GEOCHEMICAL AND PETROLOGICAL COMPARISONS OF BLEACHED CAP ROCKS FROM THE GREEN RIVER CO2 ACCUMULATION

At Green River CO₂ and CO₂- SO₄- charged fluids leak to surface up the Little Grand Wash fault, migrating into overlying aquifers. Drill-core collected in a 2012 drilling campaign, 90 m away from the fault trace, documents bleaching related to these migrating fluids. The mineralogy and Sr, S, O and C-isotope compositions of two 6-9 cm thick bleached sections of mudstone/claystone above CO₂-hosting reservoir sandstones within the Entrada Sandstone and at the base of the Carmel Formation were studied. A third, smaller, 3.1 mm wide bleached alteration zone was observed at the uppermost contact between a primary gypsum bed and red siltstone bed within the Carmel Formation. All of these reaction fronts represent late-stage diagenetic events where CO₂, SO₄ and H₂S from the migrating CO₂-charged fluids have had time to diffuse into the fine-grained impermeable rocks. Geochemical and petrological/petrographic comparisons of the alteration zones show the major reactions are the dissolution of hematite, dolomite and feldspar, and the precipitation of Fe-dolomite and clays. Pyrite was observed, precipitating as disseminated grains in the Entrada cap rock and as bedding parallel veins in the basal Carmel cap rock. Porosity and permeability measurements show reservoir porosity and permeability has not been significantly affected in all three cap rocks. Multicomponent numerical and one component analytical modelling of each reaction front reproduced the major mineralogical changes seen. The timescale for the development of the 6-9 cm hematite reaction fronts were calculated to be in the order of 100,000 years, while the 3.1 mm reaction front was on the order of 5-10 years. The Entrada cap rock and the gypsum reaction front were better modelled by a single pulse of SO₄-rich, CO₂-saturated fluid. While the Carmel cap rock was modelled by two pulses of SO₄-rich, CO₂-saturated fluid with an intervening pulse of CO₂-poor SO₄-rich fluid. Observations and modelling of these three cap rocks show that in clay-, silt-, and mud-rich beds in contact with mildly reducing CO₂-saturated fluids, reactions take tens of thousands of years to progress just centimetres, with reaction rates slowing with time. Saline aquifers with clay-, silt-, and mud-rich cap rocks are therefore ideal as CO₂ storage sites.