'’W 4 PROFIT RF-^/w 1990-1994

PROJECT SUMMARY REPORTS RESERVOIR CHARACTERIZATION NEAR WELL FLOW

Program for Research On Field Oriented Improved Recovery Technology

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Edited by:

Jem Olsen, Snorre Olaussen, Trond B. Jensen, Geir Helge Landa, Leif Hinderaker

Norwegian Petroleum Directorate Stavanger 1995 DISCLAIMER

Portions of this document may be illegible in electronic image products. Images are produced from the best available original document PROFIT - RESERVOIR CHARACTERIZATION Geometry of calcite cemented zones in shallow marine sandstones

Olav Walderhaug, Edward Prestholm and Ingrid E.L0xnevad Rogaland Research, Stavanger

Abstract thought to belong to concretions. The difference between the geometry of calcite Calcite cementation in the Jurassic shallow cementation in the Ula Formation and in the marine sandstones of the Bearreraig Formation, Bridport Sands is thought to be due to a the Valtos Formation, the Bridport Sands and relatively uniform rate of siliciclastic deposition the Bencliff Grit occurs as continuously for the Ula Formation having led to a more cemented layers, as stratabound concretions and uniform distribution of biogenic carbonate as scattered concretions. All three geometrical compared to the Bridport Sands where laterally forms of calcite cementaton may occur within extensive layers of biogenic carbonate formed the same formation, whereas in other cases a during periods of very low siliciclastic formation may be dominated by only one or deposition. Based on the results of the core and two of these modes of calcite cementation. outcrop studies, a tentative identification key Calcite cemented layers and layers of for calcite cemented zones encountered in cores stratabound concretions in the studied is suggested. sandstones have lateral extents from a few metres to more than 3 km and thicknesses from around a decimetre to two metres, whereas Introduction roughly spherical scattered concretions have diameters from a few centimetres to 3.2 m. Shallow marine sandstones on the Norwegian There is no general relationship between shelf and in other sedimentary basins thickness and lateral extent for the studied commonly contain calcite cemented zones calcite cemented layers, and prediction of the varying in shape from concretions with geometry of calcite cemented intervals diameters less than a few metres to layers with encountered in wells must therefore be based lateral extents of many kilometres I-5. The upon a combination of sedimentological calcite cemented zones are typically tightly reasoning, understanding of the nucleation and cemented and may thus form impermeable growth mechanisms for calcite cement, and data barriers that have a profound effect on fluid from outcrops. Calcite cemented intervals in flow and reservoir performance. Study of the cores from the Rannoch Formation in the geometry of calcite cemented zones in outcrops Gullfaks Field are very similar to scattered and the development of methods for concretions in the Bencliff Grit and in parts of establishing the geometry of calcite cemented the Valtos Formation, and biogenic carbonate zones encountered in wells is therefore of was probably originally present as interspersed considerable practical interest and economic shells or shell fragments in the cores, not as significance. laterally extensive biogenic carbonate-rich layers. Calcite cementation in the studied In order to understand and predict the Rannoch Formation cores is therefore thought geometry of calcite cemented zones it is to take the form of scattered concretions. necessary to determine the source of the calcite Despite a strong superficial similarity with cement and the growth mechanisms involved. It laterally extensive bioturbated calcite cemented has long been suspected that the dominant layers in the Bridport Sands, and in parts of the source of calcite cement in shallow marine Bearreraig Formation, calcite cemented intervals sandstones is biogenic carbonate originally in the Ula Formation in the Ula Field are present within the sandstone 6 , and this view has 75 now been confirmed through numerous recent 5180 and 513C were determined in five calcite .studies 7 '10. The mechanisms whereby biogenic cement samples. carbonate is redistributed and forms diverse geometrical cementation patters such as laterally continuous layers, scattered Outcrop studies concretions of varying shape and size, stratabound concretions, patchy or The Valtos Formation microconcretionary calcite cement and various combinations of these geometric end members The Bathonian Valtos Formation 18> 19 was have also recently been established 11,14. Based examined in cliff exposures along the east coast on these theoretical break-throughs regarding of the Trottemish peninsula on Skye and on the the formation of calcite cemented zones, the Isle of Eigg in the Inner Hebrides (Figure 1). In present project has focused on: 1) performing Trottemish, the 240 m long and up to 8 m high outcrop studies in order to increase the existing studied exposures comprise a coarsening data base concerning the geometry of calcite upwards sequence of well-sorted fine-, medium- cemented zones; 2) extending and refining and more rarely coarse-grained subarkosic methods for predicting the geometry of cored sandstone capped by 0.3-0.8 m thick sandy calcite cemented zones; and 3) applying and bioclastic limestones with abundant Neomiodon illustrating the use of these methods by shells. The sandstones typically contain low studying calcite cementation in shallow marine angle cross-stratification with low angle reservoir sandstones on the Norwegian shelf. In discordances between sets and common heavy this paper we present results from the mineral-rich laminae. In addition, some wave performed field work and apply these results ripples, sets of planar cross-laminated sandstone and the criteria for recognizing various and lenses of trough cross-stratified sandstone geometrical forms of calcite cementation in occur. Based on the characteristic association of cores in a study of the geometry of calcite sedimentary structures, the sequence is cementation in the Ula Formation in the Ula considered to represent upper shoreface and Field and in the Rannoch Formation in the beach sediments. Gullfaks Field. The results from the core and outcrop studies are also integrated in a tentative identification key for cored calcite cemented zones. NORWAY

Methods The studied outcrops were photographed Eigg \ systematically, logs in scale 1:50 were made and calcite cemented zones were measured and described in detail. Cross-sections high-lighting the geometry and distribution of calcite cemented zones in the studied outcrops were produced from the photomosaics and measurements. Fifty-nine thin sections from the outcrops were examined and point counted ENGLAND (300 points), most thin sections were stained for carbonates and K-feldspar. 5180, 513C and trace element data from the studied outcrops is available in the published literature 4- 8'10> l7 . Figure 1 Location of study areas. All examined cores were logged in scale 1:20, Calcite cementation in the cliffs at Valtos logs in scale 1:200 were produced, and calcite largely occurs as pervasively calcite cemented cemented zones were described in detail. Thin spectacular concretions with diameters of up to sections, mostly stained for carbonates and K- 3.2 m located within highly porous (30-34%) feldspar, were made from thirty-six samples and sandstone (Figures 2 - 4). The concretions point counted with 300 points per section. occur scattered throughout the sandstones, but when viewed from a distance, many concretions 76 seem to be concentrated at certain levels within been subjected to temperatures in excess of the cliffs. Concretion shapes vary from roughly 80°C. and concretions arc therefore thought to spherical to ellipsoidal, and concretions have have formed at depths of less than 2 km. often merged to form composite forms. The Published 5lsOpDB-values of -18.2 to -6.2% 9- shapes of concretions have also been influenced l3 . would correspond to precipitation by tight laminae, for instance concretions with temperatures of 40 - 120°C given a marine flat tops where growth terminated against Jurassic pore water with 518Osmow of -1.2%c horizontal argillaceous laminae. Sedimentary but it seems far more likely that the concretions structures are perfectly preserved within the grew at lower temperatures in a meteorically concretions, and boundaries are sharp and influenced pore water with a more negative normally cut acros lamination. The calcite 518Osmow 7'9i 20. cemented sandy bioclastic limestones at the top of the studied coarsening upward unit have On Eigg, the Valtos Formation was studied in a lateral extents of a least 50 m. Although 100 m long and 4-5 m high cliff southwards exposure is discontinuous, it is clearly seen that from Tritigh Chlithe and in several hundred metres long and up to approximately 50 m

Figure 2. Calcite cementation in the Valtos Fm at Valtos. Calcite cemented areas are brown and stand out in relief. Note continuously cemented layer at the top of the sandstone and scattered concretions in the lower part. Length of measuring rod is 1 m. several tight sandy bioclastic limestone layers high cliffs west and north of BHtr Mdr. The occur at the top of the unit. The combined Tr&igh Chlithe exposure is similar to the effect of these layers is to form a relatively exposures at Valtos, and comprises subarkosic continuous seal at the top of the unit, but with well-sorted fine to medium-grained sandstones some "holes". The bioclastic limestones are and moderately to bimodally sorted medium- thought to represent an abandonment facies. grained sandstones capped by an The abundance of biogenic carbonate within approximately 1 m thick calcite cemented layer calcite cemented volumes, but not outside, of coarse-grained Neomiodon-rich sandstone or combined with S^CpDB-values of -1.0 to 3.0%o sandy bioclastic limestone. Sedimentary for 52 calcite cement samples 9- 15, clearly structures are mostly horizontal to indicates that biogenic carbonate was the source subhorizontal lamination plus pockets of of the calcite cement. The high porosities and trough cross-lamination and rare bioturbation lack of quartz overgrowths outside concretions suggesting deposition in a beach setting. The shows that the studied unit has probably not bioclast-rich sandstone at the top of the 77 Figure «* metre * © -* metre © •+ metre o» o — metre

3

Calcite SOUTH

cementation

in

the

Valtos /

Fm

at

Valtos.

Calcite cemented

areas

are

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Scale

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long. exposure extends all the way across the by diffusional redistribution of scattered exposure whereas the underlying sandstone biogenic carbonate, whereas the calcite contains scattered pervasively cal cite cemented cemented Neomiodon-nch layers formed from concretions with diameters up to a few metres. biogenic carbonate-rich layers n. The concretions contain perfectly preserved lamination and concretion boundaries cut The Bearreraig Formation across lamination. The cliff exposures at Bl&r The Toarcian to Bajocian Bearreraig Formation Mdr are built up of several stacked beach units 21-23 was studied in cliff exposures along the of the type described from Valtos and TrMgh east coast of the Trottemish peninsula on Skye Chlithe separated by siltstones and bioclastic and along the east coast of Raasay (Figure 1). limestones, and calcite cementation also takes However, due to the inaccessibility of most of the form of scattered large concretions, the cliff exposures, detailed work on the sometimes with a tendency to occur at certain geometry of calcite cementation could only be levels, plus horizontal calcite cemented layers performed immediately north and south of rich in Neomiodon shells (Figures 4 and 5). Bearreraig Bay on Trottemish. South of The abundance of biogenic carbonate within Bearreraig Bay, the Bearreraig Formation calcite cemented sandstone in the exposures on comprises a lower shaly unit (Dun Caan Shales) Eigg, the lack of biogenic carbonate outside overlain by well-sorted, fine- and more rarely calcite cemented concretions and layers, and 41 medium-grained subarkosic and quartzitic 8l3 CpDB-values for calcite cement close to zero sandstones of the Ollach Sandstone Member. (-6.2 to 3.3%o) 15’ 17 all indicate that biogenic Individual sandstone layers dip gently NNW, carbonate was the dominant source of calcite and sandstone beds typically show horizontal or low angle lamination, current ripple lamination cement, although the most negative 813 Cpdb- values point to derivation of some carbon from and more rarely wave ripple lamination and decomposition of organic matter. Published bioturbation.. Marine fossils such as belemnites, S^OpoB-values of -14.0 to -5.3%o 15>17 bivalves, echinoderms, gastropods and foraminifers occur. The Ollach Sandstone

10000 member was deposited in a shallow marine setting, possibly as mouth bars. O Bender Gnt • BridjpOft Sands 1000- + Valtos Fm. Skye Calcite cementation in the Ollach Sandstone A Beaoerafg Fm O Valtos Fm, Elgg Member occurs as pervasively calcite cemented concretions, lenses and continuous layers with sharp boundaries against porous sandstone n □ practically devoid of calcite cement (Figures 4 = 10: and 6). The lenses and concretions are typically located along bedding planes, and in some cases lateral transitions from continuously cemented short layers to stratabound concretions are seen. Only a few of the calcite Thickness (m) cemented volumes have lateral extents greater than 10 m although two of the calcite cemented Figure 4 Length versus thickness for 413 calcite layers can be traced laterally for 285 m and cemented zones. Lengths greater than 100 m 355 m respectively before the exposures are minimum values due to termination of terminate. The first layer is cross-bedded, and exposures. calcite cementation seems to be confined to a single set. The second laterally extensive layer correspond to precipitation temperatures of 36- contains diverse sedimentary structures such as 93°C in pore water with a 818OsMOW-value low angle and horizontal lamination, planar equal to the value for Jurassic marine water cross-bedding, current ripples and climbing (-1.2%o 20), or lower temperatures in a more meteoricaliy influenced pore water. Lack of ripples and the boundary of the cemented layer commonly cuts across lamination. There is no quartz cement within and outside calcite distinct change in grain size or mineralogy at cemented volumes and minus cement porosities the boundaries of the laterally extensive calcite of 27-34% within concretions suggest that cemented layers. This is also the case for the calcite cementation was complete before burial less laterally extensive lenses and concretions to depths of 1.5-2 km. The concretions in the which typically contain low angle and Valtos Formation are thought to have formed

79 U - 1 Sandstone

KMI Calcite cemented sandstoni

H Dolerite

/Z\ Wave ripples

Cross stratification

Trough cross stratification

ZBr~ Organic rich layers Figure 5 Calcite cementation thein Valtos Fm atBldrMdr,Eigg. Calcitecemented areas are grey. Sets* Vegetation

_ '< Base of outcrop V

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Figure 6 Calcite cementation in the Bearreraig Formation atBearreraig Bay, Skye. Scale barsare 1 m long. horizontal lamination and in some cases current sandstone (Figures 4 and 7). Most of the calcite ripples. cemented layers and layers of stratabound concretions extend throughout the 3 km of At the northern end of Bearreraig Bay the exposures, while a few less extensive (<60 m) lowermost part of the cliff exposures comprise curved calcite cemented layers with a relief of a strongly bioturbated very fine-grained few metres occur in the lowermost part of the sandstones and siltstones of the Udaim Shale formation. All calcite cemented layers and Member containing calcite cemented layers concretions are extensively bioturbated, and 0.1-0.85 m thick and layers of 0.2-1.0 m thick their boundaries do not coincide with changes stratabound concretions. The calcite cemented in grain size or type or amount of bioturbation. layers and layers of calcite cemented concretions can be traced laterally for 700 m Carbonate fossil fragments occur within the before exposure is lost. Calcite cemented layers calcite cemented volumes, and 136 of 137 and concretions are also bioturbated, and there published 513CpoB-values 4> 10 for the calcite is no change in grain size or type and amount cement are in the narrow range -2.7 to -0.5%o, of sedimentary structures or bioturbation at the observations that point to biogenic carbonate as boundaries of calcite cemented volumes. the source of the calcite cement. 5180pdb- Boundaries are sharp and cut across values of -9.9 to -3.3%o correspond to bioturbation structures. precipitation at temperatures of 25-60°C in a pore water with a marine Jurassic 518Osmow of The common occurrence of biogenic carbonate -1.2%o 20 or burial depths down to a kilometre within calcite cemented samples, but not or so. Relatively early calcite cementation is outside, suggests that biogenic carbonate was also indicated by intergranular volumes above the major source of calcite cement. Published 40% in most of the examined calcite cemented 813CpDB*values 16 for the calcite cement are samples. Stratabound concretions and between -8.7%o and 0.3%o, supporting that continuously calcite cemented layers are both biogenic carbonate was the main source of thought to have formed from laterally extensive calcite cement although with some input of and continuous biogenic carbonate-rich layers, carbon from decomposition of organic matter. continuous layers forming when the content of Moderately negative pop-values (-5.6 to biogenic carbonate was high enough to allow -2.6%o) combined with the lack of quartz concretions to merge n. The curved calcite overgrowths and intergranular volumes of 29- cemented layers at the base of the Bridport 38% in calcite cemented samples suggests Sands probably formed from biogenic calcite precipitation at relatively shallow depths, carbonate-rich layers deposited on sea floor possibly around 0.5 km. Both layers of topography, possibly formed by rip currents. stratabound concretions and continuously calcite cemented layers are considered to have The Bencliff Grit formed from biogenic carbonate-rich layers, continuous layers forming when enough The Oxfordian Bencliff Grit 26 - 27 was studied biogenic carbonate was present to allow in a 5 m high and 50 m long cliff exposure at concretions to merge. Scattered concretions Osmington Mills in Dorset (Figure 1). The formed where biogenic carbonate was more Bencliff Grit comprises well-sorted, very fine­ uniformly distributed throughout the sand n. grained hummocky-laminated quartzitic sandstones with small fragments of bivalves and The Bridport Sands other carbonate fossils and up to 0.4 m thick more or less laterally persistent mudstones with The Toarcian Bridport Sands 4< 24- 25 were Diplocraterion and other trace fossils. studied in 3 km long and up to 50 m high cliff Deposition of the Bencliff Grit took place in a exposures between West Bay and Burton Beach shallow marine setting below fair weather wave in Dorset (Figure 1). Almost total bioturbation, base. marine trace and body fossils and very fine grain size (coarse silt to very fine sand) suggest Calcite cementation in the Bencliff Grit deposition of the well-sorted subarkosic to dominantly takes the form of concretions with quartzitic sands in a shallow marine diameters up to 1.5 m (Figures 4 and 8). The environment below fair weather wave base. 20- concretions vary in shape from roughly 25 % of the exposures consist of up to 75 cm spherical to flattened with long axes parallel to thick pervasively calcite cemented layers and bedding and may have bulges and protrusions layers of stratabound concretions with sharp partly controlled by the presence of tight boundaries against almost calcitc-frce porous laminae that stopped the outwards growth of the 82 Figure 7. Calcite cementation in the Jurassic Bridport Sands at Burton Bradstock in Dorset. The calcite cemented layers stand out in relief due to their greater resistance to weathering. Cliff is approximately 40 m high. %

Figure 8. Calcite cementation in the Jurassic Bencliff Grit at Osmington Mills, Dorset. Concretions stand out in relief due to their greaterresistance to weathering. Length of hammer is 25 cm.

