Salinity variations in North Sea formation waters: implications for large-scale fluid movements

Salinity variations in North Sea formation waters: implications for large-scale fluid movements* Knut Bjerlykke and Kjetil Grant Department of Geology, Box 1047, University of Oslo, 0316, Oslo 3, Norway Received 14 May 1992;revised 4 November 1992;accepted 7 November 1992 Sodium chloride is the major salt dissolved in formation waters, even at a considerable distance away from any evaporites. Chloride anions are not consumed to any significant extent by diagenetic reactions outside the salt deposits. Chloride ions are therefore preserved in the pore waters, and the distribution of pore water salinities in sedimentary basins may help to constrain pore water flow and the transport of dissolved species of silica and carbonate minerals. In the North Sea Basin and Haltenbanken (mid-Norway), salinity gradients can be inferred from formation water samples. Measurements of pore water salinity from wireline logs are less accurate than formation water samples, but have the advantage that a much larger database can be obtained. Data from 150 wells in the North Sea and Haltenbanken show that the salinities measured from well logs range from 20000 to 300000 ppm. In the southern part of the Norwegian North Sea and Haltenbanken, there is a clear trend towards higher salinities with greater burial depth. There is a pronounced salinity increase in the 500 m of section closest to the evaporites. In the northern North Sea, where underlying evaporites are not known, there is only a slight increase in the measured salinities with depth. There are, however, no measurements of high salinity pore water (>100 000 ppm) at depths shallower than 2200 m. This pattern of salinity distribution in the pore water precludes large-scale convection of the pore water during diagenesis. It also indicates limited compaction-driven flow. Log-derived salinity data from the North Sea suggest that the pore waters in Upper Jurassic and Tertiary reservoirs are generally more saline than those of Middle and Lower Jurassic reservoirs at the same depth. The constraints on pore water flow that can be deduced from salinity variations puts severe limitations on the mechanisms of transport of dissolved solids in sedimentary basins and thus also on aspects of diagenetic models. Keywords: North Sea; formation waters; salinity variations; fluid movements

Introduction One of the most critical factors controlling diagenetic processes is the amount of mass transfer by pore water flow over long distances (>100 m) in sedimentary basins. It is very important to know the extent to which common cements such as quartz and calcite have been introduced from distant sources or redistributed by local dissolution and precipitation. Large fluxes of pore water are required to move significant volumes of solids, given the low solubilities of these minerals. At low temperatures and high flow rates the pore water may be undersaturated or supersaturated with respect to the mineral phases, in particular the silicate minerals, which have slow reaction rates. With increasing temperature, the kinetic reaction rates increase rapidly and the pore water composition will approach equilibrium with the mineral phases (Giles, 1987). The concentration of most elements in the pore * Presented at the GeologicalSocietyof Londonmeeting'North Sea Formation Waters: Implications for Diagenesis and Production Chemistry', London,UK, 15 January 1992 t Currentaddress:SagaPetroleumAS, Kj~rbovn16, 1301Sandvika, Norway

water is thus controlled by the solubility of the mineral phases present in the sediments, and is therefore not very useful for tracing pore water flow. Elements which occur primarily in evaporite minerals, however, are not constrained by mineral solubilities outside of evaporites. Chloride is normally the main anion in pore water and is only to a very small extent precipitated by diagenetic reactions outside evaporites. Its concentration will therefore be mainly a function of diffusion and dilution due to mixing by advection with less saline pore water. Close to evaporites, the chemical composition of pore water will approach equilibrium with the major evaporite minerals, and will usually decrease away from the salt layers or salt domes. High concentrations of dissolved salt in the surrounding sediments reflect effective transport by diffusion or advection from the evaporites. Steep gradients or abrupt changes in the salinity provide evidence of limited advective mixing of pore water and may indicate the presence of barriers to diffusion, such as tight shales. Effective vertical mixing of pore water or convection of sufficient intensity to move significant volumes of solids in solution would have homogenized the pore water composition to a

