Petrografia e evolução diagenética dos arenitos da formação Tacaratu, bacia de Mirandiba, Nordeste do Brasil

Abstract

TheTacaratu Formation represents the basal unit of the Mirandiba Basin and constitutes the main groundwater aquifer in the central region of the State of Pernambuco. The petrology and petrography of the sandstones that constitute this unit remains poorly understood, mainly with regard to the constituentsand diagenetic processes that acted on these rocks, modifying and altering their original composition and primary porosity. Therefore, this work aims to perform a petrographic characterization and diagenetic evolution of the sandstones of the Tacaratu Formation, and to infer the impact of diagenetic constituents on the quality of the studied sandstones as reservoirs of fluids. The petrographic description of thin sections and analysis in ScanningElectron Microscope (SEM) allowed the identifications of textural, compositional, diagenetic aspects, and types of pores. The sandstones exhibits grains varying from fine to coarse sand, and subordinately very coarse sand, with moderately to poorly sorted grains; the grainsare subangular to subrounded, with a predominance of the latter. The original compositionof these sandstones corresponds mainly to subarkose and arkose, with a slight tendencytowards a more quartzose composition, due to strong dissolution of feldspars and lithic fragments. According to the modal compositions, the Tacaratu Formation deposits are plotted in Inner Craton and Transitional Continental tectonic environments, suggesting primary sourcespredominantly composed of metamorphic rocks followed by igneous rocks. These rocks have an average porosity of 20%, reaching a maximum value of 30% (arithmetic average data); the primary intergranular porosity predominates in comparison to the secondary porosity, formed essentially of detrital grains dissolution, followed by fracture porosity on grains. Next to the shearzone, there is the occurrence of intensely fractured quartz grains, feldspars and lithic fragments, sometimes dissolved and with a mylonite appearance, as well as, deformation bands.The formation of these features support the interpretation that the shear zones had a paramountinfluence on the petrophysical properties of these sandstones. Thus, the study revealed that these deformations contributed to alter the composition and original porosity of the Tacaratu sandstones of the Mirandiba Basin. Diagenetic processes of the three stages were identified: eodiagenesis, mesodiagenesis and telodiagenesis. However, diagenetic evidence shows that the reduction and production of porosity in these sandstones occurred during the early to late-eodiagenesis stage. Iron oxide-hydroxide cementation, quartz secondary growth, and mechanical infiltration of clay are the main processes that reduced the primary porosity. However, the dissolution process also produced pore spaces by dissolving partially or totally the feldspars, lithic fragments, and heavy minerals (biotite, amphibole, and rutile), resulting in a positive effect in the porosity. However, porosity was also produced, as in most samples there are pores associated with dissolution, originated by the total and partial dissolution of feldspar grains, lithic fragments and heavy minerals (biotite, amphibole and rutile), giving a positive effect on porosity of these rocks. Although the diagenetic processes and constituents have provided changes in the original porosity of these deposits, they still have good quality for storing fluids (water, gas or oil), since they still have good porosity permeability