For natural gas, these fall naturally into two categories, according to the IEA. The first is seasonal storage sites, which comprise aquifers (including depleted oil and gasfields). salt caverns. mined caverns. and disused mines. For peak storage, gasholders - originally used to store town gas - and linepacking are used. In addition, LNG storage tanks exist for either baseload or peak-shaving duties, depending on the market. of organic sulphur compounds, typically at the rate of 30 ppm. See also odorant.