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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Bathymetry of organic buildups may determine their reservoir potential. Whether a buildup was formed on a shelf, at a shelf edge, or downslope toward a potential source basin may determine distance of hydrocarbon migration, porosity and permeability history, and reservoir "sealing" events. Although there are physical paleobathymetric indicators, the most sensitive criteria are paleobiologic in nature. Our case study example involves several buildups in the Appalachian Ordovician. Depth was initially established on sedimentologic and stratigraphic bases; paleoecologic analysis of upslope versus downslope buildups led to generalizations which may apply to other Paleozoic buildups. Shelf, upslope, and downslope buildups have features in common such as abundant and diverse ec inoderms, gray to red mud-mounds, and abundant cross-bedded grainstones (although cross-beds are of different origin in shallow and deeper locales). Significant paleoecologic differences exist. Upslope and shelf buildup communities were dominated by echinoderms and arborescent bryozoa, with red and green algae. Some encrusting red algae were binders of mud-mounds. Endolithic borers were abundant. Downslope buildups were constructed by echinoderms, and encrusting bryozoa acted as binders of mud-mounds; algae are absent, and evidence of boring is rare.
Ecologic distinction between the settings may have resulted from differences in light intensity, abundance of suspended organic detritus in bottom waters (a resource derived from shelf areas), and the height above the sediment of suspended food. In shallow water, resuspended detritus allowed suspension feeding at many levels while deeper water conditions allowed suspension feeders, chiefly bryozoa, only very near the bottom.
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