(Received 14 April 1985; revised 5 August 1985)

Transcription

1 Clay Minerals (1986) 21, DIAGENETIC CARBONATE AND EVAPORITE MINERALS IN ROTLIEGEND AEOLIAN SANDSTONES OF THE SOUTHERN NORTH SEA: THEIR NATURE AND RELATIONSHIP TO SECONDARY POROSITY DEVELOPMENT K. PYE AND D. H. KRINSLEY* Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3E, and *Department of Geology, Arizona State University, Tempe, Arizona 85287, USA (Received 14 April 1985; revised 5 August 1985) A B S T R A C T: Deeply buried (> 3.5 km) Rotliegend aeolian sandstones in the Southern North Sea Basin display a number of interesting diagenetic features including (i) zoned iron-rich carbonate cements, (ii) anhydrite, halite and baryte cements, (iii) at least two generations of authigenic illite, and (iv) significant secondary porosity created by cement and framework-grain dissolution. The creation and destruction of secondary porosity is the result of changes in porewater chemistry during burial and subsequent uplift. Three pore-fluid regimes can be identified: (1) alkaline, oxidizing conditions during shallow to intermediate burial; (2) acid, reducing conditions during intermediate to deep burial; (3) alkaline, reducing conditions during deep burial and uplift. The transition from stage 1 to stage 2 was probably caused by expulsion of waters from the underlying Carboniferous shales. The transition to stage 3 probably began when faulting associated with uplift allowed invasion by alkaline fluids derived from Zechstein sediments. The Upper Rotliegend sandstones in the Southern Permian Basin of the North Sea form an important gas reservoir rock with proven recoverable reserves of m 3 (Glennie, 1984). As a result, their depositional environment and diagenetic history have been intensively investigated. Three major facies have been identified in the Upper Rotliegend: fluvial (wadi), aeolian, and a sabkha-desert lake association (Seemann, 1982). The three facies occur in belts roughly parallel to the Variscan mountain front, but in the western part of the Basin the aeolian facies is more extensively developed at the expense of the wadi and sabkha facies. Dune sands form the main reservoir rock. Studies of the sedimentary structures have shown that both transverse and seif dunes are represented, with associated inter-dune deposits characterized by the presence of adhesion ripples (Glennie, 1972, 1983). The reservoir quality of the aeolian dune sands varies from excellent to poor, reflecting important diagenetic differences (Glennie et al, 1978; Seemann, 1982). In part this reflects varying degrees of compaction related to faulting (Glennie et al., 1978), but Seemann (1979) and Rossel (1982) showed that the nature of the diagenetic clay minerals is also an important factor governing the reservoir quality of the sandstones. In this paper we discuss other diagenetic features which may influence the reservoir properties. These include zoned carbonate cements, evaporite cements, and secondary porosity created by dissolution of both framework grains and intergranular cement. The rocks studied are all aeolian dune sandstones from present depths >3.5 km The Mineralogical Society

