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Introduction

Central Italy is a seismically active region that, over the last three decades, has experienced two devastating (M_w ≥ 6) seismic sequences: the first from September 1997 until April 1998, and the second from August 2016 until January 2017 (Chiaraluce et al., 2017

Chiaraluce, L., Di Stefano, R., Tinti, E., Scognamiglio, L., Michele, M., Casarotti, E., Cattaneo, M., De Gori, P., Chiarabba, C., Monachesi, G., Lombardi, A., Valoroso, L., Latorre, D., Marzorati, S. (2017) The 2016 Central Italy Seismic Sequence: A First Look at the Mainshocks, Aftershocks, and Source Models. Seismological Research Letters 88, 757–771. https://doi.org/10.1785/0220160221

; Fig. 1a). Moreover, this area is characterised by a regional anomaly of CO₂ degassing (Chiodini et al., 2004

Chiodini, G., Cardellini, C., Amato, A., Boschi, E., Caliro, S., Frondini, F., Ventura, G. (2004) Carbon dioxide Earth degassing and seismogenesis in central and southern Italy. Geophysical Research Letters 31, L07615. https://doi.org/10.1029/2004GL019480

; Fig. 1b). As suggested by several studies (Chiodini et al., 2020

Chiodini, G., Cardellini, C., Di Luccio, F., Selva, J., Frondini, F., Caliro, S., Rosiello, A., Beddini, G., Ventura, G. (2020) Correlation between tectonic CO₂ Earth degassing and seismicity is revealed by a 10-year record in the Apennines, Italy. Science Advances 6, eabc2938. https://doi.org/10.1126/sciadv.abc2938

and references therein), such deeply derived CO₂-rich gases (hereafter CRG) can play an important role in faulting processes and possibly even in triggering earthquakes at regional scales. In order to study the preparatory phase of seismic events and the role of fluids, the INGV (National Institute of Geophysics and Volcanology) is currently leading the deployment of a dense multiparametric monitoring network, which also includes high frequency measurements of CO₂ flux, along the Apennine chain (e.g., Chiaraluce et al., 2022

Chiaraluce, L., Festa, G., Bernard, P., Caracausi, A., Carluccio, I., Clinton, J., Di Stefano, R., Elia, L., Evangelidis, C., Ergintav, S., Jianu, O., Kaviris, G., Marmureanu, A., Sebela, S., Sokos, E. (2022) The Near Fault Observatory community in Europe: a new resource for faulting and hazard studies. Annals of Geophysics 65, DM316. https://doi.org/10.4401/ag-8778

; Caracausi et al., 2023

Caracausi, A., Camarda, M., Chiaraluce, L., De Gregorio, S., Favara, R., Pisciotta, A. (2023) A novel infrastructure for the continuous monitoring of soil CO₂ emissions: a case study at the Alto Tiberina near fault observatory in Italy. Frontiers in Earth Science 11, 1172643. https://doi.org/10.3389/feart.2023.1172643

). In this framework, identifying the sources of the CRG and developing a robust regional conceptual model of fluid circulation are crucial factors for improving our ability to correctly interpret multiparametric time series (Caracausi et al., 2022

Caracausi, A., Buttitta, D., Picozzi, M., Paternoster, M., Stabile, T.A. (2022) Earthquakes control the impulsive nature of crustal helium degassing to the atmosphere. Communications Earth & Environment 3, 224. https://doi.org/10.1038/s43247-022-00549-9

; Buttitta et al., 2023

Buttitta, D., Capasso, G., Paternoster, M., Barberio, M.D., Gori, F., Petitta., M., Picozzi, M., Caracausi, A. (2023) Regulation of deep carbon degassing by gas-rock-water interactions in a seismic region of Southern Italy. Science of The Total Environment 897, 165367. https://doi.org/10.1016/j.scitotenv.2023.165367

).

