DIAGENETIC HISTORY AND RESERVOIR QUALITY OF A BRENT SAND SEQUENCE

Transcription

1 Clay Minerals (1984) 19, DIAGENETIC HISTORY AND RESERVOIR QUALITY OF A BRENT SAND SEQUENCE G. A. BLACKBOURN Britoil plc, 150 St. Vincent Street, Glasgow G2 5LJ (Received 15 June 1983; revised 28 November 1983) ABSTRACT: The Etive Formation of the Middle Jurassic Brent Group in part of the Northern North Sea comprises dominantly clean, fine- to medium-grained sands, deposited as part of a barrier-bar complex. The overlying Ness Formation was deposited on supra- or intertidal fiats, and comprises silty channel sands with silts, muds and thin coals. The sands of both Formations are mainly quartz-rich, with up to 12% by volume of feldspar, and variable proportions of clayey matrix. Early carbonate cementation preceded a phase of quartz overgrowth, which continued during burial. Later dissolution of unstable grains, dominantly feldspars, was followed by precipitation of pore-filling kaolinite and minor late-stage mineral phases. Better permeability of the Ness sands (up to 500 md) relative to the Etive (mostly < 10 md) is mainly due to the effects of diagenesis on different lithofacies. Silty sands escaped intense quartz cementation and were thus more affected by acid groundwaters which improved permeability. The Brent Group forms the reservoir rock of numerous oil accumulations in the Northern North Sea. Despite considerable investigative work, no universally accepted sedimentological or diagenetic model has yet been proposed for the Group, and each oilfield presents its own problems. The Brent Group may be subdivided into five lithostratigraphic units, the Broom, Rannoch, Etive, Ness and Tarbert Formations (Deegan & Scull, 1977). In the area under consideration here, the Etive and Ness Formations comprise the main reservoir (Fig. 1). However as Fig. 2 shows, despite relatively good porosities (averaging ~ 15%) for a burial depth in excess of ft, permeabilities are low--varying widely up to around 500 md in the Ness Formation and seldom exceeding 10 md in the Etive Formation. In addition, permeabilities in the silty, relatively poorly-sorted sandstones of the Ness Formation are generally better than in the clean, better-sorted Etive Formation. This contrasts with the common notion that clean, well-sorted sands tend to make better reservoirs than matrix-rich poorly-sorted sands. The aims of the study reported here were: 1. To explain why relatively good porosities are associated with poor permeabilities in the Ness and Etive Formation sands. 2. To determine why silty sandstones in the Ness Formation have significantly better permeabilities than clean sandstones in the Etive Formation, despite similar porosities. 3. To establish whether variation in reservoir quality of the Ness and Etive Formation sands within the area is more dependent on original sedimentary facies, or on lateral or depth-related variations in diagenesis The Mineralogical Society

2 378 G. A. Blackbourn BRENT GROUP LITHOSTRATIGRAPHY LITHOSTRATtGRAPHIC UNITS LITHOLOG~r TARBERT NESS 13- ETIVE (5 ~_ RANNOCH I I- r,, RANNOCH II RANNOCH III + BROOM :~- Q~I :::::::::::::::::::::::::: i FIG. 1. Outline lithostratigraphy of the Brent Group in the study area. Key as in Fig. 2. The petrographic work was carried out mainly by optical microscopy, supported by scanning electron microscopy and energy-dispersive X-ray analysis. Limited X-ray diffraction analyses were also made. Etive Formation DEPOSITIONAL ENVIRONMENTS AND FACIES The Etive Formation in this area comprises mostly uniform, fine- to medium-grained buff and grey sandstones with thin clay-rich stylolitic laminae (Fig. 2). There are no visible sedimentary structures in the core through most of the Formation, although horizontal and small-scale inclined bedding becomes apparent in places. The detrital petrography, although variable, shows no significant trends either laterally or with depth. Quartz proportions range from 50 to 80% by volume, of which up to 16% are polycrystalline quartz from metamorphic rock fragments. Feldspars vary from 2 to 12%, and are mostly untwinned (probably orthoclase), although occasional microcline and plagioclase grains are present. Grains are subangular to subrounded where unaffected by overgrowth, with moderate to good sorting. Muscovite and biotite are only rarely present in more than trace amounts. Clayey detrita! matrix forms only a small proportion of the rock, and is mostly present in the stylolitic laminae. Heavy minerals recognized in thin-section include tourmaline, zircon, rutile, epidote and iron oxides. The rock is generally non-calcareous, although there are some thin zones and patches of calcite cement. The depositional environment of the Etive Formation is uncertain, due to the paucity of sedimentary structures. A multistorey sequence of fluvial channel sands or an offshore marine or barrier bar sandbody could each produce a thick sandstone succession like the Etive Formation. Attempts to enhance the appearance of any structure by X-radiography

