612 



UNITED STATES MINERAL RESOURCES 



needed to retain hydrogen sulfide in breccia masses 

 until the gas was oxidized to elemental sulfur differ 

 from deposit to deposit, consisting either of over- 

 lying anhydrite, overlying impervious strata, or of 

 alluvial clay and silt within the breccia masses. Not- 

 withstanding, many limestone breccia masses are 

 barren of sulfur, indicating that they were not 

 sealed against loss of hydrogen sulfide gas. 



The location of west Texas sulfur deposits seems 

 to have been controlled largely by structure. Depos- 

 its lying near the southwest side of the Central 

 Basin Platform occur in anticlinal highs, whereas 

 those in the western part of the Delaware Basin 

 seem to occur along or near lineaments that may 

 represent faults of small displacement. Notwith- 

 standing, no clear-cut controls have been found that 

 satisfactorily explain the location of the huge de- 

 posit in Culbertson County. 



SULFUR-FORMING REACTIONS 



Regarding the chemical reactions that produced 

 the deposits, it is generally agreed that anaerobic 

 bacteria reduced the sulfate ions of anhydrite. Views 

 differ, however, on the manner in which hydrogen 

 sulfide is oxidized to elemental sulfur. Three possi- 

 ble reactions have been proposed: (1) inorganic re- 

 action between the gas and calcium sulfate mole- 

 cules : (2) oxidation of the gas by free oxygen car- 

 ried in ground water; and (3) oxidation of the gas 

 by sulfide-oxidizing bacteria (Feely and Kulp, 1957 ; 

 Davis and others, 1970; and Ivanov, 1968). With 

 particular regard to sulfur deposits in the gulf 

 coast, the second proposal seems to be unfeasible 

 because ground waters there are nearly devoid of 

 free oxygen; the required volumes of water would 

 flush out the accumulated hydrogen sulfide before 

 oxidizing any appreciable amount of that gas. Nor 

 does it seem likely that aerobic, sulfur-oxidizing 

 bacteria could coexist with anaerobic bacteria in 

 the oxygen-deficient, hydrogen sulfide-saturated 

 solutions contained in cap rock. It thus seems that 

 an inorganic reaction between hydrogen sulfide and 

 calcium sulfate would be the most likely to occur. 

 Although this reaction can proceed thermodynamic- 

 ally only at slow rates, it produces both sulfur and 

 a small amount of calcite, which in fact are de- 

 posited together during the second stage of min- 

 eralization. 



SULFUR ACCUMULATIONS IN HYDROCARBONS 



Bacterial attack upon sulfate ions in stagnant 

 waters produces hydrogen sulfide that can combine 

 with organic matter in the associated sediments; 

 the sulfur-bearing molecules can then be incorpor- 



rated into hydrocarbons formed from such parent 

 material. Most crude oils, tar sands, asphalts, and 

 organic-rich shales therefore contain sulfur even if 

 only in low concentrations. The sulfur content of 

 petroleum can range from traces to as much as 14 

 percent, although crude oil containing more than 5 

 percent sulfur is relatively uncommon. Two-thirds 

 of the petroleum produced in the United States con- 

 tains less than 1 percent sulfur, and only about 

 one-tenth contains more than 2 percent sulfur (Mc- 

 Kinney and Shelton, 1967). 



The geologic conditions leading to the wide range 

 of sulfur in hydrocarbons are not well known. It 

 can be assumed that coarser grained sediments will 

 retain less hydrogen sulfide than finer grained sedi- 

 ments. The concentration of sulfate ions available 

 for reduction also can differ, being lowest in seas 

 near the mouths of large rivers and highest in basins 

 undergoing evaporation. Still another variable is the 

 iron content of source beds and connate waters, 

 which will determine how much hydrogen sulfide will 

 be withdrawn from the system as iron sulfide. All 

 these variables probably played a part in deter- 

 mining how much sulfur was incorporated into the 

 parent material of any given crude oil. 



As petroleum matured and migrated, heavier 

 sulfur-bearing molecules could have been filtered 

 out and hydrogen sulfide perhaps was generated as 

 constituent molecules were reconstituted, both 

 mechanisms leading to a reduction of sulfur content 

 of the liquid fraction. Variations in sulfur content 

 also have been noted within given oil pools, but in 

 the aggregate all such changes can only modify the 

 amount of sulfur originally accumulating with the 

 parent organic material. 



The sulfur in tar sands is probably higher than 

 the original concentration if it can be assumed that 

 liquid and gaseous fractions containing less sulfur 

 migrated elsewhere, whereas the sulfur in shales 

 rich in organic matter must be that generated in 

 place. 



Theories of origin for hydrogen sulfide in natural 

 gas are diverse. Hydrogen sulfide and methane can 

 be generated simultaneously in fresh-water swamps, 

 for example, and perhaps both gases were distilled 

 together as sulfur-bearing petroleum underwent 

 chemical reconstitution at high pressures and tem- 

 peratures. Sulfate ground water coming in contact 

 with petroleum activates anaerobic bacteria and re- 

 sults in generation of hydrogen sulfide, and the 

 same reactions can take place where petroleum is 

 trapped under beds of anhydrite. Although most 

 sour natural gas contains only a few percent hydro- 

 gen sulfide, some contains more than 80 percent of 



