sedimentologija • sedimentology •...

Jolanta Čyžienė, Nicolaas Molenaar, Saulius Šliaupa8

Čyžienė J., Molenaar N., Šliaupa S. Clay-induced pressure solution as a Si source forquartz cement in sandstones of the Cambrian Deimena Group. Geologija. Vilnius. 2006.No 53. P. 8–21. ISSN 1392-110X

The Middle Cambrian Deimena Group sandstones are the main oil reservoir in Lithuaniawith 18 oilfields discovered so far in the western part of Lithuania. The sandstones showa great variability in reservoir quality, making the reservoir exploration and the constructionof its models difficult. The porosity and permeability show a complicated variability evenwithin a single oilfield; closely spaced wells within a single oilfield may have a differentreservoir quality, whereas in the vertical sense their properties also vary greatly within thereservoir bodies. Quartz cementation is the main diagenetic process influencing and control-ling the petrophysical properties of Cambrian sandstones. The amount of quartz cement, andinversely its porosity and permeability, are only weakly correlated with the burial depth andpalaeotemperature within particular oilfields. Evidently, factors other than general physicalconditions and overall chemical conditions related to burial depth must locally have influ-enced the processes involved in quartz cementation and controlled the location and amountsof quartz cement. The main factor is the reservoir architecture. The amount of quartz cementpositively correlates with the amount of shale structures in sandstone bodies. Clay-inducedchemical compaction and pressure solution of detrital quartz at shale–sandstone contacts andwithin thin shale lamina is the main process yielding silica for local quartz cement. Thesupply of silica for quartz cement in the sandstones is thus dependent on the number of thinclay lamina and clay intercalations within the reservoir sandstones. The sandstone/shale ratioand the thickness of sandstone bodies control the location and degree of quartz cementationand thus the reservoir quality.

Key words: sandstone, quartz cementation, Cambrian, Baltic Basin, pressure solution

Received 14 November 2005, accepted 30 December 2005

J. Čyžienė, Geological Survey of Lithuania, S. Konarskio 35, 03123 Vilnius, Lithua-nia. E-mail: [email protected]

N. Molenaar. Institute of Environment & Resources, Technical University of Denmark,Building 115, DK-2800 Kgs. Lyngby, Denmark

S. Šliaupa, Institute of Geology and Geography, T. Ševčenkos 13, 03223 Vilnius,Lithuania

Sedimentologija • Sedimentology • Седиментология

Clay-induced pressure solution as a Si source forquartz cement in sandstones of the CambrianDeimena Group

Jolanta Čyžienė,

Nicolaas Molenaar,

Saulius Šliaupa

GEOLOGIJA. 2006. T. 53. P. 8–21© Lietuvos mokslų akademija, 2006© Lietuvos mokslų akademijos leidykla, 2006

INTRODUCTION

Quartz cementation has been long recognized as a maindiagenetic process in quartz-rich sandstones (e.g.,McBride, 1989), influencing and largely controlling thepetrophysical properties. Quartz cement controls to a

large degree the properties in many hydrocarbon reser-voirs such as in the North Sea (Bj¸rlykke & Egeberg,1993; Oelkers et al., 1996) and in the Palaeozoic BalticBasin (Lashkova, 1979; Vosylius, 2000). Because ofthe economic interest to predict reservoir properties forenhancing exploration, a large number of studies have

Clay-induced pressure solution as a Si source for quartz cement in sandstones of the Cambrian Deimena Group 9

dealt with the processes involved in precipitation ofquartz cement. Nevertheless, the source of silica forquartz cement is still a matter of debate. A number ofstudies concluded silica sources placed outside the re-servoir bodies to implicitly involve a mechanism for alarge-scale advective flow of diagenetic fluids. On thecontra, other studies suggest silica sources within or ata short distance from the reservoir sandstone bodies,avoiding the difficulties of large-scale transport due tothe limited silica solubility. As stated quite early (Blatt,1979), because of the low solubility of silica, largevolumes of diagenetic fluids would be needed for silicasupply from the outside. This may be attainable undernear-surface conditions where a hydraulic head can formthe mechanism for flow, but it certainly is not a realis-tic scenario under deeper subsurface conditions (Bj¸r-lykke & Egeberg, 1993).

Under burial conditions, a large-scale flow may beinvolved with faulting and ascendance of hot fluids andinduced by thrusting, although in isolated cases suchmechanisms may supply silica externally, the commonwidespread and in fact regional occurrence of quartzcement, e.g., in the Jurassic Brent group sandstones inthe North Sea (Bj¸rlykke et al., 1992) and in the Camb-rian sandstones in the Baltic Basin, the subject of thecurrent study. This contradicts ascending cooling fluidsas a common source for quartz cement and suggeststhat quartz cementation is not related to specific tecto-nic activity.