3 Diagenetic effects on Brent reservoirs 379 PERMEABILITY 0 (me) soo o_ (~) 2s Z 0 I- < HORIZONTAL PERMEABILITY 0 (rod) ~EFG~-Fr6~-O-S-r~-V- O_ (~l 25 0 Z 0 < n- O W > W ] SANDSTONE ~'~ INCLINED BEDDING ] SILTSTONE ~ HORIZONTAL BEDDING (--) BEDDINGINDISTINCT HORIZONTAL I COAL ~ IRREGULAR BEDDING PLANT FRAGMENTS C{~ BIOTURBATION.~1 MUDSTONE CLASTS ~ BURROWING /VV STYLOLITES ~ NO VISIBLE STRUCTURE FIG. 2. Sedimentological logs and porosity/permeability data (from core analysis) of parts of the Ness and Etive Formations in two representative wells from the study area.

4 380 G. A. Blackbourn of cores produced little success, apart from highlighting rare and indistinct subvertical burrows (Figs 3 and 4). The underlying Rannoch Formation is interpreted as a large-scale coarsening-upwards sequence produced by the progradation of a wave-dominated delta front, and the boundary between this and the Etive Formation is gradational rather than abrupt. The Etive Formation may therefore have been open to some marine influence rather than being wholly terrestrial; it probably represents barrier-bar or similar shoreline sands. This agrees with the model proposed by Budding & Inglin (1981) for the Etive Formation of the Southern Cormorant Field (Fig. 5). Further support for this comes from a poor palynofacies assemblage comprising restricted marine microplankton together with fresh-water algae. The general lack of primary sedimentary structure in the sands is attributed to a combination of original good sorting, a low proportion of detrital clay and possible extensive bioturbation. Ness Formation The Ness Formation comprises a sequence of sandstones, siltstones and claystones with occasional coals. The sands are similar in some respects to those in the Etive Formation. They are buff and grey, fine- to medium-sand grade, with sporadic stylolitic laminae, although these are less common than in the Etive. The Ness sands tend, however, to have a more pervasive silt and clay matrix than in the Etive Formation; sub-vertical and horizontal burrows are more common, and permeabilities are significantly higher (Fig. 2). An indistinct cross bedding and irregular wavy silty lamination are present in places. The distribution of clay in the sand is best seen by X-radiography of cores (Figs 3 and 4). The siltstones and claystones are mostly dark grey and black. They occur as beds of variable thickness, as well as 'rip-up' clasts within the sandstones. In places they are highly pyritic, and contain variable amounts of plant material and other unidentifiable carbonaceous matter, so that they grade into thin beds of impure coal. Rare traces of rootlets are present in silty sands below the coals. Sedimentary structures are mostly indistinct, but include fiat and wavy bedding, irregular ripple lamination, and convolute bedding attributed to gravitational slumping and collapse of wet mud. The detrital petrography of the sands in the Ness Formation is very similar to that described above for the Etive. The chief differences are the higher proportions of silt and clay matrix, and a greater proportion of muscovite and biotite, reaching 3% by volume in the Ness Formation. Grains are mostly subangular to subrounded, with poor to moderate, FIGS 3 and 4. Prints of X-radiographs of Ness and Etive Formation sands. Because of widely varying densities, each radiograph is a mosaic of different exposures. Circular and rectangular white patches are sites of core-plug removal. Divisions on scale bar are 5 cm long. Fig. 3: Etive Formation. Clays (appearing dark) are mainly restricted to stylolitic laminae or bundles of stylolites (which are over-represented here relative to the Etive as a whole). Diffuse horizontal and inclined lamination is due to minor grain-size variations. Typical Etive sand is rather clean and structureless, as in the lower half of the third column. Indistinct vertical structure in the fourth column is probably a rare burrow. Fig. 4: Ness Formation. Similar to Etive, but silt and clay matrix has been intimately mixed with sand by extensive bioturbation, producing a reticulate pattern like that in the lower halves of the first two columns. This matrix prevents total silica cementation of the Ness Sands.

