Changes

==What outcrops of meander belt sediments indicate about connectivity==

==What outcrops of meander belt sediments indicate about connectivity==

Well-exposed outcrops of meander belt sediments can be used to get an understanding of the reservoir connectivity in three dimensions. Connectivity can result from the vertical incision of one point bar into an older underlying point bar, creating multistory sand bodies (Figure 176f). Point bars commonly connect with each other across the shallow, sandy course of the river where the tips of point bars overlap on opposite banks. Crossovers like this can create a connected system of point bars which Donselaar and Overeem (2008) describe as a string-of-beads sandstone body. Crevasse splays may also link up one point bar with another.

Well-exposed outcrops of meander belt sediments can be used to get an understanding of the reservoir connectivity in three dimensions. Connectivity can result from the vertical incision of one point bar into an older underlying point bar, creating multistory sand bodies ([[:file:M91FG176.JPG|Figure 5f]]). Point bars commonly connect with each other across the shallow, sandy course of the river where the tips of point bars overlap on opposite banks. Crossovers like this can create a connected system of point bars which Donselaar and Overeem<ref name=Donselaarandovereem_2008 /> describe as a string-of-beads sandstone body. Crevasse splays may also link up one point bar with another.

Nevertheless, it probably does not take much for the connectivity between the various sand bodies in a meander belt to be disrupted. The connections between the various macroforms are likely to be through apertures of limited cross-sectional area such as erosional windows, crossovers, and crevasse splay-point bar intersections. Carbonate cementation of the basal lag by circulating groundwater can create permeability barriers at the base of individual point bars. Precipitation of carbonate cement may be accentuated where calcrete fragments form part of the basal lag (Mckie and Audretsch, 2005).

Nevertheless, it probably does not take much for the connectivity between the various sand bodies in a meander belt to be disrupted. The connections between the various macroforms are likely to be through apertures of limited cross-sectional area such as erosional windows, crossovers, and crevasse splay-point bar intersections. Carbonate cementation of the basal lag by circulating groundwater can create permeability barriers at the base of individual point bars. Precipitation of carbonate cement may be accentuated where calcrete fragments form part of the basal lag.<ref name=Mckieandaudretsch_2005 />

Abundant mud chips caked along the base of point bars have the potential to attenuate communication. For example, Chapin and Mayer (1991) found that the vertical connectivity between stacked point bars was severely impeded by mudstone-rich lags at the base of individual point bars in the reservoir of the Peoria field in Colorado. Doyle and Sweet (1995) found that mudclast lags at the base of point bar sandstones have a patchy distribution in the Gypsy Sandstone of Northern Oklahoma. They consider them more likely to form baffles to flow instead of continuous barriers. Shanley (2004) noted that where basal lags contain abundant mudclasts, they can be mistaken for shales on the gamma-ray log. Caution should be taken where a shale-like wireline log response is seen within thick multistory fluvial sandstones.

Abundant mud chips caked along the base of point bars have the potential to attenuate communication. For example, Chapin and Mayer<ref name=Chapinandmayer_1991 /> found that the vertical connectivity between stacked point bars was severely impeded by mudstone-rich lags at the base of individual point bars in the reservoir of the Peoria field in Colorado. Doyle and Sweet<ref name=Doyleandsweet_1995 /> found that mudclast lags at the base of point bar sandstones have a patchy distribution in the Gypsy Sandstone of Northern Oklahoma. They consider them more likely to form baffles to flow instead of continuous barriers. Shanley<ref name=Shanley_2004 /> noted that where basal lags contain abundant mudclasts, they can be mistaken for shales on the gamma-ray log. Caution should be taken where a shale-like wireline log response is seen within thick multistory fluvial sandstones.

Coals commonly act as significant flow barriers where they occur in more humid fluvial systems. The precursor peat deposits to coal occur as thick mats of flexible and intertwined plant material and these can withstand strong erosive forces to stay substantially intact.

Coals commonly act as significant flow barriers where they occur in more humid fluvial systems. The precursor peat deposits to coal occur as thick mats of flexible and intertwined plant material and these can withstand strong erosive forces to stay substantially intact.

 

 

==See also==

==See also==