DEPOSITIONAL SYSTEMS IN THE LOWER CRETACEOUS MORRO DO CHAVES AND COQUEIRO SECO FORMATIONS, AND THEIR RELATIONSHIP TO PETROLEUM ACCUMULATIONS, MIDDLE RIFT SEQUENCE, SERGIPE-ALAGOAS BASIN, BRAZIL

In the Sergipe-Alagoas Basin, northeast coast of Brazil, the Lower Cretaceous lacustrine, middle rift sequence is composed of the Morro do Chaves and Coqueiro Seco Formations. Subsurface analysis permitted recognition and mapping of four principal types of depositional environments: Morro do Chaves carbonate platform, and Coqueiro Seco fluvial-deltaic, fan delta, and slope systems.;Coqueiro Seco fluvial-deltaic and fan delta facies display high sand/shale ratios and blocky to massive E-log patterns; slope facies display serrate to digitate E-log patterns and is less sandy. Delta plain channel-fill facies and coarse-grained meanderbelt fluvial facies are dominant in fluvial-deltaic systems of the Alagoas Sub-basin; and proximal to medial conglomerates and coarse conglomeratic sandstones are dominant facies in fan delta systems of the Rio Sao Francisco Sub-basin. Slope facies are composed of sublacustrine fans formed by fine- to medium-grained sandstones enveloped by subbituminous shales, and lacustrine limestones. Coqueiro Seco coarse clastic systems prograded across the basin and burried Morro do Chaves carbonate platforms in response to cyclic tectonic pulses related to rift development.;Evaluation of petroleum occurrences in relationship to defined depositional systems permitted recognition of several types of plays characterized by unique structural and stratigraphic relationships exhibited by reservoirs, source beds and structure. The Coqueiro Seco slope play, formed by updip pinchout of turbidite fans, is judged the most promising in the sequence.;Morro do Chaves lacustrine carbonate sediments were deposited in positive areas flanking the principal point sources (rivers), and are composed of high energy limestones. Contemporaneous deep-water euxinic and bituminous lacustrine shales were deposited under starved basin conditions. Sublacustrine canyon excavation attested to the presence of a destructional slope episode.