ALBERT

Description: The Amik Basin is an asymmetrical composite transtensional basin developed between the seismically active left-lateral Dead Sea Fault (DSF) splays and the left-lateral oblique-slip Karasu Fault segment during neotectonic period. The relationship between the DSF and the East Anatolian Fault Zone is important as it represents a triple junction between Arabian Plate, African Plate and Anatolian Block in which the Amik Basin developed. The basin was formed on a pre-Miocene basement consisting of two rock series: Paleozoic crustal units with a Mesozoic allochthonous ophiolitic complex and ~1300 m thick Upper Miocene-Lower Pliocene sedimentary sequence. Plio-Quaternary sediments and Quaternary volcanics unconformably overlie the deformed and folded Miocene beds. Quaternary alkali-basaltic volcanism, derived from a metasomatized asthenospheric or lithospheric mantle, is most probably related to the syn-collisional transtensional strike-slip deformation in the area. Active faults in the region have the potential to generate catastrophic earthquakes (M〉7). Nineteen samples of cold and thermal groundwaters have been collected over the Amik Basin area for dissolved gas analyses as well as two samples from the gas seeps, and one bubbling gas from a thermal spring Samples were analysed for their chemical and isotopic (He, C) composition. On the basis of their chemical composition, three main groups can be recognized. Most of the dissolved gases (16; Group I) collected from springs or shallow wells (〈 150 m depth), contain mainly atmospheric gasses with very limited H2 (〈 80 ppm) and CH4 (1– 2700 ppm) contents and minor concentrations of CO2 (0.5–11.2 %). The isotopic composition of Total Dissolved Carbon evidences a prevailing organic contribution with possible dissolution of carbonate rocks. However the CO2-richest sample shows a small but significant deep (probably mantle) contribution which is also evidenced by its He isotopic composition. Further three samples, taken from the northern part of the basin close to Quaternary volcanic outcrops and main tectonic structures, also exhibit a small mantle He contribution (Fig. 1). The two dissolved gases (Group II) collected from deep boreholes (〉 1200 m depth) are typical of hydrocarbon reservoirs being very rich in CH4 (〉 78 %) and N2 (〉 13%). The water composition of these samples is also distinctive of saline connate waters (Cl- and B-rich, SO4-poor). Isotopic composition of methane (δ13C ~ -65‰) indicates a biogenic origin while He-isotopic composition points to a prevailing crustal signature for one (R/Ra 0.16) of the sites and a small mantle contribution for the other (R/Ra 0.98) (Fig. 1). The three free gas samples (Group III), taken at two sites within the ophiolitic basement west of the basin, have the typical composition of gas generated by low temperature serpentinisation processes with high hydrogen (37–50 %) and methane (10–61 %) concentrations. While all three gases show an almost identical δD-H2 of ~ -750‰, two of them display an isotopic composition of methane (δ13C ~ -5‰; δD ~ -105‰) and a C1/[C2+C3] ratio (~100) typical of abiogenic hydrocarbons and a significant contribution of mantle-type helium (R/Ra: 1.33). The composition of these two gasses is comparable to that of the gasses issuing in similar geologic conditions (Chimera-Turkey, Zambales-Philippine and Oman ophiolites). The gas composition of the other site evidences a contribution of a crustal (thermogenic) component (δ13C-CH4 ~ -30‰; δD-CH4 ~ -325‰; C1/[C2+C3] ~ 3000). Such crustal contribution is also supported by higher N2 contents (40% instead of 2%) and lower He-isotopic composition (R/Ra 0.07) (Fig. 1). These first results highlight contributions of mantle-derived volatiles possibly drained towards shallow levels by the DSF and other parallel structures crossing the basin showing a tectonic control of the fluids circulating within the Basin .

