Quartz cement is a main factor influencing reservoirquality in siliciclastic sandstones. Quartzose sandstonesoften have quartz as themain cementmineral that destroys
⁎ Corresponding author.E-mail address: [email protected] (N. Molenaar).
porosity and permeability. For predicting and modellingreservoir properties, the mechanisms and the maincontrols and constraints on the precipitation of quartzcement thus must be understood. Evidently a source ofsilica must exist together with a mechanism to transportsilica towards the site of cement precipitation, as well as aprecipitation mechanism involving suitable nucleationsites. Much research has been devoted to physical and
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chemical conditions during quartz cementation, lessattention has been paid to local, facies-related controlson the degree and distribution of quartz cement.
Existing diagenetic models predict that reservoirquality of quartzose sandstones generally decreases withincreasing age of the deposits and increasingmaximum orpresent day burial depth (e.g., Byrnes, 1994). Superposedon general trends, which often are only valid for specificsets of local conditions, large variation usually exists inactual petrophysical properties and reservoir quality. Thisimplies that the general trends are indeed local empiricaltrends without solid fundamental underlying processcontrols, or that local conditions, products of other dia-genetic processes or oil emplacement either enhanced orinhibited silica sourcing, transport or the precipitation ofquartz cement. The occurrence of reservoirs in old anddeeply buried sandstones necessitates better understand-ing of petrophysical properties and reservoir quality as afunction of primary depositional processes and post-depositional diagenetic processes.
The objective of the paper is to test if internal sourcescan be a likely main source of silica for quartz cemen-tation. If so, depositional facies is expected to play animportant role in the degree of quartz cementation. Thiscan explain much of the usual variability in the degree ofquartz cementation and thus in reservoir properties andquality. This will be done by analysing the Middle Camb-rian sandstones of the Deimena Group sandstones in westLithuania where quartz cement is the main control onreservoir properties.
Fig. 1. Location of the Palaeozoic Baltic Basin and Lithuania (modified
In Lithuania and adjacent areas, the Lower and MiddleCambrian Deimena Group sandstones are the main oilreservoir. Several economically important hydrocarbonreservoirs occur in the Lithuanian, Kaliningrad and Polishoffshore and onshore where the Cambrian succession isconsidered to be the most prospective target for petroleumexploration in the region (Zdanavičiūted and Sakalauskas,2001). Fifteen oil fields have been discovered so far on-shore in west Lithuania within the Deimena Group sand-stones. These oil fields are confined to burial depthsgreater than 1.6 km. The sandstones show great variabilityin reservoir qualitymaking exploration difficult. Themainpetrophysical properties, such as porosity and permeabil-ity, have a complicated distribution evenwithin a single oilfield and closely spaced wells within a single oil field mayhave different reservoir quality.
2. Geological setting
The Baltic Basin is the largest tectonic feature of thewestern margin of the Eastern European Craton. TheBaltic Shield, the Latvian Saddle and the Mazury–Belarus High confine this basin (Fig. 1). The sedimentarysuccession consists ofVendian toQuaternary sedimentaryrocks resting on a Precambrian crystalline basement.Cambrian siliciclastic deposits are widespread in thebasin.
The thickness of the Cambrian succession (Fig. 2)ranges from several metres in east Lithuania and northEstonia to a maximum of up to 150 m in west Lithuania.
after Lazauskiene et al., 2002). TTZ — Teisseyer–Tornquist Zone.
Fig. 2. Stratigraphic scheme of the Lithuanian Cambrian (after Jankauskas, 1994).
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It comprises laterally variable alternations of sandstone,siltstone and shale that were deposited in shallow marineand shelf environments in Lithuania during minor sealevel fluctuations (Laškova, 1987; Jankauskas, 1994).Depositional environments range from shallow marineinner shelf, partly tidally influenced, to outer shelf withstorm-induced currents as the dominant transport anddepositional mechanism. Fine-grained shale drapes andshale intercalations are common within sandstonebodies. The sandstones are interbedded with and fingerout distally into marine shales. Deposits on the mid-shelf are alternations of sandstone and shale becomingshalier towards more distal, outer shelf environments.The sandstone/shale ratio and the thickness of sand-stone-dominated successions thus systematically de-crease towards the outer shelf. The depositional faciesare shown in Fig. 3. The detrital composition of thesandstones is dominated by quartz (99%) mostly veryfine-grained (ranging from very fine to medium sandsized, on average medium sorted), with rounded to well-rounded quartz grains. The sandstones thus can beclassified as quartz arenites (cf. Folk, 1964). Shales aredominantly composed of illite with variable amounts ofkaolinite, minor interlayered smectite–illite and chlorite.
The Lower Cambrian succession is 80 m thick andconsists of fine-grained light grey, rare brown and
greenish grey sandstones, which alternate with siltstonesand shales in the west. Siltstone and shale are grey,greenish grey and brown in colour. Middle Cambriansediments were deposited in a somewhat regressed basinas evident by the dominance of sandstones. In centraland western Lithuania, the Middle Cambrian reaches athickness of 70–80 m. It consists of light greysandstones with intercalated shales and siltstones(Fig. 2). Two lithologically distinct parts are identifiedin the Middle Cambrian succession. The lower one(about 10 m thick) is mainly composed of siltstones andshale and is attributed to the Kybartai Formation. Theoverlying 60 m thick succession of quartzose sandstoneswith rare siltstones and shale layers represents the mainoil reservoir in Lithuania. This layer is defined as theDeimena Group, which is subdivided into threesedimentation cycles: Giruliai, Ablinga, Pajûris Forma-tions from top to bottom successively. The overlyingPaneriai Formation is only locally present, rather thinand is unimportant as reservoir rocks. Upper Cambriandeposits are absent in Lithuania; sedimentation wasconfined to the westernmost part of the Baltic Basinduring that time (Jankauskas and Lendzion, 1992).Some of the variations in thickness can be attributed topost-depositional erosion of sediments during LateCambrian.
Fig. 3. Description of sedimentary facies of the Cambrian deposits in west Lithuania.
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Because of the abundance of shale and shaleintercalations, it is expected that this sedimentarysystem is more or less isochemical and diagenesis isconstrained by facies, although externally controlled byeffective pressure and temperature. If this assumption istrue, then prediction of the degree of quartz cementationand the reservoir properties and quality must be relatedto facies.
