Changes

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 [[:File:M91Ch6FG42.JPG|Figures 4]] and [[:File:M91FG108.JPG|5]]). 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 [[:File:M91Ch6FG42.JPG|Figures 4]] and [[:File:M91FG108.JPG|5]]). 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.

The degree to which the lower part of the shoreface is swept by water will depend on the magnitude of the vertical permeability within the lower shoreface. In the Middle Jurassic Brent Province of the UK North Sea, bypassed oil is often found within the lower shoreface facies association (Rannoch Formation). The overlying upper shoreface (Etive Formation) is typically an interval of water overrun.<ref>Thomas, J. M. D., and R. Bibby, 1991, The depletion of the Rannoch-Etive sand unit in Brent sands reservoirs in the North Sea, in W. Linville, ed., Reservoir characterization III: Tulsa, PennWell Books, 1008 p.</ref> This behavior can be reinforced by a zone of mica concentration at the top of the lower shoreface, which acts as a baffle to vertical flow (Wetzelaer et al., 1996). Horizontal wells have been drilled in several Brent Province fields to target bypassed oil in the lower shoreface (Braithwaite et al., 1989; Black et al., 1999). The success of these wells depends on the presence of low vertical permeabilities at the top of the lower shoreface interval in order to prevent water coning down from the swept upper shoreface interval.

The degree to which the lower part of the shoreface is swept by water will depend on the magnitude of the vertical permeability within the lower shoreface. In the Middle Jurassic Brent Province of the UK North Sea, bypassed oil is often found within the lower shoreface facies association (Rannoch Formation). The overlying upper shoreface (Etive Formation) is typically an interval of water overrun.<ref>Thomas, J. M. D., and R. Bibby, 1991, The depletion of the Rannoch-Etive sand unit in Brent sands reservoirs in the North Sea, in W. Linville, ed., Reservoir characterization III: Tulsa, PennWell Books, 1008 p.</ref> This behavior can be reinforced by a zone of mica concentration at the top of the lower shoreface, which acts as a baffle to vertical flow (Wetzelaer et al., 1996). Horizontal wells have been drilled in several Brent Province fields to target bypassed oil in the lower shore face.<ref>Braithwaite, C. I. M., J. D. Marshall, and T. C. Holland, 1989, Improving recovery from the Dunlin field, U.K. northern North Sea: Presented at the 54th Annual Technical Conference of the Society of Petroleum Engineers, San Antonio, Texas, SPE Paper 19878, 18 p.</ref><ref>Black, R. C., H. J. Poelen, M. J. Roberts, and S. E. Roddy, 1999, Tern field development: A marriage of new technologies for business benefit, in A. J. Fleet and S. A. R. Boldy, eds., Petroleum geology of northwest Europe: Proceedings of the 5th Conference, Geological Society (London), p. 1063–1073.</ref> The success of these wells depends on the presence of low vertical permeabilities at the top of the lower shoreface interval in order to prevent water coning down from the swept upper shoreface interval.

Shallow marine sandstones can contain extensive stratiform carbonate-cemented bands and nodules. Taylor et al. (1995) found widespread cemented intervals below major flooding surfaces in the Upper Cretaceous Blackhawk Formation in Utah. The reduced sedimentation rates associated with these surfaces are thought to have allowed more time for cements to form. The cemented horizons have the potential to form baffles.

 

Shallow marine sandstones can contain extensive stratiform carbonate-cemented bands and nodules. Taylor et al.<ref>Taylor, K. G., R. L. Gawthorpe, and J. C. Van Wagoner, 1995, Stratigraphic control on laterally persistent cementation, Book Cliffs, Utah: Journal of the Geological Society, v. 152, p. 225–228.</ref> found widespread cemented intervals below major flooding surfaces in the Upper Cretaceous Blackhawk Formation in Utah. The reduced sedimentation rates associated with these surfaces are thought to have allowed more time for cements to form. The cemented horizons have the potential to form baffles.

==Beach sandstones==

==Beach sandstones==