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Salinity variations in North Sea formation waters: K. Bjcrlykke and K. Gran saturation in the formation. The formation water large extent. The distribution of pore water salinities in resistivity is then converted to salinity (ppm NaC1 sedimentary basins may therefore help to constrain equivalents) using charts provided by the wireline pore water flow and thus also diagenetic reactions. service companies. A more detailed discussion of the determination of pore water salinities from wireline Geochemistry of formation water samples logs is given by Gran et al. (1992). A database comprising salinity information from Formation water samples are available from most nearly 150 wells in the Norwegian North Sea and from reservoirs, but the quality of samples and analyses Haltenbanken (mid-Norway) shows that the salinities varies widely (Egeberg and Aagaard, 1989). Still, these measured from well logs range from 20000 to are the best data available and must be used, albeit with 300000 ppm. The salinities are from analyses of caution regarding possible sources of contamination. formation water samples and wireline logs. In the Pore water samples are normally limited to reservoir Central Graben area (56-59 ° N) and Haltenbanken rocks and are not taken from all wells and usually from (mid-Norway), there is a clear trend towards higher only one depth. Water may be produced from both salinities with greater burial depth (Figure 1A and 1C). above and below the oil-water contact and the There is a pronounced increase in salinity in the 500 m irreducible water in the oil zone may differ from that in of section closest to the evaporites, where the salinities the water-saturated zone. approach halite saturation. Much of the scatter in the Several workers have suggested that pore water salinity values shown in Figure 1A is due to salt diapirs composition in sedimentary basins is often a in the Central Graben, which give wide ranges in two-component mixture of evaporite residue and salinities at the same depth. The high salinity pore meteoric water (Kharaka et al., 1985). Egeberg and water overlying the evaporites may be due to expulsion Aagaard (1989) suggested that the pore water composition in the North Sea Basin was due to mixing between meteoric water and a residual pore water after evaporation. This is the 'connate' water of the A evaporites which is forced out during compaction. IJ I 300000 Evaporites may have initial porosities as high as 50% I i I 250000 (Sharp et al., 1988). In addition, the dissolution of 200000 evaporites and transport by diffusion would have m • =..E contributed to the salinity gradient, but it is difficult to 150000 -. ... estimate the relative contribution from these two 100000 sources. ".-:,.% ; : ~ ' . . : . .E 50000 In the northern Viking Graben and the East Shetland I I I I I L Basin, there is no record of underlying evaporites 1000 2000 3000 4 0 0 0 5 0 0 0 6000 (Glennie, 1990). The salinities of the formation waters Depth (mRKB) in the reservoirs in the Viking Graben are in some instances less than half that of sea water, clearly B indicating a meteoric water influence. However, if all the connate water was flushed out by meteoric water, it g 300000 250000 would be difficult to explain the salinities observed in the absence of evaporites. 200000 E The influence of meteoric water on the formation 150000 water is clearly documented in Jurassic reservoirs from lOOOOO the northern North Sea, particularly in the Brent , ;,':",".'_.. ......Group, indicated by low salinities and negative 6~80 •50000 I I I L values (Egeberg and Aagaard, 1989). The extensive I000 2000 3000 4000 5000 6000 feldspar leaching observed in the Brent Group is Depth (mRKB) evidence of Jurassic meteoric water flushing (Bj0rlykke et al., 1992). To what extent the present day brackish pore water represents modified Jurassic meteoric water and whether there have been later meteoric water g 300000 recharges into these reservoirs is at present not known. 250000 High overpressures in several of these reservoirs z 200000 E would, however, preclude recent meteoric water flow 150000 (Buhrig, 1989). 100000 • 50000 : .-"~# .. Salinity profiles derived from wireline logs EE

I

Measurements of pore water salinity from wireline logs are less accurate than analyses of formation water samples, but have the advantage that a much larger database can be obtained. This method also makes it possible to analyse the salinity distribution vertically in each well. Formation water salinity is estimated by calculating the formation water resistivity (Rw), primarily using resistivity and density logs, and assuming 100% water 6

1000

I

I

I

2 0 0 0 3 0 0 0 4000 Depth (mRKB)

I

5000

I ,

6000

Figure 1 Formation water salinity as a function of depth, based on well log data and formation water analyses published by Egeberg and Aagaard (1989). (A) Central Graben (56-59 ° N), (B) northern North Sea (59-62 ° N) and (C) Haltenbanken. Note the absence of a significant increase in salinity with depth in the northern North Sea region where underlying evaporites are not known

