Source of molten elemental sulfur and hydrogen sulfide from the Inigok well, northern Alaska

Elemental sulfur (S0) and high-pressure hydrogen sulfide (H2S), the first recorded occurrence of both from a well in northern Alaska, were encountered in the Husky 1 Inigok exploratory well in 1978. Located about 100 mi (160 km) southwest of Prudhoe Bay, the well was targeting an anticlinal structure on the northeast flank of a large late Paleozoic basin beneath the coastal plain and foothills region of the eastern National Petroleum Reserve in Alaska. The sulfur and gas first appeared at a depth of 17,570 ft (5355 m) while drilling in dark-gray to black organic carbon–depleted Carboniferous limestones (the Lisburne Group) at a temperature of 347F, well above the melting point of S0. Solidification of the molten sulfur by cooling during the drilling operation caused the drillstem to stick and delayed drilling operations for several months before being released. To determine the nature and source of the sulfur and H2S and to establish any relationship between them, we performed chemical analyses on S0, drilling mud, and cuttings. Numerous holes and vesicles in some sulfur samples indicate the presence of gas, but other samples, without vesicles, contain sharp edges, fractures, and x-ray diffraction patterns characteristic of orthorhombic and not amorphous sulfur, thus arguing against a molten source in these samples. Isotopic values suggest that the sulfide in widely separated mud samples has a common source, and that the 34S values for S0 are slightly more enriched in 32S in all deeper samples. The S0 from the drill pipe at 17,240 ft (5254 m) has a 34S value similar to that of the sulfur in mud samples. The results appear to indicate that H2S is the source for the S0. The source of the original H2S is an enigma. We consider several possibilities, among them, formation by biological sulfate reduction, thermal sulfate reduction during burial, a volcanic source, or generation in the deeply buried sediment from thermal disproportionation of pyrite. Here, we evaluate several possible origins based mostly on the properties of the enclosing limestone or shale host rocks, the downhole temperatures, and the stable isotope ratios measured and conclude that pyrite is the most probable source.