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The CO₂ in the pre-Apennines and Apennines belt has been attributed to thermometamorphic processes, degassing of carbon-rich upper mantle sources, or crustal assimilation of carbonate rocks into silicate melts at shallow depth (Chiodini et al., 2004

Chiodini, G., Cardellini, C., Amato, A., Boschi, E., Caliro, S., Frondini, F., Ventura, G. (2004) Carbon dioxide Earth degassing and seismogenesis in central and southern Italy. Geophysical Research Letters 31, L07615. https://doi.org/10.1029/2004GL019480

; Frezzotti et al., 2009

Frezzotti, M.L., Peccerillo, A., Panza, G. (2009) Carbonate metasomatism and CO₂ lithosphere–asthenosphere degassing beneath the Western Mediterranean: An integrated model arising from petrological and geophysical data. Chemical Geology 262, 108–120. https://doi.org/10.1016/j.chemgeo.2009.02.015

; Lustrino et al., 2020

Lustrino, M., Ronca, S., Caracausi, A., Bordenca, C.V., Agostini, S., Faraone, D.B. (2020) Strongly SiO₂-undersaturated, CaO-rich kamafugitic Pleistocene magmatism in Central Italy (San Venanzo volcanic complex) and the role of shallow depth limestone assimilation. Earth-Science Reviews 208, 103256. https://doi.org/10.1016/j.earscirev.2020.103256

). However, the origin of CO₂ degassing remains debated to this day. Along with CO₂, minor, yet significant, amounts of light hydrocarbons and noble gases (such as helium, hereafter He) are discharged (Italiano et al., 2009

Italiano, F., Martinelli, G., Bonfanti, P., Caracausi, A. (2009) Long-term (1997-2007) geochemical monitoring of gases from the Umbria-Marche region. Tectonophysics 476, 282–296. https://doi.org/10.1016/j.tecto.2009.02.040

; Tassi et al., 2012

Tassi, F., Fiebig, J., Vaselli, O., Nocentini, M. (2012) Origins of methane discharging from volcanic-hydrothermal, geothermal and cold emissions in Italy. Chemical Geology 310–311, 36–48. https://doi.org/10.1016/j.chemgeo.2012.03.018

). Light hydrocarbons in such gas emissions have been poorly studied and their origin is currently ascribed to a combination of CO₂ reduction (abiogenesis) and thermal degradation of organic matter (thermogenesis). The geochemical characteristics of light hydrocarbons in natural gases are controlled by multiple factors, such as genetic processes and conditions, migration history, and secondary alteration reactions (Ricci et al., 2023

Ricci, A., Cremonini, S., Severi, P., Tassi, F., Vaselli, O., Rizzo, A.L., Caracausi, A., Grassa, F., Fiebig, J., Capaccioni, B. (2023) Sources and migration pathways of methane and light hydrocarbons in the subsurface of the Southern Po River Basin (Northern Italy). Marine and Petroleum Geology 147, 105981. https://doi.org/10.1016/j.marpetgeo.2022.105981

). Therefore, the study of them may hold clues to better understand the source and fate of the CRG.

With the goal of understanding the source(s) of the CO₂ degassing in Central Italy, we investigated the origin and geological controls on the geochemistry of light hydrocarbons in gas emissions located along a 140 km long NNW-SSE transect (Fig. 1) in the inner sector of the Umbria-Marche Apennine. The analysis also includes major gas chemistry, He isotopes and the carbon isotopic composition of CO₂. Sampling and analyses were carried out following standard procedures and are reported in the Supplementary Information along with the extended description of the results. The geological framework of the studied area is notably heterogeneous, encompassing a non-volcanic pre-Apennine domain in the north, followed by the Pleistocene intra-Apennine volcanic complex of San Venanzo and extending southward to the northeastern margin of the Pleistocene Vicano-Cimino Volcanic District. This regional variability is also evidenced by the He isotopes, which indicate a dominant crustal-radiogenic component in the north and minor mantle contributions in the south (Italiano et al., 2009

Italiano, F., Martinelli, G., Bonfanti, P., Caracausi, A. (2009) Long-term (1997-2007) geochemical monitoring of gases from the Umbria-Marche region. Tectonophysics 476, 282–296. https://doi.org/10.1016/j.tecto.2009.02.040