83 concretions. Lateral merging of concretions gives rise to composite calcite cemented 7/12-5 4 km volumes with lateral extents of up to several metres. Concretions may possibly be concentrated at certain levels, but the general / X impression is one of randomly scattered 7/12-6* X concretions. Boundaries of calcite cemented V \ 7/12-A15 \ volumes are sharp, and almost no calcite is \ present outside the concretions. Except when ^7/12-34 •7/12-2 concretions terminate against tight laminae, \ O \ \ boundaries cut across bedding. 7/12-3 \ The common occurrence of biogenic carbonate within the concretions, but not outside, suggests X< 7/12-4 that biogenic carbonate was the main source for 1 km X calcite cement. However, positive 513Cpdb- X values 4 (11.0 to 17.1%o) indicate that a large part of the carbon in the calcite was derived Figure 9. Location of studied wells in the Ula from CO2 produced from organic matter via Field. fermentation reactions 28. 8180pdb-values for gastropods, foraminifers and sponge spicules) samples from the outermost part of concretions are common within calcite cemented intervals. are in the range -10.6 to -9.1%o 4, which, given Partly or totally dissolved carbonate fossils, a 518Osmow of -1.2%o, suggests that calcite mostly as rims of insoluble material defining cementation was completed at temperatures of the outlines of shells, are also common outside 58-65°C or depths of less than 1.5 km. This is calcite cemented intervals. The marine trace and supported by the high minus cement porosities body fossils and extensive bioturbation point to in the calcite cemented samples (>38%) and the deposition in a shallow marine setting bellow unconsolidated nature of the calcite-free sand fair weather wave base. which suggests that maximum burial probably did not exceed 2 km. The concretions in the Table 1. Studied core intervals, Ula Formation Bencliff Grit probably formed from biogenic Well Depth (mRKB) carbonate distributed rather uniformly within the sandstone since such a distribution of 7/12-2 3385-3477 biogenic carbonate would be expected to give rise to scattered calcite cement nuclei which 7/12-3 3612-3628 grew into scattered concretions at the expense of the biogenic carbonate in the surrounding 7/12-3A 3642-3730 sandstone u. 7/12-4 3447-3525 7/12-5 3844.5-3905.2 Core studies 7/12-6 3404-3518.6 The Ula Formation, Ula Field 7/12-A15 3501-3645 A toal of 593.3 m of Ula Formation 29 cores Calcite cementation in the studied Ula from wells 7/12-2, -3, -3A, -4, -5, -6 and -A15 Formation cores takes the form of numerous up in the Ula Field 30 (Figure 1) were studied to 2.7 m thick pervasively calcite cemented at depths between 3385 mRKB and 3905.2 intervals with sharp to somewhat diffuse mRKB (Table 1). Spacing between the wells boundaries against porous calcite-free varies from 500 m to almost 6 km (Figure 9). sandstone. The calcite cemented intervals are The cored intervals consist of bioturbated and dominantly totally bioturbated, boundaries cut more rarely massive very fine-, fine- and across bioturbation structures, are not associated medium-grained well-sorted subarkosic to with erosion surfaces and do not coincide with arkosic sandstone with rare remnants of changes in grain size or type or degree of horizontal or subhorizontal lamination. A few bioturbation. A few calcite cemented intervals conglomeratic layers with thicknesses of up to a containing faint horizontal or subhorizontal few centimetres occur. Carbonate fossils lamination also occur. These intervals are (bivalves, brachiopods, echinoderm fragments, typically around 0.1 m thick, and some of them 84 can be seen to be small concretions. Most of the of calcite cemented interval in the Ula bioturbated calcite cemented intervals extend Formation cores is very similar to what the across the cores, but some of them only extend laterally extensive bioturbated calcite cemented partly across. Only a few of the calcite layers in the Bridport Sands and in part of the cemented intervals occur at the same level in Bearreraig Formation would look like in a core, different wells even when well spacing is only which was one of the reasons for selecting the 500 m (e.g. 7/12-2 and 7/12-A15). Ula Formation for study. However, several observations suggest an opposite interpretation. The large amounts of calcite cement within Firstly, some of the cored up to 0.4 m thick calcite cemented samples (30-40%) combined bioturbated calcite cemented intervals do not with the lack of quartz overgrowths suggests extend across the cores and are obviously part that calcite cementation in the Ula Formation of concretions. Secondly, very few of the calcite cores took place at depths of less than 2 km. cemented intervals occur at the same levels even Measured 5180pDB-values for calcite cement in in closely spaced wells, e.g. 7/12-2 and 7/12- the studied cores are in the range -11.3 to A15 located 500 m apart, whereas lateral -9.6%o (Table 2), suggesting calcite extents of bioturbated calcite cemented layers precipitation at temperatures of 60-75°C if the in the studied exposures are several kilometres. oxygen isotopic composition of the pore water It may also be significant that the calcite during calcite precipitation was equal to the cemented layers in the Ula Formation are more oxygen isotopic composition of Jurassic marine coarse grained than the laterally extensive water, i.e. 518Osmow = -1.2%o 20. The bioturbated calcite cemented layers in the measured 5180pDB-values, may, however, not be Bridport Sands and parts of the Bearreraig representative for all calcite cemented intervals Formation. It is therefore tentatively concluded in the cores. Transport of the calcium contained that calcite cementation in the Ula Formation in in the calcite cement in the Ula Formation from the Ula Field is dominated by concretions, sources outside the formation by fluid flow is although the presence of some calcite