Geological Survey of Denmark and Greenland Bulletin 41, 2018, 9-12 9 Oil and gas production from siliciclastic reservoirs has hith- erto been in the Danish Central Graben mostly from Pal- aeogene and Middle Jurassic sandstone. The Ravn field was the first Upper Jurassic field to start operation. The reservoir is composed of sandstone of the Heno Formation. Produc- tion takes place at a depth of 4000 m, which makes Ravn the deepest producing field in the Danish North Sea. The Heno Formation mainly consists of marine shoreface depos- its, where foreshore, middle and lower shoreface sandstones constitute the primary reservoir. The results of this study of the diagenetic impact on the mineralogical composition, po- rosity and permeability are presented here. Microcrystalline quartz has preserved porosity in the sandstone, whereas illite, quartz overgrowth and carbonate cement have reduced both porosity and permeability. Geological background The Ravn Member of the Heno Formation is located on the Heno Plateau in the Danish Central Graben (Fig. 1; Johannes- sen 2010). The Ravn field was discovered in the Ravn-1 well in 1986 and subsequently evaluated in the Ravn-2 well in 1987. In 2010, the Ravn-3 well was drilled to test the location of the oil–water contact and to evaluate the reservoir quality of the south-western flank of the field. Oil was found at several inter- vals and the oil–water contact was located at a depth of 4572 m. The Ravn Member was deposited during an overall trans- gression of the Heno Plateau during the Kimmeridgian. The member consists of up to 100 m thick marine shoreface de- posits (Johannessen 2010) where foreshore, middle and lower shoreface sandstones constitute the primary reservoirs (Fig. 2). The sediments are strongly bioturbated and are dominated by very fine- to fine-grained or muddy sandstones with occasional white, grey and light brown siltstones. Methods Sedimentological description of the Ravn-3 core was made and 18 thin sections were prepared from samples from mid- dle, lower and foreshore sandstones (Fig. 2). Petrographical investigations of the thin sections were undertaken with transmitted light microscopy. Mineral abundances were quantified by point counting of minimum 500 grains. Addi- tional information was obtained from scanning electron mi- croscopy (SEM) of gold-coated rock chips and carbon-coated thin sections using a Phillips XL 40 SEM with a tungsten filament operating at 17 kV and 50–60 µA. Porosity and per- meability were measured on core plugs according to the API RP-40 standard (American Petroleum Institute 1998) at the Geological Survey of Denmark and Greenland. Results The porosity and permeability of sandstone reservoirs re- f lect, among other things, depositional environmental, mineralogical composition and post-depositional diagenetic changes. In order to understand what affected porosity and permeability, these factors were investigated. Detrital components – Quartz is the dominant component in all sandstones. The feldspar group consists of K-feldspar and minor albite. K-feldspar is typically partially dissolved and Diagenetic impact on reservoir sandstones of the Heno Formation in the Ravn-3 well, Danish Central Graben Simone Pedersen, Rikke Weibel, Peter N. Johannessen and Niels H. Schovsbo Ringkøbing–Fyn High Feda Graben Ål Basin 25 km Salt structure Normal fault Reverse fault Well R-1R-3 R-2 A 56°N 4°E Salt Dome Province Gert Ridge NL G UK Central Graben Mid North Sea High Mandal High Outer Rough Basin Inge High Heno Plateau Tail End Graben National border N Structural high DK B Fig. 1. A: Present structural framework of the Danish sector of the Central Graben. R-1, R-2, R-3: Ravn-1, -2 and -3 wells. B: Overview of the North Sea area. Green: Land. Modified from Johannessen (2010). © 2018 GEUS. Geological Survey of Denmark and Greenland Bulletin 41, 9–12 . Open access: www.geus.dk/bulletin http://www.geus.dk/bulletin 1010 minor mica, rock fragments and chlorite grains are present. Accessory minerals are tourmaline, zircon and Fe-Ti oxides. Detrital clay occurs as tangential coatings on detrital grains and as deformed clay clasts. Diagenetic phases – Sandstones are occasionally dominated by abundant sparry Fe-dolomite and ankerite cement (Fig. 3A; Pedersen 2017). Sporadic calcite inclusions occur enclosed in the Fe-dolomite-ankerite cement. Calcite from shell fragments was recognised in one sample. Small amounts of Fe-dolomite- ankerite rhombs are present in samples where cement is not abundant. Microcrystalline quartz coatings are common in several samples independent of depositional environment (Fig. 3B). Occasionally, excessive microcrystalline quartz also occurs in the intergranular pore space (Fig. 3C). In a few sandstones, the detrital grain surfaces of quartz are only partly covered by microcrystalline quartz giving rise to growth of larger quartz overgrowths (Fig. 3C). The amount of quartz overgrowths var- ies from 0.2 to 10.8 vol%. Illite is present in all samples and depositional environments and occurs as fibrous and honey- comb-structured coatings (Fig. 3D). Authigenic illite occurs as protruding fibres growing from honeycomb-structured illitic- smectitic clay. Illite fibres alternate with quartz overgrowths, and are at times enclosed in quartz overgrowth (Fig. 3D). Porosity versus permeability – The sandstones with highest porosity and permeability are dominated by microcrystalline quartz coatings and only little diagenetic illite is present to- gether with a small amount of detrital clay (Fig. 4; Pedersen 2017). These sandstones are from the upper, middle and low- er shoreface. Two groups of sandstones are defined based on intermediate porosity and low to intermediate permeability. Of these two groups, sandstones with quartz overgrowths and minor illite have slightly higher permeability than sand- stones with microcrystalline quartz coatings and high illite and high detrital clay contents (Fig. 4). These latter samples are from lower and middle shoreface. Also the Fe-carbonate- cemented sandstones, which have the lowest porosity and permeability in the Heno Formation (Fig. 4), represent lower and middle shoreface samples. Comparison between the Ravn-1, Ravn-2 and Ravn-3 wells – The Ravn-3 well was correlated with the Ravn-1 and Ravn-2 wells based on available core and well log data (Fig. 2). The various diagenetic phases in the Ravn-3 well can be recog- nised in the other Ravn wells. Variations occur, such as quartz overgrowth and illitisation of detrital clay being more common in the Ravn-1 well, compared to authigenic illite in the Ravn-3 well, but the reservoir units can still be recog- nised. The variations seen in the Ravn-1 cores are also pre- sent in the Ravn-2 cores together with additional fractures filled with barite and ankerite. The porosity and permeabil- ity in the Ravn-1 and Ravn-2 wells lie within the same range as the sandstones in the Ravn-3 well (Fig. 4). Cl Si Sand Pbl Lithology Depositional environment Sandstone Clay or siltstone Conglomerate Lower shoreface Middle shoreface Foreshore Structures Disconnected wave ripples Structureless due to bioturbation Carbonate cemented sandstone Thin section samples4619 m Cl Si Sand Pbl Cl Si Sand Pbl Legend Ravn-1 Ravn-3 Ravn-2 4280.77 m 4155 m 0 10 20 0.001 0.1 10 0 10 20 0 10 20 0.01 1 10 0.01 1 10 PHI (%) Kh (mD) PHI (%) Kh (mD) PHI (%) Kh (mD) 0 20 40 60 80 100 120 H ei gh t ab ov e ze ro (m ) Offshore Low angle cross stratification Fig. 2. Correlation panel of the cored parts of the Ravn-1, -2, and -3 wells. The Ravn-1 and Ravn-2 logs are modified from Johannessen (2010), whereas the Ravn-3 core was logged for this study. The depositional environment described in the Ravn-3 well (Panterra 2011) is based on ichnofacies. PHI: He- porosity. K h: horizontal permeability. Cl: clay. Si: silt. Pbl: pebble. 11 Discussion Early carbonate cement – Intergrown sparry Fe-dolomite and ankerite cement (Fig. 3A) is interpreted to be sourced from dissolved calcite from shell fragments. Calcite inclusions still occur between Fe-dolomite and ankerite. This is supported by quartz grains appearing to be ‘f loating’ in the carbonate cement, which indicates the previous presence of an early car- bonate cement or fossils. Fe-carbonates are considered more stable than calcite during late diagenesis and often replace earlier phases of carbonates (Worden & Burley 2003). Early microcrystalline quartz – When early diagenetic mi- crocrystalline quartz is present in the sandstones only mi- nor quartz overgrowth has precipitated (Fig. 3B). A biogenic opal CT phase, which has been dissolved without trace, may have resulted in supersaturated pore waters that sustained nucleation of microcrystalline quartz. Grain-coating micro- crystalline quartz has previously been proposed to preserve reservoir quality by impeding quartz overgrowth, which oth- erwise may occlude intergranular porosity and reduce per- meability (Aase et al. 1996; Jahren & Ramm 2000; Weibel et al. 2010). The random growth of microcrystalline quartz may retard further development of both new microcrystal- line quartz and quartz overgrowth ( Jahren & Ramm 2000; Weibel et al. 2010). When microcrystalline quartz does not