Insights on the Formation of Vapor-Dominated Geothermal Systems

Abstract

Although many reservoirs have shallow vapor-dominated zones, only four fields are known to have low pressure (< 7 MPa) vapor-dominated zones extending to below sea level (The Geysers, Larderello, Kamojang, and Darajat). Under-pressured gas reservoirs in petroleum-bearing basins at such depths are even rarer. A review of the physical characteristics of these four fields indicates that The Geysers and Larderello have probably evolved differently to Kamojang and Darajat. The former two fields have very large reservoir areas (~100 km2), low heat flow intensities (< 1 MW/km2), and long lifespans (> 105 years for the vapor-dominated conditions; >106 y for the hydrothermal-magmatic system). The order of magnitude greater heat flow intensity at Kamojang and Darajat implies a much great infiltration rate of water (> 30 kg/s) and a shorter life for the vapor system as we presently know it (< 104 y). At The Geysers and Larderello, naturally low permeability metamorphic rocks surrounding the much of the reservoir limits water infiltration rates, and factors such as reservoir dilation and steam loss to the surface can cause and/or sustain low pressures in the system. Volcanic-hosted Kamojang and Darajat may have evolved from a magmatic-hydrothermal system analogous to Alto Peak (Philippines; Reyes et al., 1993), where a vapor chimney is surrounded by a liquid-dominated geothermal system. After multiple cycles of magmatic-hydrothermal activity, the permeability of the host volcano may become sufficiently small that the high pressure vapor chimney decays into an under-pressured vapor-dominated system. Because of a requirement for prolonged seal integrity, such systems may only evolve in volcanic arcs where the adjacent subduction is perpendicular to the trench/fore-arc region.