Undiscovered Gas Resources in the Alum Shale, Denmark, 2013
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Undiscovered Gas Resources in the Alum Shale, Denmark, 2013 6° E 7° E 8° E 9° E 10° E 11° E 12° E 13° E 14° E 15° E 16° E 17° E 18° E FINLAND NORWAY RUSSIA sing a geology- NORWAY SWEDEN ESTONIA Ubased assessment Area of LATVIA 58° N Assessment LITHUANIA methodology, the U.S. IRELAND Geological Survey U.K. POLAND estimates a mean Kattegat GERMANY undiscovered volume of NORTH SEA FRANCE 6.9 trillion cubic feet of 57° N natural gas in the Alum SWEDEN Shale in Denmark. DENMARK Jylland 56° N BALTIC SEA Danish Alum Shale Sjaelland 55° N Onshore AU Offshore AU Figure 1. Map showing assessment units (AUs) in the 0 30 60 KILOMETERS Alum Shale Total Petroleum 0 30 60 MILES GERMANY POLAND System (TPS) in Denmark. 54° N Introduction thermally immature in all but the deepest 1,000,000 square kilometers (km2) parts of the Norwegian-Danish Basin, (Nielsen and Schovsbo, 2011). The U.S. Geological Survey (USGS) where some Jurassic strata associated In northern Denmark the Alum recently completed a geology-based with salt structures may be marginally Shale can exceed 180 meters (m) in assessment of continuous gas resources mature with respect to oil (Petersen and thickness (Nielsen and Schovsbo, 2007). of the lower Paleozoic shales in Denmark others, 2008). The Mesozoic shales are, Southward it thins to less than 20 m, (fig. 1). Stratigraphically equivalent strata therefore, not likely to contain significant probably as a result of syndepositional have been tested by several wells in petroleum resources and are not uplift and erosion near the margins of the Scania (southern Sweden), with largely considered further in this study. Baltic craton. Consequently Paleozoic negative results. However, Paleozoic Lower Paleozoic black shales of shales are not considered to be potentially shales in Jylland and on Sjælland have Cambrian, Ordovician, and Silurian age productive south of the Ringkøbing-Fyn not been tested by drilling and various lines of geological evidence suggest that may be potentially productive. Of these, High in Denmark. significant natural gas resources may be the Cambrian to Lower Ordovician In Late Ordovician and Silurian preserved in certain tilted fault blocks in Alum Shale is the thickest and richest time, before the Caledonian tectonic the subsurface of Denmark. (Schovsbo and others, 2011); it is the collision, the Baltic passive margins principal subject of the assessment. were transformed into foreland basins Geology of Potentially Productive The Alum Shale was deposited in as the Laurentian and Avalonian cratons an epicontinental sea that covered the approached Baltica (Schovsbo, 2003). Danish Shales passive margins of the Baltic craton from Thick successions of sedimentary Organic-carbon-rich rocks ranging Middle Cambrian into Early Ordovician strata buried the Alum and other lower in age from Middle Cambrian to Early time. During maximum flooding in the Paleozoic shales to depths of 4 to 5 Cretaceous are present in the subsurface Early Ordovician, organic-rich mudstones kilometers (km), bringing them to of Denmark. Mesozoic shales are accumulated over an area of more than thermal maturity for oil and then for U.S. Department of the Interior Fact Sheet 2013–3103 U.S. Geological Survey December 2013 gas (greater than 2-percent graptolite In Denmark, the Alum Shale probably occurred in Cretaceous to early Paleogene time. Modeling suggests that reflectance; 1.6-percent oR , vitrinite- comprises three members that differ in reflectance-equivalent maturity) in most composition, texture, and distribution. the thermal rank reached during the early areas (Buchardt and others, 1997; Petersen The Middle Cambrian lower member Paleozoic was never exceeded during and others, 2013). Given the thickness and was deposited in an oxygenated the reburial phases. Therefore, a second richness of the shales, this burial history environment and has the lowest organic episode of gas generation is considered almost certainly resulted in generation of matter content of the three. The Upper unlikely except in the deeper parts of large volumes of hydrocarbons. Cambrian (Furongian) middle member the Danish-Norwegian Basin where the Regional uplift, normal faulting, and accumulated under conditions of present-day depth of the lower Paleozoic erosion in the Early Devonian (Mogensen oxygen depletion. The Furongian is exceeds 7 km (Lassen and Thybo, 2012). and Korstgård, 2003) affected the lower the most organic-rich and is the most Nevertheless, because of sparse data and Paleozoic shales throughout Denmark likely source-rock system reservoir modeling uncertainty, the possibility and adjacent areas, bringing the shales considered in this assessment (fig. 3). exists that lower Paleozoic shales to depths of less than 1,000 m in some It is thickest in northern Denmark. The have retained hydrocarbon-generating areas. Discontinuous subsidence was upper member, of Early