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Salinity variations in North Sea formation waters: K. Bjcrlykke and K. Gran of connate brine in the evaporites. In the northern North Sea (59-62 ° N), where the underlying evaporites are not known to occur, there is no significant increase in the measured salinities with depth (Figure 1B). The data show considerable scatter, which may be partly due to the uncertainty of the method, but there is no significant increase in salinity with depth. The log-derived salinities agree well with formation water analyses from the same depth (Gran et al., 1992). Where evaporites are present, the salinity gradient for the first few hundred metres away from the evaporites probably reflects rates of diffusion and therefore indirectly the diffusion coefficient of the lithology. Locally, advective flow due to compaction may also have contributed to salt dispersal, and the abnormally high salinities measured at intermediate burial depths may reflect such transport from the underlying evaporites. There are no measurements of high salinity pore water (>100000 ppm) from depths shallower than 2200 m, however. The pattern of salinity distribution in the pore water precludes recent large-scale overturning and convection of the pore water because it would have homogenized the salinity within the sequences involved. If convection had occurred earlier, highly saline pore water would have been distributed over the whole depth range of convective flow. To establish new salinity gradients as observed today would require dilution, e.g. by meteoric water flow or from mineral transformations (dehydration). As the uppermost 2.2 km of the sequence contains no high salinity pore water, it might be natural to assume that any intense vertical mixing must have occurred before this sequence was deposited. However, when the evaporites were shallower, the overall salinity gradient from the seafloor would have been greater, producing a more stable density gradient. A rather moderate salinity increase with depth (>25000 ppm/km) is sufficient to counteract the thermal expansion of water and therefore remove the reverse density gradient which is the driving force for thermal convection (Bj0rlykke et al., 1988). The salinity gradients observed away from the Zechstein salt in the North Sea are often much higher than this (Gran et al., 1992). It is not clear to what extent recent meteoric water flow into the basin has influenced the salinity distribution. In the Ekofisk area, overpressure starts at about 1.3 km (van den Bark and Thomas, 1981) and it is unlikely that the hydraulic head of the meteoric water would have overcome significant overpressures. The degree of overpressure increases gradually with depth in this region and this will produce unidirectional compaction-driven pore water flow. We do not, however, know when overpressure started to build up. The fact that the reservoir rocks in the North Sea do not occur on exposed land areas in Norway and Shetland reduces the potential for Neogene meteoric water flow into the basin. Tertiary reservoirs have relatively high salinities compared with the Jurassic reservoirs (Figure 2), suggesting that there is little meteoric water recharge in the lower Tertiary section. Petrographic evidence suggests that Tertiary sandstones were subject to less intensive meteoric water flushing after deposition than Middle Jurassic reservoirs (Bj0rlykke and Aagaard, 1992). Recent meteoric water recharge in the shallowest reservoirs

0,45

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o'"

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~oO o~ -=• - O

•

•

U

0,05 t

I

I

I

I

1000

2000

3000

4000

5000

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Figure 2 Formation water resistivity (Rw) for northern North Sea wells versus burial depth. The Rw values are converted to a reference temperature of 20°C. Formation water resistivities for the Jurassic reservoirs are shown for individual formations, indicating that the lower salinities in the Middle and Lower Jurassic reservoirs (Brent Group, Dunlin Group and Statfjord Group) compared with the Upper Jurassic Viking Group and Tertiary formations at similar burial depths. (~) Tertiary; (A) Viking Group; ( . ) Brent Group; (G) Dunlin Group; (&) Statfjord Formation; and ([]) Triassic

would have produced extensive biodegradation. Our data suggest that salinity/depth trends depend to a certain extent on the stratigraphic age of the reservoirs (Figure 2). This suggests that the pore water in this basin is to some extent compartmentalized and this is also evidence of restricted pore water flow. In other sedimentary basins which have evaporites near the base of the sequence, salinities often increase in a systematic and almost linear way (Dickey, 1979). Around salt domes, the salinity distribution usually reflects the flow of saline pore water up along the salt dome, continuing horizontally into overlying beds (Bray and Hanor, 1990). This results in an inverse salinity gradient and some degree of downward flow (Ranganathan and Hanor, 1988; Bray and Hanor, 1990). One of the reasons that complete convection cells do not develop is that overpressure produces forced fluid flows which are stronger than the forces driving the thermal convection. Calculations of fluid flow and mass transfer The steep salinity gradients in rocks overlying the evaporites (Figure 1A and 1C) suggest that the saline pore water has been subsiding at almost the same rate as the salt. When the salt was shallower (i.e. 2 km depth) it would probably also have had highly saline pore water overlying it. During subsidence down to 4-5 km, the saline pore water may have remained in nearly the same position relative to the salt. The net movement of the pore water is therefore clearly downwards at almost the same rate as the subsidence of the basin. This is reasonable, as the contribution of compactional water from the underlying Permian sequence is probably rather small. Where the evaporites lie relatively horizontal, the salinities in the overlying sediments may record degrees of relative upwards pore water flow. Much of the scatter in the salinity distribution versus burial depth in the Central Graben area is due to salt doming. The salinity therefore correlates better with the inverse distance to

