Luncheon – July 6, 2011

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Talk: Solution-Subsidence Control on Trends and Facies Architecture of Basal-Morrow Fluvial Valleys, Southwest Kansas

Speaker: Charles E. Bartberger, QEP Energy Company

Location: Denver Marriott City Center

Date: July 6, 2011

Description: Published in the July 2011 Outcrop

Lower-Pennsylvanian basal-Morrow valleys in southwest Kansas formed primarily by fluvial incision in response to relative lowering of sea level. Locally across a multi township region, however, solution subsidence influenced the trends of these valleys, facies architecture of valley-fill deposits, and location of hydrocarbon traps. Wireline logs, cores, and seismic data suggest that localized subsidence was caused by dissolution of anhydrite in the St. Louis Formation (Mississippian) 250 feet below the exposed Mississippian erosional surface. In some areas, subsidence occurred prior to major fluvial incision, creating isolated topographically low areas (sinkholes). Some of these sinkholes subsequently became linked together with valley segments created by fluvial incision resulting in a through-going basal-Morrow valley system. In other areas, much of the subsidence occurred later during transgression when valleys became backfilled with estuarine sediments. This syndepositional subsidence within existing incised valleys resulted in locally thick accumulations of valley fill consisting of fluvial/bayhead-delta and/or estuarine-barrier deposits. Subsidence-controlled variations in facies architecture of valley-fill deposits helped create complex traps for hydrocarbons in valley-fill reservoirs.