The U.S. Geological Survey (USGS) in cooperation with the Geological Survey of Denmark and Greenland (GEUS), recently assessed gas resources in the Lower Paleozoic Alum shale of Denmark. Two assessment units (AUs), one onshore and one offshore, were defined. Assuming unrestricted application of best practice technology, recoverable resources of 0 to 4.8 TCFG (mean = 2.5 TCFG) were estimated onshore and 0 to 8.5 TCFG (mean = 4.4 TCFG) were estimated offshore. The wide range of these estimates reflects the sparse data and geological uncertainty inherent in this untested play. The USGS estimates reflect a recovery efficiency of about ten percent of original gas in place. The AUs each consist of Paleozoic shales in tilted fault blocks at depths of 1.5 to 7 km. The fault blocks, identified on seismic data and sparse well control, occur onshore in Jylland and Sjlland and in adjacent offshore areas. "Sweet spots" were defined as fault blocks that contain both TOC-rich Furongian Alum Shale and thick Silurian strata, indicating minor Late Paleozoic uplift and erosion and thus higher probability of gas retention, which is the main technical risk to the play. Large volumes of oil and then gas were likely generated during late Silurian-Devonian burial in foreland basins on the margins of the Baltic craton. In most areas the Alum Shale attained a maturation rank of dry gas. Shale oil resources are thus not expected to occur in Denmark. The Alum Shale has been tested in southernmost Sweden with negative results, probably because non-associated gas was lost in areas subject to intensive late Paleozoic uplift and erosion. However, recoverable gas may be preserved in areas where renewed subsidence took place in the Permo-Triassic with maximum reburial during the Cretaceous to early Paleogene. Uncertainty in petroleum system modeling allows for the possibility that some shales retained generation potential despite Paleozoic burial yielding additional hydrocarbons during the Mesozoic and Cenozoic.