Significance of Deep Zones of Intense Bleaching and Silicification in the Los Humeros High-Temperature Geothermal Field, Mexico: Evidence of the Effects of Acid Alteration

摘要

The Los Humeros geothermal field is a high-temperature, wells produce high steam fraction, with a large resource potential. However, despite the drilling of more than 40 deep wells, many of which encountered ＞ 300°C steam, it has an installed electrical generating capacity of 40 MWe. Low permeability reservoir rocks and acid fluids that cause accelerated corrosion have hampered development. It has been suggested that acid fluids occur throughout the reservoir. However, this is inconsistent with the marked absence of alteration minerals in the volcanic rocks typical of low pH fluids. Instead the mineralogical evidence indicates that the hydrothermal alteration of the reservoir rocks is typical of f neutral to alkaline pH waters. Thus it appears that since the reservoir has apparently transitioned from being water-dominated to being vapor-dominated in the geologically recent past. An example of local acid leaching that formed a bleached, intensely silicified, zone is found at a depths of about 2000 m in a non-productive but very hot (＞370°C) well. These rocks retain the textures of a lithic crystal tuff. The altered rock consists almost entirely of micro-crystalline quartz, with sparse relic pseudomorphs of plagioclase phenocrysts and traces of chlorite and pyrite. This zone is approximately 160 m thick; and the deeper rocks lack this silicification. Smaller similarly bleached silicified zones in adjacent wells have been recognized in drill cuttings at different depths. These observations are interpreted to result from lateral flow of corrosive fluids. The origin of these acid fluids is still a subject of debate. They may represent emanations from a magma chamber or be due to post-exploitation processes (e.g. reaction of water and salts forming hydrogen chloride by hydrolysis at high temperatures). The very high boron content of the fluids produced by the Los Humeros wells suggests that the ultimate source of the acid gases is most likely magmatic. These acid gases have not reacted widely with the rocks. We suggest that the silicified zones are forming locally where descending waters encounter superheating steam containing acid gas and form low pH liquids that react and leach the rocks.