Published in Petroleum Transactions, AIME, Volume 207, 1956, pages 246–251. Abstract A reservoir mechanism of sulfur recovery by the Frasch process is presented. Improving the economics of recovery appears to be largely a well, rather than a reservoir problem. A most important factor is the limitation of the lateral extraction of sulfur about the wells due to the almost vertical flow of the injected hot water. Model studies are described which confirm the mechanism of sulfur recovery by presently used methods. Studies on a five-spot system indicate improved economics regarding recovery, recovery rate, thermal efficiency, and well spacing. Introduction Sulfur mining by the Frasch process is one of the most important methods of sulfur production. Developed by Herman Frasch in 1894, it enjoyed a rapid growth and presently supplies 40 per cent of the world's sulfur and about 80 per cent of the U. S. sulfur. The Frasch process of mining the underground native sulfur deposits involves melting the sulfur in place by introducing super-heated water into the sulfur bearing formations, and producing the sulfur as a liquid. Many problems are encountered in mining these native sulfur deposits, and sulfur production depends chiefly upon the nature of the sulfur deposit and the underground thermal efficiency of the heat transfer system. The Recovery Mechanism Sulfur is typically found in salt dome cap rock as crystal aggregates occurring within the cavities, seams, and pores of limestone and gypsum formations. Sulfur-bearing limestone is typically cavernous, vugular, and fractured. Porosity averages about 20 per cent and sulfur saturation averages approximately 25 per cent by weight. The sulfur bearing formations are very heterogeneous in nature and physical characteristics vary widely within small distances. Total cap rock thickness may vary from 50 to more than 1,000 ft, and net productive thicknesses from a few feet to a few hundred feet. In most cases the upper portion of the limestone formation is barren of sulfur, varying from 5 to 2,000 ft in thickness. The immediately overlying sediments and the underlying dense anhydrite formation provide permeability barriers to the flow of water from the cap rock section.