484
Stratigraphic and Tectonic Controls on the Distribution and Architecture of FlUVial Oil and Gas Reservoirs
formed during glacial lowstands of sea level (Figs.
14.3, 14.4).
Several important petroleum plays in the Cordilleran foreland basin produce from paleovalleys incised into regional unconformities generated during
the Early Cretaceous (Mannville fields, Alberta;
incised valley fills of Lower jurassic age are important reservoirs in the Ordas Basin of China. There is
much ongoing debate about whether the unconformities in the Western Interior Basin are tectonic
or eustatic in origin (Weimer 1986; Dolson et al.
1991).
14.2.2 Geometry of Reservoir Bodies
The second criterion for the classification of fluvial
petroleum reservoirs is that of the geometry of the
reservoir body. Two similar classifi cations have been
used. Friend ( 1983) suggested that fl uvial sandstones
occur primarily in two modes, the laterally extensive
sheet and the more laterally restricted ribbon or lens.
Galloway (1981) adopted the geomorphic classifi cation of Schumm (1977) in his review of Gulf Coast
fluvial systems, and this approach has been used
by several other workers (e.g., Davies et al. 1991,
1993). The classifi cation suggests three architectural
modes, (a) multilateral channel deposits formed by
bedload rivers, with high net-to-gross sandstone ratios and good interconnectedness; (b) multistory,
mixed sandstone-shale deposits with intermediate
values for interconnectedness, formed in mixedload fl uvial systems; and (c) multistory ribbon
bodies with low net-to-gross ratios and poor
interconnectedness, formed in suspended-load rivers. The first and third types are similar to the two
classes of Friend (1983), which are the subdivisions
used here.
14.2.2.1 Sheet Sandstones
Deposits of this type are commonly formed by steepgradient) bed-load systems such as braided rivers,
where channels comb across broad areas of the valley floor. Sheet sandstones typically form laterally
extensive reservoir bodies that may require a structural component to generate a petroleum trap. Excellent examples are parts of the Permian-Triassic
Sadlerochit Group of Prudhoe Bay, Alaska (Lawton
et al. 1987; Atkinson et al. 1990), the braided-fluvial
zone 2 of the Kekiktuk Formation in Endicott field,
Alaska (Melvin 1993), the Lower Cretaceous Sarir
Sandstone of Libya (Clifford et al. 1980), the
unnamed Cambtian sandstone of the Oued Mya basin, Algeria (Balducchi and Pommier 1970), the
Sherwood sandstone (Triassic) at South Morecambe
field in the Irish Sea (Stuart and Cowan 1991; G.
Cowan 1993), and many of the Hutton Sandstone
fields of Eromanga Basin, Australia (Moore et al.
1986; Wecker 1989). In most of these examples broad
sheets are limited at their margins by lateral
pinchout or channel incision. The Peco oilfield of
Alberta produces from a single braided channel
complex 20 m deep and up to 3 km wide enclosed in
impermeable floodplain deposits (Gardiner et al.
1990; Putnam 1993). The Travis Peak formation of
East Texas includes several separate channel belts up
to 10 km wide (Tye 1991).
Sheet sandstones can also accumulate where
lower-slope and lower-energy rivers, such as some
meandering systems, develop in areas of low subsidence rate, so that ch�nnel deposits are incised into
and superimposed upon each other and the preservation potential of fine-grained floodplain deposits
is low (Miall 1980). Examples include at least some of
the Permian fluVial reServoirs of Cooper Basin, Australia (Stuart et al. 1988; Stanmore and Johnstone
1988).
Slightly different is the style of the lake-margin
fluvial reservoirs in the Green River Formation of
Red Wash field, Uinta Basin. Here, individual producing sandstones are extensive, but much less so
than the area of the entire field. Isopachs of the
producing sandstones suggest that they include thin,
fluvially dominated, deltaic sheets (Chatfield 1972;
Castle 1990). Examples of fields producing from
sheet-like reservoir bodies are described in Sect.
15.3.
14.2.2.2 Sandstone Ribbons and Lenses
These form by the accumulation of sandstone bodies
within channel systems that are isolated within the
fine-grained sediments of the overbank environment. Point bars are the best known of such deposits;
they form distinctive lens-shaped reservoir bodies,
many of which preserve the fme-grained fill of the
abandoned channel around the margin of the lens
(Berg 1968; Busch 1974; Werren et al. 1990; Davies et
al. 1991, 1993; Chapin and Mayer 1991). Not all
braided fluvial systems form sandstones with a sheet
geometry. In Wytch Farm oilfield, England, sandstones interpreted as braided channel deposits and
Stratigraphic and Tectonic Controls on the Distribution and Architecture of FlUVial Oil and Gas Reservoirs
formed during glacial lowstands of sea level (Figs.
14.3, 14.4).
Several important petroleum plays in the Cordilleran foreland basin produce from paleovalleys incised into regional unconformities generated during
the Early Cretaceous (Mannville fields, Alberta;
incised valley fills of Lower jurassic age are important reservoirs in the Ordas Basin of China. There is
much ongoing debate about whether the unconformities in the Western Interior Basin are tectonic
or eustatic in origin (Weimer 1986; Dolson et al.
1991).
14.2.2 Geometry of Reservoir Bodies
The second criterion for the classification of fluvial
petroleum reservoirs is that of the geometry of the
reservoir body. Two similar classifi cations have been
used. Friend ( 1983) suggested that fl uvial sandstones
occur primarily in two modes, the laterally extensive
sheet and the more laterally restricted ribbon or lens.
Galloway (1981) adopted the geomorphic classifi cation of Schumm (1977) in his review of Gulf Coast
fluvial systems, and this approach has been used
by several other workers (e.g., Davies et al. 1991,
1993). The classifi cation suggests three architectural
modes, (a) multilateral channel deposits formed by
bedload rivers, with high net-to-gross sandstone ratios and good interconnectedness; (b) multistory,
mixed sandstone-shale deposits with intermediate
values for interconnectedness, formed in mixedload fl uvial systems; and (c) multistory ribbon
bodies with low net-to-gross ratios and poor
interconnectedness, formed in suspended-load rivers. The first and third types are similar to the two
classes of Friend (1983), which are the subdivisions
used here.
14.2.2.1 Sheet Sandstones
Deposits of this type are commonly formed by steepgradient) bed-load systems such as braided rivers,
where channels comb across broad areas of the valley floor. Sheet sandstones typically form laterally
extensive reservoir bodies that may require a structural component to generate a petroleum trap. Excellent examples are parts of the Permian-Triassic
Sadlerochit Group of Prudhoe Bay, Alaska (Lawton
et al. 1987; Atkinson et al. 1990), the braided-fluvial
zone 2 of the Kekiktuk Formation in Endicott field,
Alaska (Melvin 1993), the Lower Cretaceous Sarir
Sandstone of Libya (Clifford et al. 1980), the
unnamed Cambtian sandstone of the Oued Mya basin, Algeria (Balducchi and Pommier 1970), the
Sherwood sandstone (Triassic) at South Morecambe
field in the Irish Sea (Stuart and Cowan 1991; G.
Cowan 1993), and many of the Hutton Sandstone
fields of Eromanga Basin, Australia (Moore et al.
1986; Wecker 1989). In most of these examples broad
sheets are limited at their margins by lateral
pinchout or channel incision. The Peco oilfield of
Alberta produces from a single braided channel
complex 20 m deep and up to 3 km wide enclosed in
impermeable floodplain deposits (Gardiner et al.
1990; Putnam 1993). The Travis Peak formation of
East Texas includes several separate channel belts up
to 10 km wide (Tye 1991).
Sheet sandstones can also accumulate where
lower-slope and lower-energy rivers, such as some
meandering systems, develop in areas of low subsidence rate, so that ch�nnel deposits are incised into
and superimposed upon each other and the preservation potential of fine-grained floodplain deposits
is low (Miall 1980). Examples include at least some of
the Permian fluVial reServoirs of Cooper Basin, Australia (Stuart et al. 1988; Stanmore and Johnstone
1988).
Slightly different is the style of the lake-margin
fluvial reservoirs in the Green River Formation of
Red Wash field, Uinta Basin. Here, individual producing sandstones are extensive, but much less so
than the area of the entire field. Isopachs of the
producing sandstones suggest that they include thin,
fluvially dominated, deltaic sheets (Chatfield 1972;
Castle 1990). Examples of fields producing from
sheet-like reservoir bodies are described in Sect.
15.3.
14.2.2.2 Sandstone Ribbons and Lenses
These form by the accumulation of sandstone bodies
within channel systems that are isolated within the
fine-grained sediments of the overbank environment. Point bars are the best known of such deposits;
they form distinctive lens-shaped reservoir bodies,
many of which preserve the fme-grained fill of the
abandoned channel around the margin of the lens
(Berg 1968; Busch 1974; Werren et al. 1990; Davies et
al. 1991, 1993; Chapin and Mayer 1991). Not all
braided fluvial systems form sandstones with a sheet
geometry. In Wytch Farm oilfield, England, sandstones interpreted as braided channel deposits and