5 Diagenetie effects on Brent reservoirs ! ~!i~ ii?i~~ ~ i i ~ I %: 6~ ~ ~!~i~i~

6 382 G. A. Blackbourn RANNOCH I l NESS J~ ~ETIVE FIG. 5. Schematic depositional environments of the Ness and Etive Formations. After Budding & Inglin (1981). rarely good, sorting. Calcite cement is generally absent, but irregular patches of carbonate are present in places, with an apparently random distribution. X-ray diffraction analyses of claystones showed that kaolinite is the major component, with less abundant illite, fine-grained quartz, and minor amounts of pyrite and feldspar. As in the Etive Formation, the poor definition of sedimentary structures in the Ness Formation makes environmental interpretation difficult. The presence of thin coals associated with rootlets is, however, unequivocal evidence for subaerial emergence. By analogy with previous work on other Ness sequences with better preserved structure (e.g. Parry et al., 1981; Budding & Inglin, 1981), the Ness Formation in this area can be ascribed to a delta-top environment (Fig. 5). Distributary channels and associated crevasse splays laid down sand in an area of swamps and possibly shallow lakes. Marine microplankton in some of the claystones suggest that the interdistributary areas were subject to marine inundation, although these species may have been blown in from offshore. DIAGENESIS Despite the difference in reservoir quality, there is little appreciable difference in the inferred diagenetic sequence between the Ness and Etive sands. Neither is there any noticeable variation with depth of burial, the lower part of the Etive Formation in the deepest well lying only 700 ft deeper than the upper part of the Ness Formation in the shallowest well. The Ness and Etive Formations of all the wells studied lie within the oil zone. The diagenetic histories of both formations are thus discussed together, and can be divided broadly into four successive stages: 1. Soil processes and other results of very shallow burial. 2. Loss of porosity, due primarily to progressive silica cementation during burial.

