Importance of depositional texture in pore characterization of subsalt microbialite carbonates, offshore Brazil
M. F. Rezende, M. C. Pope, 2015. "Importance of depositional texture in pore characterization of subsalt microbialite carbonates, offshore Brazil", Microbial Carbonates in Space and Time: Implications for Global Exploration and Production, D. W. J. Bosence, K. A. Gibbons, D. P. Le Heron, W. A. Morgan, T. Pritchard, B. A. Vining
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Microbialite carbonates (e.g. stromatolites, thrombolites, shrubs and spherulites) are sedimentary deposits highly influenced by their environmental settings such as water depth, water chemistry and relative energy. Lower Cretaceous subsalt microbialite carbonates, in the Santos Basin (Brazil), have complex pore systems produced by their growth framework, which are related to carbonate precipitation by biotic and abiotic processes and also influenced by subsequent cementation and dissolution. Complex pore systems and high spatial reservoir heterogeneity result in reservoirs having total porosity ranging from 2 to 27% and permeability from less than 0.01 milidarcys to 4.9 darcys. Differences in textural characteristics such as shrub size, sorting and packing lead to different pore systems that subsequently control the petrophysical properties. Cements and dissolution also modify these texturally controlled pore systems by respectively reducing or enhancing the pore volume and pore-throats. The shrub size is a primary control on changes in the pore size and affects the permeability, whereas the shrub sorting influences the primary porosity, and secondarily the permeability. Packing acts as a secondary control on porosity. As result, a sample with small shrubs, well-sorted and tight packing has lower permeability for the same range of porosity than a sample with the same characteristics, but larger shrubs.
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Microbial carbonates (microbialites) are remarkable sedimentary deposits. They have the longest geological range of any type of biogenic limestones, form in the greatest range of different sedimentary environments, oxygenated the Earth’s atmosphere and produce and, furthermore, store large volumes of hydrocarbons. This Special Publication provides significant contributions at a pivotal time in our understanding of microbial carbonates when their economic importance has become established and the results of many research programmes are coming to fruition.
It is the first book to focus on the economic aspects of microbialites and in particular the giant pre-salt discoveries offshore Brazil. The volume contains papers on the processes involved in the formation of both ancient and modern microbialites and the diversity of style in microbial carbonate build-ups. Structures and fabrics from both marine and non-marine settings are discussed from throughout the geological record.