e2019430103-01 High porosity is a key factor for good reservoir sandstones for both hydrocarbon and geothermal energy exploitation. The porosity of sandstones generally decreases with increased burial depth due to compaction and cementation. However, some sandstones in the North Sea show higher porosity than expected for their burial depth, due to the presence of micro­ quartz coatings (e.g. Aase et al. 1996; Hendry & Trewin 1995; Jahren & Ramm 2000; Maast et al. 2011). Siliceous sponge spicules have been documented to be an internal source of silica that promotes microquartz coatings (e.g. Hendry & Trewin 1995; Aase et al. 1996). Siliceous sponge spicules, the solid ‘skeleton’ of sponges, consist of opal­A and will dissolve when exposed to higher temperatures, thereby causing supersaturation of the formation water with respect to opal­CT and quartz, resulting in nucleation of numerous small (1–5 µm) quartz crystals (Williams et al. 1985; Hendry & Trewin 1995). To predict reservoir quality it is important to understand the distribution of porosity­preserving mic­ roquartz in clastic deposits, and yet this is still poorly un­ derstood. To address this, our study presents petrographical analyses of cored sandstone sections from wells of various depositional environments, including back­barrier, estua­ rine, shoreface and gravity flows, as well as various present­ day burial depths across the Danish Central Graben. Geological background The Upper Jurassic sandstones in the Danish Central Gra­ ben were deposited as part of the syn­rift sedimentation associated with Late Jurassic rifting of the area. Sedimenta­ tion was dominated by mudstones of the Lola and Farsund formations, with local and episodic deposits of intercalated Distribution of porosity-preserving microquartz coatings in sandstones, Upper Jurassic Danish Central Graben Margrethe T. Nielsen*1, Rikke Weibel1, Jens Therkelsen2 and Henrik Friis3 GEUS Bulletin is free to individuals and institutions in electronic form. The author(s) retain copyright over the article contents. RESEARCH ARTICLE | OPEN ACCESS GEUS Bulletin Vol 43 | e2019430103 | Published online: 22 July 2019 https://doi.org/10.34194/GEUSB-201943-01-03 Jeppe-1 Tail End Graben Salt Dome Province Heno Plateau Outer Rough Basin Inge High Ål Basin Arne-Elin Graben Feda Graben Mid North Sea High Mandal High Gert Ridge Gertrud Graben/ Plateau Coffee Soil Fault Ringkøbing–Fyn High Søgne Basin Ravn-1 Rita-1 25 km Mid North Sea High Outer Moray Firth Ringkøbing–Fyn High Central Graben UK N D DK 55oN M ads H igh Ravn-2 W-1 Eg-1 Diamant-1 Gert-4 Gwen-2 Gert-2 Hejre-2 Bertel-1 Wessel-1 Saxo-1 Iris-1 Tabita-1 Lulu-2 NW Adda-1 Deep Adda-1 Gert-1 ‘Outer Rough Sand’ Heno Formation Gravity flows Ophelia-1Ophelia-1 Hejre-2 Gwen-2 Gert-2 Gert-1 Danish Central GrabenSW NE Ringkøbing–Fyn High Stage Ryazanian Volgian Kimm. Oxfordian Callovian Farsund Fm C re ta ce ou s Ju ra ss ic Se rie s Sy st em U pp er Lo w er Mid North Sea High Outer Rough Sand Lola Fm Heno Fm Ravn Mb Gert Mb Cromer Knoll Lulu Fm Fig. 1. Part of the Danish sector of the Central Gra­ ben showing the 21 sampled wells penetrating Upper Jurassic sandstones. Sandstones with microquartz coatings are indicated by blue dots. Modified from Johannessen (2003). https://doi.org/10.34194/GEUSB-201943-01-03 e2019430103-02 sandstones (Andsbjerg & Dybkjær 2003; Johannessen 2003; Fig. 1). The Heno Formation, deposited between the Lola Formation and the Farsund Formation, is subdivided into the Gert and Ravn members, and is intersected locally by the Lola Formation. The Gert Member consists of mainly estuarine and back­barrier sandstones, while the Ravn Mem­ ber comprises mainly shoreface sandstones. Both units were deposited in the central part of the Danish Central Graben (Fig. 1). The Farsund Formation sandstones comprise shore­ face sandstones deposited along the Mid North Sea High (‘Outer Rough Sand’ in Fig 1) and deep marine gravity­flow sandstones along the Coffee Soil Fault (Fig. 1). Since deposi­ tion, the Upper Jurassic sediments in