cemented not feasible due to the enormous amounts of layers cannot be excluded. The difference in fluid required 8, li. The source of the calcium the geometry of calcite cementation in the Ula must therefore be sought for within the Ula Field and in the Bridport Sands may be the Formation, and the common occurrence of result of biogenic carbonate having been more or less dissolved carbonate fossils within deposited as laterally extensive layers during the studied cores suggests that biogenic periods of very low siliciclastic deposition in the carbonate was the dominant calcium source. Bridport Sands, whereas siliciclastic deposition Measured 813CpDB-values vary from slightly may have been more continuous in the Ula negative (-5.8 to -3.1%o) to strongly negative Formation leading to a more uniform (-22.3 to -15.1%o) which shows that the carbon admixture of biogenic carbonate rather than in the calcite cement is a mixture of carbon concentration in well-defined biogenic from biogenic carbonate and from carbonate-rich layers. When biogenic carbonate decomposition of organic matter. The amount occurs in certain layers, the concentration of of precipitated calcite, was, however, limited by dissolved calcite necessary for nucleation of the available calcium from biogenic carbonate. calcite cement will first be achieved in these Table 2. SI3C and 5I8 Ofor calcite cement layers since the biogenic carbonate is the source of the calcite cement. Calcite cement nuclei will Well Depth 513 Cpdb 5180pdb thus be concentrated in the biogenic carbonate- (mRKB) (%,) (%,) rich layers, and as the nuclei grow, the biogenic carbonate will be transformed into concretions 7/12-2 3431.15 -15.1 -9.7 that eventually merge to form continuously cemented layers with a lateral extent determined 7/12-2 3445.57 -3.1 -9.6 by the lateral extent of the biogenic carbonate- 7/12-3 A 3680.70 -5.8 -11.3 rich layer. If, on the other hand, biogenic carbonate occurs scattered through a sandstone, 7/12-A15 3579.70 -3.0 -9.8 as may have been the case for the Ula Formation, supersaturations necessary for 7/12-A15 3644.60 -22.3 -10.1 calcite cement nucleation will not be restricted to certain levels, but will occur more or less uniformly within the sandstone. Calcite cement Although grain size is typically coarser and nuclei will thus have a scattered distribution, thickness sometimes greater, the dominant type and as the nuclei grow and biogenic carbonate 85 is consumed, scattered concretions form n. The intervals cannot be seen to occur at the same difference between the geometry of calcite level within different wells. cementation in the Bridport Sands and the Since the Rannoch Formation in 34/10-4 is probable geometry of calcite cementation in the presently at its maximum burial depth, calcite Ula Formation of the Ula Field is thus regarded cementation must have been completed prior to as a result of different distributions of biogenic burial to 1.6 km below the sea floor. Early carbonate within the two formations, a calcite cementation is also indicated by the high difference which in turn was caused by slight contents of calcite cement within the calcite differences in depositional environment and cemented intervals (approximately 45%) and different relative rates of supply of siliciclastic by the moderately negative 5180pDB-values. material and biogenic carbonate. 5180pDB-va Iues for calcite cement from The Rannoch Formation, Gullfaks 1852.00 mRKB and 1863.70 mRKB in well Field 34/10-4 are -5.1%o and -5.8%o respectively 8, suggesting precipitation temperatures of 35 - Cores from the Rannoch Formation 31 were 40°C if precipitation took place from pore studied in wells 34/10-4, 34/10-14 and 34/10-16 waters with a 518OsMOW-value of -1.2%o. from the Gullfaks Field 32 (Figure 1) where the Assuming Jurassic shallow sea floor Rannoch Formation is cored at depths of 1826- temperatures of 15-25°C 7 , this corresponds to 1909 mRKB, 2002-2047 mRKB and 3431- burial depths of around half a kilometre. 3465 mRKB respectively. The three wells are Significant sources of calcium within the located along a NNE-SSW line with 34/10-14 Rannoch Formation in the studied cores were furthest north, 34/10-4 3.5 km to the SSW of probably restricted to biogenic carbonate, 34/10-14, and 34/10-16 12 km SSW of 34/10-4 which is still present in some cores 32, since within Gullfaks South. Each cored interval there is no evidence for the former presence of consists of a coarsening upwards sequence of significant amounts of other calcium-bearing well-sorted moderately micaceous subarkosic minerals such as plagioclase. Published sandstone where grain size increases upwards S^Cpog-values for the mentioned calcite from very fine to fine sand. The sandstone is cement samples are in the range 5.3 to 5.8%o,- dominantly low angle- to horizontally indicating that carbon from biogenic carbonate laminated with some wave ripple lamination and and/or from fermentation reactions 28 accounts fairly common bioturbation in the most fine­ for a major part of the carbon in the calcite grained intervals. Lamina-thickness is typically cement. a few millimetres. A few syn- and antiforms are present within the low angle-laminated sets, and The calcite cemented intervals in the Rannoch set boundaries form low angle discordances. Formation are very similar to the concretions The sedimentary structures, including possible within the very fine-grained hummocky- hummocky lamination, suggest deposition in a laminated Bencliff Grit and the concretions middle to upper shoreface setting. The within the low angle to horizontally laminated Rannoch Formation in the Gullfaks Field has fine-grained sandstones of the Valtos previously been interpreted as wave-dominated Formation. Depositional environment and type delta front deposits 32. of sedimentary structures within and outside calcite cemented volumes are very similar, Calcite cementation occurs in 34/10-4 