7 Diagenetie effects on Brent reservoirs Development of secondary porosity by partial dissolution of clastic grains. Kaolinite authigenesis. 4. Late-stage authigenesis of, for example, quartz overgrowths, dolomite and fibrous illite. These stages should not be taken as distinct events, as there is much gradation and overlap between them, but they do give an approximate order for the different phases of diagenesis, both temporal and in depth of burial. The diagenetic history suggested below applies to both the Ness and Etive Formations, unless stated otherwise. The chief differences between the formations appear to relate to the response during diagenesis of different lithologies, as described below. Shallow-burial diagenesis This stage includes all the processes occurring within the first few tens of metres of burial depth. Evidence for these processes has largely been obscured by later events. Thin pellicles of well crystallized clay occur on some detrital grains, where they have been preserved below later overgrowths. These are interpreted as recrystallized detrital clay rims, although their preservation below overgrowths prevents their observation under the SEM. Irregular patches and small aggregates of calcite and siderite rhombs within the Ness Formation, concentrated particularly within the finer-grained fractions, may be related to soil processes occurring within a subaerial swamp environment. Commonly, these carbonate cements are partly enclosed within later quartz overgrowths. Disseminated and framboidal aggregates of pyrite are also common in the Ness Formation, usually associated with organic material. They are found in places partially or completely within quartz overgrowths, and are probably also of early origin. Decomposition of detrital biotite to clay minerals, iron oxides and sporadic siderite is also attributed to this early stage of diagenesis. A phase of quartz overgrowth on some grains is largely corroded and possibly abraded. It was clearly formed at an early stage, and may even have been present on the grains before deposition, having survived transport from an older rock. Syntaxial quartz overgrowths and epitaxial cements have, however, in places preserved detrital mica flakes from compaction-crushing, suggesting that a phase of quartz overgrowth did occur during shallow burial. Cementation related to progressive burial In addition to early quartz overgrowths which have apparently preserved micas from compaction-crushing, other overgrowths surround micas which have suffered severe crushing, distortion and partial decomposition (mostly to kaolinite and opaque oxides). This is taken as evidence that the overgrowth continued during burial. In many of the clean sands, almost all of the original intergranular porosity has been occluded by syntaxial quartz and minor feldspar. These cleaner sands are characteristic of the Etive Formation, where good sediment sorting is thought to have been achieved by high-energy nearshore and beach processes. The fluvial Ness sands have a higher average silt and clay content. This silt fills up much of the intergranular space, and reduces the available volume for overgrowth to take place. Although stylolites are quite common in the core, especially in the upper part of the Etive Formation, pressure solution on a microscopic scale is not a major feature of the

8 384 G. A. Blackbourn rock. Sutured contacts between quartz grains are present, and are well developed in places, but contacts between adjacent grains are more often due to the meeting of overgrowths which have formed in the originally intervening pore space. An early framework of grains and overgrowths would have prevented the high pressures required at grain contacts for extensive quartz pressure solution to have taken place. This is illustrated by the well-developed sutured contacts which are formed sporadically between grains where one or both of them lies partly within a clayey matrix, and is not so well supported by surrounding grains. Development of secondary porosity and associated authigenesis Both the Etive and Ness sands have undergone a phase of dissolution of grains and cements, producing secondary porosity. The most abundant component to have been affected are the feldspars, grains of which are variably altered from almost complete dissolution to a barely perceptible 'clouding'. Quartz grains have been much less affected, and only a slight pitting and rounding of the original grain surfaces and overgrowths can be attributed to this phase. The porosity resulting from this phase of dissolution has been partially filled by compact 'book-like' and vermicular authigenic kaolinite, formed as a degradation product of feldspar simultaneously with or shortly after dissolution. This leaching associated with kaolinite precipitation has been recorded by authors working on the Brent Group elsewhere. It has been attributed (Sommer, 1978; Hancock & Taylor, 1978; Hancock, 1978) to the action of near-surface groundwaters circulating during the Late Jurassic-Early Cretaceous Cimmerian earth movements, when the Brent Group was uplifted and partially eroded. Morton & Humphreys (1983), working on the Murchison Field, rejected this explanation. By counting heavy minerals they showe~that unstable detrital minerals, particularly apatite, had been selectively leached from two particular horizons. Apatite was absent from the top of the Etive Formation, and the top of the Middle Ness sands, but gradually increased in proportion downwards (Fig. 6). They argued that large-scale groundwater movementp originating at the surface and percolating through the whole Brent Group would affect the whole succession, not particular stratigraphic units. They therefore suggested that leaching took place during or immediately after deposition, and occurred in two phases corresponding to the two most leached horizons. A third interpretation, that all or most of the leaching occurred during deep burial, is considered here as best able to explain the observed data. The model for generation of secondary porosities in sandstones by circulating acid groundwaters at depths of the order of ft has been described by Schmidt & McDonald (1979). Carbonic and organic acids are considered to be produced during depth-related maturation of organic matter in mudrocks. Morton & Humphreys (1983) agreed that this model might explain some of the mineral solution in the Murchison Field, but thought that synsedimentary leaching was dominant. However, although their concept of two early phases of leaching is plausible, it is a remarkable coincidence that the periods of leaching should take place immediately after deposition of the two most permeable sandstones. These two sandstones were deposited in different depositional environments, and there is no supporting evidence for a prolonged period of non-deposition at the top of either. If contemporaneous leaching of apatites did take place, the thin sands lying between low-ph, organic-rich soil horizons in the Ness Formation should have been particularly susceptible, but leaching of these sands was not

