There is indeed evidence for deep methane on Earth: it vents from the mid-oceanic ridges, presumably arising from the mantle, though the amount vented is relatively small - less than the amount emitted annually in cow farts (incidentally, there are persuasive biotic explanations for the origin of this vented methane).
A new study by the US Department of Energy and Lawrence Livermore Lab. suggests that there may be huge methane deposits in Earth’s mantle, 60 to 120 miles deep. But today oil companies are capable of drilling only as deep as six miles, and this in sedimentary rock; in igneous and metamorphic rock, drill bits have so far penetrated only two miles. In any attempt to drill to a depth remotely approaching the mantle, well casings would be thoroughly crushed and melted by the pressures and temperatures encountered along the way. Moreover, the DOE study attributes the methane deposits it hypothesizes to an origin different from the one Gold described.
More to the point, Gold also claimed the existence of liquid hydrocarbons - oil - at great depths. But there is a problem with this: the temperatures at depths below about 15,000 feet are high enough (above 275°F) to break hydrocarbon bonds. What remains after these molecular bonds are severed is methane, whose molecule contains only a single carbon atom. For petroleum geologists this is not just a matter of theory, but of repeated and sometimes costly experience: they speak of an oil “window” that exists from roughly 7,500 feet to 15,000 feet, within which temperatures are appropriate for oil formation; look far outside the window, and you will most likely come up with a dry hole or, at best, natural gas only. The rare exceptions serve to prove the rule: they are invariably associated with strata that are rapidly (in geological terms) migrating upward or downward.
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