Permeabilities were measured on core plugs from stylolite-bearing chalk of the Gorm field in the Danish North Sea. Air and liquid permeabilities were measured in directions parallel to and perpendicular to the stylolite surface. Permeability was measured with sleeve pressure equal to in-situ reservoir stress. Permeabilities of plugs with stylolites but without stylolite-associated fractures were equal in the two directions. The permeability is equal to the matrix permeability of non-stylolite-bearing chalk. In contrast, when fractures were associated with the stylolites, permeability was enhanced. The enhancement was most significant in the horizontal direction parallel to the stylolites.

Summary. Medium-grained, moderately well-sorted sandstones of fluvio-deltaic to shallow marine origin have been investigated for fine production and for variations in porosity/permeability. Porosity and permeability change within small horizontal and vertical changes in sampling points. Also, air permeability depends on flow direction, Klinkenberg correction, and cleaning procedure. Kinkenberg-corrected permeability and brine permeability were found not to be comparable. Brine flow caused fines of kaolinite and quartz to be produced from the compacted sandstones, while uncompacted sandstones produced kaolinite, illite, and detrital minerals. Chlorites and mixed-layer clays were not produced as fines, although they were present in some samples. Reservoir damage occurred when formation brine was replaced by KCl-brine, at increase in flow velocity, and at pauses in flow. Changes in flow direction caused severe fines production, renewed bridging, and blocking by fines. Severe damage occurred when the samples were flushed with deionized water. Compared to field experiments, the laboratory experiments give the worst case of damage. An inverse relation was noted between porosity and the amount of fines produced. Introduction Low-enthalpy geothermal exploration in Denmark has been carried out in the Danish sub-basin with the drilling of four wells. The sandstone reservoir units studied are the Triassic Skagerrak formation, Upper Triassic to Lower Jurassic Gassum formation, and the Middle Jurassic Haulager formation. Pertinent data are given in Table 1. Tests of the Skagerrak formation in the Thisted-2 well showed a progressive decrease in transmissivity (permeability-thickness product) with time, from about 80 to 20 darcy - m [263 to 66 darcy-ft] when the test was terminated. The decrease is believed to be due to plugging as a result of fines migration. The Gassum formation in the Farso - 1 well experienced similar decreasing transmissivity, probably caused by sand production with intermittent plugging and unplugging o the perforations. In contrast, no problems were experienced when the formation was tested in the Thisted-2 well. A geothermal demonstration plant has been constructed at Thisted. The Haldager formation has been tested only in Aars-1. No test problems were experienced; however, petrographic analyses show a potential risk for fines migration because authigenic pore-filling kaolinite is abundant. An experimental core study on samples from the Gassum and Haldager formations was initiated to investigate the reservoir damage caused by migration and/or swelling causing pore blocking and to determine whether the damage was caused by fines of quartz and feldspar (mineral fines), by blocking clays or swelling clays. The study included petrographic analyses of core samples, clay analyses, and determination of the fines produced. Basic measurements on cores were those of porosity, air permeability, and grain density. Core Analysis Porosity, Air Permeability, and Grain Density. Core analyses have been performed on three different plug sets (see Fig. 1 and Tables 2 through 4). Plug Sets 1 and 2 were used to demonstrate changes in analytic results with horizontal and vertical shifts in sampling points. Plug Set 3, unconsolidated sand from the Gassum formation at shallow depth, was used only in the brine flow experiments. The specific permeability to gas, k, was measured in both forward and reverse directions by flowing nitrogen through the plugs. The air permeability is reported as an average and as a Klinkenberg-corrected permeability, k1 (see Table 3). Results. Table 4 shows that the samples cover a wide porosity and air permeability range. The analytic results vary with small horizontal and vertical changes in sampling points, with core preparation procedures, and with the permeability, which varied with changed flow direction and the method of calculating results. The horizontal heterogeneity is most clearly seen in the permeability data (Tables 2 and 3) particularly for Sample 3 vs. Sample 103, and Sample 6 vs. Sample 106, while other sampling points give more consistent results (agreement of less than +/- 10%). The variation in porosities between samples is generally less than +/- 5 %. The horizontal heterogeneity in the data can either be a result of lithological variations, or, when minor, differential drilling mud invasion. SPEFE P. 168^

The investigations of the glacial sequence in the Randers area include mapping of the surface layers, study of bore profiles and detailed examination of outcrops, concentrating on the determination of transport directions in the deposits.The following stratigraphic succession has been established:Highest: NE tillTebbestrup Formation (melt water deposits)SE tillHaldum Formation (melt water deposits)Lowest: NE tillThe two uppermost units can with certainty be referred to Weichselian. Arguments are presented that the three lowermost units are of Saalian age.The origin of the sequence is discussed. The highest hills in the area (Ølst, Lysnet) were probably formed by the Saalian ice from the NE, since they appear to have influenced flow directions during the deposition of the Haldum Formation. This formation is considered to represent a sandur formed in front of the advancing ice from the SE. In the Weichselian, prior to the formation of the Tebbestrup Formation, the landscape obviously had a varied relief. The formation which infills the lower parts of this landscape is considered as a sandur, formed in front of the advancing ice from the NE. This sandur, together with the prominent hills from the Saalian, formed the substrate for the ice from the NE. This ice modified, but did not destroy the old hills.

The present report comprises primary data and analyses of potential geothermal reservoirs in Denmark. All available well data and seismic data from the Danish onshore area are included. Based on core material and petrophysical measurements, the following formations are described: Cambrian quartsitic sandstone, Carboniferous limestone, Rotliegendes sandstone, Zechstein carbonates, Bunter Sandstone Formation (Triassic), Ørslev Formation (Triassic), Falster Formation (Triassic), Tønder Formation (Triassic), Gassum Formation (Triassic-L. Jurassic), Haldager Sand (M. Jurassic), and Frederikshavn Member (U. Jurassic-L. Cretaceous). Generalized depth maps have been constructed for Near top Bunter Sandstones Formation, Near top Triassic, Base Haldager Sand, and Base Upper Cretaceous Limstone, from seismic two-way time maps of the relevant horizons. lsotherm maps of the corresponding horizons, based on measured values (BHT) and new, calculated model values, are presented. The reservoir parameters - porosity, permeability, net sand, and depth - have been analysed and evaluated from core material and petrophysical measurements. These data are presented in isopach maps of each formation, and in tables. The regional distribution of potential reservoirs in the depth interval 2000-3000 m is described, and the geothermal reserves are evaluated, with regard to energy.

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