and boundaries of calcite cemented zones cut across 34/10-16, but not in 34/10-14, in the form of lamination, composition of the sandstones are 0.1-1.5 m thick pervasively calcite cemented similar and biogenic carbonate was probably intervals extending across the cores. The calcite the limiting factor regarding the amount of cemented intervals contain perfectly preserved precipitated calcite cement in all three cases. low angle and horizontal lamination, and The available empirical data thus suggests that boundaries against porous calcite-free the studied calcite cemented intervals are sandstone are sharp or somewhat diffuse and concretions, and this point of view is very cut across lamination. The calcite cemented strongly supported by reasoning concerning intervals are not located at well-defined the original distribution of biogenic carbonate boundaries between different sandstone units within the cored sandstones and its subsequent characterized by different mineralogies, transformation into calcite cement. Within sedimentary structures, degree or type of shoreface sandstones that are hummocky- bioturbation, nor at erosion or hiatus surfaces. laminated throughout and within uppermost The number of calcite cemented intervals is shoreface or beach sandstones with pervasive different in the three wells, and calcite cemented 86 low angle lamination that do not contain may not be simple criteria for differentiating laterally extensive surfaces of erosion or non- between all types of calcite cemented zones. depositien, biogenic carbonate would be The suggested identification key should expected to occur dispersed within the therefore only be regarded as preliminary, and sandstones rather than concentrated as laterally does not apply to all types of calcite extensive biogenic carbonate-rich layers. As cementation in shallow marine sandstones, for discussed for the Ula Formation, when instance patchy calcite cementation 12 dispersed biogenic carbonate is the source of calcite cement in a sandstone, it is not to be The starting point of the suggested expected that calcite cement nuclei should be classification system is a division of cored confined to a single plane or a thin zone, as calcite cemented intervals into three main would be the case if the biogenic carbonate classes: calcite cemented intervals with occurred concentrated in a layer, since the preserved physical sedimentary structures (Class supersaturations necessary for calcite nucleation Cl), homogeneous calcite cemented intervals will occur throughout the sandstone and not where neither physical sedimentary structures just locally within biogenic carbonate-rich nor bioturbation are recognizable (Class C2), layers 11. Calcite cement nuclei uniformly and, totally bioturbated calcite cemented distributed within a sandstone will grow into intervals (Class C3). A calcite cemented interval scattered concretions as biogenic carbonate is is still assigned to the class Cl even if some consumed n, and calcite cementation in the bioturbation is present, the class C3 is restricted studied Rannoch Formation cores would to strongly bioturbated intervals. therefore also on a theoretical basis be expected The class Cl is subdivided according to type of to occur as scattered concretions, not as laterally preserved sedimentary structures and extensive calcite cemented layers. association with erosion surfaces or boundaries Although both nucleation and growth theory of sandstone units. Class C3 is subdivided and empirical data from outcrop studies according to indications of time and duration strongly indicate that the calcite cemented of cementation, signs of exposure at the sea intervals in the Rannoch Formation cores are floor, association with erosion surfaces and part of concretions, it is important to keep in presence or absence of correlatable intervals in mind that this is not a general statement about nearby wells. calcite cementation in the Rannoch Formation. Where the Rannoch Formation consists of several coarsening upwards sequences, laterally Conclusions extensive calcite cemented layers may occur at the boundaries between these units since Calcite cemented zones in the studied outcrops laterally extensive biogenic carbonate-rich have very variable geometries, including layers may have formed at these levels. continuously cemented layers, stratabound concretions and scattered concretions. All these forms of calcite cementation may occur within the same sandstone. Lateral extents of calcite Identification of cored calcite cemented zones vary from a few centimetres to cemented zones several kilometres. No simple general relationship exists between thickness and lateral Ideally it would be desirable to summarize the extent of calcite cemented zones, but the lateral obtained knowledge concerning the geometry extent of cored calcite cemented zones can be of cored calcite cemented zones in shallow understood and predicted based on an marine sandstones in an identification scheme understanding of the original distribution of that would allow different genetic classes of biogenic carbonate in the sandstone, the calcite cemented zones to be correctly nucleation and growth mechanisms of calcite identified by a geologist without extensive cementation, and geometrical data collected experience of this problem. An identification from outcrops. In practice, this implies that the scheme of this type has been tentatively lateral extent of cored calcite cemented intervals established (Figure 10), but it has proved can be constrained based on type of preserved impossible to make the classification scheme sedimentary structures, degree of bioturbation, both simple and comprehensive. This is largely association with surfaces of erosion or non­ due to lack of data concerning some types of deposition, indications of hardground calcite cemented zones and to the fact that there cementation and presence or absence of