the Danish North Sea area have been progressively buried to a maximum depth of 5400 m and temperatures of 160°C (Weibel et al. 2019). Methods Samples were selected from cores of 21 wells penetrating Up­ per Jurassic sandstones in the Danish Central Graben (Fig. 1). Plugs were taken perpendicular to the cores, including in one deviated well, and thin sections were made from plug cut­offs. A total of 234 polished thin sections were studied by optical microscopy and by scanning electron microscopy (SEM). The relative mineral abundances are assigned as follows: Dominant (>50%), abundant (15–50%), common (5–15%), minor (1–5%) and rare (<1%). Analyses of crystal 100 µm100 µm 2 µm 50 µm C D A B cQ Sp mQ dQ Sp Sp Fig. 2. A: Sand laminae rich in sponge spicules (Sp) preserved in mud­rich facies (Iris­1 well). B: SEM­SE micrograph showing mouldic porosity from the dissolution of siliceous sponge spicules (arrows) with microquartz coatings (Rita­1 well). C: SEM­SE micrograph showing a detrital quartz grain (dQ) coated by cryptocrystalline quartz (cQ), and microquartz (mQ, Rita­1 well). D: Optical microscopy image under crossed nicols, showing interlocking quartz overgrowths (arrows, Gert­1 well). e2019430103-03 morphologies and paragenetic relationships were performed on gold­coated rock chips by SEM. Relative chemical com­ positions of mineral phases were obtained using an energy dispersive X­ray spectrometer. Plug porosity and perme­ ability were measured according to the API RP­40 standard (API 1998). Porosity and permeability measurements were performed on sandstones of the Heno Formation and from the ‘Outer Rough Sand’ of the Farsund Formation, only. Results Detrital composition: In general, quartz grains dominate (>50%) the detrital composition of the Upper Jurassic sand­ stones, though some samples contain abundant–dominant (i.e. >15%) calcite shell fragments. Feldspar, comprising both K­feldspar and albite, varies in abundance from rare (<1%) to common (5–15%). Mica rock fragments, organic matter and biogenic particles such as sponge spicules (Fig. 2A) and shell fragments are generally rare or minor (1–15%) constituents. Heavy minerals such as rutile, zircon, chromite, apatite, Fe­ Ti­oxides and tourmaline are rare. Detrital clays are present in varying amounts. Diagenetic changes: Early diagenetic (eogenesis) cement in­ cludes pyrite, sporadic anatase, cryptocrystalline and micro­ crystalline quartz coatings. The coatings are occasionally associated with mouldic porosity – a form of secondary po­ rosity formed by the dissolution of siliceous sponge spicules. We also observed chlorite coatings in samples with detrital volcanic particles, pores filled by calcite cement in samples with abundant detrital carbonate particles and secondary porosity from the dissolution of feldspar and kaolinite. Later diagenetic (mesogenesis) phases include illite, rare feldspar overgrowths, and dolomite and ankerite overgrowths on detrital carbonate clasts. Also observed were quartz over­ growths, late­patchy calcite cement, rare siderite, barite and sphalerite. Siliceous authigenic phases: Microcrystalline quartz coat­ ings are observed in a number of wells (Fig. 1). Microquartz typically occurs as a coating on detrital quartz grains (Figs. 2C, D), and as a coating on detrital carbonate grains in sand­ stones from the NW Adda­1 well. Cryptocrystalline quartz is observed between the detrital grains and the microquartz coating as shown in Fig. 2C. Sponge spicules, replaced by the quartz variant chalcedony, and mouldic porosity after dis­ solved sponge spicules are occasionally present in samples with microquartz coatings (Figs 2A, B). Microcrystalline silica is very abundant and has lithified the sandstones from the Tabita­1 and Deep Adda­1 wells, which contain abun­ dant siliceous sponge spicules. Quartz overgrowths vary in size from discrete incipient overgrowths, which may have started as small outgrowths, to large interlocking quartz ce­ mentation as in sandstones from the Gert­1 well (Fig. 2D). Distribution of