2 444 K. Pye and D. H. Krinsley PREVIOUS WORK ON THE DIAGENESIS OF ROTLIEGEND AEOLIAN SANDSTONES Marie (1975) noted that the quality of Rofliegend gas reservoir sandstones in the southern North Sea reflects the nature of facies variations, early cementation, depth of burial and tectonics. The primary porosity of the aeolian dune sands (probably around 40%) was initially higher than that of the wadi and sabkha facies which contained more intergranular matrix. Destruction of porosity and permeability in good primary reservoirs has occurred where the sandstones have been buried to >3.5 km, as in the Sole Pit, Broad Fourteens Basin and Cleveland Hills high. Marie noted evidence of two major episodes of authigenic mineral precipitation during burial diagenesis, the earlier resulting in the formation of illite, chlorite and quartz, and a later episode of dolomite and anhydrite cementation. The diagenetic history of the Rotliegend aeolian dune sandstones in the Leman Bank and Sole Pit areas was discussed by Glennie et al. (1978). These authors recognized the following main diagenetic events: (i) dissolution of Fe-bearing silicates and formation of red iron oxide and mixed-layer clay coatings on detrital grains, together with some calcite and gypsum precipitation, at shallow burial depths; (ii) kaolinization of plagioclase feldspars, dolomitization of calcite and conversion of gypsum to anhydrite at shallow to intermediate burial depths ( m); (iii) leaching of K-feldspars and partial reduction of iron oxides during intermediate to deep burial ( m); (iv) formation of authigenic iuite, chlorite and quartz around the time of maximum burial (4000 m); (v) precipitation of further quartz, dolomite and anhydrite during uplift in the late Cretaceous. A similar sequence of diagenetic events was described by Nagtegaal (1979). Rossel (1982) showed that the nature of the authigenic clay minerals in the aeolian sandstones is a function of four factors: (i) the original composition of the sands; (ii) the maximum burial depth and tectonic setting; (iii) the thickness of gas-generating Carboniferous strata underlying the Rotliegendes; (iv) the facies distribution of the overlying Zechstein. Rocks buried to <3000 m contain kaolinite and varying amounts of detrital feldspar, while those buried to greater depths contain only illite and chlorite. The acid conditions necessary for kaolinization of feldspars probably resulted from the expulsion of CO2-rich waters from the underlying gas-generating Carboniferous Coal Measures. The relative proportions of illite and chlorite in the more deeply buried rocks appear to be related to the Mg2+/K~ ratio of Rotliegend brines, which in turn reflected the composition of the overlying Zechstein facies. Little attention was paid in these studies to the carbonate and evaporite cements, or to secondary porosity development. Preferential development of dolomite and anhydrite close to major fault zones and in fractures which acted as conduits for late-stage alkaline fluids was noted by Marie (1975), Butler (1975) and Glennie et al. (1978), but these authors did not report the occurrence of halite ~s an important intergranuiar cement. The compositional and textural variety of the carbonate cements also appears not to have been fully recognized. METHODS SEM has been used widely during the past fifteen years to document the nature of the pore structures and authigenic cements in reservoir sandstones (Weinbrandt & Fatt, 1969; Pittman & Duschatko, 1970; Pittman, 1972, 1979; Stalder, 1973; Wilson & Pittman, 1977;

3 Diagenetic minerals in Rotliegend sandstones 445 Neasham, 1977; Whitaker, 1978; Pittman & Thomas, 1979; Schrank & Hunt, 1980; Wilson, 1982). Most of these studies have used the secondary electron (SE) mode to study pore casts or fracture surfaces of rocks, but the backscattered electron (BSE) mode is now being used increasingly to produce atomic-number-contrast images of polished uncovered sections (White et al., 1984; Pye & Krinsley, 1984; Callender & Dahl, 1984). In this investigation fracture surfaces and polished sections were examined by SEM using the SE and BSE modes respectively. Samples were analysed in two different microscopes equipped with four-element solid-state annular BSE detectors. Mineral phases were identified in the microscope by qualitative and semi-quantitative energy-dispersive X-ray microanalysis (EDXRA). uantitative compositional data were obtained by electron microprobe analysis (EMPA) as described by Long (1977). Samples were impregnated with epoxy resin (dyed blue to make porosity more visible), thin-sectioned and polished mechanically using 1 #m and 0.3/~m alumina and diamond paste. After preliminary optical microscope examination the sections were mounted on i A B I H D B, Y 4~0 30 2~0 I~0 ~ FIG. 1. Bulk sample XRD traces of (A) a dune top sandstone and (B) a dune base sandstone (Cu-Ktt radiation). = quartz, A = anhydrite, I = illite, S = siderite. D = dolomite/ferroan dolomite, H = halite, B = baryte.

4 446 K. Pye and D. H. Krinsley aluminium stubs and coated with approximately 200 A of carbon for SEM and microprobe analysis. Sub-samples of each specimen were analysed by XRD to determine the bulk mineralogical composition. RESULTS XRD results showed that the rocks described in this paper are composed mainly of quartz with smaller amounts of illite, dolomite, halite, anbydrite and baryte (Fig. 1). Calcite, chlorite, feldspars and kaolinite were not detected in the bulk sample XRD traces, although SEM and EDXRA showed that authigenic K-feldspar and chlorite are present in small amounts. Detrital feldspars, micas, sporadic lithic grains and kaolinite occur in samples which have been buried to <3 km, but these are not considered in detail in this paper. uartz framework grains More than 95% of the framework grains are quartz. Most are #m in size and are subrounded or subangular (Fig. 2). Grain-size lamination is well-developed in the fineand medium-grained sandstones which are interpreted as dune-top deposits (cf. Nagtegaal, 1979). Horizontally-laminated sandstones, which are interpreted as dune-base or sand sheet deposits, are coarser, slightly less well-sorted, and display less-pronounced grain-size lamination. Most of the quartz is monocrystalline, with up to 25% polycrystalline grains. There is abundant evidence of pressure solution at grain contacts (Fig. 3), particularly in samples with lower porosity. The porosity varies between samples and even between adjacent laminae; average values of 10.3 and 14.6% have been reported for similar dune base and dune top sandstones in the southern North Sea (Nagtegaal, 1979). A high proportion of the quartz framework grains show evidence of partial dissolution, both marginally and internally. uartz dissolution is considered more fully below. Authigenic clays Illite is the only important clay mineral in the rocks studied, and occurs in several different forms. It is present as a 5-10 ~tm thick coating on the surfaces of the framework grains (Fig. 3). The coatings are absent at grain contacts, indicating that they formed postdepositionally. The coatings are typically two-layered, with a more compact inner layer oriented parallel or tangential to the framework gram surfaces, and an outer, more open layer composed of laths growing radially into the pores (Fig. 4). Illite of the latter type fills also some of the intergranular pores and occurs in voids within quartz framework grains (Fig. 5). Small amounts of fibrous illite are also found on the surfaces of evaporite cement minerals which, as discussed below, are considered to be late diagenetic. Granular aggregates of illite mixed with small crystals of authigenic quartz are also present (Fig. 6). These aggregates are interpreted as altered feldspars which have been replaced by illite, possibly via an intermediate kaolinitic stage. The aggregates have been deformed to varying degrees by compaction, indicating that alteration of feldspars to clay probably occurred before the time of maximum burial. Some of the illitized feldspar grains

5 Diagenetic minerals in Rotliegend sandstones 3 FIG. 2. BSE micrograph showing the subrounded and subangular character of the detrital sand grains. The grain-size lamination runs across the micrograph from top left to bottom right. The large poikilitic crystal is siderite. FIG. 3. Over-size pore filled by late diagenetic ferroan dolomite (1), siderite (2), and anhydrite (3). uartz grains show evidence of pressure solution at points of contact and are coated with authigenic illite (BSE image). FIG. 4. Intergranular pore lined with authigenic illite (SE image). FIG. 5. Radiating laths of illite growing on authigenic quartz crystals within an intergranular pore (SE image). FIG. 6. Granular aggregate of illite, interpreted as a former kaolinized feldspar grain. Small rhombs of authigenic carbonate and authigenic quartz crystals also occur within the aggregate. Note evidence of quartz framework grain dissolution (BSE image). 447

6 448 K. Pye and D. H. Krinsley are surrounded by a tangential rim of illite, indicating that the first phase of iuite formation pre-dated, or was contemporaneous with, feldspar alteration. The second phase of iuite precipitation probably occurred during deep burial and/or uplift. Microprobe data show that the illite infilling pores and replacing detrital grains is predominantly a K-rich variety, low in Mg and Fe (Table 1). Comparison with structural data in Merino & Ransom (1982) and Ireland et al. (1983) shows that the illite in these sandstones is more similar to muscovite than typical low-temperature authigenic illite. Iron-rich chlorite occurs only rarely in these deeply buried Rotliegend sandstones. Where present, it normally occurs in association with altered detrital micas and lithic grains. It is virtually absent as a pore-lining cement. A typical analysis is given in Table 1. Carbonate minerals Carbonate minerals occur in the Rotliegend dune sandstones as blocky void-filling cements (Fig. 3), as large poikilitic crystals, and as individual rhombs or more rounded grain replacements (Fig. 7). The poikilitic crystals, which are up to 3 mm in size, are commonly composed of siderite or ferroan dolomite. uartz framework grains within such TABLE 1. Chemical composition of pore-filling illite and chlorite by electron microprobe analysis (wt% oxide). Illite Chlorite SiO A FeO MgO CaO Na K TiO MnO Total Numbers of ions calculated on the basis of 22 oxygens (illite) and 28 oxygens (chlorite) Si AI* AI'~ Fe:[: Mg Na Kw Ti Mn * Tetrahedral AI. t Octahedral A1. $ Total Fe calculated as Fe 2+. w The high K content of the chlorite suggests some interlayered illite is present.

7 Diagenetic minerals in Rotliegend sandstones FIG. 7. Marginally replaced quartz framework grains contained within a zoned poikilitic siderite crystal (BSE image). FIG. 8. BSE micrograph showing two different styles of carbonate zonation. Grains labelled 1 and 2 consist of siderite (light) and magnesian siderite (dark). Zoned crystal core labelled 3 consists of non-ferroan and ferroan dolomite; outer zone labelled 4 is ankerite (BSE image). FIG. 9. Concentrically-zoned rhomb showing alternations of non-ferroan dolomite (dark) and ferroan dolomite (light). Anhydrite (1), ankerite (2) and authigenic quartz (3) also occur in this pore (BSE image). FIG. 10. Non-ferroan dolomite (1) overgrown by ferroan dolomite (2), ankerite (3) and siderite (4), Also note partially dissolved quartz framework grain (BSE image). F~G. 11. Coalesced dolomite--ankerite--siderite rhombs have almost completely replaced this illitized detrital grain (BSE image). Fio. 12. BSE micrograph showing patch zonation and internal (dissolution?) voids in nonferroan dolomite cement. The later overgrowths and void fillings include ferroan dolomite (1), ankerite (2) and siderite (3). 449

8 450 K. Pye and D. H. Krinsley crystals show evidence of marginal replacement. Much of the carbonate cement is zoned. Several different styles of zonation are present, often within the same pore (Figs 8, 9, 10, 11). Equant concentrically zoned rhombs present the simplest case (Figs 8, 9). Each zone represents a significant variation in carbonate composition. The number of zones ranges from 2->10 (average 5). Microprobe analysis and EDXRA carried out in the SEM revealed that a majority of zoned rhombs have a core of non-ferroan or only slightly ferroan dolomite, surrounded by progressively more iron-rich carbonate towards the margins. In most cases the outer zones consist of ankerite or siderite. Following Deer et al. (1966), ankerite is defined here as having Mg:Fe <4:1. Other concentrically zoned rhombs show more complex alternations of dolomite and ankerite, or ankerite and siderite. Representative compositional data are presented in Table 2. A ternary plot of Mg 9 Ca : Fe ratios shows that two distinct compositional solid-solution series are present (Fig. 13). The first is a dolomite-ankerite series, the second a siderite-magnesian siderite series. The ankerites have a maximum of 30% Mg positions occupied by Fe, while the siderites have up to 38% Fe positions occupied by Mg. There is little substitution for Ca in the dolomite-ankerite series. Similarly, the Ca content of the siderites is uniformly low. Up to 1% Mn is found in both ankerites and siderites, and some analyses also indicated substantial Cu and Zn (Table 2). The concentrically zoned rhombs are interpreted as having formed as a result of direct precipitation under conditions of changing pore-fluid composition. The zonations indicate that dissolved iron concentrations generally increased, although in a somewhat irregular manner, with time. No convincing evidence has been found to suggest that the zoned dolomite-ankerite-siderite cements formed by in situ replacement of calcite or other carbonate. TABLE 2. Chemical composition of carbonate cements determined by EMP analysis. Dune top sandstones SiO FeO MgO CaO MnO Total Dune base sandstones SiO z FeO MgO CaO MnO CuO ZnO Total Total iron reported as FeO. SiO~ may be present as amorphous silica or microquartz impurities.

9 Diagenetic minerals in Rotliegend sandstones Co 451 ferroan dolomite 9 siderite Mg FIG. 13. Mg:Ca:Fe ratios of the late diagenetic carbonate cements. Fe Coalesced aggregates of zoned carbonates almost completely fill some pores and replace altered detrital feldspar grains (Fig. 11). The latter observation indicates that these carbonates post-date feldspar alteration. A second type of zonation, consisting of patches and mottles rather than concentric zones (referred to here :as patch zonation), is also found (Figs 8, 12). The patchy appearence commonly results from differences in the Mg:Ca:Fe ratios of a single carbonate species (e.g. siderite and magnesian siderite), but in some cases patches of cement contain internal voids partly filled with a different carbonate mineral (Fig. 12). Authigenic quartz is also sometimes found in these voids. Episodic carbonate precipitation, with intervening periods of dissolution, may be one way in which patch zonation is formed; gradual replacement of feldspar or quartz framework grains during a period of changing pore-fluid composition is another. The zonation within the poikilitic siderite crystal (Fig. 7) probably results from progressive marginal replacement of included quartz grains as the pore-fluids became enriched in Fe or depleted in Mg. Evaporite minerals Anhydrite and baryte occur as localized pore-filling cements, and commonly as large poikilitic crystals (Fig. 14). Both minerals sporadically completely infill over-size pores

10 452 K. Pye and D. H. Krinsley TABLE 3. Analyses of sulphate cements by EMP analysis. Anhydrite Baryte SiO AI " FeO -- O CaO Na K SO BaO SrO Total and partially replace illitized feldspar and lithic grains (Fig. 15). uartz grains contained within poikilitic sulphate crystals usually display deeply embayed margins due to peripheral replacement (Fig. 16). Such grain replacement is one way in which over-sized pores may be produced if cement is later dissolved. Representative microprobe analyses of the anhydrite and baryte cements are given in Table 3. Sr substitutions in the anhydrite are usually low (< 1%), but the baryte contains up to 4% SrO. Halite occurs as a dispersed pore-filling cement and takes the form both of small (10-30 gin) hexahedral crystals and massive, waxy coatings (Fig. 17). Traces of carnallite are associated with halite. Textural relationships indicate that the evaporite cements are late diagenetic. The surfaces of the crystals are generally fresh and are coated only by minor fibrous illite and prismatic authigenic quartz. The evaporites were probably precipitated at approximately the same time as the zoned carbonates, although in certain pores textural relationships suggest that the evaporites formed slightly later. As noted by Marie (1975) and Glennie et al. (1978), the preferential development of carbonate and evaporite minerals in proximity to faults and fractures suggests that they were precipitated after the onset of uplift during the Cretaceous. The main source of ions was probably the overlying Zechstein evaporites, although some may have been derived from lacustrine and sabkha facies within the Rotliegendes. Development of secondary porosity Secondary porosity is an important, if not the dominant, type of porosity in many sandstones (Heald & Larese, 1973; Hayes, 1979; Schmidt & McDonald, 1979a, b; Pittman, 1979; Surdam et al., 1984; Moncure et al., 1984; Lundegard et al., 1984; Siebert et al., 1984). Schmidt & McDonald (1979a) identified five main types of secondary porosity formed by (1) fracturing; (2) shrinkage; (3) dissolution of sedimentary grains and matrix; (4) dissolution of authigenic pore-filling cement; (5) dissolution of authigenic replacive minerals. Each genetic type is characterized by distinctive textural features. The existence of substantial mouldic, fracture or secondary intergranular porosity can substantially improve the yield of reservoir sandstones (Pittman, 1979). It is therefore important to identify the generic types present.

