Results from spectroscopy well logs in volcanic formations in the Basin of Mexico

Water supply for Mexico City is a challenge due to groundwater overexploitation and land subsidence in the Basin of Mexico. To improve the geologic and hydrogeologic knowledge of the deep formations in the basin, the local and federal governments began in 2014 a deep exploration program by drilling wells up to the depth of 2000 m. To date five deep wells have been drilled, which have provided a wealth of new geologic, geophysical and hydrogeologic data. Here we present the results of the interpretation of the well geophysical logs, through a multivariate analysis of the spectroscopy register and other sensitive registers to parameters that allow identifying lithology and mineralogy. Cross plots between spectroscopy log and neutron-density were constructed to identify the corresponding lithology, grade of alteration and main mineralogic characteristics. The interpretation and characterization of the geophysical logs were supported by observations made during the drilling process, by lab tests conducted in a few cores, and the analysis of drill cuttings at selected depths in each of the wells. Finally, a range of values and limits in thorium, uranium and potassium are proposed to determine the lithology and its alteration grade in the volcanic setting.