated that seismic resolution of
stratigraphic features is increased in areas of high
sediment supply, because the deposits are thicker
and therefore more readily resolvable. Also, deposits
that develop by lateral progradation are much more
readily subdivisible into their component units than
those which accumulate by vertical aggradation, because of the greater scale of depositional units in the
horizontal dimension. Cartwright et al. (1993)
pointed out that the improved resolving power available fr om modern seismic acquisition and processing techniques, relative to those available when
seismic stratigraphy was first introduced in the
1970s, has led to an increase in the demonstrable
complexity of stratigraphic units and to a need for
The Stratigraphic Architecture of Fluvial Depositional Systems
REFLECTION
COEFFICIENTS
. �
r
'
'
WAVELET
ZERO PHASE
10·15·45·50 HZ
NORMAL POLARITY
'
'
·, ,,·,
0
I ::::: .5
I .....
1·:_:_:_:_:
lnm
II It I
1.0
Fig. 9.43. Synthetic seismogram for a well through the
Fort Union Formation and related deposits, Powder River
Basin, Wyoming. (Ray 1982, reprinted by permission)
Paleosols may be characterized by distinctive
petrophysical properties, and may therefore be
mappable using wireline logs (Sect. 9.5.2) and seismic methods. Paleosols are commonly distinctive
stratigraphic markers (Sect. 9.2) and in some cases
are used to define sequence boundaries (Sect.
13.3.1). They can therefore be of considerable stratigraphic importance. Hanneman et al. (!994)
mapped calcic paleosols in some Miocene beds in
Wyoming using seismic methods and wireline logs
and demonstrated that their petrophysical visibility
was caused by their high densities and interval
velocities.
The Paleocene Fort Union Formation of the Powder River Basin, Wyoming, provides an example of a
seismic reflection survey of a fluvial-lacustrine deposit (Ray 1982). A synthetic seismogram is shown in
Fig. 9.43 and Fig. 9.44 is a seismic cross section. Five
unconformities have been recognized in this succession, at 810, 1000, 1295, 1360, and 1670 ms (depths on
the right-hand side of the section, Fig. 9.44). These
unconformities separate distinct stratigraphic sequences. A particularly instructive example of the
lateral changes in seismic facies discussed by Anstey
(1980) is shown by the sequence between the 1000and 1295-ms unconformities (sequence C). Note the
east-dipping clinoform structure of this sequence.
Note also the change in reflection character from
low-amplitude, low-frequency, low-continuity reflectors in the upper part of the section, particularly
in the we�t, to a regipn of high-continuity, highamplitude, high-frequency reflectors in the middle
of the unit (the clinoform structure is particularly
clear here), to a zone oflow- to moderate-continuity,
low-frequency, low-amplitude reflectors in the
lower part of the sequence. These changes are interpreted as lateral facies changes from discontinuous
fluvial sandstones and shales in the west, to
interbedded sandstones and shales of the lake-margin delta clinoforms, to uniform lacustrine shales in
the east.
The ideas in the preceding paragraphs, including
those of Anstey (1980), Brown and Fisher (1977), and
Ray (1982), relate to the recognition of large-scale
fluvial depositional systems within other types of
stratigraphic units. Much more difficult is the mapping of individual sandstone bodies within a fl uvial
system. As pointed out by Neidell and Beard (!985),
careful attention to processing and presentation of
seismic data can lead to considerable refinements in
our ability to map subtle stratigraphic objectives. A
knowledge of the structure of the propagating waveform and the use of color to display amplitude variations are two vital aspects of this work. Fulthorpe
(1991) demonstrated that seismic resolution of
stratigraphic features is increased in areas of high
sediment supply, because the deposits are thicker
and therefore more readily resolvable. Also, deposits
that develop by lateral progradation are much more
readily subdivisible into their component units than
those which accumulate by vertical aggradation, because of the greater scale of depositional units in the
horizontal dimension. Cartwright et al. (1993)
pointed out that the improved resolving power available fr om modern seismic acquisition and processing techniques, relative to those available when
seismic stratigraphy was first introduced in the
1970s, has led to an increase in the demonstrable
complexity of stratigraphic units and to a need for