fully cover detrital quartz, it cannot inhibit precipitation of quartz overgrowth (Aase et al. 1996; Weibel et al. 2010). Quartz overgrowths – Late diagenetic quartz overgrowths formed where the quartz grains were only partly covered by microcrystalline quartz. The quartz overgrowths prob- ably formed under low silica oversaturation, which favoured less nucleation and promoted the growth of larger crystals (Fig. 3C; Jahren & Ramm 2000). More intensive quartz ce- Ca Fe-Do + An MQ IL MQ QO MQ + IL MQ IL IL MQ 5 µm 20 µm 20 µm 50 µm A B C D Fig. 3. A: Abundant Fe-dolomite (Fe-Do) and an- kerite (An) occluding porosity and permeability. Remnants of the original early calcite (Ca) cement are present. B: Random and abundant micro- crystalline quartz (MQ ) coating detrital quartz grain, preventing quartz overgrowth (QO). Note the fibrous illite (IL). C: Microcrystalline quartz on detrital quartz and in pore space together with authigenic illite. Quartz overgrowth is partly enclosing microcrystalline quartz indicat- ing that the quartz overgrowth precipitated later. D: Abundant fibrous illite growing from honeycomb- structured illite succeeding micro- crystalline quartz and alternating with quartz overgrowth (QO). 10 1 0.1 0.01 0.001 0 5 10 15 2520 He porosity (%) G as p er m ea bi lit y (m D ) Carbonate cemented Microcrystalline quartz + illite + low detrital clay content Microcrystalline quartz + illite + high detrital clay content Thin section Ravn-1 Thin section Ravn-2 Thin section Ravn-3 Lower shoreface Middle shoreface Foreshore Quartz overgrowth Fig. 4. He porosity versus air permeability for all thin section samples from the Ravn-3 well, together with data from the Ravn-1 and Ravn-2 wells. The thin section samples follow the trends from the Ravn-3 well marked by the four ellipses, which depict the four characteristics of the diagen- esis. The purple ellipse comprises samples dominated by microcrystal- line quartz, illite and low detrital clay content. The green ellipse includes samples dominated by microcrystalline quartz, illite and high detrital clay content. The orange ellipse comprises samples dominated by quartz over- growth and the blue ellipse by extensive sparry carbonate cement. 1212 mentation would have been expected in these quartz-rich sandstones (Bjørlykke et al. 1989) as they have been buried to a depth of > 4 km and hence exposed to temperatures of 112–117°C as documented by vitrinite ref lectance. As no stylolites were observed and as quartz overgrowth pre- cipitated before and alternating with illite growth, another source for silica must have been present prior to transfor- mation of smectite to illite. The continued precipitation of quartz overgrowth was probably from a silica source from the transformation of smectite to illite and dissolution of K- feldspar (Hower et al. 1976; Boles & Franks 1979). This is supported by the honeycomb-structured smectite-illite coat- ings and partially dissolved detrital K-feldspar. Illite – Illite occurring as honeycomb structured coatings (Fig. 3D) is a strong indicator of a smectite precursor (e.g. Pollastro 1985). During burial, the percentage of illite in mixed-layer illite/smectite compared to smectite increases since smectite becomes more unstable with increasing tem- perature and pressure (Pollastro 1985), which may be the reason why only illite is present in the Ravn-3 well. The honeycomb-structured illite commonly forms nu- cleation or growth points for fibrous illite. K-feldspar is typically dissolved concomitantly with smectite dissolution, and K-feldspar can be an additional source for K+ and Al3+ for further illite precipitation (Hower et al. 1976; Boles & Franks 1979). The additional K+ and Al3+ from the dissolu- tion of K-feldspar might have led to further precipitation of the fibrous illite on illite honeycomb structures and singular precipitation in pore space. Fe-dolomite-ankerite rhombs are considered a by-product of the transition from smectite to illite, which may liberate Ca2+ and Fe2+. Conclusions The porosity and permeability of the reservoir sandstones in the Ravn-3 well are controlled by the diagenetic phases formed during early and late diagenesis. The reservoir sandstones with the highest porosity and permeability are dominated by low to moderate amounts of microcrystalline quartz, illite and detrital clay. However, the more distal lower shoreface sandstones with the same dominating diagenetic phases, but with higher detrital clay content, are considered a poor reservoir due to low poros- ity and permeability. 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