Description: Published

Description: Patras, Greece

Description: 4.5. Studi sul degassamento naturale e sui gas petroliferi

Description: open

Description: In this paper we present the first geochemical data set regarding long-term monitoring of dissolved gases in thermal waters from a seismic area. Three sites in Umbria (Central Apennines, Italy) were studied both for the chemical and for the helium isotopic composition of the dissolved gases. Data were collected during and after the seismic crisis that struck the region in 1997â 1998. The chemical composition of the dissolved gases revealed that a CO2-rich gas phase was always mixed with an atmospheric-derived component dominated by N2. A normal faulting marked the beginning of the seismic activity enhancing the release of CO2 on a regional scale. Variations in both the chemical and isotopic compositions of the dissolved gases were also observed as preseismic, synseismic, and postseismic phenomena related to the seismic shock of March 1998. Those geochemical modifications were interpreted as being the consequence of a drop in the CO2 degassing rate, in good agreement with the compressive focal mechanism of that seismic event. Furthermore, this interpretation was also consistent with the geologic and tectonic setting of the study area and induced us to postulate that changes in the local rock permeability, due to crustal deformations (i.e., coseismic deformation and postseismic release), were responsible for the geochemical modifications observed. On the basis of the foregoing, we have concluded that the geochemistry of dissolved gases in groundwaters represents a useful tool for the investigation of the relationships between circulating fluids and seismic activity.

Description: Published

Description: partially_open

Description: The first comprehensive geochemical data-set of the fluids circulating over a 14,000 km2-wide seismic-prone area of the Southern Apennines, Calabria Region (Italy), is presented here. The geochemical investigations were carried out with the two-fold aim of constraining the origin and interactions of the circulating fluids and to investigate possible relationships with local faults. We collected 60 samples of both thermal and cold waters, from which we extracted the dissolved gases . The geochemical features of the water samples display different types and degrees of water-rock interactions, irrespective of the outlet temperature. The calculated equilibrium temperatures of the thermal waters (60-160°C) and the low heat flow of the whole study area, are consistent with a heating process due to deep water circulation and rapid upflow through lithospheric structures. The composition of the dissolved gases reveals that crustal-originating gases (N2 and CO2-dominated ) feed all the groundwaters. The 3He/4He ratios of the dissolved helium, in the range of 0.03 to 0.22Rac for the thermal waters and 0.05-0.63Rac for the cold waters (Rac = helium isotope ratio corrected for atmospheric contamination), are mainly the result of a two-component (radiogenic and atmospheric) mixing, although indications of mantle-derived helium are found in some cold waters. As the study area had been hit by 18 of the most destructive earthquakes (magnitude ranging from 5.9 to 7.2) occurring over a 280-year time span (1626-1908) in Southern Apennines, the reported results on the circulating fluids may represent the reference for a better inside knowledge of the fault-fluid relationships and for the development oflong-term geochemical monitoring strategies for the area.

Description: In press

Description: 3.2. Tettonica attiva

Description: JCR Journal

Description: open

Description: The geochemical features of the volatiles dissolved in artesian thermal waters discharged over three basins (Millungera, Galilee and Cooper basin) of the Australian Great Artesian Basin (GAB) consistently indicate the presence of fluids from multiple gas sources located in the crust (e.g. sediments, oil reservoirs, granites) as well as minor but detectable contributions of mantle/magma-derived fluids. The gases extracted from 19 water samples and analyzed for their chemical and isotopic composition exhibit amounts of CO2 up to about 340 mlSTP/LH2O marked by a δ13CTDC (Total Dissolved Carbon) ranging from −16.9 to +0.18‰ vs PDB, while CH4 concentrations vary from 4.4 × 10−5 to 4.9 mlSTP/LH2O. Helium contents were between 9 and N2800 times higher than equilibrium with Air Saturated Water (ASW), with a maximum value of 0.12 mlSTP/LH2O. Helium isotopic composition was in the 0.02–0.21 Ra range (Ra = air-normalized 3He/4He ratio). The three investigated basins differ from each other in terms of both chemical composition and isotopic signatures of the dissolved gases whose origin is attributed to both mantle and crustal volatiles. Mantle He is present in the west-central and hottest part of the GAB despite no evidence of recent volcanism.Wefound that the partial pressure of helium, significantly higher in crustal fluids than in mantle-type volatiles, enhances the crustal He signature in the dissolved gases, thus masking the original mantle contribution. Neotectonic activity involving deep lithospheric structures and magma intrusions, highlighted by recent geophysical investigations, is considered to be the drivers of mantle/magmatic volatiles towards the surface. The results, although pertaining to artesian waters froma vast area of N542,000 km2, provide newconstraints on volatile injection, and showthat fluids' geochemistry can provide additional and independent information on the geo-tectonic settings of the Great Artesian Basin and its geothermal potential.

Description: Published

Description: 75-88

Description: 2T. Tettonica attiva

Description: JCR Journal

Description: restricted