Abstract
Abstract Basaltic aquifers are an important source of water supply in many regions worldwide. Because of their cooling process, flood basalts normally have complex internal structures and unpredictable permeable zone distribution. Geophysical profiling is a reliable tool used to identify the vertical variations of physical properties within the flood basalts, mainly to distinguish between low and high permeability intervals. In this study, a detailed analysis of drill cuttings, and geophysical logging of two depth wells helped to identify the typical response of main facies of flood basalts in the Serra Geral aquifer system. The evidence obtained from this work confirms that flood basalts may be classified as multilayer aquifer systems in which a highly permeable top covers a center with low permeability as a result of recurrent floods. In several cases, permeable layers are represented by a weathering horizon developed between two flood events. This study uses an empirical model to estimate the porosity based on acoustic velocity logging. A useful model based on a well-established Kozeny-Carman model was developed to predict the permeability of basaltic rocks using porosity data. The results obtained allow to identify the permeable intervals of basaltic aquifers and to estimate their hydraulic properties.
Highlights
Basaltic aquifers represent an important source of water, mainly for agricultural purposes, in many regions worldwide such as western India (Limaye 2010), the northwestern United States (Piersol and Sprenke 2015), Mexico City (Edmunds et al 2002), northeastern Australia (Locsey and Cox 2003), and southern Ethiopia (McKenzie et al 2001)
Since basalt acts as an important reservoir, this study aims to identify the parameters derived from geophysical logging that allow us to diagnose the facies variation within flood basalts
The wireline logs presented in this study strongly suggest that basalts are best described as multilayered aquifers, in which permeable zones correspond to flow tops, that alternate with low permeability intervals represented by flow interiors
Summary
Basaltic aquifers represent an important source of water, mainly for agricultural purposes, in many regions worldwide such as western India (Limaye 2010), the northwestern United States (Piersol and Sprenke 2015), Mexico City (Edmunds et al 2002), northeastern Australia (Locsey and Cox 2003), and southern Ethiopia (McKenzie et al 2001). Groundwater obtained from basalts represents the main source of water supply in volcanic islands such as Jeju Island, Korea (El-Kadi et al 2014), the Canary Islands, Spain (Cabrera and Custodio 2004), Madeira Island, Portugal (Prada et al 2005), and Hawaii, the United States (Wentworth 1951) Despite their importance, the distribution of permeable zones within flood basalts is poorly understood due to the internal complexity of these aquifers. The cooling structures create internal layers in response to distinct cooling rates within the flood, as described in Uhl and Joshi (1986), Whitehead (1992), Kulkarni et al (2000), and McGrail et al (2006) According to these studies, basalts typically display the same facies succession, consisting of three distinct units: flow top, flow interior, and flow bottom
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