The Geometry of Fluvial Reservoirs
a shee t-flood" units are lenticular; and form part of a
hete rogeneous clastic succession (Dranfield et al.
!987). According to Bentham et a!. (1993), braided
systems consisting of channel sandstone bodies
interbedded with significant proportions of floodplain fine-grained deposits constitute a distinctive
type of fl uvial assemblage that is much more important in the geological record than has hitherto been
recognized,
Many fluvial sand bodies are mapped and described as ribbons and may represent the fill of highor low-sinuosity channels. Examples are described
by Davies et a!. (1991) and Yinan et a!. (1987).
Hastings (1990) illustrated a complex of linear
sandbodies forming a low-sinuosity meander belt in
the Tyler Formatio n of North Dakota. Many may,
however, represent the amalgamation of point bars
or dther bar deposits along the channel trend, pro ...
ducing an overall ribbon geometry (Ebanks and
Weber 1982). Where sufficient well control is available, such ribbons may be broken down into their
individual bar units. Example· s of these various conditions are described later. Ribbon and lens reservoirs may form purely stratigraphic traps. The
ribbon shape of these sandstones should not be confused with the ribbon shape of many paleovalley-fill
sandstones, which are typically much wider and
thicker and represent the fill of entire, incised meander belts.
The Cherokee Group (Pennsylvanian) of Oklahoma, Kansas, and Missouri has provided many
classic examples of shoestring or ribbon sandstones.
Examples of producing sandstones in this group,
particularly the Bartlesville Sandstone (Desmoinesian), have been known since the 1920s and have
been illustrated in many classic textbooks (e.g.,
Levorsen 1967).
Another classic area of production from fields
consisting largely of stratigraphically trapped petroleum in ribbons and lenses is the Gulf Coast. Major
fluvial producers there are (a) the Upper Cretaceous
Lower Tuscaloosa Sandstone of southern Mississippi and Louisiana, which constitutes a transgressive, retrograde, fluvial�deltaic depositional systems
tract (Karges 1962; Berg and Cook 1968; Stancliffe
and Adams 1986; Hamilton and Cameron 1986;
Hamlin and Cameron 1987; Wiygul and Young 1987;
Hersch 1987; Klicman et a!. 1988; Hogg 1988; Miller
and Groth 1990i W erren et al. 1990; Garrison and
Chancellor 1991); (b) the Oligocene Frio Formation
of southwest Texas, another fluvial-deltaic system
485
which is an important gas producer (Galloway et al.
1982; Jirik 1990; Kerr 1990; Kerr and Jirik 1990); and
(c) the Lower Cretaceous Donovan/Sligo Formation
in Texas. The Citronelle field, which produces from
this unit, is an excellent example, illustrated later
(Eaves 1976).
Most of the producing zones in the Songliao and
Ordos basins, China, are lenticular or ribbon-shaped
in geometry; and a detailed knowledge of their subsurface distribution and facies is required for the
development of efficient production programs (Jin
et aL 1985; Moore et aL 1986; Yinan et al. 1987).
Likewise, fluvial-deltaic channel-and-bar sands of
the Oligocene Barail Sands of northeast India provide numerous ribbon-shaped and lenticular reservoir bodies in combined structural-stratigraphic
traps (Murty 1983; Desikachar 1984). Tertiary production in the Green River Basin (McDonald 1976)
also provides excellent examples of this type of reservoir geometry. It may be very difficult to produce
accurate maps of these ribbon sandstones. A tight
well spacing is required, as ot h erwise spurious correlations may result. This is discussed further below.
Examples of fields producing from ribbons an d
lenses are described in Sect. 15.4.
7 4.2.2.3 Stratigraphic Variations in Reservoir Geometry
In many fields the geometry of the reservoirs varies
stratigraphically, reflecting allogenic influences on
fluvial style or autogenic migration of the fluvial
system. For example, in South Belridge field, California, the Tulare Formation consists of a braid-delta
complex that prograded into a Jake. Lower Tulare
reservoir sands are lenticular or ribbon bodies deposited in meandering fluvial systems, whereas Upper Tulare sandstones were deposited in wider, more
proximal, braided channels and are more laterally
continuous (Miller et al. 1990). Similar stratigraphic
variations have been described in Prudhoe Bay and
Endicott fields, Alaska (Lawton et a!. 1987; Melvin
1993). In the Gullfaks field, in the Norwegian sector
of the East Shetlands Basin, the Statfjord Formation
is divided into three members comprising at least six
reservoir units, reflecting stratigraphic variations in
fluvial style (Petterson et al. 1990). In the nearby
Brent field, eight reservoir units have been mapped
in the Statfjord Formation (Struijk and Green 1991).