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Salinity variations in North Sea formation waters: K. BjCrlykke and K. Gran the evaporites than to the present day burial depth shallower section, and this suggests that large-scale (Gran et al., 1992). The salinity distribution also shows convection was not effective during the deposition of no evidence of fluid flow upwards along faults. This is this sequence. In the northern parts of the North Sea probably because most of the North Sea faults have not where evaporites are not recorded, highly saline brines been active since Cretaceous times (Glennie, 1990). have not been found. In Haltenbanken (mid-Norway), however, highly saline pore waters are recorded in the The relatively low solubility of the common silicate deepest parts of the section towards the underlying and carbonate minerals means that at any given time, Triassic evaporites. very little of the main components of these minerals Transport of solids in solution at a level which is will be held in solution in the pore water. If the pore significant for porosity reduction (i.e. by the water is in equilibrium with the main silicate and precipitation of quartz), requires very high fluxes of carbonate minerals, the rate of precipitation or pore water. Such fluxes (>108 cm3/cm2) below the dissolution can be calculated from the fluid flow relative influence of meteoric water flow can only be obtained to a temperature field. The volume precipitated, V, can by convective flow and highly focused compactionbe calculated from the solubility gradient, 0~T, the driven flow. Both formation water analyses and geothermal gradient, ~ T/~ Z, and the mineral density, log-derived salinity data show that the salinity P distribution is inhomogeneous, but suggest that the pore water has a crude depth stratification, implying V = FaT( 0 T/~ Z)/p limited vertical mixing. In the case of quartz, the solubility gradient (ocT) is close to 2 ppm/°C at 100°C (Wood and Hewett, 1986) and the mineral density is 2.7 g/cm 3. This means that if the geothermal gradient ( c~T/0 Z) is 3 × 10-4°C/cm, a Acknowledgements flux (F) close to 3 × 109 cm3/cm2 is required to cause This study was supported by Vista, a research total cementation of a given volume of sediment co-operation between the Norwegian Academy of ( V = 1). To 'import' 10% silica cement ( V = 0.1), a Science and Letters and Det Norske Stats Oljeselskap pore water flux of 3 × 108 cm3/cm2 is required, (Statoil). We thank BP, Conoco, Norsk Hydro, Saga, assuming that the pore water is in equilibrium with Shell and Statoil for providing the well logs. silica. Several models for diagenesis in the North Sea Basin and similar basins invoked large-scale vertical pore water flow to explain the transfer of solids. Burley (1986) suggested that acidic pore water References moved up section about 1-2 km and leached feldspars Bjorlykke, K. and Aagaard, P. (1992) Clay minerals in North Sea in shallow reservoirs. Gluyas and Coleman (1992) sandstones. In: Origin, Diagenesis, and Petrophysics of Clay proposed that the quartz cement in reservoirs from the Minerals in Sandstones (Eds D. W. Houseknecht and E. D. North Sea and Haltenbanken could be explained by Pittman), Spec. PubL Soc. Econ. PaleontoL Mineral No. 47, episodes of influx of silica from outside the reservoirs. pp. 65-80 It is difficult to understand how sufficient fluxes to Bjorlykke, K., Mo, A. and Palm, E. (1988) Modelling of thermal convection in sedimentary basins and its relevance to introduce a significant silica cement (108 cm3/cm2) can diagenetic reactions Mar. Petrol GeoL 5, 338-'351 be obtained without convection in sedimentary basins Bjorlykke, K., Nedkvitne, T., Ramm, M. and Saigal, G. (1992) which are about 5 km deep (5 x 105 cm). Diagenetic processes in the Brent Group (Middle Jurassic) Focused compaction-driven flow can explain only reservoirs of the North Sea - - an overview. In: Geology of the Brent Group (Eds A. C. Morton, R. S. Haszeldine, M. R. very local features involving limited volumes and Giles and S. Brown), Spec. PubL GeoL Soc. London No. 61, cannot be invoked to explain commonly occurring pp. 263-288 diagenetic features.

