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: The paper describes a case of a natural emission of methane from soil in an urban development area, generating a significant risk for the local population and buildings, due to gas explosiveness and asphyxiation potential. The site is located on the south-western margin of the East-European Platform in eastern Romania, in a hydrocarbon-prone area crossed by the Pericarpathian lineament and regional faults. Molecular composition of gas and stable isotopic analyses of methane (CH4〉90%, δ to the power of 13 C1: -49.4‰, δD1: -173.4‰) indicate a dominant thermogenic origin, with significant amounts of C2-C5 alkanes (~5%), likely migrating through faults from a deep reservoir. Possible candidates are the Saucesti and Secuieni gas fields, located in the same petroleum system. Two surface geochemical surveys, based on closed-chamber flux measurements, were performed to assess the degassing intensity and the extent of the affected area. Methane fluxes from soil reach orders of 10 to the power of 4 mg m to the power of -2 day to the power of -1. Gas seepage mainly occurs in one zone 30 000 m2 wide, and it is likely controlled by channeling along a fault and gas accumulation in permeable sediments and shallow subsoil. The estimated total CH4 emission is about 40 t year to the power of -1 CH4, of which 8–9 t year to the power of -1 are naturally released from soil and 30–35 t year to the power of -1 are emitted from shallow boreholes. These wells have likely channeled the gas accumulated in shallow alluvial sediment but gas flux from soil is still high and mitigation measures are needed to reduce the risk for humans and buildings.

Description: Published

Description: 311-320

Description: 3.8. Geofisica per l'ambiente

Description: JCR Journal

Description: reserved

Description: The Chimaera gas seep, near Antalya (SW Turkey), has been continuously active for thousands of years and it is known to be the source of the first Olympic fire in the Hellenistic period. New and thorough molecular and isotopic analyses including methane (approximately 87% v/v; δ to the power of 13 C1 from -7.9‰ to -12.3‰; δ to the power of 13 D1 from -119‰ to -124‰), light alkanes (C2 + C3 + C4 + C5 = 0.5%; C6+: 0.07%; δ to the power of 13 C2 from -24.2‰ to -26.5‰; δ to the power of 13 C3 from -25.5‰ to -27‰), hydrogen (7.5–11%), carbon dioxide (0.01–0.07%; δ to the power of 13 CCO2: -15‰), helium (approximately 80 ppmv; R/Ra: 0.41) and nitrogen (2–4.9%; δ to the power of 15 N from -2‰ to -2.8‰) converge to indicate that the seep releases a mixture of organic thermogenic gas, related to mature type III kerogen occurring in Palaeozoic and Mesozoic organic-rich sedimentary rocks, and abiogenic gas produced by low-temperature serpentinization in the Tekirova ophiolitic unit. Methane is not related to mantle or magma degassing. The abiogenic fraction accounts for about half of the total gas released, which is estimated to be well beyond 50 ton year to the power of -1. Ophiolites and limestones are in contact along a tectonic dislocation leading to gas mixing and migration to the Earth’s surface. Chimaera represents the biggest emission of abiogenic methane on land discovered so far. Deep and pressurized gas accumulations are necessary to sustain the Chimaera gas flow for thousands of years and are likely to have been charged by an active inorganic source.

Description: Published

Description: 263-273

Description: 3.8. Geofisica per l'ambiente

Description: JCR Journal

Description: reserved

Description: The study area is a 130 km long fast spreading graben in Central Greece bordered by active faults. Its complex geodynamical setting includes the presence at depth of a subduction slab responsible for the recent (Quaternary) volcanic activity in the area which possibly represents the northward continuation of the South Aegean active volcanic arc. To the area belongs also the western termination of the North-Anatolian fault a tectonic lineament of regional importance. The high geothermal gradient of the area is evidenced by the presence of many thermal springs with temperatures from 19 to 82 °C, issuing along the normal faults bordering the graben. In the period 2004-2012 more than 60 gas and water samples have been collected and their chemical and isotopic analysis revealed a wide range of compositions. Going from west to east the gas composition changes (Fig. 1) from CH4- to CO2-dominated passing through mixed N2-CH4 and N2-CO2 compositions, while at the same time the He isotopic composition goes from typical crustal values (0.02 R/Ra) up to 0.87 R/Ra (corrected for air contamination), showing in the easternmost sites a small but significant mantle input (up to ~ 10%). Isotopic composition of CH4-C indicates a thermogenic origin for the CH4-rich samples (δ13C from -50 to -37 ‰) and hydrothermal origin for the remaining samples (〉 -25‰). Positive δ15N values (around +2 ‰) indicate a contribution of crustal derived nitrogen for the N2-rich samples. The most pristine values of δ13C(CO2) refer to the most CO2-rich samples. These values (~ -3 ‰) point to a mixed mantle-marine carbonate source. Lower δ13C values (-10 ÷ -5 ‰) of the other sites can be explained by loss of CO2 due to dissolution processes. Also temperature and salinity of the waters shows differences along the graben increasing from west to east (Fig. 2). Two main groups can be separated on the basis of the total dissolved salts (TDS). The first, represented by dilute waters (TDS 〈 500 mg/l), is found in the westernmost sites characterised by the presence of CH4-rich and mixed N2-CH4 gases. The remaining waters display higher salinities (TDS from 9 to 35 g/l) due to the mixing with high salinity waters. The water composition can be explained by mixing of two end-members, one with low salinity of meteoric origin and the other with high salinity of marine origin. The mixing can be evidenced in Fig. 2. Low salinity waters show low chloride contents and their light water isotope composition overlaps the field of the cold groundwaters of the area confirming their meteoric origin. High salinity waters are aligned along the mixing line between the cold groundwaters and the seawater confirming the contribution of marine component. Most of the water compositions in the triangular graph of Giggenbach fall in the field of the non equilibrated waters being therefore unsuitable for geothermometric estimations. Only the easternmost sites (Gialtra, Ilion and Edipsos) falling the field of the partially equilibrated waters yield estimated temperatures in the range 150-170 °C. Silica geothermometers confirm these estimations. This study revealed that the complex geodynamic setting of the area is clearly reflected in the wide compositional range of the gases collected in the area that evidence contributions from different end-members (atmosphere, crust, mantle and hydrothermal systems). Water chemistry can be explained mainly from the mixing of a meteoric low-salinity end-member with a high-salinity marine end-member partially modified by hydrothermal water-rock interactions. The highest estimated temperatures in the hydrothermal reservoirs are in the range 150-170 °C.

