Luncheons – November 5 and 19, 2010

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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: November 5, 2010

Abstract: Published in the November 2010 Outcrop, p. 20

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.

 

Talk: Lance Channel Sandstone Petrography

Speaker: Andrew Govert

Location: Denver Marriott City Center

Date: November 19, 2010

Abstract: Published in the November 2010 Outcrop, p. 21

The Pinedale Field currently produces natural gas from an approximately 5000-6000 foot (1524-1828 m) section of the Upper Cretaceous fluvial tight-gas sandstones, primarily the Lance Formation. Petrographic and whole rock geochemical analyses on samples from the channel facies in 7 well cores in the upper and lower Lance Formation were integrated with detailed core descriptions in order to examine the relationship between porosity, depositional faces and diagenetic history.

Burial depth and depositional facies influence porosity development in the Lance. Porosity decreases with increasing depth in the Lance formation. Porosity decreases upward through an individual channel sequence (facies L to U). Detrital grain composition influences porosity. Porosity is strongly influenced by the quantity of rigid cement and ductile grains. Porosity is best preserved when there is 12 to 22 %vol total cement and a rigid cement to ductile grain (RC/DG) ratio of 0.4 to 0.8. Early cement played a significant role by prescrving IGV and preventing the collapse of pore space. There is no indication that the type of cement (quartz or carbonate) is significant to the present-day porosity.

The cement type varies with depth. Cement is also facies-dependent, with the amount of cementation increasing from the L to the U facies. Quartz cement decreases with decreasing depth, and is the most volumetrically significant cement, averaging 8% of the rock volume. Two distinct generations of quartz cement were documented by SEM-CL, with the later generation more significant. Carbonate cement increases with decreasing depth.

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