Abstract: Geometry and High-Resolution Stratigraphy of Aptian Lacustrine Turbidites - Alagoas Rift Basin, Brazil

Arienti, L. M. - Petrobras/E&P

The Aptian rift lacustrine succession from the Maceió Formation, Alagoas Basin, northeastern Brazil, was analyzed by comparing outcrop information with data from 16 wells of the Tabuleiro dos Martins oil field (logs and 718 m of cores). Facies and facies associations allowed to define four main types of deposits: fandeltas (closely related to a northeast border fault and axial fandeltas), turbidite channel/overbank (outcrops), turbidite lobe/interlobe deposits and shallow lacustrine deposits (wells).

High-resolution stratigraphy was applied to identify the main stratigraphic sequences and map their lateral and vertical continuity. The turbidite succession was divided into three 4^th order sequences: Alpha, Beta and Gamma. The Alpha Sequence (180 m thick) was divided into six 5^th order sequences, the Beta Sequence (150 m thick) into four 5^th order sequences and the Gamma Sequence (400 m thick) into eight 5^th order sequences.

Net sand maps of 5^th order sequences were used to understand the distribution of sandstone bodies across the oil field. The geometry and lateral continuity of channel and lobe deposits were compared and the internal heterogeneities, such as overbank, interlobe deposits and shale beds were mapped.

Turbidites are cyclically accumulated during high lake levels, as a consequence of continental flooding and concomitant high siliciclastic supply into the basin. The turbidites show dispersed coal debris and shales enriched in terrestrial organic matter. Low lake levels are represented by organic-rich black shales (algalic TOC up to 17 %) with mudcracks, algal mats and calcilutites. These deposits were controlled by high salinity and anoxic conditions of the shallow lake, occurring associated with evaporites.

Tectonic activity was inferred to be the main factor controlling the occurrence of 3^rd order sequences, whereas climate, since the periodic changing of humidity affected sediment influx, was inferred to be the prime control on the development of 4^th and 5^th order sequences.

The sequence stratigraphic exercise performed in a rift context shows how difficult is the application of such concepts to a basin where climatic and tectonic controls are complexely interrrelated. Nevertheless, this study has showed that an integrated approach using outcrops and subsurface data is a powerful tool to quantify and calibrate the geometry and continuity of continental-rift deposits.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil