A GLOBAL DATASET OF ONSHORE GAS AND OIL SEEPS: A NEW TOOL FOR HYDROCARBON EXPLORATION

Petroleum seeps have historically been important drivers of global petroleum exploration. Still today they can serve as direct indicators of gas and/or oil subsurface accumulations. In particular the assessment of the origin of seeping gas is a key task for understanding, without drilling, the subsurface hydrocarbon potential, genesis and quality, e.g. the presence of shallow microbial gas, deeper thermogenic accumulations, the presence of oil and non-hydrocarbon undesirable gases (CO2, N2, H2S). Low quality, biodegraded petroleum can also be recognised, before drilling, through specific geochemical features of the seeping gas. Seeps are then indicators of tectonic discontinuities (faults) and fractured rocks; they can also represent geo-hazards and sources of greenhouse gas (methane) and photochemical pollutants (ethane and propane). A new global dataset of onshore gas and oil seeps (GLOGOS) is here presented. GLOGOS includes more than 1150 seeps from 84 countries (version August 2009) and it is continuously updated and expanded. The data-set includes geographical and gas-geochemical data (molecular and isotopic composition of the main gases). Many seeps are recently discovered or never reported in other data-bases. Seeps are catalogued by country and classified in three types: gas seeps, oil seeps and mud volcanoes. All seeps have a bibliographic or www reference. GLOGOS is a unique tool for hydrocarbon exploration, assessment of Total Petroleum Systems and geo-structural studies

GLOGOS, A New Global Onshore Gas-Oil Seeps Dataset

Petroleum seeps have historically been important drivers of global petroleum exploration. Still today they can serve as direct indicators of gas and/or oil subsurface accumulations. In particular the assessment of the origin of seeping gas is a key task for understanding, without drilling, the subsurface hydrocarbon potential, genesis and quality; e.g., the presence of shallow microbial gas, deeper thermogenic accumulations, the presence of oil and non-hydrocarbon undesirable gases (CO2, N2, H2S). Seeps are then indicators of tectonic discontinuities (faults) and fractured rocks; they can also represent geo-hazards and sources of greenhouse gas (methane) and photochemical pollutants (ethane and propane). A new global dataset of onshore gas and oil seeps (GLOGOS) is here presented. GLOGOS includes more than 1150 seeps from 84 countries (version August 2009), and it is continuously updated and expanded. The dataset includes geographical and gas-geochemical data (molecular and isotopic composition of the main gases). Many seeps are recently discovered or never reported in other databases. Seeps are catalogued by country and classified in three types: gas seeps, oil seeps and mud volcanoes. All seeps have a bibliographic or www reference. GLOGOS is a unique tool for hydrocarbon exploration, assessment of Total Petroleum Systems and geo- structural studies

Mud volcanoes and microseepage: the forgotten geophysical components of atmospheric methane budget

Mud volcanoes and microseepage are two important natural sources of atmospheric methane, controlled by neotectonics and seismicity. Petroleum and gas reservoirs are the deep sources, and faults and fractured rocks serve as main pathways of degassing to the atmosphere. Violent gas emissions or eruptions are generally related to seismic activity. The global emission of methane from onshore mud volcanoes has recently been improved thanks to new experimental data sets acquired in Europe and Azerbaijan. The global estimate of microseepage can be now improved on the basis of new flux data and a more precise assessment of the global area in which microseepage may occur. Despite the uncertainty of the various source strengths, the global geological methane flux is clearly comparable to or higher than other sources or sinks considered in the tables of the Intergovernmental Panel on Climate Change

Technical guidance to prepare national emission inventories

Studies performed since 2000 have demonstrated that geologic emissions of methane are an important global greenhouse-gas source (Etiope, 2004; Kvenvolden and Rogers, 2005; Etiope al, 2008). It is recognised that significant amounts of methane, produced within the Earth crust, released naturally into the atmosphere through faults and fractured rocks. Major emissions are related to hydrocarbon production in sedimentary basins (microbial and thermogenic methane), through continuous exhalation and eruptions from more than 1 200 onshore and offshore mud volcanoes, more than 10 000 onshore and shallow marine seeps and through diffuse soil microseepage. Specifically, six source categories must be considered: mud volcanoes, gas seeps (independent of mud volcanism), microseepage (diffuse exhalation from soil in petroleum basins), submarine seepage, geothermal (non-volcanic) manifestations and volcanoes. Global emission estimates range from 42 to 64 Tg y-1 (mean of 53 Tg y-1), almost 10 % of the total CH4 emission, representing the second most important natural methane source after wetlands. Geo-CH4 sources would also represent the missing source of fossil methane recognised in the recent re-evaluation the fossil methane budget in the atmosphere (about 30 %; Lassey et al,, 2007; Etiope et al, 2008), which implies a total fossil methane emission much higher than that due to fossil fuel industry. The global geo-CH4 emission estimates are of the same level as or higher than other sources or sinks considered in the Intergovernmental Panel on Climate Change (IPCC) tables, such as biomass burning, termites and soil uptake. Recent studies indicate that Earth’s degassing also accounts for at least 17 % and 10 % of total ethane and propane emissions (Etiope and Ciccioli, 2009)

Widespread abiotic methane in chromitites

Recurring discoveries of abiotic methane in gas seeps and springs in ophiolites and peridotite massifs worldwide raised the question of where, in which rocks, methane was generated. Answers will impact the theories on life origin related to serpentinization of ultramafic rocks, and the origin of methane on rocky planets. Here we document, through molecular and isotopic analyses of gas liberated by rock crushing, that among the several mafic and ultramafic rocks composing classic ophiolites in Greece, i.e., serpentinite, peridotite, chromitite, gabbro, rodingite and basalt, only chromitites, characterized by high concentrations of chromium and ruthenium, host considerable amounts of 13C-enriched methane, hydrogen and heavier hydrocarbons with inverse isotopic trend, which is typical of abiotic gas origin. Raman analyses are consistent with methane being occluded in widespread microfractures and porous serpentine- or chlorite-filled veins. Chromium and ruthenium may be key metal catalysts for methane production via Sabatier reaction. Chromitites may represent source rocks of abiotic methane on Earth and, potentially, on Mars

Trace-gas metabolic versatility of the facultative methanotroph Methylocella silvestris

The climate-active gas methane is generated both by biological processes and by thermogenic decomposition of fossil organic material, which forms methane and short-chain alkanes, principally ethane, propane and butane1, 2. In addition to natural sources, environments are exposed to anthropogenic inputs of all these gases from oil and gas extraction and distribution. The gases provide carbon and/or energy for a diverse range of microorganisms that can metabolize them in both anoxic3 and oxic zones. Aerobic methanotrophs, which can assimilate methane, have been considered to be entirely distinct from utilizers of short-chain alkanes, and studies of environments exposed to mixtures of methane and multi-carbon alkanes have assumed that disparate groups of microorganisms are responsible for the metabolism of these gases. Here we describe the mechanism by which a single bacterial strain, Methylocella silvestris, can use methane or propane as a carbon and energy source, documenting a methanotroph that can utilize a short-chain alkane as an alternative to methane. Furthermore, during growth on a mixture of these gases, efficient consumption of both gases occurred at the same time. Two soluble di-iron centre monooxygenase (SDIMO) gene clusters were identified and were found to be differentially expressed during bacterial growth on these gases, although both were required for efficient propane utilization. This report of a methanotroph expressing an additional SDIMO that seems to be uniquely involved in short-chain alkane metabolism suggests that such metabolic flexibility may be important in many environments where methane and short-chain alkanes co-occur

Gas-water partition and gas channelling along Rn-He-CO2 bearing faults

The amount and distribution of radon (222Rn) and helium (4He) at the Earth’s surface are controlled not only by type and depth of the source, permeability of geological formations and hydrogeological setting, but also by disequilibria processes occurring during upward gas migration. In order to evaluate the latter processes, in this work we discuss Rn and He data collected during surveys performed simultaneously in different types of natural occurrence (i.e. soil-air, groundwater, gas vent and soil-atmosphere exhalation flux) located along faults bearing pressurised CO2 (Siena basin, Central Italy). The obtained results emphasize the effects of gas-water partition and gas channelling processes induced by faults on Rn and He distribution in near-surface environment; accordingly they suggest that the analysis of a single “sub-system” of the geologic environment (e.g., groundwater analysis only) may provide data not representative of gas occurrence or abundance in the investigated site

Mud volcanoes and methane seeps in Romania: main features and gas flux

Romania is one of the European countries with the most vigorous natural seepage of methane, uprising from pressurised natural gas and petroleum reservoirs through deep faults. The largest seepage zone is represented by large mud volcanoes, with CH4 >80% v/v, occurring on the Berca-Arbanasi hydrocarbon-bearing faulted anticline, in the Carpathian Foredeep. Smaller mud volcanoes have been identified in other areas of the Carpathian Foredeep, in the Transylvanian Depression and on the Moldavian Platform. New surveys carried out in Transylvania allowed us to discover the richest N2 mud volcano zone in the world (N2>90% v/v), with a remarkably high He content and a helium isotopic signature which highlights a contribution of mantle-derived source. The large mud volcanoes are generally quiescent, with rare explosive episodes and provide a methane flux in the order of 102-103 t km−2 y−1. Independently from mud volcanism, a remarkable dry macroseepage, however, has been found, with a degassing rate up to three orders of magnitude higher than that of mud volcanoes (i.e. 103-105 t km−2 y−1). The total gas flux from all investigated macroseepage zones in Romania is estimated in the range of 1500-2500 t y−1. The emission from microseepage, pervasively occurring throughout the hydrocarbon-prone basins, has yet to be assessed and added to the total gas output to the atmosphere

Oceanographic signals at the Benthic Boundary Layer in the Mediterranean Sea

The Benthic Boundary Layer (BBL) is considered a quite homogeneous environment where a wide variety of processes (chemical, physical, geological and biological) occur often producing front structures or inducing turbulence phenomena. The typical stratification of these zones can be interrupted by episodic events which effects can diffuse to the ocean interior exploiting by local current and mixing processes. According to hydrodynamic definition, the BBL thickness may vary from few millimetres up to 100 metres depending on the friction intensity with the sea bed and the stability of water column above it. Generally in deep-sea condition, the BBL thickness is defined by the ratio between the friction velocity and the Coriolis parameter according to the Ekman scale. In the latest years several experiments have been carried out in the deep water of Mediterranean Sea, focusing on the survey and study of benthic processes following a multidisciplinary approach. Benthic observatories, such as SN-1 and GEOSTAR, allow to record long time-series of geochemical, seismological, geomagnetic, geodetic and oceanographic data and allow to understand the dynamics and evolution of the processes though comparison and interpolation of different types of signals. From a oceanographic point of view, the technology of these benthic observatories brings the possibility to observe and measure directly the hydrological properties at the seafloor collecting data for long-time series and with high sampling rate. The observatories deployed in Mediterranean Sea, have provided good information about variations and oscillations of hydrological parameters in deep water where the monitoring is almost lacking. In some cases it has been possible to link these deep-sea datasets with upper data collected by ship-handled system during the same period or during different cruises. This allows to have a more complete idea of the linkage between surface, intermediate and bottom sea. Hence the multidisciplinary approach represents a very important aspect for this kind of study, because it allows not only a cross check of functionality among all the instruments but also an important tool to recognise and better understand possible nonphysical- oceanographic phenomena

Gridded maps of geological methane emissions and their isotopic signature

Methane (CH4) is a powerful greenhouse gas, whose natural and anthropogenic emissions contribute ∼20&thinsp;% to global radiative forcing. Its atmospheric budget (sources and sinks), however, has large uncertainties. Inverse modelling, using atmospheric CH4 trends, spatial gradients and isotopic source signatures, has recently improved the major source estimates and their spatial–temporal variation. Nevertheless, isotopic data lack CH4 source representativeness for many sources, and their isotopic signatures are affected by incomplete knowledge of the spatial distribution of some sources, especially those related to fossil (radiocarbon-free) and microbial gas. This gap is particularly wide for geological CH4 (geo-CH4) seepage, i.e. the natural degassing of hydrocarbons from the Earth's crust. While geological seepage is widely considered a major source of atmospheric CH4, it has been largely neglected in 3-D inverse CH4 budget studies given the lack of detailed a priori gridded emission maps. Here, we report for the first time global gridded maps of geological CH4 sources, including emission and isotopic data. The 1∘×1∘ maps include the four main categories of natural geo-CH4 emission: (a) onshore hydrocarbon macro-seeps, including mud volcanoes, (b) submarine (offshore) seeps, (c) diffuse microseepage and (d) geothermal manifestations. An inventory of point sources and area sources was developed for each category, defining areal distribution (activity), CH4 fluxes (emission factors) and its stable C isotope composition (δ13C-CH4). These parameters were determined considering geological factors that control methane origin and seepage (e.g. petroleum fields, sedimentary basins, high heat flow regions, faults, seismicity). The global geo-source map reveals that the regions with the highest CH4 emissions are all located in the Northern Hemisphere, in North America, in the Caspian region, in Europe and in the East Siberian Arctic Shelf. The globally gridded CH4 emission estimate (37&thinsp;Tg&thinsp;yr−1 exclusively based on data and modelling specifically targeted for gridding, and 43–50&thinsp;Tg&thinsp;yr−1 when extrapolated to also account for onshore and submarine seeps with no location specific measurements available) is compatible with published ranges derived using top-down and bottom-up procedures. Improved activity and emission factor data allowed previously published mud volcanoes and microseepage emission estimates to be refined. The emission-weighted global mean δ13C-CH4 source signature of all geo-CH4 source categories is about −49&thinsp;‰. This value is significantly lower than those attributed so far in inverse studies to fossil fuel sources (−44&thinsp;‰) and geological seepage (−38&thinsp;‰). It is expected that using this updated, more 13C-depleted, isotopic signature in atmospheric modelling will increase the top-down estimate of the geological CH4 source. The geo-CH4 emission grid maps can now be used to improve atmospheric CH4 modelling, thereby improving the accuracy of the fossil fuel and microbial components. Grid csv (comma-separated values) files are available at https://doi.org/10.25925/4j3f-he27.</p