Conclusions Formation water salinities derived from wireline logs agree well with analyses of formation water samples and provide a better picture of the vertical and regional salinity distribution in a sedimentary basin. The log-derived data for pore water salinity provide a better basis for studying vertical salinity gradients, as several measurements can be made at different depths in each well. The highly saline pore water overlying the evaporites has been subsiding at almost the same rate as the basin. The observed salinity gradients in the sequences above the evaporites in the North Sea and Haltenbanken preclude large-scale intensive convective flow, as this would have homogenized the pore water salinity. The gradients are also evidence of limited vertical flow and mixing of the pore water. The absence of high salinity pore water (>100000 ppm) shallower than 2.2 km shows that compaction-driven flow from the deeper part of the basin does not reach into the 8

Bray, R. B. and Hanor, J. S. (1990) Spatial variations in the subsurface pore fluid properties in a portion of southeast Louisiana: implications for regional fluid flow and solute transport Trans Gulf Coast Assoc. GeoL Soc. 40, 53-64 Buhrig, C. (1989) Geopressured Jurassic reservoirs in the Viking Graben: modelling and geological significance Mar. Petrol GeoL 6, 31-48 Burley, S. D. (1986) The development and destruction of porosity within Upper Jurassic reservoir sandstones, Outer Moray Firth, North Sea Clay Miner. 21,649-694 Dickey, P. A. (1979) Petroleum Development Geology, PPC Books Division of the Petroleum Publishing Company, Tulsa, OK, 398 pp Egeberg, P. K. and Aagaard, P. (1989) Origin and evolution of formation waters from oil fields on the Norwegian shelf AppL Geochem. 4, 131-142 Giles, M. R. (1987) Mass transfer and problems of secondary porosity creation in deeply buried hydrocarbon, reservoirs Mar. Petrol GeoL 4, 188-200 Glennie, K. W. (1990) Introduction to the Petroleum Geology of the North Sea, Blackwell, London, 402 pp Gluyas, J. and Coleman, M. (1992) Material flux and porosity changes during sediment diagenesis Nature 356, 52-54 Gran, K., BjOrlykke, K. and Aagaard, P. (1992) Fluid salinity and dynamics in the North Sea and Haltenbanken derived from well log data. In: Geological Application of Wireline Logs II (Eds A. Hurst, C. M. Griffiths and P. F. Worthington), Spec. PubL GeoL Soc. London No. 65, pp. 327-338

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S a l i n i t y v a r i a t i o n s in N o r t h Sea f o r m a t i o n w a t e r s : K . B j c r l y k k e a n d K. Gran Gulf of Mexico sediments. In: Diagenesis, II (Eds G. V. Chilingarian and W. H. Wolf), Developments in Sedimentology, Elsevier, Amsterdam, pp. 43-113 Van den Bark, E. and Thomas, O. D. (1981) Ekofisk: first of the giant oil fields in western Europe Am. Assoc. Petrol. GeoL Bull. 65, 2341-2363 Wood, J. R. and Hewett, T. A. (1986) Forced fluid flow and diagenesis in porous reservoirs - - controls of the spatial distribution. In: Roles of Organic Matter in Sediment Diagenesis (Ed. D. L. Gautier), Spec. Pub/. Soc. Econ. Paleontol. Mineral. No. 38, pp. 181-189

Kharaka, Y. K., Hull, R. W. and Carothers, W. W. (1985) Water-rock interactions in sedimentary basins. In: Relationship of Organic Matter and Mineral Diagenesis (Eds D. C. Gautier, Y. Kharaka and R. Surdam), Soc. Econ. Paleontol, Mineral. Short Course No. 17, pp. 79-272 Ranganathan, V. and Hanor, J. S. (1988) Density driven ground water flow near salt domes Chem, GeoL 74, 173-188 Sharp, J. M., Galloway, W. E. Jr., Land, L. S., McBride, E. F., Blanchard, P. E., Bodner, D. P., Dutton, S. P., Farr, M. R., Gold, P. B., Jackson, T. J., Lundegard, P. D., MacPherson, G. L. and Milliken, K. L. (1988) Diagenetic processes in Northwestern

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