More evidence against large-scale transport is givenby the fact that formation waters are often diagenetical-ly evolved seawater that remained in place (e.g., Hogget al., 1995). Other studies concluded, however, thatmeteoric water has been involved and mixed or repla-ced the original marine pore water (e.g., Williams,1997). The latter would point to a larger-scale flow butnot necessary implies that the silica is also externallydelivered during the same phase of flow. The presenceof the original pore water strongly suggests that silicais supplied through diffusion from internal or short dis-tance sources. It seems therefore likely that quartz ce-mentation can take place regardless of the type of for-mation water present.

If silica was externally supplied, advective flow beinginvolved in silica transport, then quartz cement wouldbe expected as the most interconnected and at that timemost permeable parts of the sandstone bodies. The con-trary is often true, and the thicker sandstone units oftenhave better petrophysical properties and form hydrocar-bon reservoirs. Also, model calculations show that whensilica is transported by advection, quartz cementationwould be more pervasive in coarser-grained and morepermeable sandstones, allowing for high flow rates (Ca-nals & Meunier, 1995).

The regional as well as local distribution of quartzcement, and inversely the petrophysical properties, in-trinsically contain information about the processes in-volved in quartz cement precipitation, including silica

sources and the transport mechanism of dissolved silicatowards the place of cementation. Regional distributionpatterns may point to general threshold conditions and/or confining physical and/or chemical conditions forquartz cementation. Local quartz cement variability, ho-wever, is more likely to be linked to small-scale varia-bility in sedimentary parameters possibly associated withdepositional facies.

This will be tested by a case study of the Cambrianmarine siliciclastic sandstones in the Lithuanian part ofthe Palaeozoic Baltic Basin. The Cambrian sandstonesare widespread in Lithuania and are an example of qu-artz arenites with dominant quartz cement in the west(Lashkova, 1979; Vosylius, 2000). The petrophysical pro-perties show a rather large variability even on a localscale. The study focuses on West Lithuania where anumber of producing oilfields occur at a present dayburial depth from about 1700 to 2200 metres.

METHODS AND DATA

Because of the petroleum exploration and productionactivities, sample petrophysical data, geophysical welllogs and core material are available for research. Thenatural gamma ray logs are used to calculate the shalecontent.

Polished thin sections of about 200 samples wereused to study the texture and mineralogical composi-tion by polarized light microscopy. The components in83 sandstone samples were quantified by image analy-sis using microphotographs from backscattered electron(SEM-BS) microscopy, cathode luminescence scanningelectron (SEM-CL) and cold cathode (CL) luminescen-ce microscopy using a standard point counting and theimage ImagePro Plus analysis software. The mineralo-gical composition was confirmed by backscattered elec-tron imaging (SEM-BS) and EDX microscopy and bulkX-ray diffractometry (XRD). XRD was also used forclay mineral determinations in 62 samples (orientedmounted fraction <2 mm, non-treated, ethylene glycola-ted and stepwise heated to 350 and 500 ˚C) accordingto standard methods.

Porosity and permeability data from existing indust-rial reports, listing nearly 10,000 measurements of sand-stones from special core analyses, provided consistentinformation on the lateral changes of the reservoir pro-perties. In addition, the porosity and permeability ofthe new samples were measured. Helium porosity wasdetermined by the standard gas expansion method us-ing a HGP100 helium porosimeter. The permeabilitywas measured using a digital gas permeameter DGP200corrected for the Klinkenberg effect.

BURIAL HISTORY

Lithuania is situated on the south-eastern flank of theBaltic sedimentary basin, which is the largest tectonicfeature of the western margin of the Eastern European


Craton (Fig. 1). The sedimentary succession consists ofVendian to Quaternary sedimentary rocks resting on aEarly Proterozoic crystalline basement. The thicknessof the sedimentary fill ranges from 0.2 km in East to2.3 km in west Lithuania. Cambrian deposits representthe basal part of the sedimentary cover. They are over-lain by a carbonaceous-shaly succession of Ordovicianage. The thickness of the Cambrian ranges from a fewtens of meters in the east to 170 m in the west. Thesuccession comprises alternating beds of sandstones, silt-

stones and shales with different proportions across thebasin, which were deposited in shallow-marine and shelfenvironments in Lithuania during minor sea level fluc-tuations (Laškova, 1987; Jankauskas, 1994). The depo-sitional environment ranges from shallow-marine innershelf, which is partly tidally influenced, to the outershelf with storm-induced currents as the dominant trans-port and depositional mechanism. Fine-grained shale dra-pes and shale intercalations are common within sand-stone bodies. The sandstones are interbedded and fin-ger out distally into marine shales. Deposits on the midshelf are alternations of sandstone, and shale becomesmore dominant towards the distal, outer parts of theshelf. The sandstone/shale ratio and the thickness ofsandstone-dominated successions thus systematically de-crease towards the outer shelf.

