Geometry of Calcite Cemented Zones in Shallow Marine Sandstones

'’W 4 PROFIT RF-^/w 1990-1994 PROJECT SUMMARY REPORTS RESERVOIR CHARACTERIZATION NEAR WELL FLOW Program for Research On Field Oriented Improved Recovery Technology oismsunoN of ihb documeot is iwumiteo % 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,

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