YOUNG PROFESSIONAL
caprock is associated with large quantities of native sulfur. One example for such a setting is the Spindletop salt diapir, which triggered the Texas oil boom. Once oil production at Spindletop was exhausted, the caprock of the salt dome was mined for sulfur.
S
tinky ulfur
Amanda Labrado, SA-3896
When my love affair with geology began I was too young to even know it as a branch of science. Any outdoor activity meant collecting a new rock, questioning their origin and staring at their beauty in awe. As I grew older, science became my pas- sion and my appreciation for the natural world grew. Geology was presented to me as a possible major by my college advisor during orientation, and I was completely hooked when he told me hiking and camping were required to earn my degree. Little did I know that there was also something unexpected, my love for the outdoors would soon turn into love for smelly, slimy stuff!
During my undergraduate studies, I became fascinated with the interconnectedness of living and non-living worlds and wanted to understand how these connections work. A huge opportunity presented itself when I was offered a Master’s project on how microbial activity affects cave formation and the limits of life since these “bugs” thrive in a subsurface environ- ment with no sunlight and little oxygen. The project required me to get my hands dirty in caves, and as I become more experi- enced with speleology/caving, I was once again able to combine hobbies with my wish to learn how the bright, white, sticky microbial biofilms, or “tiny bug slime” that sometimes exist in caves, work. The white stuff turned out to be accumulations of tiny native sulfur globules formed by the oxidation of sulfide, which gives one a nose full of rotten-egg smell! The toxic sulfur compound is a lesson for a lifetime, or one would think…
As it turns out, climbing, crawling, traversing, and wiggling my way through the large Frasassi cave system (near Genga in east central Italy) while carrying about 20 pounds of gear and being covered in mud during my Master’s did not turn me away from inhaling the stench of sulfur. Instead of being fed up, I began to seek it out. Shifting my focus from cave systems to salt diapirs for my Ph.D. research, I quickly encountered an old friend, native sulfur. In the top and sometimes flanking position of salt diapirs, an assemblage of lithologies, referred to as caprock, can be found and generally consists of anhy- drite located next to the salt, then gypsum, and occasionally carbonate-dominated lithologies. Sometimes, the carbonate
Today, carbonate caprocks are relevant in manifold ways for oil and gas exploration. They can act as reservoirs, traps, seals, or conduits for hydrocarbons and pose possible drilling hazards. Furthermore, the spatial and temporal presence or absence of caprocks, as well as their lithological composition, record the geological history of a salt diapir. This record pro- vides essential information about kinematics, fluid migration and composition, and thermal history. Presumably, carbon- ate caprock on salt diapirs forms in two stages. First, salt- undersaturated water preferentially dissolves halite, leading to the underplating of less soluble anhydrite to the base of older caprock. Second, microbes or thermochemical sulfate reduc- tion mediate the replacement of sulfate minerals, i.e. gypsum and anhydrite, with carbonate minerals and native sulfur by coupling the oxidation of hydrocarbons to sulfate reduction. While the mechanistic details of these transformations are not known, there is a consensus regarding the overall processes. In contrast, the last step in the formation of limestone-sulfur caprock assemblages, namely the genesis of native sulfur, is controversial.
The currently best accepted model for the genesis of native sulfur in limestone-sulfur caprock assemblages postulates that sulfate-reducing bacteria produce sulfide, which is then
oxidized by molecular oxygen (O2) to native sulfur. What bugs me (pun intended) about this explanation is that to generate large native sulfur deposits there needs to be an ample, well-
balanced supply of O2, which must meet the following three conditions: (i) be abundant enough to allow for the oxidation of sulfide to native sulfur, but (ii) sufficiently scarce enough to prevent further oxidation of native sulfur to sulfate, and (iii) can be kept away from the sulfate-reducing bacteria for whom
O2 is toxic. From my observations in the cave systems – where air is available – I understand that comparably small amounts of native sulfur can be generated in this way, but it is diffi- cult to conceive how this can be achieved on the large scale of native sulfur deposits associated with caprock on salt domes.
Putting simultaneously hydrocarbons, O2 and sulfate-reducing bacteria into one box leads to contradictions, as O2 and sulfate- reducing bacteria must be kept apart to avoid poisoning the bacteria, and O2 and hydrocarbons must be kept apart to avoid
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Geology was presented to me as a possible major by my college advi- sor during orientation, and I was completely hooked when he told me hiking and camping were required to earn my degree. Little did I know that there was also something unex- pected, my love for the outdoors would soon turn into love for smelly, slimy stuff!
direct consumption of the O2 during hydrocarbon oxidation. Of course, explanations that make this scenario feasible can
be found, such as fluctuations between hydrocarbon and O2 supply. During one phase sulfate is reduced to sulfide, whereas
during the next phase sulfide is oxidized by O2 to native sulfur.
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