). The geothermal gradients in the former and latter areas are ∼30 °C/km and up to ∼100 °C/km, respectively (Cinti et al., 2014

Cinti, D., Tassi, F., Procesi, M., Bonini, M., Capecchiacci, F., Voltattorni, N., Vaselli, O., Quattrocchi, F. (2014) Fluid geochemistry and geothermometry in the unexploited geothermal field of the Vicano–Cimino Volcanic District (Central Italy). Chemical Geology 371, 96–114. https://doi.org/10.1016/j.chemgeo.2014.02.005

; Bonini et al., 2023

Bonini, M., Bicocchi, G., Montanari, D., Ruggieri, G., Tassi, F., Capecchiacci, F., Vaselli, O., Sani, F., Maestrelli, D. (2023) Small-magnitude earthquakes triggering fluid vents in a pressurised CO₂ system, Caprese Michelangelo (Central Italy). Tectonophysics 847, 229693. https://doi.org/10.1016/j.tecto.2022.229693

). In the investigated area, crustal extension is driven by a set of low angle detachments dipping ENE, the youngest and easternmost of these being the Alto Tiberina fault, and the associated high angle normal faults (Chiaraluce et al., 2007

Chiaraluce, L., Chiarabba, C., Collettini, C., Piccinini, D., Cocco, M. (2007) Architecture and mechanics of an active low‐angle normal fault: Alto Tiberina Fault, northern Apennines, Italy. Journal of Geophysical Research: Solid Earth 112, B10310. https://doi.org/10.1029/2007JB005015

). Circulation and intense degassing of geogenic fluids are evidenced by gas emissions with estimated CO₂ fluxes of up to tens of tonnes/day, extra-atmospheric CO₂ and He dissolved in groundwater and deep CO₂ overpressurised reservoirs at 3.5–5 km depth (Chiodini et al., 2004

Chiodini, G., Cardellini, C., Amato, A., Boschi, E., Caliro, S., Frondini, F., Ventura, G. (2004) Carbon dioxide Earth degassing and seismogenesis in central and southern Italy. Geophysical Research Letters 31, L07615. https://doi.org/10.1029/2004GL019480

; Italiano et al., 2009

Italiano, F., Martinelli, G., Bonfanti, P., Caracausi, A. (2009) Long-term (1997-2007) geochemical monitoring of gases from the Umbria-Marche region. Tectonophysics 476, 282–296. https://doi.org/10.1016/j.tecto.2009.02.040

).

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Sources and Formation Mechanisms of Light Hydrocarbons

The main component of the gas samples is CO₂ (>75 vol. %), with the only exception of the Nogna site where CH₄-dominated (∼92 vol. %) gases are discharged. Gases have high ⁴He/²⁰Ne ratios (>30), suggesting negligible (<1 %) air contributions (Fig. 2). Gases discharged at higher latitude show high He concentrations and ³He/⁴He ratios ranging from 0.01 to 0.03 Ra (where Ra is the atmospheric ³He/⁴He ratio), values consistent with those typical of crustal fluids (Fig. 2). In contrast, southern gases exhibit lower He contents and ³He/⁴He ratios ranging from 0.1 to 0.7 Ra (Fig. 2). This spatial distribution is in accordance with previous investigations (e.g., Italiano et al., 2009

Italiano, F., Martinelli, G., Bonfanti, P., Caracausi, A. (2009) Long-term (1997-2007) geochemical monitoring of gases from the Umbria-Marche region. Tectonophysics 476, 282–296. https://doi.org/10.1016/j.tecto.2009.02.040

) and confirms a southward increase in the mantle component (up to ∼10 % sub-continental lithospheric mantle at Montecchie).

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In most of the studied CRG, methane has concentrations <0.5 vol. % and it is consistently the most abundant hydrocarbon. Methane is found in both crustal and mantle natural fluids. Furthermore, it can have multiple origins, being produced by microbial activity, thermal cracking of organic matter and abiotic synthesis (Stefánsson et al., 2024

Stefánsson, A., Ricci, A., Garnett, M., Gunnarsson-Robin, J., Kleine-Marshall, B.I., Scott, S.W., Lelli, M., Dantas Cardoso, C., Pik, R., Santinelli, C., Ono, S., Barry, P.H., Broadley, M.W., Byrne, D., Halldórsson, S.A., Fiebig, J. (2024) Isotopic and kinetic constraints on methane origins in Icelandic hydrothermal fluids. Geochimica et Cosmochimica Acta 373, 84–97. https://doi.org/10.1016/j.gca.2024.03.015

and references therein). The relative contributions of crust and mantle gases to CH₄ can be evaluated by comparing measured CH₄/³He ratios and δ¹³C values of CH₄ with those of crust-mantle mixing curves (Fig. 3a). Most samples have CH₄/³He ranging between 10⁸ and 10¹⁰, in accordance with values found in continental extensional settings with medium–high heat fluxes (Fig. 3a), and suggesting a negligible (≤0.1 %), if any, contribution of abiotic CH₄ from mantle sources. A predominantly thermogenic origin from crustal sources is instead evidenced by high values of the CH₄/³He ratio and δ¹³C-CH₄ systematically higher than −50 ‰ (Fig. 3a). Furthermore, the δ¹³C-CH₄ of the CRG increases towards the south, potentially indicating a source organic matter of progressively higher thermal maturity.

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The Uppiano gas departs from this trend. Furthermore, it shows CH₄ with high concentrations (∼5 vol. %) and enriched in ¹²C and ¹H, relatively high CH₄/(C₂H₆ + C₃H₈) (∼180) and C₂H₆/C₃H₈ (∼18) ratios, and C₂₊ hydrocarbons enriched in ¹³C. These features strongly suggest minor mixing with microbial gas (mainly CH₄) and the occurrence of C₂₊ biodegradation. At Uppiano, gases are diffusively emitted over a relatively large area and discharge rates appear significantly lower compared to other CRG sites. This may have allowed sufficient time for microbial activity to partially alter the composition of deeply sourced hydrocarbons.

In contrast to the CRG, hydrocarbons discharged at Nogna are microbially formed via hydrogenotrophic methanogenesis, as evidenced by the CH₄-dominated gas composition, CH₄/(C₂H₆ + C₃H₈) > 2000, ¹³C- and ²H-depleted isotopic composition of CH₄, and δ¹³C-CO₂ of −12 ‰ (Fig. 3). This indicates that the CRG and the Nogna emission have distinct sources both for hydrocarbons and CO₂, and separate gas feeding systems.

Inspection of Figure 3b reveals that, except for the microbially modified/formed gases (i.e. Uppiano, Nogna), the northern CRG (i.e. Fungaia and Umbertide; hereafter NCRG) show δ¹³C and δ²H values of CH₄ comparable to those commonly found in confined systems, such as natural gas reservoirs in sedimentary basins (Fig. 3b), and strikingly similar to those measured in the hydrocarbon accumulations located 100–170 km to the north, in the Northern Apennines (NA) and in the Southern Po River Basin (SPRB). Furthermore, like the NA-SPRB hydrocarbons, the C₁-C₄ n-alkanes of the NCRG exhibit a positive carbon isotopic trend with increasing molecular weight (i.e. δ¹³C-CH₄ ≤ δ¹³C-C₂H₆ ≤ δ¹³C-C₃H₈ ≤ δ¹³C-nC₄H₁₀; Fig. 3d). This type of isotopic ordering is typical of thermogenic gases in petroleum reservoirs associated with sedimentary basins and fold-and-thrust belts (Ricci et al., 2023

Ricci, A., Cremonini, S., Severi, P., Tassi, F., Vaselli, O., Rizzo, A.L., Caracausi, A., Grassa, F., Fiebig, J., Capaccioni, B. (2023) Sources and migration pathways of methane and light hydrocarbons in the subsurface of the Southern Po River Basin (Northern Italy). Marine and Petroleum Geology 147, 105981. https://doi.org/10.1016/j.marpetgeo.2022.105981

). Light hydrocarbons from the NA-SPRB areas are commonly considered to have formed by thermal cracking of marine organic matter (type II kerogen) having initial δ¹³C ranging from −32 to −26 ‰ and hosted in Mesozoic carbonate rocks (Fig. 3c; Ricci et al., 2023