Sequence concepts which may explain these variations are discussed below.
a shee t-flood" units are lenticular; and form part of a
hete rogeneous clastic succession (Dranfield et al.
!987). According to Bentham et a!. (1993), braided
systems consisting of channel sandstone bodies
interbedded with significant proportions of floodplain fine-grained deposits constitute a distinctive
type of fl uvial assemblage that is much more important in the geological record than has hitherto been
recognized,
Many fluvial sand bodies are mapped and described as ribbons and may represent the fill of highor low-sinuosity channels. Examples are described
by Davies et a!. (1991) and Yinan et a!. (1987).
Hastings (1990) illustrated a complex of linear
sandbodies forming a low-sinuosity meander belt in
the Tyler Formatio n of North Dakota. Many may,
however, represent the amalgamation of point bars
or dther bar deposits along the channel trend, pro ...
ducing an overall ribbon geometry (Ebanks and
Weber 1982). Where sufficient well control is available, such ribbons may be broken down into their
individual bar units. Example· s of these various conditions are described later. Ribbon and lens reservoirs may form purely stratigraphic traps. The
ribbon shape of these sandstones should not be confused with the ribbon shape of many paleovalley-fill
sandstones, which are typically much wider and
thicker and represent the fill of entire, incised meander belts.
The Cherokee Group (Pennsylvanian) of Oklahoma, Kansas, and Missouri has provided many
classic examples of shoestring or ribbon sandstones.
Examples of producing sandstones in this group,
particularly the Bartlesville Sandstone (Desmoinesian), have been known since the 1920s and have
been illustrated in many classic textbooks (e.g.,
Levorsen 1967).
Another classic area of production from fields
consisting largely of stratigraphically trapped petroleum in ribbons and lenses is the Gulf Coast. Major
fluvial producers there are (a) the Upper Cretaceous
Lower Tuscaloosa Sandstone of southern Mississippi and Louisiana, which constitutes a transgressive, retrograde, fluvial�deltaic depositional systems
tract (Karges 1962; Berg and Cook 1968; Stancliffe
and Adams 1986; Hamilton and Cameron 1986;
Hamlin and Cameron 1987; Wiygul and Young 1987;
Hersch 1987; Klicman et a!. 1988; Hogg 1988; Miller
and Groth 1990i W erren et al. 1990; Garrison and
Chancellor 1991); (b) the Oligocene Frio Formation
of southwest Texas, another fluvial-deltaic system
485
which is an important gas producer (Galloway et al.
1982; Jirik 1990; Kerr 1990; Kerr and Jirik 1990); and
(c) the Lower Cretaceous Donovan/Sligo Formation
in Texas. The Citronelle field, which produces from
this unit, is an excellent example, illustrated later
(Eaves 1976).
Most of the producing zones in the Songliao and
Ordos basins, China, are lenticular or ribbon-shaped
in geometry; and a detailed knowledge of their subsurface distribution and facies is required for the
development of efficient production programs (Jin
et aL 1985; Moore et aL 1986; Yinan et al. 1987).
Likewise, fluvial-deltaic channel-and-bar sands of
the Oligocene Barail Sands of northeast India provide numerous ribbon-shaped and lenticular reservoir bodies in combined structural-stratigraphic
traps (Murty 1983; Desikachar 1984). Tertiary production in the Green River Basin (McDonald 1976)
also provides excellent examples of this type of reservoir geometry. It may be very difficult to produce
accurate maps of these ribbon sandstones. A tight
well spacing is required, as ot h erwise spurious correlations may result. This is discussed further below.
Examples of fields producing from ribbons an d
lenses are described in Sect. 15.4.
7 4.2.2.3 Stratigraphic Variations in Reservoir Geometry
In many fields the geometry of the reservoirs varies
stratigraphically, reflecting allogenic influences on
fluvial style or autogenic migration of the fluvial
system. For example, in South Belridge field, California, the Tulare Formation consists of a braid-delta
complex that prograded into a Jake. Lower Tulare
reservoir sands are lenticular or ribbon bodies deposited in meandering fluvial systems, whereas Upper Tulare sandstones were deposited in wider, more
proximal, braided channels and are more laterally
continuous (Miller et al. 1990). Similar stratigraphic
variations have been described in Prudhoe Bay and
Endicott fields, Alaska (Lawton et a!. 1987; Melvin
1993). In the Gullfaks field, in the Norwegian sector
of the East Shetlands Basin, the Statfjord Formation
is divided into three members comprising at least six
reservoir units, reflecting stratigraphic variations in
fluvial style (Petterson et al. 1990). In the nearby
Brent field, eight reservoir units have been mapped
in the Statfjord Formation (Struijk and Green 1991).
Sequence concepts which may explain these variations are discussed below.