Changes

Shoreface sands are deposited along shorelines, and they generally form extensive, high-quality reservoir systems ([[Figure 191]]). Wave action and occasional storms act to deposit sand along the shoreface. The lower shoreface lies below fair-weather wave base but can be affected by storms; the sands tend to be siltier and more poorly sorted by comparison to the upper shoreface, where the sands have been subjected to wave winnowing. A shoreface deposit separated by a lagoon from the land is known as a barrier island.

Shoreface sands are deposited along shorelines, and they generally form extensive, high-quality reservoir systems ([[Figure 191]]). Wave action and occasional storms act to deposit sand along the shoreface. The lower shoreface lies below fair-weather wave base but can be affected by storms; the sands tend to be siltier and more poorly sorted by comparison to the upper shoreface, where the sands have been subjected to wave winnowing. A shoreface deposit separated by a lagoon from the land is known as a barrier island.

[[FIGURE 191.]] The photograph shows a shoreface profile on St. Cyrus Beach, Scotland. The beach is just over a hundred meters wide. Reservoir properties are influenced by the degree of wave reworking up the shoreface profile. Lower figure from McCubbin (1982). Reprinted with permission from the AAPG.

[[File:M91FG191|thumb|300px|{{figure number{{1}}|The photograph shows a shoreface profile on St. Cyrus Beach, Scotland. The beach is just over a hundred meters wide. Reservoir properties are influenced by the degree of wave reworking up the shoreface profile. Lower figure from McCubbin (1982). Reprinted with permission from the AAPG.]]

==Shoreface sands form layer-cake geometries==

==Shoreface sands form layer-cake geometries==

==Parasequences and parasequence sets==

==Parasequences and parasequence sets==

Shoreface sandstones commonly form parasequence sets (see Chapter 10, this publication). An individual parasequence can comprise a series of facies belts showing a progression from coastal to offshore sediments. For example, in the Scott field in the UK North Sea, back barrier, foreshore, upper shoreface, and lower shoreface facies belts can be mapped out in the Upper Piper Sandstone Member (see [[Figure 73]]). (Guscott et al., 2003). An analysis of parasequence stacking patterns can help the geologist to predict and map facies belts in the areas beyond the well control. For example, Spaak et al. (1999) used stacking analysis on the Jurassic shoreface sediments of the Fulmar field, UK North Sea, to help construct the depositional scheme for the reservoir.

Shoreface sandstones commonly form parasequence sets (see Chapter 10, this publication). An individual parasequence can comprise a series of facies belts showing a progression from coastal to offshore sediments. For example, in the Scott field in the UK North Sea, back barrier, foreshore, upper shoreface, and lower shoreface facies belts can be mapped out in the Upper Piper Sandstone Member (see [[Figure 73]]). (Guscott et al., 2003). An analysis of parasequence stacking patterns can help the geologist to predict and map facies belts in the areas beyond the well control. For example, Spaak et al. (1999) used stacking analysis on the Jurassic shoreface sediments of the Fulmar field, UK North Sea, to help construct the depositional scheme for the reservoir.

The basal section of individual parasequences is defined by a flooding surface that is commonly a marine shale. Shales can isolate individual parasequence shoreface cycles vertically, and they can be laterally extensive for several hundreds of meters or more. Fluid flow communication may occur between parasequences where the shales are absent as a result of erosion or nondeposition. It can be useful to produce vertical flow barrier maps for parasequence boundaries (see [[Figure 102]]). The localized presence or absence of bounding shales can be a critical feature in the flow geology characterization of a shoreface reservoir (Larue and Legarre, 2004).

The basal section of individual parasequences is defined by a flooding surface that is commonly a marine shale. Shales can isolate individual parasequence shoreface cycles vertically, and they can be laterally extensive for several hundreds of meters or more. Fluid flow communication may occur between parasequences where the shales are absent as a result of erosion or nondeposition. It can be useful to produce vertical flow barrier maps for parasequence boundaries (see [[Figure 102]]). The localized presence or absence of bounding shales can be a critical feature in the flow geology characterization of a shoreface reservoir (Larue and Legarre, 2004).