For a number of reasons the Baltic Basin forms anatural laboratory for studying the influence of quartzcementation and chemical compaction on petrophysicalproperties and reservoir quality, and the controls on thesemain diagenetic processes. The various Cambrian de-positional sequences and sandstone bodies can be tracedover wide areas, which have undergone different burialhistories. From north to south and east to west, the presentday burial depth increases from outcrops along theEstonian Baltic seacoast towards 2.3 km burial depth inSW Lithuania and 3.5 km in the Kaliningrad enclave(Fig. 4A). This present day burial depth coincides with themaximum burial depth attained in the geological historyof the basin although the maximum burial depth isestimated to have been up to several hundreds of metresmore. The subsidence rates (Fig. 4B) and geometry of theBaltic Basin have changed throughout the Phanerozoic
(McCann et al., 1997; Poprawa et al., 1999). During theCambrian and Ordovician, deposition took place in apassive margin setting with total subsidence of 150–300 m during the Cambrian and 50–250 m during theOrdovician. The geodynamic situation changed duringthe Silurian into a converging continental margin settingwith intense subsidence ranging from 100–200 m in theeast up to 4–5 km in the westernmost part of the BalticBasin in the central Poland and NE Germany. Themagnitude of Silurian subsidence in west Lithuania is700–900 m. Intense subsidence persisted during theDevonian during which period about 1–1.3 km thickterrigenous and calcareous sediments accumulated in thebasin centre. Subsidence drastically decelerated duringthe earliest Carboniferous givingway to uplift and erosionof the basin flanks during Carboniferous–earliest Perm-ian; in some place themagnitude of erosion is estimated tobe as much as 0.5–2 km. Numerous diabase intrusionspenetrated the Cambrian–Silurian succession in thecentral part of the Baltic Sea. During the succeedingPermian–Cainozoic times, only central, southern andwestern parts of the basin were involved in subsidence inthe range of 0.1 km (east) to 1.5 km (southwest).
The present day geothermal gradient in Lithuania is2–4 °C/100 m with a maximum burial Tof the Cambrian
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succession modelled at about 80–90 °C in west Lithua-nia (Fig. 4C). The temperatures of the Cambrian success-ion increase concordantly with increasing depth from 7–20 °C in the shallow basin periphery to 80–90 °C in thewest (Sliaupa and Rasteniene, 2000). The highest tempe-ratures of the Cambrian are reported in southwest Lit-huania and are related to amaximum in heat flow. The oilfields of west Lithuania are located where the temper-ature of the Cambrian exceeds 65 °C. The thermalhistory of the Cambrian sediments was modelled usingavailable RockEval Tmax estimates and reflectance ofvitrinite-like matter in selected Lithuanian wells. Thepresented wells Ramuciai-3 and Rusne-1 (Fig. 2B)characterise west Lithuania. The thermal maturity of theLower Palaeozoic organic matter increases to the south-west, following the pattern trend of the present tempe-ratures. A few available RockEval Tmax estimates of theCambrian and Ordovician rocks of central Lithuania arein the range of 425–432 °C. More extensive RockEvalstudies of Cambrian andOrdovician successions from 19wells in west Lithuania indicate maximum palaeotem-peratures in the southwest and along the coast of theBaltic Sea, where Tmax values vary from 445 °C to455 °C (Zdanavičiūtė and Swadowska, 2002). Theydecrease to the north to 435–427 °C (Fig. 4D). It indi-cates that most of the west Lithuanian territory is placed
Fig. 4. A. Present day burial depth of the top Cambrian in the Lithuanian parintervals). B. Burial modelling of wells Ramuciai-3 and Rusne-1 (west Lreflectances Ro (centre) and thermal evolution (right) are calculated for the MC. Map showing present day temperature isotherms of the top Cambrian witmaturity of Cambrian and Ordovician organic matter of west Lithuania. Tmax
in the oil window. The pattern of Tmax correlates with theheat flow intensity. Thermal maturity of the LowerPalaeozoic vitrinite-like organic matter and chitinozoas(Zdanavičiūted and Swadowska, 2002; Lazauskiene andMarshall, 2002) bear some uncertainty due to difficultieswith calibrating measured reflectance values to standardvitrinite reflectance. The interpretation of measuredvalues was improved by fitting these data to Tmax
estimates that show a similar pattern. The most intensepalaeothermal regime is identified in the southern part ofwest Lithuania where R0=0.9–1.05 (Fig. 4D). Itdecreases to R0=0.7–0.55 in the north and the east. Noconsistent data have been received from the eastern halfof Lithuania, though aforementioned RockEval dataindicate eastward decreasing palaeotemperatures.
Thermal modelling of selected wells of west Lithuaniashows that the present temperatures are too low to explainthe high thermal maturity of the organic matter. Thestratigraphic data do not suggest any significant differ-ences in palaeodepths compared to present burial con-ditions. Magmatic activity took place in the whole regionduring (Middle) Devonian and Carboniferous–Permiantimes. The thermal event in the Baltic Basin has beendatedMiddleDevonian according to radiometric dating ofauthigenic illites (Sr/Rb isotope data) (Kirsimae et al.,1999; Kirsimäe and Jorgensen, 2000; Srodon and Clauer,
t of the Baltic Basin, with location of studied wells (isobaths at 100 mithuania). Oil window is indicates on burial graphs (left). Vitriniteiddle Cambrian reservoir containing interlayers of organic-rich shales.h location of studied wells (isotherms at 10 °C intervals). D. Thermal(left) and vitrinite reflectance (right). Locations of wells are indicated.
Fig.4(con
tinued).
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Fig. 4 (continued ).
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2001). Some evidence however points to a much laterJurassic age for the thermal event (Sliaupa et al., 2001).This event correlates to the extensive igneous activity inthe central part of the Baltic Sea and Mazury High (northPoland) that occurred 355–295 Ma (Depciuch et al.,1975; Sliaupa et al., 2001). 1D modelling calibrated tovitrinite reflectance and Tmax data revealed an increase in
the heat flow for 15–17 mW/m2 during the latestPalaeozoic (355–290 Ma). This is in agreement withestimates received from the Baltic Sea area (Sliaupa et al.,in press). The maximum temperatures of 115–125 °Cwere attained in the southwestern part of Lithuania duringCarboniferous time (Fig. 4B), whereas they reached 80–90 °C in the northern half of west Lithuania.
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3. Methods, sample and data material
The study is focussed on central and west Lithuaniawhere most of the producing oil fields occur and con-sequently most drilling took place and core material andgeophysical well logs were available for this research.Less core material was available in the east but this alsohas been studied. The petrophysical and lithologicaldatabase is from measurements on core samples from 50wells in the Lower and Middle Cambrian deposits(Fig. 4A).
The lithofacies was studied in detail in combinationwith petrophysical analyses. Polished thin sections of149 samples from 50 wells were used for studying thetexture and mineralogical composition by polarized lightmicroscopy. The components of 45 samples from 20representative wells were quantified by pointcountingand image analyses software of microphotographs fromcathodoluminescence scanning electron (SEM-CL) andcold cathodoluminescence microscopy. Mineralogicalcompositions were confirmed by backscattered electronimaging (SEM-BS; 27 samples). Roentgen diffractom-
Table 1Results of pointcounting detrital and diagenetic features in thin sections of t
Only sandstones from the Deimena Group are incorporated in this table.The IGV is calculated according to Paxton et al. (2002) but corrected for seMost of the clay is authigenic illite and kaolinite.Sandstones with significant detrital clay through related to burrows have no
etry (XRD) has been used for clay mineral determina-tions (oriented mounted fractionb2 μm, non-treated,ethylene glycolated and heated to 350 and 500 °C;N=62) according to standard methods. Helium porosity,gas permeability (horizontal and vertical), and bulk andgrain density data of almost 3000 sandstone sampleswere available for this study. Most of the data are fromexisting hydrocarbon industry reports with different setsof core samples used for the various analyses. Therefore,new core samples have been taken (148 for this study) forcombining petrophysical data with petrographic analy-sis. The porosity was determined by standard gas expan-sion method using a helium porosimeter HGP100. Thepermeability was measured using a digital gas permea-meter DGP200 and corrected for Klinkenberg effect.XRF bulk chemical data from 67 sandstone samples andgrain size data based on standard sieve analyses werealso available. The number of data points may be diffe-rent for the various parameters since requirements forplug and sample size can be different and not met be-cause of limited core sample size, and not all measure-ments have been done paired.
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4. Sandstone petrographic description
Petrographic investigation of Cambrian sandstones,results are summarized in Table 1, revealed that detritalcomponents are mainly quartz grains (mono- and poly-crystalline) with negligible amounts of feldspar, lithicgrains, andminor minerals (mainly glauconite and heavyminerals). Glauconite occasionally is an abundant mi-neral in the lower part of the Cambrian succession, itsoccurrence being related to sequence boundaries. Heavyminerals such as opaque minerals, zircon, garnet andrutile are accessory detrital minerals occurring dispersedor in thin lamina. Sandstones can be considered asmineralogically very mature as the detrital frameworkconsists of 99% quartz. This is confirmed by other stu-dies reporting 95–99% detrital quartz (Laškova, 1987;Kilda and Friis, 2002) and by bulk chemical analysis(Table 2). The sandstones are on average very fine-grained and grain supported, lack matrix, and thereforeare quartz arenites (cf. Folk, 1964). Detrital quartz grainsare very fine to medium sand sized, occasionally coarse-grained and well-rounded sand. The average sorting isgood (S0=1.33±0.15; N=87), locally with bimodalvery fine to medium or medium to very coarse sand sizedlamination. The contacts between detrital quartz grainsare point contacts. Elongate and angular grains withsutured grain contacts, common in the shales and alongshale–sandstone contact surfaces, are characterised byshale linings and indicate pressure dissolution.
The shales are composed mainly of illite, with va-riable amounts of kaolinite and minor iron rich chloriteand occasionally minor amounts of a mixed layer smec-tite–illite. The shales contain variable amounts of quartzand some K-feldspar grains of very fine-grained sandand silt size.
The sandstones are to a variable degree lithified bycompaction and quartz cement. Quartz is the main ce-ment mineral and a main control on petrophysical pro-perties. Towards the deeper buried parts of the basin inwest Lithuania the amount of quartz cement increases.Besides prevailing quartz cement some carbonate ce-
Table 2Results of bulk chemical analysis (XRF) of 67 Deimena Group sandstone sa
The aluminium and potassium is associated with clay minerals, mainly illitedrapes.The sandstones are pure quartz arenites with grain supported framework free
ment, pyrite and authigenic illite are present in theeastern, marginal parts of the basin. The sandstones areweakly lithified with iron rich dolomite or calcite andminor quartz cement. Probably compaction also played arole in reduction of porosity and lithification. Compac-tion comprised mechanical rearrangement of grainsthroughout the sandstones as well as chemical compac-tion located along shale–sandstone contacts and withinthe shales.
4.1. Clay minerals
Shale lamina in the sandstone facies have the samemineralogical composition as shale facies with illite asdominant clay mineral, variable kaolinite content andtraces of chlorite. Authigenic illite in sandstones occursas pore filling cements composed of fibrous illite withlong axis perpendicular to pore walls. It occurs in thinzones of several tens of microns thick along shale–sandstone contacts. At places with pore filling authi-genic illite, no quartz or little quartz cement is observed,pointing to precipitation of illite just after quartz cementprecipitation started but before the main phase of quartzcement precipitation. Occasionally kaolinite was foundaggregated in pores or partially entrapped in authigenicquartz, suggesting an authigenic origin.
4.2. Quartz cement
The main cementing material in the sandstones isquartz, which occurs as syntaxial overgrowths ondetrital quartz grains in all samples and all sandstonefacies. Heterolithic sandstones may lack quartz cementwhen they are matrix supported because of bioturbation.Quartz cement completely or partly fills pore spaces andis one of the main causes of porosity loss in Cambriansandstone reservoirs in the Baltic Basin (Laškova, 1987;Vosylius, 1998; Kilda and Friis, 2002) (Fig. 5A–F).Dust rims composed of mineral and/or fluid inclusionspartly mark the contact between authigenic and detritalquartz. In strongly cemented samples all quartz grains
with less chlorite and kaolinite, occurring in thin shale lamina or shale
of detrital matrix.
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have overgrowths, but the thickest overgrowths arepresent on monocrystalline rounded to well-roundedquartz grains. The presence of overgrowths resulted inlong compromise boundaries between adjacent quartzgrains. The quartz cement is non-luminescent withoutzonation suggesting few changes in pore fluid chemistryduring quartz precipitation or a single cement precipi-tation phase. Thinner bedded sandstones (facies S2 andS3) show homogenous degree of cementation, whereasthick-bedded sandstones of facies S1 show a trend ofdecreasing quartz cement from the margins of the bedstowards the central parts.
The grains are densely packed (the packing density,PD, averages 76.3%, S.D. 4.9%, N=45) but still havepoint contacts and few samples show minor grain inter-penetration. PD, as defined by Kahn (1956), is the ratioof the sum of the grain length encountered along atraverse across a thin section to the total length of thetraverse, and PD can be expressed by the equationsPD=Σgi / t×100%, where gi is the grain intercept lengthof the ith grain in the traverse; t is the total length of thetraverse. Framework grains are without compactioninduced features such as brittle grain fracturing. Theamounts of quartz cement (average 23.0%; S.D. 6.4%;N=45) and the total intergranular volume (i.e., theintergranular volume IGV according to Paxton et al.,2002) (average 28.5%; S.D. 4.3%; N=45) point to theemplacement of the quartz cement after mechanicalcompaction by grain rearrangement but before inter-granular pressure dissolution.
4.3. Carbonate cement
Small amounts of carbonate cement, mainly Fe-richdolomite (0–3%), occur in some of the sandstones. Alsoin shales and siltstones some Fe–dolomite and ankeritecan occur. The dolomite cement occurs as isolated clus-ters of poikilotopic cement with crystals with the samesize or much larger than detrital grains. These clusters ofa few cemented quartz grains are dispersed in sandstoneotherwise cemented by quartz. The grain packing densityis low in such dolomite cement patches suggesting rela-tive early carbonate cementation before the main phase
Fig. 5. Microphotographs. A, B, C and D. Micrographs of thin sections showpores (5A — plane polarised light; 5B— idem with crossed polarization filt5D — idem 5C but SEM-BSE micrograph). Well location Lašiai-3; depth 20SEM-BSE and SEM-CL microphotograph of quartz arenite with high porosilocation Genciai-3; depth 1822.7 m; porosity 15.7%, vertical permeabilitycompaction of detrital quartz through clay-induced pressure dissolution at quaoutlines. Note also the lack of overgrowths on quartz grains. Well location SSEM-BSE micrograph showing oversized pores suggesting dissolution of dlocation Genciai-2, depth 1857.6 m.
of mechanical compaction. Some oversized pores(Fig. 5H) filled by dolomite are common in thesepatches, sometimes with quartz inclusions, suggestingthe former presence of lithic grains that were largelydissolved. The dolomite cement either pre-dates thequartz cement as is evidenced by the absence of quartzcement in carbonate cemented patches; or in other casesinterrupts quartz cement and fills remaining pores left byincomplete quartz cementation.
Further toward the basin margin in east Lithuania,occasional sandstone beds with pervasive calcite cementare intercalated in otherwise weakly quartz cementedsandstone. Most of this pervasive carbonate cement iscalcite, but ankerite and siderite also occur. In cases ofpervasive carbonate cementation, the porosity is reducedto nearly zero. Some of these layers also contain calciteshell clasts.
4.4. Compactional features
Besides mechanical compaction in the sandstonessuggested by the low IGV values, also features indicativeof chemical compaction along shale–sandstone inter-faces and within silty and sandy shales are present, aswell as penetrative stylolites in some wells (cf. Park andSchot, 1968). Pressure dissolution can occur in sand-stones along discretely spaced surfaces, such asstylolites, which often are parallel to bedding planesperpendicular to the maximum stress field (e.g., Mitraand Beard, 1980), and is most common along shaleintercalations (Fig. 5G). Although packing density ishigh, most contacts between detrital grains in thesandstones are merely point contacts. Intergranular pre-ssure dissolution in sandstones with characteristic longand concavoconvex grain contacts is uncommon and hasbeen found in few sandstone samples.
Part of the sandstone facies is characterised by nu-merous thin–thick clay laminae and intercalations aswell as some dispersed clay matrix related to burrows inbioturbated beds in heterolithic facies. Microstyloliteswere identified during thin section analysis showing thatpressure dissolution took place on a grain scale but wasrestricted to grain–shale contacts (Fig. 5G). Contacts
ing sandstone with abundant quartz cement and some reduced primaryers; length of photomicrographs is 0.96 mm; 5C-SEM-CL micrograph;65.0 m; porosity is 1.02%, permeability is below 0.01 mD. 5E and 5Fty from a reservoir interval with relative thin quartz overgrowths. Well1422.18 mD, horizontal permeability 1527.95 mD. 5G. Chemical
rtz clay interfaces. Quartz grains are largely dissolved and show suturedilute-1; depth 2081.5 m; length of photomicrograph is 1.87 mm. 5H.etrital grains. The fine-grained material is invaded drilling mud. Well
Table 3Overview of average values, standard deviation (S.D.) and range ofpetrophysical properties of the depositional facies in the studied wellsin Lithuania
The heterolithic and shale facies samples have relative high porositythat is mainly microporosity due to the clay matrix.
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between quartz grains along such microstylolites aresutured and grains have more elongated shapes, insteadof rounded to well-rounded and spherical grains within
Fig. 6. Graphs showing correlations between different petrophysical parametA. Although the sandstone porosity shows a decrease with increasing depthsamples). B. The graph shows a relatively good correlation between porosipermeability for the different sandstones facies. The thickness of the sandstonS3 facies. Although the absolute range of property values is almost the same,highest porosity and permeability values. This conforms with the expectationshale–sandstone contacts through clay induced pressure dissolution. All sandthinner bedded. S1 facies is thus also cemented but the degree of cementationrandomly (that is at regular but varying intervals), not just in the central mostCross plot of sandstone porosity in a single well (in Genciai oilfield) withcontent. A statistically significant negative correlation exists (R=0.553 fordecreasing gamma intensity or shale content, pointing to the fact that porosi
adjacent sandstone lamina. The dissolution has producedsutured boundaries at clay–quartz contacts (Fig. 5G) andstraight boundaries at mica–quartz contacts. Thisindicates that large parts of detrital grains have beendissolved along these shale–quartz contacts and withinthe thin silty–sandy shale intercalations. The removedvolume of silica from detrital quartz grains along micro-stylolites based on CL image studies is estimated to be atleast 18–25%. This very approximate estimation hasbeen made from drawing probable initial contours ofdissolved quartz grains. The intensity and degree ofdissolution in some parts of the succession have leftmonly fragments of quartz grains with clay mineralsforming the matrix. This observation is commonthroughout the succession where shale is in contactwith quartz grains. Quartz occurs as overgrowths ondetrital quartz grains in inter-stylolite regions, in sand-stones without any dissolution features due to chemicalcompaction. Aqueous silica that was produced duringdissolution may have been transported by diffusion intointer-stylolite regions where it precipitated as quartzcement on quartz grains. The parts of the sandstonesadjoining dissolution seams contain more quartz cementthan in thick-bedded sandstones without shale intercala-tions. The scale at which quartz dissolution has occurredat shale–sandstone interfaces suggests that clay-induceddissolution of quartz grains may have been the source ofthe bulk of quartz cement found on quartz grains in inter-clay and inter-stylolite regions.
5. Reservoir properties
The geometry of pore space, commonly described byparameters such as pore size distribution, shape andorientation of pores, and the nature of the connectivitybetween pores, controls the reservoir characteristics.The main type of pore is a primary intergranular porereduced mainly by quartz overgrowths (Fig. 5). In thestrongly quartz cemented samples, small remnantprimary pores occur isolated and disconnected. Samples
ers of Cambrian sandstones in the Lithuanian part of the Baltic Basin., the variation in porosity values at each depth is rather distinct (2820ty and permeability (2560 samples). C. Cross plot of porosity versuses decreases while the amount of shale interlamina increases from S1 tothe thicker bedded sandstones attain the best reservoir quality with theif silica for quartz cement is derived from the thin shale laminae and
stones are quartz cemented, but thicker bedded sandstones less so thandecreases from the margins towards the middle. The samples are takenporous parts of beds, explaining partly the range in properties of S1. D.the natural gamma radiation (well log), that is equivalent to the shalea linear correlation equation, N=140) with porosity increasing withty is indeed related to depositional facies.
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Fig. 6 (continued ).
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Fig. 7. A. Cross plot showing statistically significant negative correlation between quartz cement (%) and porosity (%) (R=0.683). The numericavalues were obtained by point counting between 1000–2000 points on digital combined SEM-CL and SEM-BS micrographs (average values fromseveral micrograph of a single sample). B Cross plot of porosity against sorting (according to Folk and Ward) (N=133). Porosity evidently is nodependent on depositional grain size distribution.
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l
t
Fig. 8. Graph shows absence of correlation between bulk carbonate content (Fe–dolomite %) and porosity (%) A (N=744), and carbonate content andhorizontal permeability B (N=516). Two data points with carbonate content (calcite) above 15% have been omitted in the graphs. The dolomitecement evidently has no significant influence on the reservoir properties.
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Fig. 9. Petrophysical properties of sandstones for selected oilfields in Lithuania. A. Graph shows porosity versus depth for all wells in the Genciaioilfield in west Lithuania (793 samples). The distribution has a large spread in porosity values that is typical for such a local scale. B. The porosity–permeability distribution for all wells in the Genciai oilfield in west Lithuania shows a positive correlation (793 samples).
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with porosity lower than about 5% have negligiblepermeability, below 1 mD. Secondary pores do occur.They are oversized pores (Fig. 5H), a typical feature forsecondary pores (e.g., Schmidt and McDonald, 1979).They are related to post-depositional leaching of detritalgrains, with rounded forms suggesting dissolution ofmicrocrystalline quartz grains or with elongated andangular forms suggesting dissolution of detrital feldsparor bioclastic carbonate grains. The dissolution ofmineral components will clearly lead to redistributionof pore space and change in pore size distribution, butnot necessarily to an increase in porosity, since disso-lution may lead to precipitation of authigenic mineralselsewhere in the system (Giles, 1987; Giles and de Boer,1990). Secondary pores account on average for 16% ofthe total porosity.
A petrophysical data set of about 3000 porosity,permeability and density values of Cambrian sandstonesin Lithuania has been used for detailed analysis. Theaverage values of main petrophysical properties aresummarized in Table 3. As expected, porosity and per-meability in the Cambrian sandstones generally decreasewith depth in the whole Baltic Basin whereas bulkdensity shows an opposite trend (Fig. 6A). Porosity ispositively correlated with permeability (Fig. 6B). Graindensity remains fairly stable with an average value of2.633 gm/cm3 for the sandstones reflecting the quartzarenite composition.
In most cases (86.9%) the horizontal permeabilityequals or exceeds the vertical permeability of the sand-stones, which is typical for laminated and thin-beddeddetrital sediments. The anisotropy is present in bothshallow and deep parts of the basin. Some of the sand-stones show higher vertical permeability that is causedby either fracturing or to dominancy of verticallyoriented Skolithos trace fossils.
Porosity is inversely correlated with the amount ofquartz cement (Fig. 7A), and does not show any corre-lation with grain size or sorting (Fig. 7B). Dolomitecement has no influence on reservoir properties (Fig. 8).The amount of dolomite is very small, 97.6% of thesamples have less than 3% dolomite; N=762, and thereis no evidence for dissolution. In fact the outer marginsshow some zonation by slightly variable Fe contents.The present reservoir properties thus depend on thedegree of modification through diagenetic processes,and the observed variability is the result of differentialmodification, i.e. a different degree of compaction and/orquartz cementation.
Looking in detail at porosity and permeability distri-bution with depth, it is apparent that the data show a greatvariation independent of depth. Large variations exist
even within a single field at similar burial depths(Fig. 9A, B). Fig. 6C shows an example from an oilfieldwell with a clear correlation between sandstone porosityand depositional facies of sandstones. High porosity andpermeability values are restricted to thick-beddedsandstones (S1) and in particular the central parts ofsuch beds or amalgamated beds have highest porosityand permeability. The total amount of shale in the mainsandstone bodies in the upper part of the Cambriansuccession also correlates with the porosity (Fig. 6D).The porosity is evidently related to depositional facies, inparticular the amount of intercalated shale laminae.
The presence of a generalized trend of decreasingreservoir quality with increasing depth would suggestthat either temperature or loading because of overburden,although not necessarily equivalent to effective pressure,is in some way related to the increasing effect of dia-genesis, in particular to the amounts of quartz cement.However, this does not explain the local variation inpetrophysical properties superposed on the generaltrend. The observed variation suggests that factorsother than those strictly related to burial depth influencedquartz cementation during geological history, such asfacies-related amounts of intercalated shale laminae.
6. Discussion
For successful forward modelling of reservoir pro-perties, the effects of diagenetic processes need to betaken into account since they largely influence anddetermine these properties although the effect may besuperimposed on the depositional properties. Thediagenetic processes having most influence on reservoirproperties are mechanical compaction, quartz cementa-tion and chemical compaction. For modelling theseprocesses and their effects on reservoir properties thefundamental controls must be understood and a mass-balanced model is essential. Therefore the import andexport, or the openness of the sandstone diageneticsystem, need to be understood in addition to the effects ofprocesses in adjacent or interbedded shales. However,sandstone diagenetic systems are still not well under-stood in terms of major element mobility. Some studiessuggest that sandstones are open diagenetic systems forseveral major elements (e.g., Milliken et al., 1994).
6.1. Mechanical compaction
The average total intergranular porosity (total per-centage cement and porosity) of 28.5% (with a rangefrom 17.1 to 37.5%) is significantly lower than might beexpected for a medium sorted clean sand (Beard and
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Weyl, 1973). In general, although the rate and degree ofporosity reduction through mechanical compaction isevidently more important in lithic sandstones containingductile detrital grains (e.g., Nagtegaal, 1978; McBrideet al., 1991), mechanical compaction through grainrearrangement is also a significant process in quartzosesandstones (Paxton et al., 2002). Although the exactinfluence of mechanical compaction is still debated, it isclear from the literature that in well-sorted quartz-richsandstone, rearrangement of detrital grains alone canresult in reduction of depositional porosity of around40% to values of about 34% (Lundegard, 1992; Wilsonand Stanton, 1994) or even 26% (Paxton et al., 2002).Ductile grains are absent in the Deimena Group sand-stones and grain interpenetration or brittle fracturing ofquartz grains has only been observed in few sampleswith very low IGV values (17–23%). Since detritalcomposition is invariable, the observed variation in deg-ree of compaction must be related to differences indepositional texture and grain size distribution.
Although the primary porosity of sands is not wellestablished, an initial porosity of about 40% may beassumed for well-sorted to about 38% for moderatelywet-packed, well-sorted marine sands (Beard and Weyl,1973). The measured average IGV of 28.5% is in
Fig. 10. The intergranular pore volume (IGV) versus quartz cement graphcorrelation is statistically significant (R=0.760).
agreement with the literature (e.g., Wilson and Stanton,1994) as approaching a minimum value possible solelyby pure mechanical compaction in quartzose sandstones.This implies that the bulk of the quartz cement has beenintroduced after mechanical compaction, before theintroduction of quartz cement could counteract compac-tion by stabilising the detrital sandstone framework. IGVis positively correlated (R=0.760) with the amount ofquartz cement (Fig. 10). The observed variation in IGV,being lowest in thick-bedded sandstones (Table 1), maypartly depend on detrital texture but could also point to aslightly different time and burial depth for introductionof quartz cement. The introduction of even small a-mounts of cement is enough to retard further mechanicalcompaction and preserve IGV (e.g. Paxton et al., 2002).Textural variations in grain size distribution (sorting andinitial packing density) may have been responsible forinitial differences in IGVor caused differences in the rateand final degree of mechanical compaction.
6.2. Quartz cementation model
The reservoir properties of the Cambrian successionare strongly controlled by the amount of authigenicquartz cement as in other quartzose sandstones such in
(47 samples). Although the graph shows some variation, the positive
154 N. Molenaar et al. / Sedimentary Geology 195 (2007) 135–159
as the North Sea. Prediction of reservoir properties musttherefore be based on understanding quartz cementation.A diagenetic model for quartz cementation must explainthe supply of silica in terms of processes yielding silicain solution and the transport mechanism towards andwithin the sandstone body. A diagenetic model furthermust explain the timing, distribution and amount ofquartz cement and the effects on petrophysical proper-ties. Concomitantly, the paragenetic succession andtiming of quartz cementation, as well as the location ofquartz cement within the sandstone body, can help inunderstanding the silica supply mechanism.
Nucleation sites are abundantly available since det-rital clayey matrix is rare in the main sandstone facies,being restricted to burrows or burrowed intervals. Detri-tal clay coatings are absent, and the little illite cementthat is exclusively adjacent to shale intervals. Clay ce-ments may shield nuclei and are supposed to preventprecipitation of quartz cement. Availability of nucleationsites therefore is not a major control on the location ofquartz cement.
Many models attempting to explain quartz cementa-tion still conclude that external sources of silica areneeded to explain the observed quantity of quartz cement(see review byMcBride, 1989; Dutton and Diggs, 1990).However, realistic silica sources and transport mechan-isms are generally lacking and the increase in silicacontent is difficult to explain (e.g., Gluyas and Coleman,1992) unless it is related to compaction of the pertinentsandstone succession.
Present day burial depth may not always reflect themaximum attained burial depth; nor the effective stress,which is much more difficult to ascertain from basinanalysis. The trend with increasing temperature could berelated to simple reaction kinetics, including the rates ofsilica yielding processes, such as mineral dissolutionand clay transformations, and quartz precipitation. Thesolubility of minerals such as feldspar may increase withtemperature but so does the solubility of megaquartz andother forms of silica. Temperature may be a thresholdfor illitization of smectite, a transformation that suppo-sedly yields silica (Boles and Franks, 1979). Theseprocesses, however, are not merely controlled by tempe-rature since potassium supply, for instance, might alsocontrol the onset and rate of illitization (Heling, 1974).The trend with increasing age is usually ascribed tolonger time for processes to proceed even under lower Tconditions, and possibly a higher chance for the sand-stone to be in the appropriate diagenetic window withrespect to favourable physical and chemical conditionsfor quartz cementation. The duration of burial or the ageof the deposits are not likely to be main factors in quartz
cementation since some studies have pointed out thatquartz cement precipitation phases can occur in rela-tively short periods (Robinson and Gluyas, 1992b;Gluyas and Oxtoby, 1995).
Large volumes of diagenetic fluids are needed forquartz cementation (Blatt, 1979; Girard and Deynoux,1991), although they become increasingly smaller withincreasing P and in particular T. However, the meansavailable to generate and in particular to transport suchfluids in the deeper subsurface are lacking. Under surfaceconditions of meteoric water infiltration and rechargesufficient amounts of fluid may be moved, leading tolocally megaquartz cemented sandstones, such as theFontainebleau sands (Thiry and Marechal, 2001).However, this cannot account for quartz cement volumesseen in most sandstones, nor for the enhanced tempera-tures measured in fluid inclusions during main cementprecipitation phases. The mechanism to move diageneticfluids at some depth is lacking. The main flow mecha-nism in basins is supposed to be compaction drive(Bjørlykke, 1993). This mechanism is only operative incases of shaley sediments and sandstones with sufficientductile components susceptible for rearrangement intodenser grain packing and mechanical deformation ofgrains and framework collapse (Nagtegaal, 1978). Mostof the fluids are expelled during early compaction in thefirst hundreds of metres of burial (Bjørlykke, 1994).Chemical studies of pore fluid composition suggest thatno large-scale fluid flow took place in the North SeaBasin, limiting also the effect of compaction-driven flow(Bjørlykke and Gran, 1994). Moreover, later compactionand expulsion of fluids is dependent on local pressuresand interconnectivity of more permeable units such asporous sandstone bodies, since shales increasinglybecome physical barriers for fluid flow. For externalsources it is either crucial that coarse-grained permeablesandstone bodies are interconnected allowing for flow ofdiagenetic fluids, or that mechanical compaction andexpulsion of interstitial fluids from shales is not yetfinished. Large-scale transport and supply of silicathrough basin scale transport mechanism is thus unlikely,unless that would be facilitated by open faults or thrustfaults and pressure gradients as induced by tectonicactivity. Although tectonic deformation took place du-ring the Caledonian Orogeny, the Baltic Basin and theLithuanian study area is distant from the deformationfronts. Uplift of basin margins and minor fault activitytook place during Devonian times. In the case of externaladvective supply, it would be expected that the thickerand most permeable intervals would become mostintensely cements. The contrary is the case in the Dei-mena Group sandstones. Moreover, the Cambrian sand-
155N. Molenaar et al. / Sedimentary Geology 195 (2007) 135–159
rich systems are not or only partly interconnected to-wards the deeper parts of the basin, which would alsopoint to internal transport.
External sources are therefore unlikely to be a majorcontributing agent for quartz cement in the DeimenaGroup and in general. The most likely scenario is someform of nearby or internal silica source with either di-ffusion or local flow as the transport mechanism (e.g.,Bjørlykke and Egeberg, 1993). Quartz cementationwould thus take place in an isochemical diagenetic sys-tem. Quartz diagenesis could either be a continuous or amultiphase process, depending on local constrainingconditions as well as on the overall physical conditionsand changes therein given by the basin history. Multi-phase quartz cement has been observed in a number ofwell documented studies (e.g., Girard et al., 2001).
Some of the hypothetical sources are unlikely to havebeen responsible for significant amounts of quartz ce-ment. However, even small amounts of quartz cement, ifprecipitated during early diagenesis– for instancethrough dissolution of siliceous microfossils such asradiolaria that appeared in latest Precambrian– couldinfluence the evolution of petrophysical propertiesthrough the stabilising effect on the sandstone frame-work. As pointed out by the intergranular volume, thiswas not the case in the Deimena Group sandstones.
Dissolution of detrital silicate grains, such as feld-spars or lithic grains may locally play a role, dependingon the original detrital composition as related to depo-sitional facies. Facies and detrital composition are relatedthrough selective deposition as a result of sorting ongrain size, form and density and specific grain size po-pulations of detrital grain types. However, oversizedpores are only 1% so that detrital silicate dissolutioncannot have been a significant silica source.
Alternative silica sources during burial are localsupply from surrounding shales through illitization ofsmectite clays or through chemical compaction (alsocalled pressure dissolution) of quartz within the shales,or internal supply through chemical compaction withinthe sandstones or along sandstone/shale interfaces.These types of local or internal supply are controlledby depositional facies and architecture.
The original amount of smectite cannot be ascer-tained. The presence of interlayered smectite–illite inthe Cambrian shales along the northern margin of thebasin (Kirsimae et al., 1999), however, suggests thatoriginally smectite was present as a detrital clay mineralin the Cambrian basin, although smectite grain size andselective transport and accumulation may have causedcertain increasing trends in the more distal parts of thebasin.
The actual process of smectite illitization of smectiteprocess is still under discussion but most studies suggesta dissolution–reprecipitation process, which may yieldsilica amongst other elements (Boles and Franks, 1979).This silica source would be restricted to detrital shalegrains, drapes and intercalations. The latter are the mostvoluminous and represent the most evident form ofsilica. However, the diagenetic openness of such shalesintercalations is questionable. Studies on fine-grainedrock fractions (e.g., Lynch, 1997; Land et al., 1997;Lynch et al., 1997) suggest that shale diagenesis is anopen-system process that required addition of K2O andAl2O3, and resulted in loss of SiO2. In the particular casestudied the amount for silica through shale diagenesiswould be sufficient to account for the amount of quartzovergrowths in the associated sandstones. More reliablewhole rock studies by Bloch and Hutcheon (1992) andWintsch and Kvale (1994) suggest that thick shales mayact as diagenetic systems closed for silica, leading toauthigenesis of clay minerals and/or quartz within theshales, whereas thin shales could be open and exportsilica towards the intercalated sandstones.
If export of silica occurred as a result of illitization ofsmectite, it is likely controlled by the architecture of theshale–sandstone succession, so the effects of silicasupply would be facies-related, being more or lessrestricted to parts of the sandstone successions withshale intercalations.
6.3. Chemical compaction induced by sheet silicates
Thin section observations show that in wells in thecentral and western parts of Lithuania where quartzcement is abundant and has significant effect on reser-voir properties, the detrital quartz grains have beendissolved extensively along shale–quartz contacts leav-ing truncated and elongated remnant grains. Theremoved volume of quartz along microstylolites on CLimages is estimated to be 18–25%. Such enhancement ofdissolution of quartz by clay or sheet minerals has beennoticed in a number of studies (e.g., Engelder and Mar-shak, 1985; Renard et al., 1997; Sheldon et al., 2003). Insome deep wells also, true stylolites within sandstonebeds occur, but even these are usually restricted tooriginal thin shale intercalations. However, chemicalcompaction resulting in detrital quartz grain interpene-tration is rare. A good agreement exists between thesandstone facies, mainly determined by bed thicknessand amount of shale lamina, and the reservoir properties(i.e., porosity and permeability) (Fig. 6D). The thinnerbedded sandstones with more abundant shale laminaehave the lowest porosity and permeability and are
156 N. Molenaar et al. / Sedimentary Geology 195 (2007) 135–159
homogeneously quartz cemented. Thick-bedded sand-stones, with bed thickness above about 10 cm, showhigher porosity and permeability towards the centre ofthe beds and lower amounts of quartz cement. Chemicalcompaction of detrital quartz (also called pressuredissolution) is common along shale–sandstone contactsand is therefore considered to be the logical source ofsilica for quartz cement, but not necessarily the onlysilica source. Illitization of smectite and chemical com-paction within thin shale intercalations certainly couldhave supplied silica as well. Local facies-related hetero-geneities such as shale drapes and lamina will lead todifferential diagenesis comprising localised chemicalcompaction and quartz cementation within the samesandstone body.
Theoretical model calculations show that silica re-leased by pressure dissolution of quartz in shale succes-sions through diffusion could cause quartz cementationin adjacent sandstone beds (Thyne, 2001), dependent onmud mineralogical composition and quartz content(Mullis, 1992). The effects would be confined to somedecimetres into of the sandstone. So far, only uncon-vincing evidence for this has been presented in theliterature (e.g., Füchtbauer, 1979).
The process of pressure solution is described by thesolution of quartz at a point of grain contact as a result ofstress, generally caused by effective load pressure (Ren-ton et al., 1969). It is often regarded as a mechanism ofductile rock deformation in natural rocks (Rutter, 1983)and it is considered an important process in deformationduring metamorphism and compaction during diagene-sis (James et al., 1986; Tada and Siever, 1989). It hasbeen proposed that clay promotes the process of pressuresolution by providing a microenvironment of high pH atphyllosilicate quartz grain contacts. The silica would be
Table 4Silica budget
Mainfacies
Shale content(gamma logs)
Thickness %(total 100%)
Sandstone insuccession
Quartz cement infacies according tothin section analyses
The system could be in balance with enough silica released by pressure solutiocement in the sandstones.A conservative estimate of 20% (from SEM-BS micrographs) pressure solucalculation.This dissolved quartz is assumed to migrate into the sandstones for quartz cThe shale–sandstone contents for the various facies is derived from naturalThe amount of detrital quartz has been estimated from SEM–BS micrograph
deposited as authigenic quartz overgrowths as a result ofa decrease in pH (Thomson, 1959; Renton et al., 1969).The possibility of clay promoting the process of pressuresolution has been proposed by a number of workers.Heald (1959) suggested that clay might act as a catalystaccording to the association between presence of clayand stylolites in sandstones, whereas Weyl (1959)favoured enhanced diffusion through the clay betweenthe quartz grains. However this is in contradiction withthe common presence of chemical solution in carbonateswhere increased pH would have an opposite effect oncarbonate mineral solubility (Sheldon et al., 2003).
It is likely that clay minerals (sheet silicates in general)can promote mineral dissolution through ‘water film dif-fusion’ because a thicker water film can be preserved attheir surface despite of high pressures (Renard et al.,1997). Clays can have structural electric charges spreadon their surface, which stabilise the water film and allowthe solutes to diffuse easily into the pore space (Renardand Ortoleva, 1997; Renard et al., 2001). On the otherhand, in areas where clay coatings already exist, silicaprecipitation is hindered because, as stated by Ramm(1992) there is loss of clean grain surfaces for nucleationof quartz. Thus, clay coatings inhibit the nucleation ofquartz overgrowths and thereby act to maintain relativelysmall grain contact areas on which stress is more effec-tively concentrated than on contacts that have been en-larged by overgrowth precipitation (Houseknecht, 1988).
Clay-induced pressure dissolution can be regarded asa self-propagating mechanism since the process leads toincreasingly shielded quartz grains that cannot serve asnucleation sites, promoting the diffusion away of thegenerated silica. Moreover the process is also associatedwith reduction of pore space and outward movement offluid from the volume that is pressolved. The process
otal 5.0 quartzeeded forementation
Shale insuccession
Detritalquartz %in shale
Total 5.0 quartz releasedfrom assuming about 20%detrital quartz dissolution
.4 0.6 60 0.1
.0 1.0 60 0.1
.4 3.2 60 0.4
.2 4.5 55 0.549.0 40 3.9
n in shales and along sandstone–shale contacts to provide all the quartz
tion of detrital quartz in the shale intervals and lamina is used in the
ementation.gamma logs.s.
157N. Molenaar et al. / Sedimentary Geology 195 (2007) 135–159
might be controlled by the rate of diffusion as dependenton the chemical potential differences and temperature aswell as tortuosity and diffusion path length.
The driving force of chemical compaction is thechemical potential through heterogeneous anisotropicloading of framework grains (Sheldon et al., 2003). Theprocess is related to effective pressure and temperature,which is correlated to diffusion rates, and also to thereaction kinetics of detrital quartz dissolution and pre-cipitation of quartz cement. However, chemical com-paction is dependent not merely on the physical andtextural conditions of the sandstone but also on the porefluid chemistry (Niemeijer et al., 2002). Diffusioncontrols chemical compaction at great burial depth asa limiting factor as evidenced by experimental work,whereas at moderate burial depth and low temperaturesthe process is controlled by reaction kinetics. Thustemperature is a crucial parameter in controlling the rateof chemical compaction (Renard et al., 1997).
The Deimena Group sandstones is characterised bychemical compaction mainly along sandstone–shaleinterfaces and within shale intercalations as well as truestylolites (cf. Park and Schot, 1968) in the deepestburied sandstones. The scale at which quartz dissolutionhas occurred suggests that clay-induced pressuredissolution might have been the sources of the bulk ofquartz cement found on quartz grains in adjacentsandstones (Fig. 7D). A general estimate of the amountof silica potentially yielded by pressure dissolutionsuggests that this would be sufficient to account for allthe quartz cement in the sandstones (Table 4). Thus, theclay-enriched fine-grained sandstone layers played therole of efficient silica exporters to neighbouring well-sorted sandstones. This proposed model explains thelarge variation in properties in sandstone depositionalsystems in relation to different degrees of quartzcementation as an integral part of facies-related features.The conclusion is that reservoir properties and diagen-esis are to a large degree controlled by facies archi-tecture, and that diagenesis acts differentially uponoriginal properties according to facies architecture.
7. Conclusions
For predicting reservoir quality in quartzose sand-stones it is necessary to understand the controls onporosity and permeability development through diagen-esis and in particular the source or sources of silica forquartz cement that in combination with mechanicalcompaction is the main factor in porosity reduction andcontrolling reservoir quality. Knowing the source andtransport mechanism of silica would also allow under-
standing of the amount and distribution of quartz cementwithin sandstone bodies and the effects on reservoirproperties.
Cambrian quartz arenites have been studied becauseof their regional occurrence over an area with a largevariation in burial depth and T, changes in sandstone/shale ratio, but deposited by similar depositional me-chanisms and with the same detrital provenance. TheCambrian sandstones are shelf-storm deposits and partlytidally influenced inner shelf deposits, implying fine-grained clayey toe-sets and clay drapes intercalatedwithin reservoir sandstone bodies, leading to differentialdissolution–precipitation.
At a local scale, no obvious trend exists betweenporosity and burial depth in the central and western partsof the basin or for single oil fields. Even within a smallarea, or within the same oil field, large variation exists inreservoir properties. This suggests local controls bysedimentary structures and the vertical and lateralsediment architecture. The petrophysical properties aremainly related to the amount of quartz cement, which iscorrelated with the depositional facies. The amount ofquartz cement increases with increasing amount ofinterlayered shale lamina in the sandstones and conse-quently with decreasing sandstone bed thickness. Thedata show that chemical compaction through pressuredissolution of detrital quartz grains along shale–sand-stone interfaces and within sandy–silty shales was acommon process likely to represent the main source ofsilica for quartz cementation.
The results of the present study indicate that sand-stones or sandstone–shale successions may be diage-netic closed systems with respect to silica and quartzcement. The correlation between shale intercalations,clay-induced pressure dissolution of detrital quartzgrains and location and amount of quartz cement ininterbedded sandstone–shale successions suggests acausal relationship. In such lithological successions,silica is generated locally and related to depositionalfacies. Diagenesis is thus constrained by facies, althoughexternally controlled by pressure and temperature. Astrong correlation between facies and final reservoirproperties and quality therefore exist. Once a system isunderstood in terms of diagenetic processes and theireffects on petrophysical properties, prediction of thedegree of quartz cementation and the reservoir quality ispossible within the frame of a facies model.
Acknowledgements
Special thanks are due to the generous financial sup-port by the Nordic Energy Research (The Petroleum
158 N. Molenaar et al. / Sedimentary Geology 195 (2007) 135–159
Technology Council), The Nordic Ministry Council, the‘Vera og Carl Johan Michaelsens fond’ and DONG'sjubilæumslegat that was essential for enabling the re-search project. We also would like to thank H. Friis andS. T. Paxton for their valuable comments.
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