Paleoenvironment, Cyclicity, and Reservoir Quality of Chaetetid ÒReefÓ Mounds of the Amoret Limestone Member (Upper Desmoinesian) in Southeastern Kansas

Michael Lichtenwalter and C. D. Burke. Wichita State University, Department of Geology

Wichita, KS 67260-0027  [email protected]

Chaetetes is an extinct demosponge that was prolific in Kansas throughout the Desmoinesian (Middle Pennsylvanian). Chaetetid-bearing beds, exposed in southeastern Kansas, are only reported within the Marmaton Group and include the Altamont, Pawnee, and Fort Scott Limestones. Rock characteristics of the Amoret Limestone Member of the Altamont Limestone were analyzed for potential depositional cyclicity and reservoir quality. 

Characteristics in the Amoret Limestone Member exposure suggest two cycles of chaetetid ÒreefÓ development. Each cycle is composed of a lowermost phylloidal algae unit overlain by a well defined chaetetid unit. Between each cycle is a characteristic growth-disruption surface of chaetetid colonies. The bounding surfaces of the Altamont Limestone Member are represented by subaerial exposure, which may result in subunconformity traps. 

The cyclicity during the Middle Pennsylvanian allowed for excellent chaetetid growth. Chaetetes lived in within the photic zone of a shallow tropical sea of normal salinity below wave base, where occasional storm waves may have disrupted the sea floor. The top of the Amoret suggests a period of rapid sea-level rise as indicated by presence of deeper-water marine organisms and an overlying black shale. Chaetetid colonies in this unit have good to excellent secondary porosity; however, the lower matrix porosity results in marginal reservoir quality. 

Depositional cyclicity within the Amoret Member may have resulted from rapid eustatic sea-level fluctuations. Periodic influxes of sediment during lowstands probably stifled chaetetid growth in the lowermost Amoret, whereas the uppermost chaetetids may have been out-competed as sea level rose. Based on this site, prediction of reservoir location and quality may be delineated by depositional cyclicity based on lithologic characteristics.