Alteration of Alluvium by Natural Gas in the Pyramid Lake Area, Nevada

Abstract

Some of the chemical and physical processes associated with the migration of natural gas and their impact on engineering properties of soils are not well known. In the vicinity of the Truckee River Delta, gas (mostly methane with some hydrogen sulfide) was noted in natural seeps and in shallow test holes in sandy Holocene deltaic alluvium and in ancient Lake Lahontan clays and silts. Shiny black cores of Lahontan silt, after exposure to air, become buff from the surface inward. The black color is caused by iron sulfide pigmentation which disappears upon oxidation. Seasonal color changes from black (when submerged) to buff (when air-exposed) were observed in outcrops of Lahontan silts in the riverbed. They indicate periodic oxidation and reduction of the pigment by diffusion of either oxygen or hydrogen sulfide. Iron sulfides change the plasticity, shear strength, and other engineering properties of sediments and, together with gas pressure, may be partially responsible for underwater landslides in the delta. Oxidation of iron sulfides in ground water near the ground-water table probably caused development of a limonitic “rusty” stain on pebbles and sand near the top of deltaic alluvium. Isotopic composition of carbon in the methane seems to indicate its “marsh” origin. Hydrogen sulfide is probably created by bacterial ( Desulfovibrio desulfuricans ) reactions with methane and sulfates.