Ricci, A., Cremonini, S., Severi, P., Tassi, F., Vaselli, O., Rizzo, A.L., Caracausi, A., Grassa, F., Fiebig, J., Capaccioni, B. (2023) Sources and migration pathways of methane and light hydrocarbons in the subsurface of the Southern Po River Basin (Northern Italy). Marine and Petroleum Geology 147, 105981. https://doi.org/10.1016/j.marpetgeo.2022.105981

and references therein). Such rocks, as proven by deep drilling, indeed host the deep CO₂ overpressurised reservoirs that likely feed the Fungaia and Umbertide gas manifestations. This indicates that hydrocarbons in the NCRG may have formed by thermogenesis at conditions and from a source rock similar to those encountered in the confined accumulations of the NA-SPRB (Fig. 3b,c). Following this reasoning, the relatively low values of the CH₄/(C₂H₆ + C₃H₈) ratio measured in the NCRG may indicate a thermally mature source of hydrocarbons at depth (Fig. 3c). For the Umbertide gas, which shows CH₄ compositions relatively enriched in ¹³C and ²H (Fig. 3), a type II-III kerogen (typical of near shore marine depositional settings) cannot be excluded.

In contrast to the NCRG, southern gas emissions (hereafter SCRG) exhibit δ¹³C-CH₄ and δ²H-CH₄ values consistent with those of CH₄ in hot fluids from volcanic-hydrothermal systems worldwide (Fig. 3b). Here, CH₄ is generally interpreted as the product of open-system thermal cracking of organic matter at elevated temperatures (Fiebig et al., 2019

Fiebig, J., Stefánsson, A., Ricci, A., Tassi, F., Viveiros, F., Silva, C., Lopez, T.M., Schreiber, C., Hofmann, S., Mountain, B.W. (2019) Abiogenesis not required to explain the origin of volcanic-hydrothermal hydrocarbons. Geochemical Perspective Letters 11, 23–27. https://doi.org/10.7185/geochemlet.1920

). The hydrocarbons from the SCRG depict a trend of increase in δ¹³C-CH₄ and CH₄/(C₂H₆ + C₃H₈), similar to the pattern of the NA-SPRB thermogenic gases but shifted to significantly more ¹³C-enriched CH₄ isotopic compositions, apparently reaching an asymptotic δ¹³C value of around −18 ‰ (Fig. 3c). Highly ¹³C-enriched kerogen (≥−15 ‰) is required to replicate the geochemical features of these hydrocarbons via thermal maturation. However, the δ¹³C of organic carbon sources typically ranges between −34 and −18 ‰, ruling out thermal maturation of an unconventional kerogen as the hydrocarbon formation mechanism.

The attainment of isotopic equilibrium between CO₂ and CH₄ at high temperatures (350–450 °C; Fig. 3c) may account for the observed ¹³C-enrichment measured in the SCRG. However, the occurrence of CO₂-CH₄ isotopic equilibrium is commonly considered unlikely due to the sluggish reaction kinetics and the complex reaction steps (Xia and Gao, 2022

Xia, X., Gao, Y. (2022) Validity of geochemical signatures of abiotic hydrocarbon gases on Earth. Journal of the Geological Society 179, jgs2021-077. https://doi.org/10.1144/jgs2021-077

) and, hence, the isotopic compositions close to equilibrium may be coincidental. Fiebig et al. (2019)