Description: Published

Description: Patras, Greece

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

Description: open

Description: This thematic issue of Geofluids includes 11 papers representing the three main topics discussed in the 10th edition of the International Conference on Gas Geochemistry (ICGG-10): (i) gas in petroleum systems and seepage, (ii) gas in geothermal systems and volcanoes and (iii) gas, seismicity and geohazards. ICGG-10 was held in 2009 in Romania, a country extraordinarily rich in surface gas manifestations, that offers innumerable opportunities for innovative studies on gas geochemistry. We briefly describe the present knowledge on gases occurring both in petroliferous sedimentary basins and geothermal areas of Romania. The 11 contributions of this special issue, which include data from eight countries, are then summarised. Based on these papers and other works presented at the ICGG-10, we find that significant advances in analytical capabilities, data treating and interpretation have led to innovative insights into the origin, distribution and environmental impact of gases migrating to the Earth’s surface. It is increasingly clear, in particular, that gas geochemistry can be more effective for petroleum exploration, volcano-tectonic, geodynamic and environmental studies, if multiparametric studies are performed and the data are interpreted in the geological context.

Description: Published

Description: 457-462

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

Description: JCR Journal

Description: restricted

Description: Gas seepage from petroleum basins is the second largest natural source of methane to the atmosphere, after wetlands. The uncertainty in global emission estimates should be reduced by extending the flux database which is fundamental for defining the emission factors and the actual area of seepage adopted for up-scaling. As a contribution to this goal, we report a new seepage data-set for the Transylvanian Basin, one of the largest natural gas producing regions of Europe, that is characterized by the widespread occurrence of natural leakages of gas at the surface, including at least 73 mud volcanoes and gas seeps. In this study, methane flux was measured using closed-chambers, from 12 seepage sites, in correspondence with focused gas vents (mud volcano craters, bubbling pools, and flammable gas leaks), in the soil surrounding the vents, and at 15 sites located far from macroseep zones but close to gas fields. Fluxes from individual vents (macro-seeps) were found to reach orders of kg CH4 m)2 day)1 (up to 12 kg m)2 day)1) and diffuse fluxes from soils (miniseepage) were found to be up to a few g CH4 m)2 day)1. Far from seep zones, positive CH4 fluxes (microseepage) may occur locally, typically on the order of tens to hundreds of mg m)2 day)1. The values, as well as the occurrence of seepage even far from vent zones and in mud volcanoes that are apparently extinct, are coherent with results obtained in other countries. Gas fluxes from macro-seeps and soils may change seasonally, but the interannual variation of the average emission factor was found to be minimal. The total CH4 output for Transylvania macro-seeps is estimated conservatively to be around 680 t year)1; the total geo-CH4 seepage emission from the Transylvania petroleum system could be approximately 40 · 103 t year)1, and at least 100 · 103 t year)1 for all Romanian petroleum systems, that is roughly 10% of the total anthropogenic CH4 emission in the country.

Description: Published

Description: 463-475

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

Description: JCR Journal

Description: restricted

Description: Natural gas seeps in the Alpine region are poorly investigated. However, they can provide useful information regarding the hydrocarbon potential of sedimentary Alpine units and related geofluid migration, typically controlled by pressurized gas accumulations and tectonics. A gas seep located near Giswil, in the Swiss Northern Alps, was investigated, for the first time, for molecular and isotopic gas composition, methane flux to the atmosphere, and gas flux variations over time. The analyses indicated that the gas was thermogenic (CH4 〉 96%; d13C1: )35.5& to )40.2&) and showed evidence of subsurface petroleum biodegradation (13C-enriched CO2, and very low C3+ concentrations). The source rock in the region is marine Type II kerogen, which is likely the same as that providing thermogenic gas in the nearby Wilen shallow well, close to Lake Sarnen. However, the lack of d13CCO2 and d13C3 data for that well prevented us from determining whether the Wilen and Giswil seeps are fed by the same reservoir and seepage system. Gas fluxes from the Giswil seep, measured using a closedchamber system, were significant and mainly from two major vents. However, a substantial gas exhalation from the soil occurs diffusely in an area of at least 115 m2, leading to a total CH4 output conservatively estimated to be at least 16 tonnes per year. Gas flux variations, monitored over a 1-month period by a special tent and flowmeter, showed not only daily meteorological oscillations, but also an intrinsic ‘pulsation’ with periods of enhanced flux that lasted 2–6 h each, occurring every few days. The pulses are likely related to episodes of gas pressure build-up and discharge along the seepage system. However, to date, no relationship to seismicity in the active Sarnen strike-slip fault system has been established.

Description: Published

Description: 476-485

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

Description: JCR Journal

Description: restricted