11 Diagenetic minerals in Rotliegend sandstones FIG. 14. Secondary mouldic pore (3) formed by quartz dissolution post-dates surrounding carbonate and evaporite cement. (1) = siderite, (2) = ankerite, (4) = dolomite, (5) = anhydrite. Late-stage authigenic iuite partly infilling the pore (BSE micrograph). FIG. 15. Baryte (white) infiuing a secondary over-sized pore (1) and partly replacing an illitized feldspar(?) grain (2) (BSE image). FIG. 16. Embayed margins on quartz framework grains created by baryte (white) replacement (BSE image). Fro. 17. Halite pore-filling cement (SE image). Fro. 18. BSE micrograph showing dissolution of a quartz framework grain and partial infilling of the resultant secondary porosity by authigenic illite (1) and anhydrite (2). Note also the twolayered nature of the residual illite coating (3). FIG. 19. Late-stage authigenic illite and quartz within a secondary mouldic pore formed by framework-grain dissolution (SE image). 453

12 454 K. lye and D. H. Krinsley The Rotliegend aeolian dune sandstones described here display secondary porosity belonging to genetic types 1,3,4 and 5, but types 3 and 4 appear to be the most significant seen in thin section. There is textural evidence of two periods of framework-grain alteration and/or dissolution. At least one phase of intergranular cement dissolution has also occurred. The distinction between framework-grain alteration (FGA) and framework-grain dissolution (FGD) is an important one (Siebert et al, 1984). In the former case, the products of grain dissolution are reprecipitated as authigenic minerals close to the site of dissolution; as a result, only microporosity is created. In the case of FGD, the products of dissolution are not precipitated within the immediate vicinity and substantial mouldic macro-porosity is created; this may, however, be filled by authigenic minerals at a later date. In the Rotliegend, feldspars and lithic framework grains appear to have experienced in situ alteration to kaolinite at shallow to intermediate burial depths (Glennie et al., 1978; Rossel, 1982). Much of the kaolinite in the deeper samples (>3 km) has subsequently been altered to illite and quartz, but with little, if any, increase in the microporosity (e.g. Fig. 6). Kaolinization of the feldspars probably occurred when the sandstones were invaded by acid CO2-rich waters expelled by compaction of shales in the underlying Carboniferous Coal Measures (Rossel, 1982). A similar process has been invoked to explain feldspar dissolution and kaolinization in many other sandstones (Curtis, 1983, 1984; Irwin & Hurst, 1984). Invasion of the Rotliegend by acid shale waters probably also resulted in the almost complete dissolution of early diagenetic carbonate and sulphate cements. Very little evidence of these early cements remains, but the high intergranular porosities, over-sized intergranular pores and limited pressure solution seen in some samples provide evidence of their former existence. Small patches of etched non-ferroan dolomite, such as that shown in Fig. 12, might be relics of this early cement. The second phase of FGD involved dissolution of detrital quartz grains. Authigenie quartz overgrowths appear not to have been affected (Fig. 10). All stages in the dissolution of quartz grains can be seen in BSE micrographs (Fig. 18). There is some evidence that polycrystalline and strained monocrystalline grains have been more affected than nonstrained monocrystalline grains. The mouldic porosity created has subsequently been filled by varying amounts of authigenic illite, but only rarely do the moulds contain carbonate or evaporite cements (Fig. 14). This indicates that the main phase of quartz FGD occurred slightly later than evaporite cementation. During the last stage of diagenesis, small authigenic quartz crystals, filamentous illite and minor authigenic K-feldspar formed within intergranular and secondary mouldic pores (Figs 14, 19). CONCLUSIONS The diagenesis of these rocks appears to have taken place under three different porewater regimes at different times during the burial history. The main diagenetic effects may be summarized as follows: Stage 1. Alkaline, oxidizing porewaters at shallow to intermediate burial depths: (a) intrastratal alteration of iron-bearing silicates; (b) formation of iron oxide-clay coats on detrital grains; (c) limited development of authigenic quartz and K-feldspar overgrowths; (d) formation of non-ferroan carbonate and sulphate cements; some replacement of silicate grains.

13 Diagenetic minerals in Rotliegend sandstones 45 5 Stage 2. Acid, reducing, CO2-rich porewaters during intermediate to deep burial (probably expelled from underlying Carboniferous Coal Measures): (a) dissolution of early carbonate and evaporite cements, creating secondary intergranular porosity; (b) leaching and kaolinization of feldspars, micas and lithic grains; some secondary mouldic porosity created, but most destroyed by precipitation of authigenic clays and quartz; (c) reduction of iron oxide in grain coatings; limited formation of pyrite; (d) main phase of primary porosity reduction by compaction and pressure solution. Stage 3. Alkaline, reducing porewaters during deep burial and uplift (probably introduced via fractures and faults from the overlying Zechstein): (a) illitization of authigenic kaolinite; (b) limited formation of authigenic chlorite; (c) precipitation of ferroan carbonate cements; some replacement of framework grains and formation of overgrowths on relicts of earlier carbonate cement; (d) precipitation of anhydrite, halite and baryte cements; (e) dissolution of quartz framework grains, creating significant secondary mouldic porosity; (f) formation of further pore-lining and pore-filling illite, authigenic quartz and limited K-feldspar. ACKNOWLEDGMENTS We thank BP Development for provision of core samples. KP acknowledges financial support from the Royal Society and NATO grant no. 039/84. Cambridge Earth Sciences Contribution 575. REFERENCES BUTLER J.B. (1975) The West Sole gas field. Pp in: Petroleum and the Continental Shelf of North West Europe. 1 Geology (A. W. Woodland, editor). Applied Science Publishers, Barking, Essex, UK. CALLENDER C.A. DAHL H.M. (1984) Characterization of petroleum reservoir rocks by scanning electron microscopy. Scanning Electron Microscopy 1984/IV, CURTIS C.D. (1983) The link between aluminium mobility and destruction of secondary porosity. Am. Assoc. Petrol. Geol. Bull. 67, CURtiS C.D. (1984) Geochemistry of porosity enhancement and reduction in clastic sediments. Pp in: Petroleum Geochemistry and Exploration of Europe (J. Brooks, editor). Geol. Soc. London. Spec. Pub. 12, Blackwell, Oxford. DEER W.A., HOWIE R.A. & ZUSSMAr~ J. (1966) An Introduction to the Rock-Forming Minerals. Longman, London, 528 pp. GLENNIE K.W. (1972) Permian Rotliegendes of Northwest Europe interpreted in light of modern desert sedimentation studies. Am. Assoc. Petrol. Geol. Bull. 56, GLENNIE K.W. (1983) Lower Permian Rotliegend desert sedimentation in the North Sea. Pp in: Aeolian Sediments and Processes (M. E. Brookfield and T. S. Ahlbrandt, editors). Elsevier, Amsterdam. GLENNIE K.W. (1984) Early Permian--Rotliegend. Pp in: Introduction to the Petroleum Geology of the North Sea (K. W. Glennie, editor). Blackwell, Oxford. GLEr~m K.W., MUDD G.C. & NAGTEGAAL P.J.C. (1978) Depositional environment and diagenesis of Permian Rotliegendes sandstones in Leman Bank and Sole Pit areas of the UK Southern North Sea. J. Geol. Soc. Lond. 135, HAYES J.B. (1979) Sandstone diagenesis--the hole truth. Pp in: Aspects ofdiagenesis (P. A. Scholle and P. R. Scbluger, editors). SEPM Spec. Pub. 26, Tulsa, Oklahoma.

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