Exploring the influence of microbialite microfabrics and diagenetic evolution on reservoir quality: the key role of detailed petrography

Abstract

The Yacoraite Formation (Maastrichtian – Danian, Argentina) is a hydrocarbon reservoir unit developed in a mixed carbonate-siliciclastic lagoon system during the post-rift stage of the Salta Group basin. Microbialite facies have been previously reported but here, a stromatolite level (M2) from the Maimará locality is described in detail for the first time, focusing on depositional and post-depositional processes and their effect on reservoir quality. At outcrop scale, the level has a biostrome structure composed of domes up to 30 cm. At mesoscale, it exhibits two sectors with distinct structure and composition. The lower sector is calcitic-dolomitic and has both columnar and continuous-laminated structures, whereas the upper sector is strongly silicified and has columnar structure. The macro and mesostructures of the stromatolites suggest a shallow-water environment with moderate energy, under the influence of waves and tidal currents. At microscale, the predominant microfabric is micrite-microsparite laminae with remnants of primary micropeloidal texture, indicating a microbially induced origin. Additionally, sparite laminae were probably formed by relatively early post-mortem precipitation and fibrous calcite laminae probably had a stronger abiotic and hydrochemical control. Locally, trapping and binding processes were also responsible for agglutinated laminae. Therefore, the main factors influencing the depositional microstructures were the growth and decay of microbial communities, hydrochemistry and hydrodynamics. Post-depositional processes have been determined, mainly cementation (calcite, dolomite, barite, and quartz), recrystallization, silicification and dissolution, which have modified the primary microfabrics. As a consequence, porosity (up to 10%) developed, creating a possible microbialite reservoir, at least in its lower sector.