From Yangtze: Nature, History, and the River, by Lyman P. Van Slyke (Stanford Alumni Assn., 1988), p. 95:
While the whole Yangtze Valley east of the Three Gorges was supplied with salt from the Liang-huai district, Szechwan produced its own salt in an industry of great antiquity and technological sophistication. Salt in Szechwan is the result of its geological history. As we have seen, prior to the collision between Indian and Asian crustal plates, Szechwan was submerged by a great primordial ocean. As the land uplifted, Szechwan became an inland sea, then finally took on the mountain-girt basin character it has possessed throughout historic times. This process, dating from Triassic times 250 million years ago, produced large underground brine or solid salt deposits and, from the dense vegetation of many millions of years, large pockets of natural gas and extensive beds of coal.
The underground deposits lie at various depths, though a few brine pools can be found on the surface. We do not know when this salt was first exploited, but the earliest wells are claimed for the third century B.C. and attributed to Li Ping, the engineering genius who conceived All-Rivers Weir above Chengtu…. In early times, these wells must have been quite shallow, but within a few hundred years records indicate dozens of deep wells in the most productive regions. The brine obtained from these wells was evaporated by boiling, with wood (or charcoal) from the abundant forests as the principal fuel. At a later date, coal was also used for this purpose. In a few fortunate locations, even shallow wells brought in natural gas as well as brine. This enabled the brine to be boiled with gas carried by bamboo pipes.