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 [[:File:M91FG117.JPG|Figure 6]]). | 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 [[:File:M91FG117.JPG|Figure 6]]). |