9 Diagenetie effects on Brent reservoirs /19-3 ~ 211/19-4 l===~=a A x 1 q E NESS x x I C ETIVE [ - - J B2 RAN- NOCH [ i F 1 I~ 10% Apatite No apatite in sample B~ A BROOM FIG. 6. Distribution of apatite (expressed as proportion of total heavy mineral suite) in two wells from the Murchison Field. From Morton & Humphreys (1983). NESS ETIVE RANNOCH [] [] MOST PERMEABLE SANDS LESS PERMEABLE SEDIMENTS ACID GROUNDWATERS CONVECTING FROM HOT, DEEP SOURCE F~G. 7. Postulated groundwater movements in the Brent Group. Hot circulating fluids would flow preferentially along the tops of more permeable sands (see text for explanation). reported by Morton & Humphreys (1983). The non-micaceous sands generally form the best Brent Group reservoirs, and in the wells studied by Morton & Humphreys they are present only in the Etive Formation and Middle Ness sands. Dissolution at depth by circulating acid groundwaters can explain the results of Morton & Humphreys, since fluid movements in the more permeable sands would be faster than in less permeable sands and shales (Fig. 7). This would result in a greater supply of hydrogen ions, and more efficient removal of the products of dissolution. Acid groundwaters moving from masses of deeply-buried maturing organic matter will rise into cooler rock, and will

10 386 G. A. Blackbourn tend to flow along the tops of permeable layers. This may explain why dissolution of apatite in the permeable sands increases upwards. Quartz overgrowths have surrounded many feldspars prior to dissolution, and the shape of the original grain has been preserved. This is further evidence that mineral dissolution is a late-stage event. Although Morton & Humphreys' (1983) results are considered here not to demonstrate very early leaching, they do not support leaching associated with Cimmerian earth movements (as proposed by Sommer (1978) and others cited above). Near-surface leaching results mainly from the downward movement of meteoric water, and this would not explain the observed dissolution profile. Curtis (1983) argued that descending meteoric waters are likely to lose acidity rapidly, largely by the conversion of carbonic acid into neutral metal bicarbonate ions. Enormous quantities of water would be required for significant porosity enhancement by this method. While accepting that better documentation is required on porewater movements in subsiding basins, Curtis considered that very much smaller volumes of acid water moving from a deeply buried organic-rich source would achieve the same degree of mineral dissolution. If the secondary porosity had been formed during shallow burial, there would have been plenty of opportunity for its mineralization later in its burial history. Assuming that diagenetic activity effectively ceased on invasion by oil during the early Tertiary (Goff, 1983), there would have been about 80 million years and 6000 ft of burial following the Cimmerian episode for further mineralization to have taken place, while only about 20 million years were available before it. However, other than through kaolinization, which is mostly the product of feldspar dissolution, and the minor late-stage events described in the next section, the pores have remained open. A simple explanation is that secondary porosity was produced during deep burial, by acid waters moving from an area of organic maturation. Progressively deeper burial resulted in the generation of oil, which migrated into the Brent sands and prevented further chemical diagenesis from taking place. Late-stage diagenesis Diagenesis continued to a minor extent after the dissolution stage. Authigenic illite clays are discernible under the scanning electron microscope; they appear mainly to be alteration products of kaolinite plates, although they also occur rarely in a neomorphic fibrous form on pore walls. Their abundance appears to increase slightly with depth, although this is difficult to quantify. This may be related to increasing illite stability relative to kaolinite, and possibly other minerals, with depth, or to a slight change in the original sedimentary composition which has affected subsequent clay authigenesis. Alternatively, since the Ness and Etive Formations in all the wells studied are within the oil zone, illitization may have been terminated by the progressive extension of oil saturation down through the reservoir, as proposed by Hancock & Taylor (1978) for the Brent Group elsewhere in the Viking Graben. Thus, lower parts of the reservoir were out of the oil zone for longer than higher parts, and there was more time for illitization to take place. However, illite is nowhere abundant. Another widespread but small-scale late-stage event is the overgrowth of quartz grains into the pores produced by dissolution of less stable grains. These overgrowths are distinctive in being euhedral and fresh, having formed after the phase of pitting and corrosion which affected to some extent all the other grains and diagenetic cements. Small

