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2018 AAPG International Conference and Exhibition

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Brazil Santos Campos Widespread Chemogenic Lacustrine Carbonate Seismic Facies

Abstract

Exploration efforts began in the Campos Basin in 1968. This basin now produces over 1.7 MMBOEPD, with a significant contribution from the pre-salt play, baptized by Statoil as the “Perfect Play System”, with lacustrine enhanced reservoir quality carbonates growing on basement highs, locally sourced from underlying lacustrine shales and sealed by thick evaporites, allowing large accumulations which can be filled down to base salt spill. The Santos Basin has seen a similar success story with huge discoveries including the 8 BBO Lula Field. Extensive evaluation carried out by Paul Wright in the Santos Basin has indicated that the Aptian Barra Velha Formation is not a microbialite as has been cited in numerous publications. Facies and isotopic analyses as well as thermodynamic evidence all point to a shallow, connected and uniform lake model with a carbonate formed chemogenically by the precipitation of magnesium silicate. Despite ample evidence, this model has been faced with major opposition from seismic models which favour microbial differentiated platforms, similar to the Statoil model, with deep lake basins in between, due to significant structural relief at this level. In order to investigate the discrepancy between observations at micro and macro scales, a seismic facies analysis was carried out using a regional 2D seismic dataset covering the Santos Basin. The seismic sequence associated with the Barra Velha Formation was found to have a distinctive low frequency, relatively uniform, from high amplitude to semi opaque character. It was seen thinning onto structurally high areas and potentially missing at various structural culminations. It lies directly beneath the base of a thick evaporite sequence and at its base is usually marked by an unconformity which seems to truncate the sequence below. All these observations led to an improved understanding of the depositional model of the Barra Velha and how it relates to Paul Wright’s model, recently published in Search and Discovery. The major revelation came from flattening at the base of the evaporite sequence. The Barra Velha’s distinctive character was seen to be extensive and relatively uniform across both present day highs and lows. This is surprisingly in agreement with and largely supports the shallow water lake model. An extensive shallow water lake which can be mapped with seismic data has huge implications on the future exploration of this prolific play type.