GROUNDWATER-FLOW, LATE CEMENTATION, AND PETROLEUM ACCUMULATION IN THEPERMIAN LYONS SANDSTONE, DENVER BASIN

The gray diagenetic facies of the Permian Lyons Sandstone, which is as sociated with all known petroleum accumulations in the formation, form ed late in the history of the Denver basin as an alteration product of the formation's red facies. The red facies that makes up most of the sandstone contains iron oxide coatings, quartz overgrowths and calcite cements. The gray facies, which occurs locally in the deep basin, is distinguished by pore-filling dolomite and anhydrite cements and by a lack of iron oxide and calcite. The dolomite and anhydrite cements ove rlie bitumen that was deposited by migrating oil, and hence formed aft er oil was first generated in the basin, late in the Cretaceous or ear ly in the Tertiary. The isotopic composition of oxygen in the dolomite ranges to such light values that the cement must have formed deep in the basin in the presence of meteoric water. The gray facies likely fo rmed in a regime of groundwater flow resulting from Laramide uplift of the Front Range during the Tertiary. In our model, saline groundwater flowed eastward through the Pennsylvanian Fountain Formation and then upwelled along the basin axis, where it discharged into the Lyons San dstone. The saline water mixed with more dilute groundwater in the Lyo ns, driving a reaction that dissolved calcite and, by a commonion effe ct, precipitated dolomite and anhydrite. The facies' gray color result ed from reduction of ferric oxide in the presence of migrating oil or the Fountain brine. Underlying source beds by this time had begun to g enerate petroleum, which migrated by buoyancy into the Lyons. The asso ciation of the gray facies with petroleum accumulations can be explain ed if the Fountain brines discharged across aquitards along the same f ractures that transmitted oil. As petroleum accumulated in the Lyons, the newly formed cements prevented continued migration, as is observed in shallower strata, by sealing oil into the reservoirs from which it is produced today.