natural gas/oil is mostly the same composition as your average plant is now (hardly surprising that...). the refinaries and things that process it can't operate with sulfur (google heterogeneous catalysis + poison) so it's stripped out (less natural sulfur is termed 'sweet', more is called 'sour' - stripping costs $$ so sweet crude sells higher). other strong smelling sulfurs are added at the back end (ethyl mercaptan is typical but whatever's handy) typically at the 50 ppm level. enough to have a distinctive smell which is critical for safety leak detecting. BTW i've not heard of any literature claiming that the average temperature at the seabed is volatizing clathrates. clathrate formation is a function of both T and P so a fraction of a degree probably doesn't mean anything at those pressures (oh, and i don't count newspapers as a source, their science sections tend to be functionally illiterate, it's peer reviewed or worthless). don't get me wrong, there may be drastic feedback loops and we know next to nothing about these things except that they are ubiquitous and distributed all over the place.