==Vertical permeability profiles in shoreface sandstones==

==Vertical permeability profiles in shoreface sandstones==

Shoreface sandstones characteristically show upward-increasing permeability profiles. This in turn reflects increasing grain size and better sorting higher up the shoreface profile (see also [[Figures 42]], [[108]]). A contrast in rock properties is characteristically seen between the lower and upper shoreface intervals. Upper shoreface beach facies associations generally show higher permeabilities than lower shoreface sediments. When a shoreface sand is subjected to a waterflood, the water tends to edge ahead through the high-permeability tops of these cycles by viscous forces. Gravity and capillary action will then draw the water down through the shoreface cycle into the lower units, displacing oil upward. Sweep efficiencies can be high as a result.

Shoreface sandstones characteristically show upward-increasing permeability profiles. This in turn reflects increasing grain size and better sorting higher up the shoreface profile (see also [[Figures 42]], [[108]]). A contrast in rock properties is characteristically seen between the lower and upper shoreface intervals. Upper shoreface beach facies associations generally show higher permeabilities than lower shoreface sediments. When a shoreface sand is subjected to a waterflood, the water tends to edge ahead through the high-permeability tops of these cycles by viscous forces. Gravity and capillary action will then draw the water down through the shoreface cycle into the lower units, displacing oil upward. Sweep efficiencies can be high as a result.

==Beach sandstones==

==Beach sandstones==

Beach sandstones form as single belts or accrete laterally to form strand plains many kilometers long and several kilometers wide. Modern strand plains such as the Nayarit strand plain of western Mexico show a ridge and swale topography on their surfaces (McCubbin, 1982). Mud fills in the interridge swales can act as permeability barriers to lateral flow in the subsurface. One example in the Frio Formation of south Texas is known to have caused the compartmentalization of a beach ridge interval containing several million barrels of recoverable oil (Reistroffer and Tyler, 1991). Water ingress may preferentially occur along the low-lying swales (Tyler and Ambrose, 1986) (see also [[Figure 117]]).

Beach sandstones form as single belts or accrete laterally to form strand plains many kilometers long and several kilometers wide. Modern strand plains such as the Nayarit strand plain of western Mexico show a ridge and swale topography on their surfaces (McCubbin, 1982). Mud fills in the interridge swales can act as permeability barriers to lateral flow in the subsurface. One example in the Frio Formation of south Texas is known to have caused the compartmentalization of a beach ridge interval containing several million barrels of recoverable oil (Reistroffer and Tyler, 1991). Water ingress may preferentially occur along the low-lying swales (Tyler and Ambrose, 1986) (see also [[Figure 117]]).

==Tidal channels==

==Tidal channels==

Barrier islands form thick, well-sorted sand bodies with a tabular geometry ([[Figure 192]]). They typically comprise a composite of beach, dune, and upper shoreface sandstones (Galloway, 1986). Barrier islands can be continuous for tens of kilometers along strike but may only be a few kilometers wide. Local heterogeneity can be provided by tidal channel inlet deposits. These form crosscutting lenticular pods, disrupting the layer-cake continuity of the barrier island body. Recent barrier island sediments on the South Carolina coast provide a modern analog and are described in detail by Sexton and Hayes (1996).

Barrier islands form thick, well-sorted sand bodies with a tabular geometry ([[Figure 192]]). They typically comprise a composite of beach, dune, and upper shoreface sandstones (Galloway, 1986). Barrier islands can be continuous for tens of kilometers along strike but may only be a few kilometers wide. Local heterogeneity can be provided by tidal channel inlet deposits. These form crosscutting lenticular pods, disrupting the layer-cake continuity of the barrier island body. Recent barrier island sediments on the South Carolina coast provide a modern analog and are described in detail by Sexton and Hayes (1996).

[[FIGURE 192.]] Generalized map and cross sections showing major environments and facies associations of a barrier island-lagoonal system (from McCubbin, 1982). Reprinted with permission from the AAPG.

[[File:M91FG192.JPG|thumb|300px|192. Generalized map and cross sections showing major environments and facies associations of a barrier island-lagoonal system (from McCubbin, 1982). Reprinted with permission from the AAPG.]]

Ambrose et al. (1997) gave an example from an oil field in Venezuela where sweep has resulted from preferential water encroachment along the sandstone-rich core of the barrier island depositional axis with bypassed oil remaining along the landward pinch-out edge.

Ambrose et al. (1997) gave an example from an oil field in Venezuela where sweep has resulted from preferential water encroachment along the sandstone-rich core of the barrier island depositional axis with bypassed oil remaining along the landward pinch-out edge.