The present day burial depth in the study area inWest Lithuania coincides with the maximum burial depthattained in the geological history (Fig. 2). The subsi-dence rates and geometry of the Baltic basin have chan-ged throughout the Phanerozoic (Poprawa et al., 1999;McCann et al., 1997). During the Cambrian and Ordo-vician, deposition took place in a passive margin set-ting with a total subsidence of 150–500 m during theCambrian and 50–250 m during the Ordovician. Thegeodynamic situation changed during the Silurian intoa converging continental margin setting with an intensesubsidence ranging from 100–200 meters in the east upto 4–5 km in the westernmost part of the Baltic basinin Central Poland and NE Germany. The magnitude ofthe Silurian subsidence in West Lithuania is 700–900m. Intense subsidence persisted during the Devonianduring which period about 1 km thick terrigenous andcalcareous sediments accumulated in the basin centre.

Fig. 1. Location of the Baltic Basin and Lithuania1 pav. Baltijos baseino ir Lietuvos paplitimo schema

Fig. 2. Burial models of Girkaliai-2 and Naumiestis-1 wells (representative for West Lithuania). The oil window and thewindow of quartz cementation are indicated on burial graphs2 pav. Girkalių-2 ir Naumiesčio-1 gręžiniams sudarytas uolienų grimzdimo modelis (būdingas Vakarų Lietuvai). Naftos gene-racija ir kvarco cementacija pavaizduoti grimzdimo grafike


The subsidence drastically decelerated during the earli-est Carboniferous giving way to uplift and erosion ofthe basin flanks during the Carboniferous – earliest Per-mian; in some places the magnitude of erosion is esti-mated to as much as 0.5–2 km. Numerous diabase in-trusions penetrated the Cambrian–Silurian succession inthe central part of the Baltic Sea. During the succee-ding Permian–Cainozoic times only central, southern andwestern parts of the basin were involved in the subsi-dence in a range of 0.1 km (east) to 1.5 km (south-west).

CAMBRIAN STRATIGRAPHY

The whole Cambrian succession, Lower-Middle Camb-rian, has a thickness ranging from 10.5 (uplifted anderoded Cambrian succession in East Lithuania) to 177metres in Southwest Lithuania with an average thicknessof 116 m (No = 154). Traditionally, the succession issubdivided into groups and formations based on welllogs (Figs. 3 and 4). The detrital composition of thesandstones is dominated by quartz (95–99%) (Laškova,1987; Kilda & Friis, 2002). Detrital grains are mostlyvery-fine to fine-grained sand-sized (on average well sor-ted), with rounded to well-rounded quartz grains. Besi-des the dominant mono- and polycrystalline quartz grains,there are also negligible amounts (less than 1%) of feld-spar, lithic grains, and heavy minerals (opaque minerals,zircon, garnet and rutile). Glauconite grains occasionallyare abundant and related to sequence boundaries. Thesandstones can be considered as mineralogically very ma-ture and are classified as quartzarenites (cf. Folk, 1964).Shales are dominantly composed of illite or interlayeredsmectite-illite with variable amounts of kaolinite, and mi-nor chlorite. The shales are silty-sandy and contain va-

riable amounts of detrital quartz and minor K-feldsparwith very fine-grained sand and silt size. The shales ha-ve a matrix-supported framework.

The Lower Cambrian succession is up to 80 m thickand consists of fine-grained light grey, rarely brown andgreenish grey sandstones, which alternate with siltstonesand shales. Siltstones and shales are grey, greenish greyand brown in colour. The lowermost Cambrian “BlueClay” formation occurs in the eastern part of Lithuania.The Middle Cambrian succession, dominated by light greysandstones with intercalated shales and siltstones, rea-ches a thickness of 70–80 m in Central and West Lithu-ania (Fig. 3). The lower part of the Middle Cambriansuccession (The Kybartai Formation, about 10 m thick)is mainly composed of siltstones and shales. The over-lying, around 60 m thick, succession of quartzose sand-stones with a number of shaly sandstones-siltstones andshale layers represents the main oil reservoir in Lithua-nia, which is defined as the Deimena Group. This isclassically subdivided into Giruliai, Ablinga, Pajūris For-mations from top to bottom successively (Fig. 4). TheKybartai Formation and Deimena Group are attributed tothe Paradoxides oelandicus trilobitic zone of the MiddleCambrian. Further west in the Polish offshore area, thesedeposits grade into deeper-marine siltstones and shales.The overlying Paneriai Formation of the Paradoxides pa-radoxissimus zone is only locally presented in a fewwells in Eastern Lithuania, constituting of small isolatedresidues of sandstones and siltstones several meters thick.It is widely distributed in the east on the western flankof the Moscow palaeobasin (up to 20–30 m thick) andthe western periphery of the Polish Basin. Upper Camb-rian deposits are absent in Lithuania; sedimentation wasconfined to the westernmost part of the Baltic basin du-ring that time (Jankauskas & Lendzion, 1992).

