Active Processes on a Mixed Clastic Carbonate Brazilian Shelf Margin: Importance for Hydrocarbon Exploration in Turbidites

CAINELLI, CESAR, Petrobras, Rio de Janeiro, Brazil

The search for subtle hydrocarbon accumulations in turbidite systems requires additional approaches for more successful

exploration, particularly when direct recognition on seismic lines is difficult. This includes the determination and understanding of processes controlling sand distribution on the shelf and the mapping of sand pathways from the shelf to the slope/basin that can guide efforts to look for more favorable sites for turbidite sandstone deposition. The approach can be exemplified in the Sergipe-Alagoas basin, on the Brazilian Atlantic passive margin. The section analyzed is the Piacabucu Formation, a thick seaward prograding wedge composed of coastal sandstones and shelf edge carbonates on a narrow shelf and slope-basin shales with turbidite lenses.

Waves and currents control the redistribution of sediments transported to the shelf by rivers. More wave energy is expended in ten hours in the Sao Francisco delta than in an entire year in the Mississippi delta. Such environment precludes deposition of mud on the shelf, but it stimulates the development of shelf edge carbonates. Rimmed carbonates along the shelf break serve as a barrier for downslope movements of coarse-grained sediment, where turbidites are oil targets. The search for gaps in the carbonate barrier which can tap the behind-barrier sands is critical for sand-rich turbidite development. It is believed that canyons create these gaps and act as active turbidity current routes. As an example, a large deep-sea fan is developing off the Sao Francisco river connected by a de ply incised canyon. Turbidite composition is dictated by how much canyon incision takes place. Carbonate-rich turbidites or debris flows would occur off young canyons whose indentation would only reach the upper slope or shelf edge. Sand-rich turbidites would occur off mature canyons that incise deeply into the shelf.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)