Stratigraphic evaluation of reservoir and seal in a natural CO2 field: Lower Paleozoic, Moxa Arch, southwest Wyoming

Abstract

The Moxa Arch in the Greater Green River Basin, southwestern Wyoming, hosts two potential reservoirs for CO2 sequestration. The Bighorn Dolomite and Madison Limestone are interpreted to be independent reservoirs based on differing CO2 compositions and production histories; the two reservoirs are separated by Devonian carbonates, siliciclastic rocks, and evaporites. On the Moxa Arch, the Bighorn ranges in thickness from 67 to 120 m with porosities from 3 to 15 percent. The massive buff-colored Steamboat Point Member comprises the bulk of the subsurface Bighorn in southwest Wyoming. Mottled dolostone (light-colored patches with higher porosity and dark-colored patches with lower porosity) is very common and is presumably the result of preferential early dolomitization of bioturbation. Core study suggests that this factor affects gas saturation and storage in this extensive reservoir. The Lower Member of the Jefferson Formation is the most probable seal within the Devonian stratigraphy. Strata of this member are interpreted to have been deposited in a shallow basin semi-isolated from the deeper marine environment to the west. 87Sr/86Sr isotopic analyses of anhydrite sampled from Moxa Arch well cuttings support the interpretation of a depositional environment exposed to a mixture of seawater and freshwater. High-frequency relative sea level fluctuations superimposed on a gently sloping shelf produced alternating layers of marine carbonates, peritidal siliciclastic rocks, and evaporites. The evaporites are interpreted to seal CO2 in the Bighorn Dolomite from the overlying Madison Limestone. The Lower Paleozoic strata on the Moxa Arch provide an effective trap-reservoir-seal combination for naturally occurring CO2 with potential applications to future studies at analogous locations in the central Rocky Mountain Region.