Iron Precipitation in a Natural CO2 Reservoir: Jurassic Navajo Sandstone in the northern San Rafael Swell, UT, USA

Sally Potter-McIntyre, Jessica Allen, Majorie Chan, Weon Shik Han, Si-Yong Lee, and Brian McPherson

The Jurassic Navajo Sandstone extends across much of the Colorado Plateau. Both broad-scale and regional reservoir properties have been extensively studied, particularly in southern Utah where the unit is well exposed. The purpose of this study is to infer diagenetic fluid chemistries of the Navajo Sandstone in the northern San Rafael Swell. In Justenson Flats, a lithologically controlled iron (oxyhydr)oxide reaction front (45 m long X 5 m tall) extends through a cross bed set and abuts a bounding surface at the top. This study documents the mineralogy and geochemistry (via QEMSCAN, whole rock analysis, x-ray diffraction and petrography) in vertical and horizontal transects across the reaction front. Dolomite and ferroan dolomite are the most abundant cements; iron (oxyhydr)oxides, kaolinite, illite and minor gypsum are also present. In nearby Ten Mile Graben fault system, CO2- and hydrocarbon-charged saline waters emanate from the Navajo Sandstone in modern springs and geysers. Comparison of bulk geochemistry from the Justenson Flats reaction front and water chemistry from the modern springs shows good correlation of principle ions. This suggests that fluids similar to the modern reservoir fluids (i.e, CO2- and hydrocarbon-charged saline water) were responsible for the cementation in Justenson Flats. Cements in the reaction front precipitated sequentially. 1. Syndepositional gypsum and some dolomite evaporite minerals precipitated from a near surface water table. 2. Dolomite and ferroan dolomite and clays precipitated from a CO2 and hydrocarbon-charged reducing fluid. 3. Late stage overprinting of iron (oxyhydr)oxide minerals precipitated when an oxidizing fluid infiltrated along fractures. CO2 injection into saline aquifers would precipitate clays; however, the Navajo Sandstone contains only small amounts of feldspar and cannot produce enough clay to affect porosity. Dolomite precipitation does significantly reduce porosity. Diagenetic fluid chemistry is variable both spatially and temporally on a reservoir scale.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012