11 Diagenetic effects on Brent reservoirs 387 areas of euhedral prismatic apatite occur in places and are also considered to have formed late in diagenesis, as they are too delicate to be detrital, and are unaffected by the phase of dissolution. EFFECTS OF DIAGENESIS ON RESERVOIR QUALITY One of the main aims of this study was to determine how diagenesis has affected reservoir quality, and in particular to explain why the permeabilities of the Ness sands are so much better than those of the Etive sands. In terms of reservoir quality, diagenesis can be either constructive or destructive. In the model for the creation of secondary porosity at depth mentioned above and described by Schmidt & McDonald (1979), diagenesis tends progressively to destroy porosity and permeability down to a certain depth (generally ~ ft, but varying widely according to local factors). At this point, the geochemical environment becomes corrosive, and certain minerals are dissolved with a consequent porosity increase. The precise method by which this secondary porosity is produced is still unclear, but it seems (Curtis, 1983) to be related to the generation of acidic groundwaters during organic maturation. Naturally, different minerals are variably affected by the fluids which produce secondary porosity, and the relative stability of minerals appears to be similar to that observable during normal weathering at the earth's surface. Thus, for example, quartz grains are far less susceptible to secondary dissolution than feldspar grains. Fig. 2 shows that, although porosities of sands in the Ness and Etive Formations in the area are broadly similar (averaging ~15%), permeabilities in the Etive Formation are mostly poor, while in the Ness Formation they are generally higher, varying widely up to several hundred millidarcies. This is at variance with the common notion that relatively clean, well-sorted sands, like the majority of the Etive ones, make better reservoir rocks than silty, less-well-sorted sands like those in the Ness Formation. Fig. 8 shows schematically the apparent reason for the observed permeability distribution. The left-hand column shows a clean sand, while on the right is a sand with a pervasive silt and clay matrix. The grains are mainly quartzitic, with occasional feldspars. On deposition, the open pores of the clean sand (representing the majority of the Etive) create a good porosity and permeability, contrasting with the poor quality of the dirty (Ness) sand. During burial, quartz overgrowths reduce poroperm values of both lithologies, although quartz precipitation is inhibited to some extent in the silty sand by the pervasive matrix. On secondary dissolution, the stable quartz grains and overgrowths are only slightly affected, whereas feldspars and other relatively unstable minerals are dissolved. This allows individual unstable grains in the clean sand to be removed, increasing porosity, while the pore-throats, largely blocked by quartz overgrowths, are little affected. Permeability therefore is little altered. In the silty sands, however, not only are sand-grade feldspar grains removed, but components of the matrix, such as iron oxides, carbonates and silt-grade feldspar may be dissolved. The remaining insoluble matrix may be redistributed throughout the newly-created pore space, perhaps aided by a deflocculating effect of the acid waters on any clays present, and permeabilities may be substantially increased. The petrography of the Brent sands described here suggests that this is the process mainly responsible for the high permeabilities of the silty sands in the Ness Formation relative to the Etive, despite similar porosities. Although carbonates are not abundant in the Brent Group of this area, they are present

