Datapages, Inc.Print this page

Microbial Boundstone Slope Shedding—a Model for Carbonate Platform Growth


Kenter, Jeroen A. M.1, Paul (Mitch) Harris2, Giovanna Della Porta3 (1) Vrije Universiteit, Amsterdam, Netherlands (2) ChevronTexaco Energy Technology Company, San Ramon, CA (3) Universität Potsdam, Potsdam, Germany


Characteristics of two prograding steep, high-relief margins fronting deep basins pro­vide a depositional model which may apply elsewhere. Seismic and well data from Tengiz, one of the larger fields in the Pricaspian Basin characterized by Latest Visean and Serpukhovian progradation, corroborate outcrop patterns of Serpukhovian to Moscovian progradation in Asturias of northern Spain. These margins show progradation of up to 5 km and more than 10 km, respectively, despite the high-relief (up to 600 m) and their steep (~20-32°) nature.

Both examples share a highly productive microbial boundstone slope extending from the platform break to nearly 300 m (or more) depth and a lower slope dominated by (mega)breccias and grain flow deposits derived from the margin and slope itself. The broad depth range of microbial and cement boundstone “factory” increases the potential for pro­duction during both lowstands and highstands of sea level and thereby facilitates prograda­tion. Rapid in situ lithification of the boundstone provides stability to the steep slopes, but also leads to readjustment through shearing and avalanching. Remarkable observations are the contrasts with the Bahamian highstand shedding depositional model, little control by fluctuations in sea level or by paleo-wind directions due to their self nourishing nature, and the accretion rates of in-situ boundstone.

This new model of “slope” shedding has implications for slope readjustment, slope architecture, sequence stratigraphic models, reservoir characterization, and reservoir modeling, especially given that the isotropic character of microbial boundstone will reduce the potential for coherent seismic reflections to develop and possibly invoke, under certain stress regimes, shattering and fracturing thereby generating significant non­matrix permeability.