You should read Thomas Gold's book "The Deep Hot Biosphere" to understand what is going on with Methane underground. Other western sources are required to cowtow to the ridiculous biogenic theory. See also this link for a quick presentation in a popular style.
The unchallanged scientific dogma in the west has been that the hydrocarbons in the Earth are a product of biology. For methane, this ridiculous idea is no longer tenable even in the west, but the whole thing has long been known to be false in the Soviet Union, where Stalin made a post-war push for oil exploration and production, and had an all-Soviet research project to determine the source of petrolium hydrocarbons.
The correct theory was worked out by Kudryavtsev, although I am getting all my information from Thomas Gold, who wrote about this in English in the West (his book that I recently read in dead-trees form is "The Deep Hot Biosphere"). As Gold shows, oil hydrocarbons are thermodynamically favorable at crust pressures of 5000 atm or higher, at temperatures of around 1000 degrees celsius or higher, and they are formed under these conditions of temperature and pressure in the mantle from upwelling abiogenic methane left over from the Earth's formation.
The hydrocarbons float up in solution in the dense methane (it is past it's critical point at these pressures, and it is essentially a liquid, although it has no boiling point). As the hydrocarbons go up, they progressively dehydrogenated by passing through rocks of progressively greater oxygen content, so that light oil is found below, and heavy oil on top. Finally, the oil wanders through pores in sedimentary rock, slowly carbonating sediments into coal as the hydrogen in the hydrocarbons are stolen by oxygen atoms. The fossils in the sediments are preserved in this process of coal formation.
This theory explained the following:
- Coal is associated with oil and other hydrocarbons. This wouldn't be surprising, except that in the biogenic theory, the two types of deposits are formed in completely different ways: oil is fossilized sea plants, coal is fossilized land plants. Gold shows the distribution of coal and oil, and the overlap is precise and striking.
- There is Helium association with hydrocarbons! He is a deep Earth gas, produced from radioactive decay in the mantle and core. It is completely inert, and must be carried around from place to place by dissolving and moving, since it makes no chemical bonds with anything. Yet practically all commercial He production on Earth is from petrolium fields. How did the He get there? Gold realized that the oil is coming from below, since this association is completely inexplicable from biogenic petrolium theory.
- There are heavy metals in the oil, and in the coal, including radioactive elements. YOu find gold, silver, Vanadium, Iridium, Nickel, Uranium, and other metals in hydrocarbons with metallic substitutions. These cannot be biological. The explanation from biogenic theory is replacement, but the replacement must be in deep Earth, since these elements, particularly iridium, are rare in the crust. The elemental composition of petroleum impurities is also an unequivocal source of deep-Earth origin. The heavy impurities make coal burning annoyingly radioactive
- Coal and oil are found in codeposition with metallic deposits, like seams of gold, silver, nickel, and other metals. This association is puzzling, but is explained by Gold's contention that these deposits are produced by metallic hydrocarbons pumping and concentrating the metals as they flow through the Earth. This theory has the advantage that these metals are all found in hydrocarbon solution in significant quantities, while their deposition through water flow (which is the mainstream theory of depositing) has never been shown to work.
- Oil wells can be drilled in deep nonsedimentary bedrock, which has led to ridiculous theories of oil migration and seepage in the biogenic literature. The seepage is a just-so story, the oil is just formed under bedrock. Gold dug and found commercially insignificant but non-trace amounts of oil below 6km of broken bedrock sitting atop only a few feet of sediments, in Sweden. His results were dismissed by journals, because "you can't find oil in nonsedimentary rock".
- The methane outgassings in the theory are required to close the Carbon budget of the Earth. Carbon is locked up in limestone deposits in the ocean.
- the abiogenic theory explains the CO2 emissions of volcanos, and the methane emissions associated with eruptions.
- Oil fields refil: the predictions for the Earth running out of oil have consistently failed since 1973, when the first ones were made. Partly, this is due to new exploration, but mostly it is due to the fact that oil pressure is restored at old fields, leading them to produce many times the initial estimate of quantity. The process of refilling is known to producers to be "light oil coming from deep below", which is obvious in Gold's theory, but inexplicable in the biogenic view.
In the biogenic theory, the fossils in coal are recklessly misinterpreted as clues to the plants that gave rise to the coal, leading to ridiculous statements about the types of plants that fossilized to create the coal. There is no known process of removing everything but carbon from plants, you can't turn carbohydrates (plants, which are mostly polymerized sugar, in the cell wall), to hydrocarbons. Hydrocarbons are common in the universe, but in biology, they are just found in oils.
I had to go into detail, because abiogenic oil is laughed at in the west, despite the fact that it was accepted in the Soviet Union. The only evidence for biogenic oil is the following:
- There are biological markers in oil, including carbon chains holding metal atoms that resemble chlorophyll in structure.
This last part, the clear biological tracers, are explained by Gold's bold idea that there are bacteria active at depths of 20km. He was able to culture previously unknown strains of thermophillic anaerobic bacteria from his oil dig in Sweden, although these were cultured in an ordinary petri-dish using sugar, not under high pressure with petroleum. Who knows what other bacteria are down there.
His idea is very plausible today, considering the extent of vent-life and thermophillic archaea. It is generally believed today that the most ancient forms of life were found at reasonably high temperatures, under conditions of little or no atmospheric oxygen, and these are the conditions that Gold proposes for the deep hot biosphere. The contamination of oil with biological residues is relatively slight. As someone on the internet said, a ketchup stain on a tie doesn't prove that the tie is made of tomatoes.
For your question, the natural gas leaks out of subduction zones as natural oil seeps (see here, but the association with sedimentary rocks is something stupid they made up because they don't want to step on biogenic toes: http://oils.gpa.unep.org/facts/natural-sources.htm), where methane and petrolium come out in tremendous volume. This is the type of emissions that you are asking about. I repeat, it has nothing to do with sedimentary rock, as oil and gas are abiogenic. But this is most of the methane emmissions in the world. It has generally been conceded, even in the west, that these emissions are abiogenic, as their volume is too large to be plausibly associated with any ancient plant matter.
The methane emissions of the oceans and land are ubiquitous, and include mud-fields, and volcanic eruptions. The methane from these emmissions usually does not get hot enough to explode or burn (although there are exceptions, listed by gold, of flames erupting from the ground, and from volcanoes), rather it usually enters the atmosphere and is converted into CO2 and H2O over a period of some week.
As I said, I learned all this from Gold, but I do not know the Soviet prior art. I suspected something like this for a while, just from the implausiblity of generating hydrocarbons from carbohydrates under any natural conditions, but that this is actually the mainstream consensus in Russia was news to me. In case you think that this theory is useless for drilling, the Soviet Union went from an oil-poor nation in 1950 to the largest producer and exporter of petrolium in the 1980s, and Russian scientists attribute this to their superior understanding of the process of oil formation, something with which I must wholeheartedly agree.