--> --> Geochemistry of Bubbling Springs, Atlantic Rim, South-Central Wyoming

AAPG Rocky Mountain Section Meeting

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Geochemistry of Bubbling Springs, Atlantic Rim, South-Central Wyoming

Abstract

The Atlantic Rim, located along the eastern edge of the Washakie Basin in south-central Wyoming, hosts natural gas production primarily from Upper Cretaceous sandstone and coal reservoirs. In addition to this controlled subsurface production, historic records refer to gas-rich bubbling springs in the same area. Previous work spurred by coal bed natural gas interest investigated the water chemistry of these springs, however, the composition or source of the spring gas has not been investigated geochemically. As part of the Wyoming State Geological Survey’s Statemap program, all springs were identified within the Garden Gulch 7.5’ quadrangle, a study area located on the crest of the Dad Arch, a gentle west-plunging anticline on the Atlantic Rim. Of the identified springs, many were previously undocumented and approximately two-thirds were bubbling during the summer of 2018. All observed bubbling springs discharged from the Upper Cretaceous Lewis Shale or from the underlying Almond Formation. Gas and water samples were collected from five bubbling springs with various morphologies and water- and gas-discharge rates. The five springs emit a dry gas that is predominantly methane with minor amounts of nitrogen and trace amounts of ethane, oxygen, argon, and helium. Isotopic analysis of the methane yields carbon isotope ratios (δ13C-CH4) between -58.56‰ and - 47.26‰, and hydrogen isotope ratios (δD-CH4) ranging from -266.1‰ to - 289.3‰. These values indicate that both thermogenic and microbial acetoclastic sources contribute methane to the springs. A probable source for this gas is a coal-bearing unit underlying the Lewis Shale such as the Almond Formation.