Importance of depositional texture in pore characterization of subsalt microbialite carbonates, offshore Brazil

TLDR

Microbialite carbonates in the Lower Cretaceous Santos Basin form complex, highly heterogeneous pore systems whose porosity (2–27 %) and permeability (≤0.01–4.9 darcys) are governed by depositional texture, cementation, and dissolution processes. The study shows that shrub size primarily controls pore size and permeability, while shrub sorting mainly determines porosity and packing acts as a secondary influence, resulting in smaller, well‑sorted, tightly packed samples exhibiting lower permeability at a given porosity than larger‑shrub counterparts.

Abstract

Abstract 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.