Study of the geothermal potential of Tetitlan area (Nayarit, Mexico)

Abstract

The soil-gas method has been widely used to infer the nature of subsurface geology/geochemistry since gases released from active geothermal systems, can freely rise through overlying cover to be detected in the near-surface. The high mobility of some gases makes them the best pathfinders for concealed natural resources. Indeed, the gases produced and/or accumulated in geothermal reservoirs can escape towards the surface by diffusion, through transportation by rising hot fluids and by migration along fractures and faults. The Tetitlan area (Nayarit, Mexico) has been investigated by CFE (Comisiòn Federal de Electricidad) since nineties by means of exploratory wells for geothermal purposes and gravimetric prospectings. On the basis of obtained results, a potential deep geothermic reservoir has been inferred in spite of the scarcity of evident surface manifestations (e.g., hot springs, vents). A total of 154 soil-gas samples and 346 CO2 and CH4 flux measurements were collected in an area of 72 square kilometres. The performed soil-gas and flux geochemical surveys highlighted a general rising patterns linked to local fault system. Experimental variograms confirmed the presence of anisotropies highlighting different spatial domains. The contour maps elaborated on the basis of the calculated experimental variograms, demonstrated that gas emission at the surface is not spatially heterogeneous within studied area with the important implication that the highest fluxes, as well as concentrations, could be used in undeveloped geothermal systems to identify main upflow regions and areas of increased and deep permeability. Further, the total natural gas emission has been calculated in order to estimate the exploitation potential of the inferred system. The overall calculated levels of CO2 and CH4 emissions (2.35 104 t/day and 6.6 103 t/ day, respectively) from the Tetitlan system is found to remain within the range of normalized emissions measured for geothermal, volcanic, non-volcanic and hydrothermal systems worldwide.