Clay Minerals (1986), 21, 429-441

F E A T U R E S OF M I N E R A L D I A G E N E S I S H Y D R O C A R B O N R E S E R V O I R S :

A N I N T R O D U C T I O N

I N

C. V. J E A N S

Department of Applied Biology, University of Cambridge, Pembroke Street, Cambridge CB2 3DX

Mineral diagenesis* affects the quality of sedimentary hydrocarbon reservoirs from the time of their deposition until their hydrocarbons are extracted by man. The papers on diagenesis presented in this volume fall naturally into three groups. The main group of 14 contributions deals with the description and interpretation of diagenetic effects in North Sea reservoirs and associated sediments. The second group (four papers) considers how specific diagenetic problems which impair the performance of a reservoir can be circumvented or removed before or during production operations. The third and last group of contributions (two papers) discusses the principles by which reservoir description and modelling can be carried out to optimize reservoir behaviour. The publication of so much new and detailed data on the diagenesis of siliciclastic reservoirs from the North Sea makes it an appropriate moment to consider the relationship between mineral diagenesis and reservoir quality in the light of both old and newly-developing concepts. The views expressed below are my own and they must in no way detract from the interpretations and opinions expressed by individual authors.

R E G I O N A L D I A G E N E S I S IN T H E N O R T H SEA

Lithofacies-diagenetic relationships dominate the papers dealing with regional diagenesis. The effects of enhanced temperature or depth of burial on mineral diagenesis are played down or are only utilized to explain minor changes in clay mineralogy or to provide a mechanism for the migration of hydrocarbons and pore-fluids. It is becoming clear that any direct correlation between the burial depths (or maximum burial depths) and reservoir quality are at the best very restricted and are certainly not the most logical method of predicting reservoir quality: if such correlation occurs it is the indirect 'result of the interaction of lithofacies-controlled diagenesis, grain pressure and pore-fluid movement.

Lithofacies-related intrinsic diagenesis

This volume provides much additional information on lithofacies-related diagenesis in many of the main reservoir formations in the North Sea. For the Rotliegendes (Lower Permian) Sandstone of the southern North Sea Basin, Pye & Krinsley provide further details on the carbonate and evaporite cements and the development of secondary porosity, and Goodchild & Whitaker describe the relationships between the depositional facies, burial and mineral diagenesis in the Rough Gas Field. The Triassic continental Skagerrak Formation in

* The terms diagenesis, miagenesis and metamorphism are used in the sense of Jeans (1984) throughout this introduction.

�9 1986 The Mineralogical Society

430 C. V. Jeans

the Main Claymore Field is dealt with by Spark & Trewin who describe two pat terns of li thofacies-related diagenesis: an earl ier one related to the fluvial sandstones of Triassic age and a later marine one, which is restricted to a zone beneath the unconformity at the base of the late Oxfordian mar ine sediments. Jurassic sediments of marginal and brackish to freshwater facies are discussed from the Hild Field in the northern Nor th Sea (Lmaoy, Akselsen & R~nning) and from the Piper Forma t ion of the Main Claymore Field (Spark & Trewin). The diagenesis of fully mar ine Uppe r Jurassic sediments is described from the Kimmer idge Clay of both the Main Claymore Field (Spark & Trewin) and Well 14/26-1 (Jeans & Fisher), and from the Fulmar Format ion in the central Nor th Sea (Stewart).

The popular hypothesis that the kaolinite cements in the Middle Jurassic sandstones of the Brent Province result from the effects of meteoric water penetrat ion during a Cimmerian phase of subaerial exposure has rightfully been severely criticized. The most damaging evidence is that the development of kaolinite shows no special relat ionship to this unconformity (Riches, Traub-Sobott , Zimmerle & Zinkernagel). When its diagenetic development is li thofacies-related, kaolinite is of early diagenetic origin and is associated with brackish and brackish/freshwater sediments. Later kaolinite cements may occur in a much greater variety of facies but they are probably all of non-intrinsic origin. The relationship between general deposi t ional facies and the development of intrinsic clay cements and silicates is illustrated in Fig. 1.

The frequency and extent to which there is a correlation between lithofacies and pat terns of diagenesis in the Nor th Sea means that generally sediments must have stewed in their own pore-fluids and a massive flow of migrat ing fluids through the pore spaces was not as extensive as the l i terature suggests; when such through flow did occur it must have been concentrated into l imited conduits. This correlation suggests also that the extent of

FIG. 1. Schematic model of depositional facies-clay assemblages for deltaic (or estuarine) and open marine environments based partly on the non-volcanogenic Jurassic and Cretaceous sediments of the U.K. Patterns of neoformed clay assemblages of intrinsic miagenetic origin are shown in relation to (i) the general salinity of the depositional site, (ii) the differential settling of a variety of continentally-derived detrital clay minerals and amorphous material (rich in AI, Si and Fe) under the influence of flocculation, floccule type, precipitation processes and the depositional energy of the environments, and (iii) the relative abundance of highly reactive biogenic and volcanic silicate material in the marine environment. Both the marine and deltaic patterns have a two-fold division of their neoformed clay mineral assemblages: an earlier phase of Fe-rich clays (Fe 2+ and Fe 3+ within their structure) which form at or below the aerobic/ anaerobic interface at the depositional site, and a later phase in which Fe-poor clay minerals are developed.

The extent of the open marine and deltaic areas of influence are independent of each other. However, as the deltaic (or estuarine) pattern results from the influence of the marine environment on the input of a freshwater river carrying a load of fine-grained detritus, the spatial arrangement of the detrital and neoformed clay mineral assemblages will remain constant. These spatial relationships are illustrated at the bottom of this Figure. (A) shows the relationship between the main depositional environments (deltaic and open marine) and the early neoformed Fe-rich clay assemblages. (B) deals with the relationship between the depositional environments and the later neoformed Fe-poor clay assemblages. (C) shows the observed distribution pattern to be expected from these later neoformed assemblages when the dilution effects of the deposition of the fine-grained mineral detritus of the deltaic system are

taken into account.

Problem of mineral diagenesis in hydrocarbon reservoirs: an introduction 431

JURASSIC AND CRETACEOUS DEPOSITION FACIES MODEL

zone of deltaic influence

I ' freshwaler brackish water I

MARINE PATTERN J unstable sil icate material for I --=== cloy mineral neoformation I

I i DELTAIC PATTERN: EFFECTS OF DIFFERENTIAL SETTLING

�9 I i detri tot koohnite, mica, chlor i te

CLAY MINERAL-

I zone of marine influence

marine water

! I

NEOFORMED CLAY MINERAL ASSEMBLAGES OF MIAGENETIC ORIGIN I

Early miogeneti,c Berthierine J I Glouconite (sensu Iota) i

Late miogeneti~ i Kaolinite

Smectite- Mica

Mica I Smectite-Mico-sllica m|nerals

A margin of• �9 landmass r-i = _L /'~ ioege~ ~ f ~ ~ I cJ ~ r _ ~ - . - - r~ -~- . I ~ , ~ o ~%~,~

_ _ : - _ - _ - - ~ ; w = , q ~ - _ - - - - _ I ~ \ ~ , \ ~ . \ ~ I ~ o ~ . ~ ' ~ ~\\\\\~ ---------- -------- I" I' f~ o.o7~ I" I'

. . . . ---4 ate j - - - - - - ' " .OeOO o . i ' I - ------- I " -----: �9 I �9 I" Voo~ I I

~ ~ detrital cloys from the BERTHIERINE MICA margin of the landmass

GLAUCONITE(sensu I o t a ) KAOLINITE malnbulk of detrital clays of deltaic origin

S M ECTITE- MI CA

SMECTITE-MICA--slIic a minerals

_.>

detri tol vermicu l i te , mixed-h~yer c lays, smect i te I I

', ~ ~ ~ amorphous mo'teriol r ich in Si', AI and Fe

i

432 C. V. Jeans

compaction must have been insufficient to alter markedly the pathway taken by intrinsic diagenesis: Huang, Bishop & Brown describe experiments on the dissolution of feldspar and the formation of illite at various fluid : rock ratios and pH; it is quite clear that fluid : rock ratios are an important factor in determining the reaction products and their style of precipitation. Earlier, Correns (1961) had shown experimentally that the development of pseudomorphs is controlled by the concentration of reactants in the fluid phase and it is interesting in the light of this to consider Huggett's suggestion that the mica/kaolinite intergrowths and the pure kaolinite booklets in the East Midlands (UK) Westphalian Coal Measures represent respectively kaolinite replacement of mica as pseudomorphs and the precipitation of kaolinite from solution.

Overpressures, fibrous cements and septarian nodules

Some examples where lithofacies and diagenesis are clearly interdependent reflect the development of considerable overpressures (Fig. 2) in the sediment at an early stage of burial and their maintenance to a stage at which intrinsic diagenesis no longer continued. There appears to be no other mechanism than overpressuring by which similar sediments with considerably different structural settings and burial histories can maintain their own pore- fluids and similar fluid:rock ratios--and hence demonstrate remarkably similar patterns of intrinsic diagenesis. Evidence for this overpressuring is not lacking in the contributions to this volume. The Fulmar Sandstone reservoir of the central North Sea (Stewart) is at present only mildly overpressured but the presence of 30-40% shortening of thin vertical mineral veins in parts of the reservoir sandstone suggests that it must have been much more extensively overpressured for most of its diagenetic history. Jeans & Fisher present evidence from the Upper Jurassic sandstone sequence in Well 14/26-1 that the sediments were extensively overpressured during the greater part of their diagenesis. At one stage localized parts of the sandstones apparently expanded when the fluid pressure exceeded the lithostatic pressure. This expansion created numerous nucleation sites where fibrous dolomite cements rapidly crystallized. The Hild Field reservoir (L~noy et al.) is at present very highly overpressured and this seems to have developed at an early stage of its burial history,

Fibrous carbonate cements very similar to those found by Jeans & Fisher are associated with cracks in septarian nodules and it seems no coincidence that Astin, in a closely argued hypothesis concerning the origin of these enigmatic nodules, considers their development to be related to an overpressured mudstone or shale environment with low horizontal stress; such conditions being typically associated with rapid burial. The fibrous cement again is probably related to the sudden appearance of numerous new nucleation sites (on the walls of the septarian cracks) in a situation where the pore solutions are supersaturated with calcium carbonate and calcite is being actively precipitated. Is it possible that some examples of fibrous illite cements develop in similar conditions? Fibrous neoformed clay minerals are characteristic of some smectite-rich mudstones and their development may be related to a period of overpressuring in which the open mudstone texture is preserved, allowing the free growth of euhedral fibrous clays. The occurrence of such clays has been reviewed recently by Holtzapffel & Chamley (1986).

Although overpressures are of particular importance for the scientist studying the intrinsic diagenesis and miagenesis of sediments, or searching for deeply buried reservoirs which have escaped the ravages of compaction, they are a major problem for the hydrocarbon explorationist. One of the most favoured mechanisms for generating overpressure at depth in

Problem of mineral diagenesis in hydrocarbon reservoirs." an introduction 43 3

SEDIMENT COMPACTION AND INFLATION GRAIN PRESSURE AND OVERPRESSURE

I000

--- 2000

i ,4),="

~P "lO

4000

5000

average lithostatic pressure

\

over- pressure

average hydrostatic pressure " ~

- \ \ \

I

grain pressure

s e d i m e n t inflation

pore fluid pressure

0 500 I000 1500 bars pressure

FIG. 2. The hydrostatic pressure/lithostatic pressure/pore-fluid pressure/grain pressure/ ovcrpressure relationships within an imaginary packet of sediments undergoing continuous burial but experiencing a variety of different geological situations. The build up in over- pressures (= geoprcssures :pore-fluid pressure above hydrostatic) between 1500 and 2500 m is due to the increasing restriction of the fluid escape pathway from the sediment to the surface. The sediment inflation at approximately 3000 m is caused by a high pressure breakout from depth. The sudden decrease in overpressurcs between 3300 and 4300 m is related to the development of tensional tectonics and the opening of free communication between the pore- fluids of the sediment package and the surface: the resulting high grain pressures are responsible for extensive pressure solution of the mineral grains making up the framework of the sediment. The increasing overprcssures below 4300 m reflect the restriction of these fluid pathways as the

sediments enter a comprcssional situation.

434 C. V. Jeans

shales is the diagenetic conversion of smeetite to illite, and Hall, Astill & McConnell explore the role that the dehydration reactions of smectite may play in this process.

Factors controlling mineral diagenesis

The pathway taken by mineral diagenesis within a packet of recently deposited sediments (eventually to become a hydrocarbon reservoir) is not only under the control of their mineral, chemical and organic (living and dead) composition but also depends on the relationship between the regional patterns of lithostatic, pore-fluid and grain pressures developed during their post-depositional history. This relationship plays an important role in determining the chemical environment and pattern of fluid flow in which the evolving reservoir is located. As a consequence it will be responsible for the extent and pattern of compaction (and with this the rock : fluid ratios) and the movement of allochthonous pore-fluids into or through the reservoir. Intrinsic diagenesis will be best developed in sediments which are sealed at an early stage in their post-depositional history and which rapidly develop overpressures so that compaction is retarded and the characteristic rock :pore-fluid ratios are maintained. The presence of high pore-fluid pressures in a tightly sealed reservoir will greatly lessen the chances of allochthonous pore-fluids penetrating from the surrounding regions to modify the reservoir's chemical system.

The chances of a reservoir being perfectly sealed are small and there will be no doubt be a continual leakage of pore-fluids into adjacent lower pressure regions; there will also be a movement of fluids within the reservoir related to minor differences in pore-fluid pressure. If both the reservoir and adjacent sediments were deposited with porewaters of similar chemistry, leakage would cause only minor variations to the diagenetic pattern. If, however, deposition takes place in different chemical environments (e.g. ranging from the fully marine to freshwater) such as occurs extensively in marginal continental and marine deposits, these minor movements of pore-fluids within the reservoir are likely to alter drastically the detailed relationship between lithofacies and mineral diagenesis.

Non-intrinsic diagenesis

The reservoir may become part of an open system in which there is unrestricted hydraulic continuity with the surface and will thus become a conduit through which allochthonous pore-fluids will pass on their way to lower pressure zones. This will terminate intrinsic diagenesis, and subsequent mineral reactions taking place between the rock framework, its earlier intrinsic cements and the new pore-fluids will be of a non-intrinsic type. Whether this will affect greatly the pattern and style of mineral diagenesis will depend on the chemical differences between the original pore-fluids of the reservoir and those which replace them. The supermature marginal marine Lower Jurassic sandstones described by Riches et al. from the Troms 1 area of off-shore northern Norway show a very complex diagenetic pattern which is thought to have evolved mainly in an open system involving hydrothermal solutions and enhanced metamorphic effects related to deep-seated, rift-associated faults. The Hild Field (Lone~j et al.) shows the non-intrinsic diagenetic effects caused by a local gas chimney developing in the reservoir: this has resulted in localized secondary porosity in the vicinity of the chimney caused by the dissolution of carbonate cements. Burley's detailed comparison of the diagenesis of the Piper Formation sandstones in the Piper and Tartan Fields (Outer Moray Firth, North Sea) provides insight into the complexities of non-intrinsic factors

Problem o f mineral diagenesis in hydrocarbon reservoirs: an introduction 435

(tectonic setting and history, varying grain pressures, availability of aggressive solutions for secondary porosity development, entry of hydrocarbons) responsible for variations in reservoir qualities. The reservoir qualities of the marine Brent and Statfjord Formations in two wells (25/4-1 and 25/4-5) situated 3 km apart on the eastern flank of the Central Viking Graben reflect a very similar pattern of intrinsic diagenesis (Thomas). However, in Well 25/4-1 a major phase of fibrous mica cementation was terminated in the late Eocene by the introduction and accumulation of hydrocarbons in the reservoirs, which preserved good reservoir quality with permeabilities up to 500 mD. In Well 25/4-5 this fibrous mica cement continued to precipitate until the Oligocene, reducing the permeability drastically (< 5 mD).

Source and texture o f silica cements

Marine sediments containing a high proportion of unstable silica (such as biogenic silica or certain forms of volcanic ash) develop silica cements of variable mineralogy and texture as part of their early intrinsic miagenesis close to the water/sediment interface. This is in contrast to a 'normal' quartz-rich sediment with little highly unstable silica, where if quartz cements develop they do so at a much later stage of diagenesis and are derived from silica made available by pressure solution, clay mineral reactions, dissolution of feldspar or rising hydrothermal solutions rich in silica. One of the factors responsible for these early silica cements is the considerable undersaturation of seawater with respect to silica. This is caused by the extraction of dissolved silica by various organisms to form skeletons of opal-A (biogenic silica): certain sponges, radiolaria and diatoms are some of the common organisms involved with this extraction process. When this skeletal material of biogenic silica and other types of highly unstable silicate material is incorporated in the sediment it dissolves rapidly in the porewaters and subsequently becomes involved with microbiological- ly-controlled environments that lead to the precipitation of silicate mineral cements--these include silica minerals such as opal-A, opal-CT, low-temperature tridymite, low-temperature cristobalite and a-quartz, feldspars, analcime and heulandite, as well as a variety of clay minerals. Quartz, on the other hand, is a relatively stable mineral and is little affected by the undersaturated pore solutions, although it may be dissolved locally in regions of high pH associated with the precipitation of early carbonate cements. The bottom sediments of the Mediterranean Sea near Sicily illustrate this point: the AI and Si contents of the surface waters are of the order of 2/tm/1 and 200 #g/1 respectively (Mackenzie et al., 1978), whereas at a depth of 50 cm in the bottom sediments the concentration of A1 and Si in the porewaters is up to 100/~g/l and 2120 #g/l, respectively (Caschetto & Wollast, 1979). Examples of this type of early silicate cementation in the Mesozoic of both on-shore and off-shore UK are fairly common: the Corallian beds and the Upper Greensand (Upper Albian) of southern England are good on-shore examples which have been studied in some detail (Brown et al., 1969; Milodowski & Wilmot, 1984; Jeans, 1978; Milodowski et al., 1982); off-shore examples include many of the upper Jurassic fully marine shelf sandstones of the northern North Sea and the Moray Firth (Jeans & Fisher; Stewart). The Fulmar Sandstone of the central North Sea (Stewart) originally contained abundant sponge spicules, sphenasters and possibly volcanoclastic debris. The greatest proportion of all this unstable material must have dissolved in the porewaters of the sediment during the earliest stages of diagenesis without leaving any trace. However, in those parts of the sediment where the abundance of sponge spicules and sphenasters is high, silicate cements will have begun to precipitate before all the biogenic silica had dissolved. These early cements will have coated the sphenasters and

436 C. II. Jeans

spicules either protecting them from further dissolution or forming fossil moulds if sphenaster and spicule solution continued. In other situations where the concentration of silica in the pore solutions is particularly high, opal-CT pseudomorphs after the biogenic skeletal grains may have developed. Detailed petrographic investigation of similar sediments originally rich in unstable silicate material indicates that as the silica concentration in the porewater decreases with time (as the result of the precipitation of silicate cements), opal-CT type of cement gives way to chalcedonic and quartz-fringing cements and then to quartz overgrowths (Fig. 3). Such a cement stratigraphy may occur within a small group of pores; however, similar changes may occur laterally, where they represent lateral variations in varying proportions of biogenic silica within the original sediment (Fig. 4).

Most forms of early intrinsic miagenetic mineral cements derive some of their components from the dissolution of unstable parental material in the sediment. I f no vestige or evidence of the parental material remains the interpretations favoured by many petrographers for the source of the cement are often highly unlikely: there is a tendency to avoid postulating components in the original sediment of which there is no evidence. Such an approach is dangerous because if one wishes to predict the regional patterns in these early cements there will be a great difference between a pattern related to the concentration of sponge spicules in the sediment and one caused by a metamorphic reaction (pressure solution) at depth in which silica is released and has to find its way into the sediment by way of the regional pore-fluid pressure pattern.

Pressure solution of quartz grains is a popular source for the origin of quartz overgrowth cements. In many instances when this source is suggested there is little evidence to support the association. Quartz is a very stable mineral and is not readily soluble under normal earth- surface conditions even where pore-fluids are undersaturated with respect to silica. Pressure solution of quartz grains is a metamorphic process and can be correlated with the increase in grain pressure that may be associated with increasing depth of burial. Enhanced temperature and the chemistry of the pore-fluids are important factors in controlling the rate and extent of solution. Pressure solution of quartz grains may or may not be an effective source of silicon for quartz cements depending on the conditions under which it is occurring. If the pore-fluids in the vicinity of pressure solution are saturated or nearly saturated with respect to silica the addition of silicon will soon cause supersaturation and promote the precipitation of quartz. Alternatively, if the pore solutions are considerably undersaturated, in silicon, extensive pressure dissolution of quartz will be necessary to bring it into a state of super-saturation and to precipitate quartz cements. Such a state may never be reached either because grain pressures may be reduced by overpressure development or because pore-fluids are migrating through the zone of pressure solution too rapidly to become supersaturated. The most

FIG. 3. Textures in quartz-cemented marine sandstones. A. Detrital quartz grains (dQ) and feldspar grains (F) with many opaque inclusions (op) cemented by porous fine-grained quartz cement (fQ) and chalcedonic quartz chert (Ch). The small quartz crystals (average length ~ 20 #m) of the fine-grained cement are arranged commonly with their c-axes at 90 ~ to the grain surface forming a fringing and encrusting coat; they occur less commonly as micro-overgrowths or in random orientation. These quartz cements pre-date the feldspar overgrowth (Fo); elsewhere in the sediment quartz macro-overgrowth cement is the only silica cement and the initiation of this post-dated the first development of feldspar overgrowths. The outer marginal zone of a calcitic bioclast (cb) has been both silicified by fine-grained quartz and partly replaced by pyrite. B. Grains of detrital quartz (dQ) and feldspar (F) cemented by a fine-grained fringing

Problem of mineral diagenesis in hydrocarbon reservoirs: an introduction 437

pore

quartz cement (fQ). C. Detrital quartz grains (dQ) showing an early phase of quartz micro- overgrowth cement which subsequently developed into macro-overgrowths (Qo). D. Detrital quartz grains (dQ) cemented by quartz macro-overgrowths (Qo). These overgrowths have during their development included and grown over patches of fine-grained clay-rich material (cC).

438 C. V. Jeans

STYLES AND MINERALOGY OF SILICA CEMENTS 8

massive opal-CT 8 pal-CT( j o lepispheres 8

biogenic "~ silica --*-solution tmassive microquartz ._~

spongy microquartz quartz

fringing quartz -o quartz overgrowth

FIG. 4. Suggested origin and sequence of early miagenetic silica cements in marine sediments. Silica concentration in the pore solutions is thought to be the main control on mineralogy and textural style, although other factors such as the particular chemical milieu and the availability

of nucleation sites are likely to be of importance.

.E

unambiguous case of pressure solution-derived quartz cement in this volume is that described by Riches et al. from the Lower Jurassic sandstones of the Troms-I area of off-shore Norway. Here quartz-cemented zones are associated with stylolites and these alternate with relatively uncemented zones free of stylolites. Such a relationship suggests that pressure solution took place in a sediment in which there was sufficient dissolution of quartz to increase the concentration of silica in the porewater to initiate quartz crystallization only in the vicinity of the stylolites,

Secondary porosity

The development of secondary porosity is a potential method of improving the quality of a hydrocarbon reservoir. Its widespread presence in many of the North Sea reservoirs makes it difficult to understand that until Schmidt & McDonald published their papers in 1979 its importance was not fully realized. Early petrographers must have been familiar with the evidence but failed to communicate it to the oil world at large. The examples discussed by Schmidt & McDonald (1979a,b), and by many authors subsequently, have concentrated on deeply buried reservoirs and late diagenetic development of secondary porosity, a n d consequently burial depth as a controlling factor has been over-exaggerated. It is clear from various papers in this volume (e.g. Stewart; Lon~y et al. ; Pye & Krinsley; Jeans & Fisher) that the formation of secondary porosity may occur at all stages of diagenesis depending o n local conditions. This process must be considered as unexceptional and closely linked to the interrelationship between the pattern of compaction and grain pressures, the precipitation of

Problem of mineral diagenesis in hydrocarbon reservoirs: an introduction 439

frame-building and strengthening cements, and the chemical interaction between the pore- fluids and the mineral matter of the sediment. If it is assumed that the strength of a reservoir sediment is sufficient to withstand the grain pressure without collapse, a variety of reactions will lead to the development of secondary porosity which will be of more or less effect in improving reservoir quality:

(1) When the sediment contains an unstable component which rapidly dissolves in the porewaters without causing the precipitation of a new mineral phase, useful porosity will be added to the sediment.

(2) If, however, all the mineral phases which dissolve are reprecipitated as a new cement of the same or greater volume either at the same location as pseudomorphs or elsewhere, there will be no improvement in quality: this situation can be modified by the rapid migration of the porewaters out of ~he reservoir before there is time for complete precipitation of the new cement.

(3) Allochthonous pore-fluids may be considerably undersaturated in relation to some minerals of the host sediment through which they pass and may be responsible for the removal of considerable amounts of mineral matter. Such reactions involved in the potential development of secondary porosity are

fundamental to mineral diagenesis but whether useful secondary porosity develops depends very much on incidental factors such as the relationship between the strength of the sediment, grain pressure and the timing of the development of secondary porosity.

At the moment the most favoured mechanisms for developing late diagenetic secondary porosity at depth out of reach of meteoric waters are either by utilizing acidic porewaters generated in Curtis's zone of abiotic decarboxylation reactions (Curtis, 1978, 1983), or by the effects of porewaters enriched in organic acids such as carboxylic and phenolic acids derived from the maturation of kerogens (Surdam & Cressey, 1985), or by hydrothermal solutions of deep-seated origins. Indirect evidence of the roles played by these different mechanisms could be tested both experimentally and in the field by applying Morton's work on the stability and distribution of detrital apatite grains in relation to the chemistry of the depositional environment and associated porewaters. Apatite appears to be stable in sediments with neutral or alkaline pore-fluids independent of burial depth (down to at least 3800 m) whereas it is unstable in acidic porewaters. Unetched apatite occurs commonly:in fully marine sediments and in many continental 'red bed' facies in which extensive late diagenetic secondary porosity has developed and this observation does not support the idea that the aggressive solutions responsible for the secondary porosity are acidic.

F O R M A T I O N D A M A G E A N D T H E I M P R O V E M E N T OF R E S E R V O I R Q U A L I T Y

Formation damage is a very important aspect of reservoir diagenesis. It develops as a side effect of well drilling and production operations. Harper & Buller review the variety of problems which may develop and how they can be recognized and resolved. They deal specifically with fluid/rock and fluid/fluid incompatibility, departure from laminar radial flow in a homogeneous medium, mechanical deformation around the borehole or perforation tunnels, and the reduction of fluid pressure during production. The formation damage associated with the development of the Ras Budran reservoir (Nubian Sandstone, Egypt) is used as a case history to illustrate the approach that petroleum engineers should take to overcome these problems. The movement of fine particles within reservoirs is a widespread

440 C. II. Jeans

problem in production operations and the experimental results described by Gronow on the factors and mechanisms controlling the movement and filtration of two different mineral types of asbestos fibres (each with opposite surface charge) in aqueous systems has application to a proper understanding of the problem.

Kantorowicz, Lievaart, Eylander & Eigner stress the importance of making careful petrological and mineralogical study of a reservoir before planning production and they provide a flavour of the mineralogical problems that can develop by trying to improve reservoir quality by acidization, water injection and steam injection--it is clear that one may be worse off after the 'improvements' than before if sufficient care is not taken. Pittman & King provide a case history of the intimate relationship between the original sediment, the natural mineral diagenesis and formation damage caused by drilling and attempts to improve production in the Upper Cretaceous sandstone reservoirs in the Oguendjo West Block off the coast of Gabon.

R E S E R V O I R D E S C R I P T I O N A N D M O D E L L I N G

Hurst & Archer present an overview of the role played by geology and mineralogy in the three-dimensional modelling of hydrocarbon reservoirs prior to field development. They emphasise the importance of integrating studies on the actual sediments with those obtained by a variety of different logging methods. In a second paper, the same authors deal more specifically with the important and often crucial role played by clay mineralogy in this description and modelling.

F U T U R E D E V E L O P M E N T S

Although research into the problems caused by mineral diagenesis in hydrocarbon reservoirs and their associated sediments is being actively pursued both in industry and academia, we are a long way from having a satisfactory understanding of the relationship between siliciclastic sediments and their mineral diagenesis. For the North Sea--and probably for most other areas of the world--published detailed studies on such reservoirs are too few to form the database from which it is possible to distill the principles governing the prediction of reservoir quality. Only when such prediction becomes possible with an acceptable degree of success will the study of mineral diagenesis 'become of age' as a respectable part of earth sciences, with a major role to play in the search for economic accumulations of hydro- carbons. At the moment, mineral diagenetic studies tend to be either part of post-mortem investigations on reservoirs or to be utilized in planning production.

There are no short cuts to improving the numbers and quality of detailed investigations on North Sea reservoirs. Each has to be based on the most careful petrographic study and this is one of the main bottle-necks: dedicated petrographers are a rare breed and microscopy is both very time-consuming and tiring. Until this basic petrographic information is available it is not possible to sensibly plan detailed geochemical and mineralogical investigations to help substantiate the physical and chemical processes involved. Help to encourage mineral diagenesis to 'become of age' should come from academia by showing undergraduates how fascinating and useful are the post-depositional changes that take place in sediments, and from the oil industry by encouraging not only the thorough and careful examination of the sediments that make up their reservoirs and surrounds, but also the promotion of their publication and their addition to the pool of common knowledge without which progress

Problem o f mineral diagenesis in hydrocarbon reservoirs: an introduction 441

cannot be made. Help is also needed to avoid the dispersal of the published data throughout the myriad of earth science journals now available. The Clay Minerals Group and the

Petroleum Exploration Society of Great Britain must be particularly grateful for the generous financial help from the oil industry which has allowed the concentration of all these papers into a single issue of Clay Minerals. Let us hope their faith in us will be repaid by providing them in the future with a mineral diagenesis which will be even more useful.

REFERENCES

BROWN G., CATT J.A. & WEIR A.H. (1969) Zeolites of the clinoptilolite-heulandite type in sediments of south-east England. Mineral. Mag. 37, 48(~488.

CAscI-IE'r'ro S. & WOLLAST R. (1979) Dissolved aluminium in interstitial waters of recent marine sediments. Geochim. Cosmochim. Acta 43, 425-428.

COI~ENS C.W. (1961) The experimental chemical weathering of silicates. Clay Miner. Bull. 4, 257-259. CURTIS C.D. (1978) Possible links between sandstone diagenesis and depth-related geochemical reactions

occurring in enclosing mudstone. J. Geol. Soc. Lond. 135, 107-117. CORTIS C.D. (1983) Geochemistry of porosity enhancement and reduction in clastic sediments. GeoL Soc. Spec.

Publ. 12, 113-125. HOLTT-~,PFrEL T. & CHAMLEY H. (1986) Les smectites lat6es du domaine Atlantique depuis le Jurassique

superieur: gisement et signification. Clay Miner. 21, 133-148. JEANS C.V. (1978) Silicification and associated clay assemblages in the Cretaceous marine sediments of

southern England. Clay Miner. 13, 101-126. JEANS C.V. (1984) Patterns of mineral diagenesis: an introduction. Clay Miner. 19, 263-270. MACKENZIE F.T., STOFFYN M. & WOLLAST R. (1978) Aluminium in seawater: control by biological activity.

Science 199, 680-682. MILODOWSKI A.E., MARTIN B.A. & WILMOT R.D. (i 982) Petrography of the Cretaceous core from the Harwell

Research Site. Rep. Inst. Geol. Sci. ENPU 82-14, 83 pp. MILODOWSKI A.E. & WILMOT R.D. (1984) Diagenesis, porosity and permeability in the Corallian Beds (Upper

Oxfordian) from the Harwell Research Site, South Oxfordshire, UK. Clay Miner. 19, 323-341. SCrlMIDT V. & McDoNALD D.A. (1979a) The role of secondary porosity in the course of sandstone diagenesis.

Pp. 175-208 in: Aspects ofDiagenesis (P. A. Scholle & P. R. Schluger, editors). S.E.P.M. Spec. Publ. 26. SCHMIDT V. ~ McDoNALD D.A. (1979b) Texture and recognition of secondary porosity in sandstones. Pp. 209-

225 in: Aspects ofDiagenesis (P. A. Scholle & P. R. Schluger, editors). S.E.P.M. Spec. Publ. 26. StSaDAM R.C. & CRESSEV L.J. (1985) Organic-inorganic reactions during progressive burial: key to porosity

and permeability enhancement and preservation. Phil. Trans. R. Soc. Lond. A315, 135-156.