Determinação dos parâmetros petrofísicos do Sistema Aquífero Urucuia Centro-Ocidental

Abstract

The Urucuia Aquifer System (UAS) is a hydraulic unit interrelated to the lithostratigraphic units of the Urucuia Group. They correspond, indistinctly, to the Aquifers: (i) Posse (intergranular porosities) and; (ii) Serra das Araras (intergranular porosities and, locally, fissures). In hydrogeology; the porosity, permeability and hydraulic conductivity characterize the rocks regarding the storage, transmission and flow of fluids in aquifers. The methods for determining these properties were applied to five geophysical well logs and to grain-size analysis in twelve sediment samples. Additionally, the heterogeneity and anisotropy of the UAS were characterized by analyzing the geophysical logs to obtain the vertical (Kz) and horizontal (Kx) hydraulic conductivity and the anisotropy ratio (Kx/Kz). In this context, the Posse and Serra das Araras Aquifers present contrasting sedimentological, petrophysical and hydrodynamic properties according to the trend from ENE (source area) to WSW (depositional area). In this direction, there is: (i) increase in shaliness (VSH) in the Posse Formation with depth and decrease in total porosity (φ) due to mechanical infiltration of clays (pseudomatrix of aeolian sandstones). The effective porosity (φe) and intrinsic permeability (ƙ) increase due to greater selection of sediments; (ii) in the Serra das Araras Formation there is a decrease in VSH in its extension, due to clayey infiltration into the underlying unit. In the central-eastern portion of the hydrogeological basin there is a decrease in the mean values of φ, φe and ƙ due to the higher frequency and thickness of silicified sandstone levels. In principle, petrophysical parameters characterize the Posse and Serra das Araras Aquifers, which are permoporous, contiguous and segmented by silicified aquitars that provide leakege between the lithostratigraphic units. The grain-size analysis allowed to obtain the porosity from the equations Wang et al., (2017) and exponential regression 𝜑w, which were consistent and confirmed the values obtained in the literature. The application of the equation of Vukovic & Soro (1992) and exponential regression 𝜑vs overestimated the porosity of the UAS. Permeability and hydraulic conductivity are valid (i.e., SAU characterized by fine to medium sands, moderately selected and uniformity coefficient (U) less than 1) as applied to the equations: (i) Slitcher; (ii) United State Bureau of Reclamation (USBR); (iii) Kozeny-Carman and (iv) Hazen. Respectively, the permeability (k) is between 10-12 to 10-9 m2 and the hydraulic conductivity (K) between 10-5 to 10-2 m/s; the second with k between 10-11 to 10-10 m2 and K around 10-4 to 10-3 m/s, and the last two k between 10-10 to 10-9 m2 and K between 10-3 and 10-2 m/s. The Beyer and Terzaghi equations cannot be considered because k and K are overestimated. The analysis of heterogeneity and anisotropy indicates that (i) the Posse Formation is conditioned by the presence of a clayey pseudomatrix that characterizes heterogeneities in aeolian sandstones, and the anisotropy ratio between 4.62 to 11.34; (ii) the Serra das Araras Formation has heterogeneity due to the diagenetic processes involved (ie, silicified sandstones), and in the central portions of the basin, an anisotropy ratio around 1.02, and, in the eastern and western extremes, higher ratios to 1.