12 388 G. A. Blaekbourn CLEAN SAND SILTY/CLAYEY SAND (DOMINANTLY ETIVE EMN) (DOMINANTLY NESS FMN) 1. DEPOSITION GOOD POROSITY GOOD PERMEABILITY 2, BURIAL DIAGENESIS GRAIN OVERGROWTHS POOR POROSITY POOR PERMEABILITY POOR-MODERATE POROSITY POOR PERMEABILITY 3. MINERAL DISSOLUTION SECONDARY POROSITY ~~'~ VERY POOR POROSITY VERY POOR PERMEABILITY MODERATE GOOD POROSITy POOR PERMEABILITY MODERATE-GOOD POROSITY MODERATE PERMEABILITY FIG. 8. Sketch illustrating the effects of the major diagenetic events on the reservoir quality of different sandstone facies in the study area. as patchy, sometimes nodular, cements, especially in the Ness Formation of one well. They are probably of early diagenetic origin and may have escaped total dissolution by being present in sufficient quantity to neutralize the acids locally. The carbonate-cemented sands have negligible porosities, but have little effect on the overall reservoir. Kaolinite infilling pores, while obviously reducing porosity, retains a considerable microporosity. This is evident by comparing porosity measurements obtained from a kaolinitic sandstone by point-counting of a thin-section, and by helium porosity measurements on cores. A typical Brent Group kaolinitic sand gives about 6-8% porosity from point-count results, and 15% by core analysis. The main problem with kaolinite is that the individual plates are liable to become detached during oil production, and block the pore-throats, with a major reduction in permeability. Fibrous illite, although capable of destroying permeability of a reservoir, has little effect in the areas considered here, because of its low abundance. ACKNOWLEDGEMENTS I am grateful to Stuart Haszeldine for helpful discussions, and Barbara Migdal for encouragement. I thank Britoil plc and partners in the study area for permission to publish.

13 Diagenetic effects on Brent reservoirs 389 REFERENCES BUDDING M.C. & INGLIN H.F. (1981) A reservoir geological model of the Brent Sands in Southern Cormorant. Pp in: Petroleum Geology of the Continental Shelf of North-West Europe (L. V. Ilting & G. D. Hobson, editors). Heyden & Son Limited, London. CURTIS C.D. (1983) Geochemistry of porosity enhancement and reduction in clastic sediments. Pp in: Petroleum Geochemistry and Exploration of Europe. (J. Brooks, editor). Blackwell, Oxford. DEEGAN C.E & SCULL B.J. (1977) A standard lithostratigraphic nomenclature for the Central and Northern North Sea. Rep. Inst. Geol. Sci. 77/25. GOFF J.C. (1983) Hydrocarbon generation and migration from Jurassic source rocks in the East Shetland Basin and Viking Graben of the northern North Sea. J. Geol. Soc. Lond. 140, HANCOCK N.J. (1978) Possible causes of Rotliegend sandstone diagenesis in northern West Germany. J. Geol. Soc. Lond. 135, HANCOCK N.J. & TAYLOR A.M. (1978) Clay mineral diagenesis and oil migration in the Middle Jurassic Brent Sand Formation. J. Geol. Soc. Lond. 135, MORTON A.C. & HUMPHREYS B. (1983) The petrology of the Middle Jurassic sandstones from the Murchison Field, North Sea. J. Petroleum Geol. 5, PARRY C.C., WHITLEY P.K.J. & SIMPSON R.D.H. (1981)Integration of palynological and sedimentological methods in facies analysis of the Brent Formation. Pp in: Petroleum Geology of the Continental Shelf of North-West Europe (L. V. Illing and G. D. Hobson, editors). Heyden & Son Limited, London, SCHMIDT V. & McDONALD D.A. (1979) The role of secondary porosity in the course of sandstone diagenesis. S EPM Spec. Pub. 26, SOMMER F. (1978) Diagenesis of Jurassic sandstones in the Viking Graben. J. Geol. Soc. Lond. 135,