Fiebig, J., Stefánsson, A., Ricci, A., Tassi, F., Viveiros, F., Silva, C., Lopez, T.M., Schreiber, C., Hofmann, S., Mountain, B.W. (2019) Abiogenesis not required to explain the origin of volcanic-hydrothermal hydrocarbons. Geochemical Perspective Letters 11, 23–27. https://doi.org/10.7185/geochemlet.1920

demonstrated that high temperature cracking of organic matter under open-system conditions can produce CH₄ with δ¹³C ≥ −25 ‰ when relatively high thermal maturities are reached. At these conditions also flat to reverse patterns of δ¹³C values in the n-alkanes (δ¹³C-CH₄ ≥ δ¹³C-C₂H₆ ≥ δ¹³C-C₃H₈ ≥ δ¹³C-nC₄H₁₀) are obtained. Notably, C₁-C₂ isotopic reversal (δ¹³C-CH₄ ≥ δ¹³C-C₂H₆) is observed for the SCRG. Furthermore, C₂H₆ and C₃H₈ with roughly constant δ¹³C value of around −28 ‰ (Fig. 3d) are also observed, in accordance with what would be expected for high temperature open-system cracking and high thermal maturity of the source rock. An overmature kerogen is also highlighted by the high CH₄/(C₂H₆ + C₃H₈) ratio of the SCRG (Fig. 3c). At high temperature, the difference between the δ¹³C of organic carbon source and that of the generated C₂₊ hydrocarbons is expected to be small (Fiebig et al., 2019

Fiebig, J., Stefánsson, A., Ricci, A., Tassi, F., Viveiros, F., Silva, C., Lopez, T.M., Schreiber, C., Hofmann, S., Mountain, B.W. (2019) Abiogenesis not required to explain the origin of volcanic-hydrothermal hydrocarbons. Geochemical Perspective Letters 11, 23–27. https://doi.org/10.7185/geochemlet.1920

). Therefore, the kerogen for the SCRG likely has a δ¹³C value of approximately −28 ‰, well within the isotopic range of the marine organic matter hosted in the Mesozoic carbonate formations (Fig. 3d; Katz et al., 2000

Katz, B.J., Dittmar, E.I., Ehret, G.E. (2000) A geochemical review of carbonate source rocks in Italy. Journal of Petroleum Geology 23, 399–424. https://doi.org/10.1111/j.1747-5457.2000.tb00494.x

), which constitute a significant part of the tectono-stratigraphic sequence of Central Italy. This could also indicate that hydrocarbons from SCRG and NCRG originate from similar source rocks.

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Thermal Regimes Influence on Hydrocarbon Generation and Migration

As discussed in the previous section, He isotopes, CH₄/³He, δ¹³C-CH₄, C₁-C₄ isotopic distribution pattern, and CH₄/(C₂H₆ +C₃H₈) of the CRG vary along the investigated axis. Thermogenic processes at elevated temperatures and open conditions appear to explain the observed geochemical characteristics of hydrocarbons in the southern gases. Notably, while geochemical observations suggest a southward increase in thermal input to the source rocks, Figure 4 indicates that hydrocarbons with hydrothermal-like signatures (SCRG) are associated with lower heat fluxes. However, by analysing the proximity to the 100 mW/m² isoline, commonly considered a threshold value distinguishing hyperthermal from low–moderate geothermal gradient areas (Mählmann and Le Bayon, 2016

Mählmann, R.F., Le Bayon, R. (2016) Vitrinite and vitrinite like solid bitumen reflectance in thermal maturity studies: Correlations from diagenesis to incipient metamorphism in different geodynamic settings. International Journal of Coal Geology 157, 52–73. https://doi.org/10.1016/j.coal.2015.12.008

), it becomes evident that the SCRG are located nearer to regions with elevated heat fluxes (Fig. 4). In fact, the geochemical signatures of hydrocarbons align with this spatial correlation, indicating that hydrocarbons closer to the thermal anomaly display characteristics typical of volcanic-hydrothermal contexts (Fig. 4). By studying the amount of heat transported by the main springs of the central Apennine, Chiodini et al. (2013)

Chiodini, G., Cardellini, C., Caliro, S., Chiarabba, C., Frondini, F. (2013) Advective heat transport associated with regional Earth degassing in central Apennine (Italy). Earth and Planetary Science Letters 373, 65–74. https://doi.org/10.1016/j.epsl.2013.04.009

showed that the estimated heat fluxes in this region are substantially underestimated. This discrepancy arises due to the cooling effect exerted on the upper crust by the deep circulation of meteoric waters through the highly permeable Mesozoic-Cenozoic carbonate formations. Their study estimates heat flux values exceeding 300 mW/m² within the Narnese-Amerina hydrogeological structure, which is approximately congruent with the location of the SCRG. Such a pronounced thermal anomaly is likely attributable to the Quaternary magmatism within the adjacent Latium volcanic province. These findings effectively reconcile the discrepancies in estimated heat fluxes and hydrocarbon geochemistry in the area of the SCRG, thereby providing new constraints on the thermal regime of this region.

