Pinpointing Deep Geothermal Upflow in Zones of Complex Tectono‐Volcanic Degassing: New Insights from Aluto Volcano, Main Ethiopian Rift

Abstract

Active rifts release large amounts of gases from deep sources to the atmosphere by advection and diffusion processes along permeable fracture zones. The objective of this study is to develop geothermal exploration concepts for areas with little or no hydrothermal surface expressions suitable for fluid sampling and analyses (e.g., hot springs, geysers, and fumaroles). In such areas, soil gas surveys can complement established geophysical and geochemical exploration. We report CO2, 222Rn (radon), and 220Rn (thoron) emission data and ground temperatures from the Aluto volcanic complex in the Main Ethiopian Rift to improve understanding of tectonic and volcanic controls on the existing geothermal system. This suite of gas emission measurements allows us to identify major, deep‐rooted permeable structures with active fluid circulation and identify suitable drilling targets for geothermal production wells on Aluto. We show that significant differences in gas signatures (i.e., efflux and spatial pattern) can be used to identify predominantly volcanically and/or tectonically influenced compartments. Major gas emissions indicate significant fluid circulation at depth, which is typical for magmatic systems. Such high gas emissions have been observed in areas affected by major tectonic structures interacting with magmatic bodies at depth (tectono‐volcanic). Predominantly fault‐controlled sectors also show hydrothermal fluid circulation, but to a lower extent compared to tectono‐volcanic sectors. Within the Aluto volcanic complex, geothermal production wells mainly target such fault‐controlled domains, whereas results of the study indicate strongest fluid circulation in tectono‐volcanic sectors. This result should be considered for the future exploration and development strategy of the site.

Magmatic volatiles to assess permeable volcano-tectonic structures in the Los Humeros geothermal field, Mexico

“…Specific areas with obvious gas emissions are often sampled with a higher density of measurements for a more accurate detection of variations in soil gas emissions and delineation, whereas areas with less obvious gas emissions are often not taken into account (Werner et al, 2000;Chatterjee et al, 2019) despite the chance of missing permeable structures (hidden faults). Multiple studies showed the advantage of regular spaced surveys (e.g., Werner et al, 2000;Jolie et al, 2019). Systematic grid sampling is an effective and unbiased concept to collect spatially correlated data over a large area (geothermal reservoir scale) whilst avoiding interpolation artefacts (Isaaks and Srivastava, 1989) and allowing a detailed estimation of the total CO2 output (combined advective and diffuse degassing) (Lee et al, 2016).…”

“…Such low efflux values typically originate from biogenic sources (i.e. plant and microbial respiration and organic decomposition; see also Werner et al, 2000;Cardellini, 2003;Peiffer et al, 2014;Hutchison et al, 2015;Jolie et al, 2019). In fact, maximum CO2 efflux values reported for biogenic sources generally range from below detection limit of the device up to a few tens of grams per square meter and day in highly vegetated areas, but never exceed 100 g m -2 d -1 (Chiodini et al, 2007;Raich and Tufekcioglu, 2000;Widén and Majdi, 2011).…”

“…The structural control of permeability and fluid flow in volcanic-geothermal reservoirs like Los Humeros has a substantial influence on the geothermal potential of the system (Jafari and Babadagli, 2011). Hydrothermal fluid flow occurs preferentially along major subsurface discontinuities (Curewitz and Karson, 1997;Rossetti et al, 2011;Rowland and Sibson, 2004;Jolie et al, 2016Jolie et al, , 2019.…”

Geological controls on geothermal resources for power generation

“…At the most fundamental level, faults and fracture systems serve as channels for geothermal fluids [64][65][66][67] from hot deeper levels of the crust to within a few kilometres of the surface, where wells can tap the fluids for power production. Conversely, such structures also facilitate the circulation of meteoric fluids from near the surface to deeper crustal levels (termed recharge), which is essential for sustaining broad convective geothermal systems 39 .…”

Gas emissions and sub-surface architecture of fault-controlled geothermal systems: a case study of the North Abaya geothermal area

“…Although the deep (km-scale) architecture of these volcanic-geothermal systems is best imaged by magnetotellurics, high-spatial resolution (~10-50 m) gas surveys are particularly powerful at identifying permeability and pinpointing drilling locations (Jolie et al, 2019). Though many of East Africa's geothermal resources fit this model and are intimately associated with silicic caldera complexes there are various sites which show geothermal manifestations and high fluxes of magmatic volatiles, but no surface volcanism.…”