Changes

==Grainstone shoals on shelves==

==Grainstone shoals on shelves==

Grainstone shoals form large elongate sheets that can extend for tens of kilometers in length ([[:File:M91FG196.JPG|Figure 1]]). They are commonly found on the seaward edges of banks, platforms, and shelves (Halley et al., 1983). The grainstone shoals are composed of sand-size grains, which can be skeletal or non-skeletal in origin. The latter includes ooids. Ooids are coated grains with a calcareous outer cortex and nuclei that are variable in composition.<ref name=TW1990 /> Oolites are rocks formed from ooids. Where oolites are relatively uncemented and not too deeply buried, they can form world-class productive intervals such as in the Jurassic Arab-D reservoirs of the Middle East. However, oolites can undergo cementation such that the interparticle volume is pervasively cemented, whereas the ooids dissolve out to form oomoldic porosity. The ooids are typically poorly connected. One example, described from the Upper Jurassic Smackover Formation in Arkansas and Louisiana, shows 30% porosity but only one millidarcy or less permeability (Halley et al., 1983). Grainstone shoals are known to accrete laterally as a series of shingled units that may be compartmentalized by muddy barriers.<ref name=Sneider2 /> Minor lateral heterogeneity occurs where tidal channels cut the ooid shoals.

Grainstone shoals form large elongate sheets that can extend for tens of kilometers in length ([[:File:M91FG196.JPG|Figure 1]]). They are commonly found on the seaward edges of banks, platforms, and shelves.<ref name=Halley>Halley, R. B., P. M. Harris and A. C. Hine, 1983, [http://archives.datapages.com/data/specpubs/carbona3/data/a043/a043/0001/0450/0463.htm Bank Margin], in P. A. Scholle, D. G. Bebout, and C. H. Moore, eds., Carbonate depositional environments: [http://store.aapg.org/detail.aspx?id=656 AAPG Memoir 33], p. 463–506.</ref> The grainstone shoals are composed of sand-size grains, which can be skeletal or non-skeletal in origin. The latter includes ooids. Ooids are coated grains with a calcareous outer cortex and nuclei that are variable in composition.<ref name=TW1990 /> Oolites are rocks formed from ooids. Where oolites are relatively uncemented and not too deeply buried, they can form world-class productive intervals such as in the Jurassic Arab-D reservoirs of the Middle East. However, oolites can undergo cementation such that the interparticle volume is pervasively cemented, whereas the ooids dissolve out to form oomoldic porosity. The ooids are typically poorly connected. One example, described from the Upper Jurassic Smackover Formation in Arkansas and Louisiana, shows 30% porosity but only one millidarcy or less permeability (Halley et al., 1983). Grainstone shoals are known to accrete laterally as a series of shingled units that may be compartmentalized by muddy barriers.<ref name=Sneider2 /> Minor lateral heterogeneity occurs where tidal channels cut the ooid shoals.

==Subtidal and intertidal complexes==

==Subtidal and intertidal complexes==