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Finally, we hypothesise that, in the northern sector, the source organic matter experienced slow heating rates (tens of Myr), low temperatures (≤150 °C) and confined thermal cracking, yielding hydrocarbons with geochemical features comparable with those of thermogenic gases in conventional oil and gas systems. Purely crustal (0.01–0.03) R/Ra values and the high ⁴He/²⁰Ne ratios further indicate the occurrence of confined systems at depth (Caracausi et al., 2022

Caracausi, A., Buttitta, D., Picozzi, M., Paternoster, M., Stabile, T.A. (2022) Earthquakes control the impulsive nature of crustal helium degassing to the atmosphere. Communications Earth & Environment 3, 224. https://doi.org/10.1038/s43247-022-00549-9

). It is plausible that deeply derived CO₂ has migrated and accumulated in crustal reservoirs hosted in Mesozoic rocks where hydrocarbons and radiogenic He have been added to the gas phase.

In contrast, in the south, the Quaternary magmatism at depth associated with the nearby volcanic complexes may have imposed a thermal stress to the surrounding Mesozoic sedimentary rocks which, in turn, reached considerably high temperatures (>300–400 °C) in a relatively short period of time (<2 Myr). These conditions may have promoted the thermal maturation of organic matter hosted in such rocks and the formation of thermogenic hydrocarbons, which were subsequently transported to shallower crustal levels by deep CO₂. Groundwater circulation, locally enhanced by convective flows induced by the magmatism-related heat anomaly, might have favoured the expulsion of the generated hydrocarbons.

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Conclusions

Our study reveals that light hydrocarbons in CO₂-rich gas emissions from the inner sector of the Umbria-Marche Apennines of Central Italy are of predominantly thermogenic origin, with negligible contributions from abiotic sources. Geochemical data indicate that, while the source rock, predominantly composed of Mesozoic carbonates, is consistent throughout the investigated area, the conditions of hydrocarbon formation and expulsion vary, with elevated temperatures and open-system conditions prevailing in the southern zones.

In addition, we demonstrate that the geochemical characteristics of light hydrocarbons are linked with He isotopes and heat flux, providing a potential framework for identifying areas with high heat fluxes, particularly where geological phenomena influence near surface heat transfer or where thermal data are lacking.

Finally, we propose that light hydrocarbons, once formed at crustal depths (≤5–6 km), are transported towards the surface by ascending CO₂ derived by deeper sources. Our findings contribute to the development of a conceptual model of fluid origin and migration essential for interpreting variations linked to active regional seismicity, and open new perspectives for high frequency monitoring of the region.

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Acknowledgements

This research was carried out in the framework of the national PON GRINT (Geoscience Research Infrastructure in ITaly) CCI:2014IT16M2OP005, the MUSE (Multiparametric and mUltiscale Study of Earthquake preparatory phase in the central and northern Apennines) INGV project and the INGV projects Rete Multiparametrica (D.P.n.74/2020) and Progetto DL50 “Centro Italia” – Ricostruzione (D.P.76/2020). A. Caracausi appreciates support by the “Ramon´ y Cajal” research program (RYC2021-033270-I; MCIN/AEI/10.13039/501100011033 – EU “NextGenerationEU/PRTR”). Mariano Tantillo and Francesco Salerno are acknowledged for their help with chemical and isotope analysis. We acknowledge constructive reviews from two anonymous reviewers. We wish to thank Prof. Eric H. Oelkers for his careful editorial handling.

Editor: Eric Oelkers

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References

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Supplementary Information

The Supplementary Information includes:

• Materials and Methods
• Extended Description of the Results
• Tables S-1 to S-4
• Supplementary Information References

Download the Supplementary Information (PDF)