Paul Wright shares his insights on the pre-salt “Microbialite” reservoirs of the South Atlantic.
The Cretaceous Aptian Barra Velha Formation of the Santos Basin (offshore Brazil), often referred to as “Microbialite” reservoirs, has hosted over 30 discoveries, with recoverable reserves estimated as > 60 BBOE. This limestone unit, up to 550m thick, with equivalents in other offshore South Atlantic basins, is now considered perhaps the largest chemogenic (chemically formed, not microbial) carbonate deposystem in Earth history, covering at least a third of a million square kilometers. Besides having no modern or ancient analogues, much of the porosity is the result of the dissolution of magnesium clays.
Two opposing views are held as to where these carbonates formed.
One view, based on sedimentological and geochemical evidence, has interpreted the reservoirs as having been deposited in hyper-alkaline shallow evaporitic lakes, affected by some syn-depositional tectonism, but later significantly affected by post-depositional deformation immediately before, during and after salt deposition. This model interprets the local relief on the top of the reservoir of often 1km or more, as structural in origin, with age-equivalent carbonates in down-thrown areas as being of the same facies.
The other model interprets the relief as reflecting the formation of the reservoir carbonates as isolated carbonate build-ups separated by deep lake deposits likely lacking reservoir-prone facies.
A consequence of this second model is that platforms are regarded as areally differentiated with various companies populating reservoir models with different facies assemblages. In contrast, in the shallow lake model, individual facies are envisaged as being laterally very extensive and layer cake. The crux of the controversy seems to be to what extent seismic geometries should be interpreted as expressions of sedimentological features, versus where a detailed structural analysis, linked closely to detailed sedimentological and geochemical analyses, has been carried out. The implications for exploration and reservoir development are enormous.
Discussion of this topic will feature prominently in the GeoLogica field course – Modern and Ancient Carbonate Lakes of the Western U.S.: Lessons for Interpreting the Cretaceous Pre-Salt Reservoirs in the South Atlantic (G030) 02 – 05 November, 2020. Paul’s other upcoming GeoLogica courses include: De-risking Carbonate Exploration (G008) Houston, 15 – 18 June, 2020, and Fundamentals of Carbonate Depositional and Diagenetic Systems Field Seminar: Lessons from the Permian Basin (G007) 8 – 13 November, 2020 (co-led with Kate Giles).
Paul’s recent work has included various publications and workshops relating to the pre-salt of the South Atlantic as well as the investigation of facies stacking in Cretaceous hydrocarbon-bearing intra-platformal basins.
Recently published articles include a study of reservoir architecture in the super giant Karachaganak field in Kazakhstan (with Simon Beavington-Penney, Stuart Kennedy and Mark Covil): 2019 Integration of static and dynamic data and high-resolution sequence stratigraphy to define reservoir architecture and flow units within a ‘super giant’ gas condensate and oil field, Kazakhstan. Marine and Petroleum Geology 101 (2019) 486–501. doi.org/10.1016/j.marpetgeo.2018.11.005
Paul was also invited to write an article for GeoExpro (September 2019, 28–31) on the controversy over the seismic models used to interpret the pre-salt carbonates offshore Brazil.
In conjunction with Andrew Barnett of Shell, Paul has provided a practical methodology for characterizing the unusual textures found in the pre-salt Barra Velha “Microbialite” reservoirs of offshore Brazil (Facies, 2020 released December 2019). doi.org/10.1007/s10347-019-0591-2
