Borehole geology and hydrothermal alteration of well CHA-1, Chipilapa geothermal field, El Salvador, C.A.

Abstract

The report describes the borehole geology of the directional well CHA-1, located inside the Cuyanausul graben, in the southern part of the Chipilapa geothermal field, and approximately 8 km southeast of the Ahuachapan geothermal field. The well had a total measured depth of 2750 m and was designed to be drilled directionally in order to intersect the Cuyanausul fault. The stratigraphic sequence found was mainly alternate sequences of basalts - basaltic andesite to andesite lava flows with interlayering of lithic tuff. The formation found at the bottom of the well is basically a sequence of fine grained lithic tuff and lithic tuff with andesitic breccias. The main aquifer in well CHA-1 is at 650-734 m depth, with a measured temperature of 200 C. This aquifer is actually cased off and therefore it is not possible to exploit it. Minor feed zones were identified at 866, 1037 and 1377 m depth. No circulation losses were found below 1400 m depth and the well showed very low permeability. Furthermore, three probable minor feed zones were identified by hydrothermal alteration, at 1900, 2200 and 2550 m depth. They were not noticed during drilling and probably sealed off by mud or mineralization ...