Abstract: Upper Cretaceous Bioturbated Fine-to-Medium Sands: a Problem for Deep-Water Exploration in Campos Basin, Offshore Brazil

Mario Carminatti, Leo Zimmermann, Ricardo Jahnert, Carlos Pontes

The presence of bioturbation in oil-prone turbidite deposits has detained a better exploration of upper Cretaceous siliciclastic reservoirs in Campos Basin. Bioturbated sandstones have degraded their permo-porosity with impact in oil production. They occur associated to unbioturbated sandstones with similar seismic amplitude, becoming difficult to separate them in seismic mapping. A comprehensive study in order to reduce the exploratory risk must consider firstly the recognition of genetic facies association through cores, and secondly the calibration of sonic well-logs and seismic velocity sections with rock data. This study deals with sedimentary facies association. The range of main facies in such reservoirs includes: (1) medium-to-coarse siliciclastic sandstone with c oss stratification; (2) fine-to-medium massive sandstone with thin traction carpets, bioturbated by opportunistic ichnofabrics and, (3) bioturbated, muddy, fine-to-medium quartz-feldspatic and glauconitic sandstone over 60 m thick.

The genetic facies associations suggest that the sandwich reservoirs were formed by high density turbidite currents deposited en masse or by thin traction carpets. The bioturbated sandstone was originated by reworking of bottom currentsrich in nutrients and oxygen. Detritic and biogenic glauconite covered and/or filled bioturbations indicate a basinward movement of the bottom currents. The successive alternation of high-energy with low-energy ichnofabrics reflects cyclic variations in current velocities.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela