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Natural gas microseepage in petroleum‐bearing sedimentary basins is an important complement to geophysical methods in oil‐gas exploration and a natural source of methane (CH4) for the atmosphere. Microseepage, typically occurring in correspondence with petroleum fields throughout the world, is generally lower in summer, due to temperature‐driven methanotrophic consumption, and higher in winter. The global estimates of microseepage methane emission have, however, relatively high uncertainties because of limited amounts of flux data, leading to poor knowledge of the spatial distribution and temporal variability of the gas emission factors. We studied the seasonal variation of microseepage flux to the atmosphere from a petroleum field in China (the Dawanqi oilfield), through methane flux measurements performed in summer 2014, winter 2015, and summer 2019. Winter data refer to frozen soil conditions, with snow cover and ice thickness in the soil exceeding 60 cm. Gas concentration (CH4, CO2, C2+ alkanes) and stable C isotopic composition of CH4 and CO2 in shallow (4 m deep) boreholes confirmed the existence of thermogenic gas seepage. Methane microseepage is higher in summer and lower or nil in winter. This seasonal trend is opposite to what was observed in areas where winter soil is not or poorly frozen. Our data suggest that seasonal microseepage variation may not be univocal worldwide, being strongly dependent on the presence of ice and snow cover in winter. The regional increase of temperature due to climate change, already demonstrated for the Tarim Basin over the last 50 years, could, in the future, reduce winter ice and enhance annual methane emission to the atmosphere. Key Points: Microseepage of thermogenic gas over the Dawanqi oilfield is confirmed by molecular and isotopic hydrocarbon data in 4‐m deep boreholesContrary to previous observations in other petroleum basins, methane fluxes to the atmosphere are higher in summer and lower in winter due to relevant ice thickness in the soilMethanotrophic consumption and the ice‐snow barrier effect compete in the establishment of the seasonal microseepage pattern; microseepage may increase in future milder winters due to regional climatic warming [ABSTRACT FROM AUTHOR]

Geologic hydrocarbon seepage is considered to be the dominant natural source of atmospheric methane in terrestrial and shallowwater areas; in deep-water areas, in contrast, hydrocarbon seepage is expected to have no atmospheric impact because the gas is typically consumed throughout the water column. Here, we present evidence for a sudden expulsion of a reservoir-size quantity of methane from a deep-water seep during the Pliocene, resulting from natural reservoir overpressure. Combining three-dimensional seismic data, borehole data and fluid-flow modeling, we estimate that 18-27 of the 23-31 Tg of methane released at the seafloor could have reached the atmosphere over 39-241 days. This emission is ~10% and ~28% of present-day, annual natural and petroleum- industry methane emissions, respectively. While no such ultraseepage events have been documented in modern times and their frequency is unknown, seismic data suggest they were not rare in the past and may potentially occur at present in critically pressurized reservoirs. This neglected phenomenon can influence decadal changes in atmospheric methane. [ABSTRACT FROM AUTHOR]

The global methane (CH4) budget is becoming an increasingly important component for managing realistic pathways to mitigate climate change. This relevance, due to a shorter atmospheric lifetime and a stronger warming potential than carbon dioxide, is challenged by the still unexplained changes of atmospheric CH4 over the past decade. Emissions and concentrations of CH4 are continuing to increase making CH4 the second most important human-induced greenhouse gas after carbon dioxide. Two major difficulties in reducing uncertainties come from the large variety of diffusive CH4 sources that overlap geographically, and from the destruction of CH4 by the very short-lived hydroxyl radical (OH). To address these difficulties, we have established a consortium of multi-disciplinary scientists under the umbrella of the Global Carbon Project to synthesize and stimulate research on the methane cycle, and producing regular (~biennial) updates of the global methane budget. This consortium includes atmospheric physicists and chemists, biogeochemists of surface and marine emissions, and socio-economists who study anthropogenic emissions. Following Kirschke et al. (2013), we propose here the first version of a living review paper that integrates results of top-down studies (T-D, exploiting atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up models, inventories, and data-driven approaches (B-U, including process-based models for estimating land surface emissions and atmospheric chemistry, and inventories for anthropogenic emissions, data-driven extrapolations). For the 2003-2012 decade, global methane emissions are estimated by T-D inversions at 558 & Tg & CH4 & yr-1 (range [540-568]). About 60 & % of global emissions are anthropogenic (range [50-65 & %]). B-U approaches suggest larger global emissions (736 & Tg & CH4 & yr-1 [596-884]) mostly because of larger natural emissions from individual sources such as inland waters, natural wetlands and geological sources. Considering the atmospheric constraints on the T-D budget, it is likely that some of the individual emissions reported by the B-U approaches are overestimated, leading to too large global emissions. Latitudinal data from T-D emissions indicate a predominance of tropical emissions (~64 & % of the global budget, < & 30° & N) as compared to mid (~32 & %, 30° & N-60° & N) and high northern latitudes (~ & 4 & %, 60° & N-90° & N). T-D inversions consistently infer lower emissions in China (~58 & Tg & CH4 & yr-1 [51-72], -14 & %) and higher emissions in Africa (86 & Tg & CH4 & yr-1 [73-108], +19 & %) than B-U values used as prior estimates. Overall, uncertainties for anthropogenic emissions appear smaller than those from natural sources, and the uncertainties on source categories appear larger for T-D inversions than for B-U inventories and models. The most important source of uncertainty on the methane budget is attributable to emissions from wetland and other inland waters. We show that the wetland extent could contribute for 30-40 & % on the estimated range for wetland emissions. Other priorities for improving the methane budget include: i) the development of process-based models for inland-water emissions, ii) the intensification of methane observations at local scale (flux measurements) to constrain B-U land surface models, and at regional scale (surface networks and satellites) to constrain T-D inversions, iii) improvements in the estimation of atmospheric loss by OH, and iv) improvements of the transport models integrated in T-D inversions. The data presented here can be downloaded from the Carbon Dioxide Information Analysis Center (doi:10.3334/CDIAC/Global_Methane_Budget_2016). [ABSTRACT FROM AUTHOR]

Abstract: The “methane-led hypotheses” assume that gas hydrates and marine seeps are the sole geologic factors controlling Quaternary atmospheric and climate changes. Nevertheless, a wider class of geologic sources of methane exist which could have played a role in past climate changes. Beyond offshore seepage, relevant geologic emissions of methane (GEM) are from onshore seepage, including mud volcanism, microseepage and geothermal flux; altogether GEM are the second most important natural source of atmospheric methane at present. The amount of methane entering the atmosphere from onshore GEM seems to prevail on that from offshore seepage. Onshore sources inject a predominantly isotopically heavy (13C-enriched) methane into the atmosphere. They are controlled mainly by endogenic (geodynamic) processes, which induce large-scale gas flow variations over geologic and millennial time scales, and only partially by exogenic (surface) conditions, so that they are not affected by negative feedbacks. The eventual influence on atmospheric methane concentration does not necessarily require catastrophic or abrupt releases, as proposed for the “clathrate gun hypothesis”. Enhanced degassing from these sources could have contributed to the methane trends observed in the ice core records, and could explain the late Quaternary peaks of increased methane concentrations accompanied by the enrichment of isotopically heavy methane, as recently observed. This hypothesis shall be tested by means of robust multidisciplinary studies, mainly based on a series of atmospheric, biologic and geologic proxies. [Copyright &y& Elsevier]

Lusi is a sediment-hosted hydrothermal system located in North-East Java, Indonesia. This eruption has been actively bursting boiling mud breccia, oil, gas, and water since the 29th of May 2006 and today occupies a region of 7 km2. Over this large area are scattered thousands of active seepage sites that surround a 100 m diameter central vent characterized by a geysering behaviour. Geophysical, petrography, and geochemical investigations revealed that Lusi is fuelled by a magmatic intrusion at depth, and that hydrothermal fluids migrate from the neighbouring Arjuno-Welirang volcanic complex and into the Lusi conduit zone. A dedicated survey was designed 1) to estimate the type and the amount of gas released both from the seeps and from the central vent, and 2) to map the preferential pathways for the fluids migration. Besides aqueous vapour, CO2 and CH4 represent the main gases released from Lusi. Results show that the areas of intense degassing are along fractured zones of the NE-SW oriented Watukosek fault system. This fault system connects Lusi with the neighbouring volcanoes. The whole area surrounding the crater releases primarily CH4 from thousands of satellite seeps and by invisible diffuse seepage. In contrast, the central vent is CO2–dominated both during the quiescent and geysering phases. Our estimates show that, since the beginning of its activity, Lusi has released carbon into the atmosphere in the order of Mt units. Lusi is unique on Earth today, but has several characteristics in common with the so-called hydrothermal vent complexes from the geological past. The hydrothermal vent complexes were formed as a result of degassing in sedimentary basins affected by Large igneous provinces, and may be responsible for rapid climate changes such as the Toarcian event and the PETM. We argue that Lusi represents the only modern analogue for these palaeo systems. Therefore, quantification of the Lusi degassing holds the promise for a better understanding of the dynamics of deep time carbon degassing from sedimentary basins. [ABSTRACT FROM AUTHOR]