SULFUR 



611 



A specific sequence of geologic events must take 

 place, and specific geologic conditions must be met 

 before sulfur deposits can form ; should any one ele- 

 ment be missing, sulfur mineralization will not oc- 

 cur. Both in the gulf coast and in west Texas, the 

 basic conditions needed for sulfur mineralization are 

 (1) solution of evaporite minerals, forming porous 

 ground, (2) migration of petroleum into the pre- 

 pared ground, (3) generation and retention of hy- 

 drogen sulfide gas within the formation, and (4) 

 oxidation of the hydrogen sulfide to elemental sul- 

 fur. The required geologic steps differ between the 

 gulf coast and west Texas, as described in the fol- 

 lowing paragraphs. 



GULF COAST 



Preparation of the ground in the gulf coast en- 

 tailed formation of lenses of anyhdrite over the 

 upper surfaces of salt diapirs, which consist of im- 

 pure rock salt containing only several percent anhy- 

 drite. Ground water dissolved the salt at the upper 

 surfaces of the diapiric masses, permitting residual 

 accmumlation and subsequent lithification of the less 

 soluble anhydrite. Anhydrite accumulated in appre- 

 ciable thicknesses only where aquifers intersected 

 the apexes of salt diapirs and where the rate of up- 

 ward flow of rock salt was equalled by the rate of 

 salt solution, providing stable surfaces upon which 

 the anhydrite could accumulate. Conditions favor- 

 ing caprock development differed widely, inasmuch 

 as anhydrite lenses range in thickness from a few 

 feet to more than 1,000 feet ; more than one-third of 

 the 190 diapirs in the gulf coast are overlain by less 

 than 200 feet of anhydrite, thicknesses inadequate 

 to permit formation of sulfur deposits of commer- 

 cial size (Taylor, 1938; Goldman, 1952; Murray, 

 1961; Halbouty, 1967; Bodenlos, 1970). 



During their intrusion, salt masses dragged sur- 

 rounding strata upward and produced structures 

 favoring migration of hydrocarbons toward the 

 diapirs. Hydrocarbons entered most, but not all, 

 caprocks partly because oil sands do not underlie the 

 entire diapiric province, and partly because petro- 

 leum was too tightly held in peripheral traps to have 

 escaped in quantity. As a result, biogenic limestone 

 produced by bacterial attack overlies anhydrite as 

 lenses ranging from a few to hundreds of feet in 

 thickness. 



Hydrogen sulfide was retained and oxidized to ele- 

 mental sulfur only where host caprocks were over- 

 lain by thick beds of impermeable clay or mudstone. 

 Caprocks barren of sulfur are overlain either by 

 thin beds of clay or directly by porous sandstone, 

 which permitted hydrogen sulfide to escape. 



The incidence of sulfur deposits in the gulf coast 

 indicates how infrequently all steps leading to their 

 generation are met. Of the 190 salt diapirs in the 

 gulf coast, the caprocks of only 18 contained ore 

 bodies yielding more than 1 million tons of sulfur; 

 another three or bodies opened more recently un- 

 doubtedly contain ore bodies of substantial size. 



Although the structures produced by intrusion of 

 salt diapirs favor migration of ground water and 

 hydrocarbons toward the salt masses, the critical 

 factors needed to produce sulfur deposits seem to be 

 stratigraphic — permeable strata are needed up to 

 the apexes of diapers to permit the flow of fluids, 

 whereas impermeable strata are needed above cap- 

 rocks to retain hydrogen sulfide. It seems unlikely 

 that sulfur deposits can form in stratigraphic se- 

 quences uniformly coarse grained or uniformly fine 

 grained and impermeable. 



WEST TEXAS 



The sulfur deposits in west Texas occur in thick 

 extensive units of bedded anhydrite of evaporite 

 origin that underlie the entire Delaware Basin and 

 adjacent platforms. As bedded anhydrite is one of 

 the least permeable of rocks, greatly inhibiting 

 movement of ground water and hydrocarbons, 

 ground was prepared for sulfur mineralization by a 

 sequence of events differing from that in the gulf 

 coast. Evidently all deposits in west Texas were 

 formed in lenses or chimneys of anhydrite collapse 

 breccia, so it seems probable that ground water 

 moved upward along joints or faults from underly- 

 ing aquifers and dissolved anhydrite at the base of 

 the evaporite units. This solution process produced 

 an underground, cavernous karst system, into which 

 overlying anhydrite collapsed. Hydrocarbons as- 

 cending along the same fissures from underlying oil 

 sands then penetrated the breccias, and the broken 

 anhydrite was altered through bacterial activity to 

 a fine- to medium-grained gray limestone. There- 

 after, white calcite and elemental sulfur were de- 

 posited in the breccia voids and in the pores of the 

 gray limestone (Davis and Kirkland, 1970; Hinds 

 and Cunningham, 1970; McNeal and Hemenway, 

 1972). 



Some mineralized breccia masses are confined to 

 the lowermost parts of the anhydrite units, but 

 others extend as chimneys completely through them, 

 impinging against overlying carbonate or clastic 

 beds. Chimneys breaking through anhydrite units 

 exposed at the surface produced swallow holes that 

 diverted surface drainage, and the voids in the brec- 

 cia became clogged with very fine to coarse-grained 

 alluvium. In the west Texas deposits, the seals 