87 Homogeneous. Novisible physical sedimentary structures or bioturbation

Preserved sedimentary structures Totally bioturbated X (C3) X X X X Hummocky-laminated throughout. Located at boundary of sandstone unit, Boundaries do not coincide with boundaries Borings. Encrusting fauna. Fibrous calcite. Boundariesdo not coincide with e.g. within an abandonment facies of the homogeneous bed containing the High content of calcite cement. Oxygen isotop laterally extensive surfaces of erosion calcite cemented interval values of calcite cement close to zero or non-deposition or changes in grain size or sedimentarystructures. Lowe boundary may coincide with base ol Possibly laterally extensive layer Probableconcretion Hardground swale with up to several km lateral extent (C2A) (C1E) t Laterally extensive layer with up to several km lateral extent (C3E) Concretion/lens with less than a few! Boundaries coincide with boundaries of the metres lateral extent ; homogeneous bed containing the calcite (CIA) cemented interval Located above an erosion surface X Possible layer Located above an erosion surface Low-angle or horizontally laminated (C2B) throughout. Boundaries do not coincide with laterally extensive surfaces of erosion Base of stolpn-deposited layer ornon-deposition orchanges in grain size or sedimentary structures

Layer with a metre to a few hundred No change in grain size, bioturbation or physical Located at base of storm-deposited laye metres lateral extent sedimentary structures at boundaries. Not (C1D) associated with an erosion surface Probable concretion/lens with less than Layer with a metre to a few hundred a few metres lateral extent metres lateral extent (C1B) (C3D) Lack of possible correlatable calcite cemented intervalsin nearby wells

Laterally extensive erosion surface I Laterally extensive erosion surface Probable concretion/lens or short layer t (C3A) Possibly laterally extensive layer Possibly laterally extensive layer with several km lateral extent with several km lateral extent (C3C) (C1C) Probably correlatable with calcite cemented intervals in!1 I1Clearby wells V Laterally extensive layer orlayer of stratabound concretions with up to several km lateral extent (C3B)

FJmiw f A T/ corrclatable cal cite cemented intervals in nearby Jurassic of Wyoming and Montana, Geol. Soc. wells. Am. Bull., 84, 1685-1698, 1973. Calcite cemented intervals in the studied 3. Bryant, I.D., Kantorowicz, J.D. and Love, Rannoch Formation cores from the Gullfaks C.F., The origin and recognition of laterally Field are probably part of concretions with less continuous carbonate cemented horizons in the than a few" metres lateral extent. This is Upper Lias sands of southern England, Marine supported both by direct comparison with Petroleum Geol., 5, 108-133, 1988. similar calcite cemented zones in the Bencliff Grit and in the Valtos Formation, and by a 4. Walderhaug, O., Bjgrkum, P.A. and probable homogeneous distribution of biogenic Nordg&rd Bolfis, H.M., Correlation of calcite- carbonate in the cored Rannoch Formation cemented layers in shallow marine sandstones sandstones. of the Fensfjord Formation of the Brage Field. In: Collinson, J.D. (ed.), Correlation in Calcite cemented intervals in cores from the Ula hydrocarbon exploration, London, Graham and Formation in the Ula Field are superficially Trotman, 367-375, 1989. similar to laterally extensive calcite cemented layers in the Bridport Sands and in parts of the 5. Bj0rkum, P.A. and Walderhaug, O., Bearreraig Formation. However, calcite Lateral extent of calcite-cemented zones in cemented intervals cannot be correlated shallow marine sandstones. In: Buller, A.T. et "between closely spaced (500 m) wells, and al. (eds), North Sea oil and gas reservoirs - II, combined with the occurrence of some obvious London, Graham and Trotman, 331-336, 1990. concretions in the cores, this suggests that 6. Krynine, P.D., Petrographic studies of calcite cementation in the Ula Field is variations in cementing material of the dominated by concretions. The presence of Oriskany Sand, Pennsylvania State College some more laterally extensive calcite cemented Bull., 33, 108-116, 1941. layers is, however, not excluded. The difference in geometry between calcite cementation in the 7. Hudson, J.D. and Andrews, W.J., The Bridport Sands and in the Ula Formation may diagenesis of the Great Estuarine Group, be that, unlike the Bridport Sands where Middle Jurassic, Inner Hebrides, Scotland. In: biogenic carbonate was concentrated as laterally Marshall, J.D. (ed.), Diagenesis of sedimentary extensive layers during periods of low sequences, Geol. Soc. Spec. Publ., 36, siliciclastic deposition, rates of siliciclastic Blackwell, Oxford, 259-276, 1987. deposition were relatively uniform for the Ula 8. Salgal, G. and Bj0rlykke, K., Carbonate Formation resulting in a more homogeneous cements in clastic reservoir rocks from offshore distribution of biogenic carbonate throughout mid Norway - relationship between isotopic the sand. composition, textural development and burial depth. In: Marshall, J.D. (ed.), Diagenesis of sedimentary sequences, Geol. Soc. Spec. Publ., Acknowledgements 36, Blackwell, Oxfoid, 313-324, 1987. BP Petroleum Development of Norway A.S are 9. Wilkinson, M., Concretions of the Valtos thanked for providing access to cores from well Sandstone Formation of Skye: geochemical 7/12-A15. Snorre Olaussen, Mark Andersen indicators of palaeo-hydrology, J. Geol. Soc. and Bj0rn Lundschien are thanked for London, 150, 57-66, 1993. reviewing the manuscript. 10. 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