Ordovician potential throughout the Paleozoic renewed in the Permo-Triassic, Early age, has concentrations of organic and may have generated additional Cretaceous, and Paleogene, followed by matter intermediate between the Middle hydrocarbons during Mesozoic and uplift in the late Neogene and by glacial Cambrian and Furongian shales. Cenozoic time in some areas. erosion in the Pleistocene. Consequently, the Paleozoic shales Geological Model for Assessment Assessment Units occur today as remnants in tilted fault blocks, which include strata as young In Denmark, large volumes of Two separate continuous-type as earliest Permian. The fault blocks oil and then gas were generated from assessment units (AUs), one onshore are preserved beneath the Variscan organic-rich lower Paleozoic shales and one offshore, were defined. Each unconformity and are overlain by rocks during deep burial in pre-Caledonian consists of Paleozoic shales preserved of the Zechstein Formation and younger foreland basins on the margins of the in tilted fault blocks currently at depths strata (fig. 2). Baltic craton. In most areas the burial between 1.5 and 7 km in the subsurface This complicated geological history history resulted in temperature exposures in Denmark. The fault blocks are of burial, uplift, and erosion is known sufficient to bring organic matter to a known, on the basis of seismic data to have caused depressurization and gas maturation rank of dry gas, cracking and sparse well control, to occur in loss in Scania, where some areas have previously formed oil. As a result, northern Jylland, on Sjælland, and in been uplifted to shallow depths (Pool significant volumes of shale oil are not adjacent offshore areas in the Baltic Sea, and others, 2012). Similar gas loss is expected in Denmark. the Kattegat, and the North Sea (fig. believed to account for the absence of Although free hydrocarbons were 1). Within these two assessment units, producible hydrocarbons in a well in the lost in areas subject to intensive uplift “sweet spots” were defined as those fault Kattegat (fig.1). Uplift to shallow depths and erosion during the Devonian, blocks that contain Furongian Alum and depressurization during the late significant quantities of natural gas may Shale and thick post-Alum Paleozoic Paleozoic pose the greatest risk to natural have been preserved in certain areas. strata, indicating less intense exposure gas retention and resource potential in the Subsidence was renewed in the Permian to late Paleozoic uplift and erosion and Alum Shale of Denmark. and Triassic and maximum reburial greater probability of gas retention. Figure 2. Schematic South Oil well North cross section of 0 the geology and EXPLANATION stratigraphy in part of northern Jylland, Alum Shale Denmark, showing the 2 Shale geological situation Sandstone of lower Paleozoic shales, including the Clay and siltstone prospective Alum Chalk Shale, in a tilted 4 fault block (vertical Depth, in kilometers Basement exaggeration ~x2). Clay and sandstone (Courtesy of Geological Survey of Denmark and 5 KILOMETERS Variscan unconformity Greenland.) 6 Table 1. Key assessment input data for continuous assessment units in the Alum Shale Total Petroleum System in Denmark. [EUR, estimated ultimate recovery per well; BCFG, billion cubic feet of gas; AU, assessment unit. The average EUR input is the minimum, median maximum, and calculated mean. %, percent] Danish Alum Shale Onshore AU Danish Alum Shale Offshore AU Assessment input data Calculated Calculated Minimum Mode Maximum Minimum Mode Maximum mean mean Potential production area of AU (acres) 3,000,000 3,225,000 4,000,000 3,408,333 4,700,000 5,042,000 6,250,000 5,330,667 Average drainage area of wells (acres) 120 160 200 160 120 160 200 160 Percentage of AU in sweet spots (%) 10 20 40 23 15 30 60 35 Input data for geologic sweet spots Average EUR (BCFG, gas) 0.1 0.45 1.3 0.492 0.1 0.45 1.3 0.492 Success ratios (%) 10 50 90 50 10 50 90 50 Input data for geologic non-sweet spots Average EUR (BCFG, gas) 0.1 0.2 1 0.245 0.1 0.2 1 0.245 Success ratios (%) 10 40 70 40 10 40 70 40 Resource Summary The USGS assessed technically recoverable continuous (unconventional) resources in two AUs defined in the Alum Shale resulting in a total estimated mean of 6,935 billion cubic feet of gas (BCFG) (table 2). This includes estimated gas resources of 0 to 4,848 BCFG in the Onshore Continuous Gas AU (mean = 2,509 BCFG) and 0 to 8,492 BCFG in the Offshore Gas AU (mean = 4,426 BCFG). Natural gas liquids were assessed to be negligible. Acknowledgments Although the assessment methodology, numerical input, risking structure, and quantitative estimates reported here are the exclusive product of the USGS, the geological 150 foundation of the assessment relies on concepts, data, and interpretations Lower Ordovician 120 provided by the Geological Survey of Furongian Denmark and Greenland (GEUS). The collaboration of Niels H. Schovsbo, of 90 Middle Cambrian GEUS, is specifically and gratefully acknowledged.