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



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In the Sergipe-Alagoas Basin, along the northeast coast of Brazil, the lacustrine, middle rift sequence is composed of the Lower Cretaceous Morro do Chaves and Coqueiro Seco Formations. Subsurface analysis permitted recognition and mapping of four principal types of depositional systems that infilled the basin with more than 3,000 meters of clastic-carbonate sediments: Morro do Chaves carbonate platform, Coqueiro Seco fluvial-deltaic, Coqueiro Seco fan delta, and Coqueiro Seco slope systems. The generally poor quality of seismic profiles in this rift sequence precludes conventional seismic stratigraphic approaches. Morro do Chaves lacustrine carbonate platform sediments were deposited on shallow positive areas flanking the principal point sources (rivers), and are composed of massively bedded, high energy limestones. Contemporaneous with shallow-water sedimentation, deepwater euxinic and bituminous lacustrine shales were deposited under starved basin conditions. Sublacustrine canyon excavation attested to the presence of a destructional slope episode. Coqueiro Seco fluvial-deltaic, fan delta, and slope sediments are principally terrigenous. Fluvialdeltaic 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 are less sandy. Delta plain channel-fill facies and coarse-grained meanderbelt fluvial facies are dominant in fluvial-deltaic systems, and proximal to medial conglomerates and coarse conglomeratic sandstones are dominant facies in fan delta systems. Slope facies are composed of sublacustrine fans composed of fine-to medium-grained sandstones enveloped by thick, subbituminous shales, and thin, marly, lacustrine limestones. Coqueiro Seco clastic systems prograded across the basin and buried Morro do Chaves carbonate platforms in response to tectonic pulses related to rift development. Cyclic sedimentation occurred in the highly unstable Alagoas Sub-basin where fluvial-deltaic and slope systems are dominant, but fan delta and slope systems in the less complex Rio São Francisco Sub-basin do not exhibit cyclicity. Coqueiro Seco fluvial-deltaic, fan delta and slope sedimentation terminated because of continued basin subsidence and diminishing sediment supply as source areas were leveled. Consequently, the basin became the site of lacustrine shale deposition represented by the Ponta Verde Formation in the Alagoas Sub-basin. The rift sequence is truncated by a pre-Aptian unconformity in the Rio São Francisco Sub-basin. 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.