Investigation of the 1867 Lesbos (NE Aegean) earthquake fault pattern based on soil-gas geochemical data

Thermal waters from natural hot springs and boreholes are clear geothermal features of the city of Ourense (Galicia, Spain). The urban area of Ourense is located in the Mi&#241;o River&#8217;s valley which is characterized by two fault systems (NW&#8211;SE and NE&#8211;SW) that determine the groundwater circulation in the region. The low permeability of the granite and granodiorite only allows fluid circulation throughout faults and fractures transporting the fluid and transferring the heat to the lower elevations in the valley (Araujo 2008; Fern&#225;ndez Portal et al. 2007). During July to August 2019, an intensive soil gas geochemical survey was carried out at urban area of Ourense in order to identify the presence of vertical permeable structures and possible upflow of deep-seated gases. A total of 539 soil gas samples were taken with an average distance of &#8776;100 m between sampling sites and covering an area about 13Km2. In-situ soil CO2 efflux and soil gas 222Rn activity measurements were performed at each sampling site. In addition, soil gas samples at 40 cm depth were collected for chemical (He, Ne, H2, O2, N2, CH4 and CO2) and isotope (d13C-CO2 vs. VPDB) analysis by micro-gas chromatography and IRMS, respectively. Soil CO2 efflux and 222Rn activity values ranged from 0.7 to 92 g&#183;m-2&#183;d-1 (mean value of 16 g&#183;m-2&#183;d-1) and from 2.7 to 743 Bq&#183;m-&#179; (mean value of 73 Bq&#183;m-&#179;), respectively. Regarding soil gas He and H2 concentration, the values ranged from 5.2 to 25.0 ppmV (mean value of 6.2 ppmV) and from 0.5 to 24.9 ppmV (mean value of 1.9 ppmV), respectively. Soil CO2 concentrations showed a range between 355 and 53,766 ppmV (mean value of 7,824 ppmV) with a range of isotopic ratios from -14.1 to -28.5&#8240; vs. VPDB (mean value of -22.1 &#8240;). The binary plot of d13C-CO2 vs 1/CO2 concentration suggest the presence of small fractions of CO2 deep-seated in the soil gas atmosphere (mainly an atmospheric and biogenic gas mixture) of the city of Ourense. Soil CO2 efflux, soil gas Rn-222 activity and soil gas He, H2 and CO2 concentration contour maps were constructed using the sequential Gaussian simulation (sGs) interpolation method. Estimated diffuse CO2 emission from the study area is about 201 tons per day and about 8 tons per day could be considered deep-seated degassing. Spatial distribution analysis of the soil gas geochemical data show relatively high values of soil CO2 efflux and soil gas H2 concentration at the Chavasqueira-Tinteiro urban sector, while As Burgas and Outariz-Mui&#241;o urban sectors showed relatively high values of soil 222Rn activity. These results show the usefulness of the soil gas geochemistry to identify permeable zones and possible upflow of deep-seated gases at the city of Ourense.

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Application of High-Resolution Soil-Gas Geochemical Data to Solving Organic Contamination Problems: ABSTRACT

AAPG Bulletin ◽

10.1306/44b4bf4a-170a-11d7-8645000102c1865d ◽

1990 ◽

Vol 74 ◽

Author(s):

James H. Viellenave, James C. Hicke

Keyword(s):

High Resolution ◽

Soil Gas ◽

Geochemical Data ◽

Organic Contamination

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Characterizing Permeability from Geological and Geochemical Data in the Olkaria Domes Field in Kenya.

East African Journal of Environment and Natural Resources ◽

10.37284/eajenr.3.1.429 ◽

2021 ◽

Vol 3 (1) ◽

pp. 161-182

Author(s):

Harriet Nkatha Achini ◽

Olubunmi C Adeigbe ◽

Bernard Kipsang Rop

Keyword(s):

Research Work ◽

Geothermal Gradient ◽

Geothermal Field ◽

Soil Gas ◽

Geochemical Data ◽

Rift System ◽

Central Field ◽

Ratio Distribution ◽

North West ◽

North East

Olkaria geothermal field is located in the Kenyan Rift Valley that is a part of the Great East Africa Rift System (EARS). The geothermal field continues to be associated with a high geothermal gradient that arises from shallow magmatic activities which are ongoing in the enormous igneous province. Exploration and drilling of wells that were undertaken in the past revealed the existence of exploitable geothermal steam. The Olkaria field is divided into seven sections namely; Olkaria East field, Olkaria North East field, Olkaria North West field, Olkaria South East field, Olkaria South West field, Olkaria Central field, and Olkaria Domes field. The productivity of the geothermal wells continues to be influenced by factors such as subsurface permeability. Permeability is one of the parameters used for the characterization of geothermal fields. Other parameters used for characterization are associated with geotechnical weak zones and include features such as; fractures, vein bodies, and deformational fault systems. The research work involved geoscientific characteristics of the Olkaria Domes field based on the geological and geochemical factors to characterize the permeability of the field. The research involved studying rock types in the area by analysing drill cuttings obtained from six drilled wells in the Olkaria Domes field. Three of the six drill wells were considered for correlative description for the purpose of this paper. Correlation of the main lithologies and zones for loss of circulation in the field was also undertaken as well as the creation of mineralogical maps to capture the distribution of the minerals that were derived from hydrothermal weathering processes. The depths and formation for major loss circulation zones in the reservoir section of the field were identified and included in the description. Analysis of soil gas survey using radon as a geochemical tool in the Domes field was also carried out successfully. The relatively high levels of the soil gas ratios that were analysed captured the ratio distribution of carbon dioxide to radon at various reservoir depths. The detection of the two gases at the surface showed the existence of permeable zones which facilitated the movement of the gases through the fault-controlled structural systems of the studied Olkaria Domes field.

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