Changes

==Shoreface sands form layer-cake geometries==

==Shoreface sands form layer-cake geometries==

Shoreface sands prograde by lateral accretion with a tendency to produce layer-cake tabular geometries. Depositional dead ends are rare within individual shoreface sandstones, and sweep efficiencies are generally high as a result; for example, Tyler and Ambrose (1986) described excellent continuity and efficient simple sweep in the Carlson shoreface reservoir of the North Markham-North City Bay field of Texas. The large size and excellent lateral continuity of shoreface reservoirs give a reasonable chance that these systems will be in contact with an aquifer (Table 29).

Shoreface sands prograde by lateral accretion with a tendency to produce layer-cake tabular geometries. Depositional dead ends are rare within individual shoreface sandstones, and sweep efficiencies are generally high as a result; for example, Tyler and Ambrose<ref>Tyler, N., and W. A. Ambrose, 1986, Facies architecture and production characteristics of strand-plain reservoirs in North Markham-North Bay City field, Frio Formation, Texas: AAPG Bulletin, v. 70, no. 7, p. 809–829.</ref> described excellent continuity and efficient simple sweep in the Carlson shoreface reservoir of the North Markham-North City Bay field of Texas. The large size and excellent lateral continuity of shoreface reservoirs give a reasonable chance that these systems will be in contact with an aquifer (Table 29).

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Background deposition within the lagoon behind the barrier island is generally mud, but with some sand bodies present. These include washover fans and flood-tidal deltas ([[:File:M91FG192.JPG|Figure 7]]). Washover fans form when storms pitch sand over the barrier bar into the lagoonal area. Flood-tidal deltas develop as a result of the tidal movement of sand through an inlet into the lagoon. These lagoonal sand bodies are characterized by a pinch-out geometry into the lagoonal shales. The shales interfinger with the sandstones, commonly enveloping the sandstones and sometimes isolating them as discrete hydraulic units.

Background deposition within the lagoon behind the barrier island is generally mud, but with some sand bodies present. These include washover fans and flood-tidal deltas ([[:File:M91FG192.JPG|Figure 7]]). Washover fans form when storms pitch sand over the barrier bar into the lagoonal area. Flood-tidal deltas develop as a result of the tidal movement of sand through an inlet into the lagoon. These lagoonal sand bodies are characterized by a pinch-out geometry into the lagoonal shales. The shales interfinger with the sandstones, commonly enveloping the sandstones and sometimes isolating them as discrete hydraulic units.

Washover sandstone complexes may be rather patchy and laterally heterogeneous. Production from washover sandstones in the Glasscock reservoir of the West Ranch field in Texas is described by Galloway (1986). Waterflooding has proceeded irregularly with injected water preferentially flowing along the washover channels. As a result, the Glasscock reservoir has the lowest projected recovery factor (38%) of all the major intervals in the West Ranch field.

Washover sandstone complexes may be rather patchy and laterally heterogeneous. Production from washover sandstones in the Glasscock reservoir of the West Ranch field in Texas is described by Galloway (1986). Waterflooding has proceeded irregularly with injected water preferentially flowing along the washover channels. As a result, the Glasscock reservoir has the lowest projected recovery factor (38%) of all the major intervals in the West Ranch field.