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Abstract: Sedimentological and Geochemical Evidence of High Frequency Climatic Cycles Controlling the Deposition of Source and Reservoir Rocks in a Lower Cretaceous Rift Lake, Sergipe-Alagoas Basin, Brazil

AZAMBUJA FILHO, NILO C. DE; L. M. ARIENTI and M. R. MELLO - PETROBRAS

Summary

Integrated study of 800 m of outcrop cores and subsurface mapping using 17 wells and seismic data from a Lower Cretaceous succession in the Sergipe-Alagoas basin revealed strong climatic and tectonic controls on the sedimentary facies distribution of two major Formations in the basin. The Aptian Maceio Fm. and the Lower Aptian Morro dos Chaves Member of the Coqueiro Seco Fm. were deposited in a lake environment during the proto-Atlantic rift phase. The thickness of these formations is highly variable due to local faulted blocks, and ranges from tens of meters up to hundreds of meters.

The rift lake had frequent climatic shifts from humid to dry. When the lake had a humid climate, fan deltas and turbidites formed, but when it was arid, organic-rich shales containing up to 17% of TOC were deposited. Algal mats and mudcracked horizons ended this cycle. On the other hand, the underlying Morro dos Chaves Member has a cycle that begins with a fan delta that is transgressed by important coquinas interbedded with black shales that contains up to 8% TOC. The tops of many of these shales have mudcracks filled by either coquina or pebbles of the overlying cycle.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah