AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Late Rift South Atlantic Pre-salt Carbonate Sequence Stratigraphy
(1) New Ventures, BHP Billiton Petroleum, Houston, TX.
The Pre-Salt Microbiolite Carbonate Reservoirs found in the Giant Santos Basin Tupi Field and in the Pre-Salt Fields of the Campos Basin have led to a world wide focus on these little understood reservoirs. Algal stromatolite reservoirs are not unknown in the geologic record and are more common than many explorationists realize. The Ellenberger in West Texas, the Khuff in the Middle East, and the Caspian Tengiz Field all have as a stratigraphic component, microbiolite reservoirs. Unlike these analogs, however, Brazilian microbiolite reservoirs result from a late stage of thermal rift subsidence and lacustrine fill which terminates in the deposition of hypersaline sediments and ultimately evaporate deposition.
The Brazilian late rift carbonate sequence begins with an early fresh-water lacustrine sequence and then grades upwards to a saline lacustrine sequence dominated by microbiolite deposition, followed by anhydrite and ultimately halite deposition. The early Coquinas sequence , a lithostratigraphic description, has freshwater gastropods and ostracodes. This freshwater sequence passes into a shallow water hypersaline sequence similar to the shallow water Wyoming Green River Lake Gosiute sequence. With the exception of a discrete region in the Santos Basin, where progrades suggest water depths as great as 200m, the seismic stratigraphy, coupled with biostratigraphy and chemostratigraphy argues for Pre-Salt microbiolite deposition within shallow hypersaline lakes.
Pre-Salt microbiolite reservoirs appear to occur on rift horst blocks with muddy carbonate deposition expected on the flanks of these paleo-highs. Microbiolite morphology across the Brazilian Pre-Salt Fields appears to reflect the position of the paleo-highs with respect to the inboard rift margin. Microbiolite morphology varies from the classic stromatolite morphology in the Santos basin to a brushy stromatolite morphology in the Campos Basin Salinity and amount of clastic influence appear to vary with the position of these fields within the overall rift setting.