microquartz: Stratigraphically, micro­ quartz coatings are restricted to the Farsund Formation and Ravn Member of the Heno Formation, but absent in the Lola Formation and in Gert Member of the Heno Formation (Fig. 0 20 40 60 80 100 120 Back-barrier Estuarine Tidal Upper shoreface Middle shoreface Lower shoreface Shoreface Offshore Gravity flow D ep os iti on al en vir on m en t Number of samples 0 20 40 60 80 100 120 Ravn Mb Gert Mb Lola Fm Farsund FmLi th os tr at igr ap hy With microquartz No microquartz A B Fig. 3. Distribution of microquartz according to A: lithostratigraphy and B: depositional environment. Here, offshore environments are represented by the Lola Formation. e2019430103-04 3). Microquartz coatings are restricted to shoreface and grav­ ity­flow sandstones, whereas microquartz coatings are absent in sandstones deposited in back­barrier, estuarine and tidal environments (Fig. 3). Porosity variation: Sandstones of the Heno Formation generally plot with relatively high porosities when compared to the regional mean porosity­depth trends given by Selley (1978) and Bjørkum et al. (1998) (Fig. 4). Samples with micro quartz, shallower than 4.6 km, have overall porosi­ ties of 20.7% ± 8.0, compared with 12.2% ± 5.6 for samples without microquartz. Many samples deeper than 4.6 km also show relatively high porosities compared to regional trends, even though microquartz is rarely observed in these sand­ stone samples (Weibel et al. 2019). Discussion The porosity of the Upper Jurassic sandstones shows large variations at all depths (Fig. 4), though there is a general trend towards lower average porosity at increased depth. Microquartz has a positive influence on porosity preserva­ tion in sandstones from depths ≤4.6 km, which is the maxi­ mum depth of observed microquartz in The Danish Central Garben. This is due to a combination of mechanical stabili­ sation of the sandstone and inhibition of quartz cementation (Aase et al. 1996; Jahren & Ramm 2000; Bonnell et al. 2006; Lander et al. 2008). The porosity­preserving effect of mic­ roquartz coatings is particularly pronounced when the indi­ vidual crystals are randomly orientated. Whereas coatings with a consistent crystallographic orientation can become overgrown by macroquartz (e.g. Weibel et al. 2010; French et al. 2012; French & Worden 2013). The presence of microquartz coatings surrounding moul­ dic porosity after dissolved sponge spicules (Fig. 2B) suggests a close relationship, which is in line with previous studies by Hendry & Trewin (1995) and Maast et al. (2011), suggest­ ing that microquartz coatings in the North Sea are sourced from dissolved silicous sponge spicules. In the Upper Juras­ sic sandstones of the Danish Central Graben, microquartz coatings are restricted to sandstones deposited in gravity­ flow and shoreface environments (Ravn Member and Far­ sund Formation), while microquartz is absent in sandstones deposited in back­barrier and estuarine environments (Gert Member). This corresponds to the preferred marine habi­ tats of silica sponges (demosponges) during the Late Jurassic (Leinfelder et al. 1996). Demosponges are filter feeding or­ ganisms that consume predominantly nanoplankton, which decrease in abundance with depth, and hence, demosponges preferentially occur in shallow marine, middle and outer ramp environments (Leinfelder et al. 1996). As the sponges colonised the sea floor, died and disintegrated, the ‘skeletal’ spicules, would have been released into the environment. Sponge spicules from the shoreface environments could have been carried to deeper waters along with sand grains by gravity flow, similar to the interpretations of the Cretaceous sandstones in the UK North Sea (Hendry & Trewin 1995). Microquartz coatings are only observed in sandstones buried no deeper than than 4.6 km (Fig. 4), corresponding to maximum temperatures of c. 150oC assuming a geothermal gradient of 33oC km–1 (Evans 2003). This may be explained by increasing temperatures making larger quartz crystals in continuous overgrowths thermodynamically more stable (Williams et al. 1985). Microcrystalline quartz formed at a low temperature may coarsen during increased burial to form a thermodynamicaly stable crystal, but this thermodynamic drive is less likely to have promoted coarse microquartz crys­ tals (c. 5–10 µm) to be dissolved and replaced by syntaxial overgrowths (William et al. 1985; Hendry & Trewin 1995; Jahren & Ramm 2000). The deeper buried sandstones prob­ ably never developed significant amounts of microquartz or other early diagenetic phases, such as opal­CT, or zeolite protected the grain surfaces and were later dissolved during increased burial, thereby exposing the grain surfaces for mac­ roquartz nucleation (e.g. Hendry & Trewin 1995; Weibel et al. 2010, 2019). 2500 3000 3500 4000 4500 5000 5500 6000 D ep th  (m ) Porosity (%) 0 5 10 15 20 25 30 35 Samples without microquartz Samples with microquartz Average porosity at  200 m intervals Expected porosity (Selley 1978) Expected porosity (Bjørkum et al. 1998) Fig. 4. Porosity vs. depth for samples with (white dots) and without (black bots) microquartz. Also shown are estimated or predicted porosity­depth curves according to Selley (1978) and Bjørkum et al. (1998). e2019430103-05 Conclusions Microquartz coatings in Upper Jurassic sandstones of the Danish Central Graben are confined to offshore gravity flows and shoreface sandstones, and absent in sandstones deposited in back­barrier and estuarine environments. In the samples presented here, chalcedonic sponge spicules and mouldic porosity after sponge spicules often coincide with the presence of microquartz coatings. This observation supports the idea that the presence of microquartz is deter­ mined by silica sponge habitats and depositional processes. Improved understanding of the depositional environments likely to concentrate siliceous sponge spicules would enable prediction of sandstone reservoirs that contain microquartz coatings and exceptionally high porosity and permability. Acknowledgments This study is based on results from the multiclient project PETSYS to which numerous colleagues at GEUS have contributed. Thanks are ad­ dressed to reviewers J. Hendry and J. 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Journal of Sedimentary Petrology 55, 301–311. https:// doi.org/10.1016/0198­0254(85)93828­2 How to cite Nielsen, M.T., Weibel, R., Therkelsen, J. & Friis, H. 2019: Distribu­ tion of porosity­preserving microquartz coatings in sandstones, Up­ per Jurassic Danish Central Graben. Geological Survey of Denmark and Greenland Bulletin 43, e2019430103. https://doi.org/10.34194/ GEUSB-201943-01-03 *Corresponding author: Margrethe T. Nielsen | E-mail: mtn@geus.dk 1 Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350, Copenhagen K, Denmark. 2MOE A/S, Næstvedvej 1, DK-4760 Vordingborg, Denmark 3Department of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, DK-8000 Aarhus C, Denmark. https://doi.org/10.1306/64eda0f0-1724-11d7-8645000102c1865d https://doi.org/10.1306/64eda0f0-1724-11d7-8645000102c1865d https://doi.org/10.1306/1d9bc5cf-172d-11d7-8645000102c1865d https://doi.org/10.1306/1d9bc5cf-172d-11d7-8645000102c1865d https://doi.org/10.1016/j.sedgeo.2012.12.004 https://doi.org/10.1016/j.sedgeo.2012.12.004 https://doi.org/10.2110/jsr.2012.39 https://doi.org/10.1306/d42680cc-2b26-11d7-8648000102c1865d https://doi.org/10.1306/d42680cc-2b26-11d7-8648000102c1865d https://doi.org/10.1002/9781444304237.ch18 https://doi.org/10.1306/07160808037 https://doi.org/10.1306/07160808037 https://doi.org/10.1306/03071110122 https://doi.org/10.1306/03071110122 https://doi.org/10.1144/gsjgs.135.1.0119 https://doi.org/10.1144/gsjgs.135.1.0119 https://doi.org/10.1016/j.sedgeo.2010.04.008 https://doi.org/10.1016/j.sedgeo.2010.04.008 https://doi.org/10.1016/j.marpetgeo.2019.02.006 https://doi.org/10.1016/j.marpetgeo.2019.02.006 https://doi.org/10.1016/0198-0254(85)93828-2 https://doi.org/10.1016/0198-0254(85)93828-2 https://doi.org/10.34194/GEUSB-201943-01-03 https://doi.org/10.34194/GEUSB-201943-01-03 mailto:mtn%40geus.dk?subject=