RESULTS

Cambrian lithofaciesThe shale content and the sandstone bed thickness aretwo main parameters in distinguishing depositional fa-

Fig. 3. The West Lithuanian Cambrian succession subdividedinto groups and formations based on well logs (natural gam-ma radiation)3 pav. Vakarų Lietuvos kambro pjūvio koreliacija pagalgeofizinių tyrimų gręžiniuose kreives (gama metodas)

Fig. 4. Stratigraphic scheme of the Lithuanian Cambrian4 pav. Lietuvos kambro stratigrafinė schema


cies in the Cambrian succession as relevant for unders-tanding the reservoir properties. Based on core descrip-tions and well logs, in particular the natural gamma raylogs, a number of lithofacies with a different sandsto-ne–shale ratio can be distinguished in the Lower andMiddle Cambrian succession: sand-dominated, interme-diate heterolithic and shale-dominated lithofacies are dis-tinguished (Table 1). The further subdivision in the sand-stone subfacies was based on sandstone bed thickness.The sandstones contain variable amounts of intercalatedshale, whereas bed thickness and size of cross-beddingincrease with sandstone content. The shales, on the ot-her hand, contain variable amounts of thin or lenticu-lar-bedded sandstones and siltstones.

Reservoir bodiesThe reservoir is built from two up to four sandstone-dominated bodies (individual sandstone bodies rangingfrom around 10 to 35 metres in thickness) separated byshales or shaly sandstones. The reservoir is heterogene-

ous because of the vertical and lateral variability inporosity and permeability within the sandstone bodiesas a consequence of differential quartz cementation. Theeffective thickness of the reservoir is reduced by hea-vily quartz-cemented, low-porosity sandstone layers. Theeffective thickness shows a considerable variation, whichis largely caused by drastic changes in the content ofquartz cement, which is increasingly important in thewest. Individual layers and bodies are sheet-like or len-ticular. The thickness of the sandstone bodies partly ismodified by amalgamation, and the facies are influen-ced by the local synsedimentary tectonic activity duringthe Middle Cambrian, and the sandstone is thicker andwith less shale intercalations in the central domal partsof the reservoirs and uplifted structures in general (e.g.,the Girkaliai structure).

The detrital composition is homogeneous quartzareni-te, but the grain-size distribution and thus the initial tex-ture of the sandstones is variable. Even more importantis that the lithological succession is variable both in the

Table 1. Summary of main characteristics of the lithofacies distinguished in West Lithuania1 lentelë. Vakarø Lietuvos litofacijø apibûdinimas

Facies Subfacies Lithology Sedimentary Bed Facies Sandstone Depositionalstructures thickness thickness percentage environment

S S1 Massive Horizontal or Metres 5–20 m >95 Inner shelf;sandstone low angle amalgamated(quartz cross-lamination storm and stormarenite) surge deposits

S2 Thick- Wave ripples. Decimetre- 2–8 m 88–95 Inner shelf;bedded Small-scale metre storm depositssandstone hummockywith few crossshale lenses stratificationor laminaat the top

S3 Thin-bedded Horizontal or Decimetre- 5–10 m 80–88 More distal innersandstones cross-bedded centimetres, shelf: intermediatewith shale lamination. 0.10–10 mm distal stormlamina Rare shale deposits and super-

bioturbation imposed wavecurrent activity

H Thin inter- Lenticular- Centimetres 5–15 m 50–80 Mid shelf: distalbedded sand- wavy storm deposits andstones and bedding, wave activity andshales horizontal biological activity

lamination,wave andcurrent ripples:up to stronglybioturbatedsome

SH Shales with Horizontal Millimetrs- 1.5–5 m – <50 Outer shelf, belowsandstone lamination to centimetres 20 m normal storm-wavelaminas and lenticular base; low energylenses bedding; and intense

intense biological activitybioturbation


vertical and the lateral sense. Sandstone bed thicknessand the content of shale show a wide range (Fig. 5).

Sandstone diagenesisPore reduction by mechanical compaction, as evidentfrom the reduced IGV values (Table 2) and quartz ce-ment are the main controls of the petrophysical proper-ties of the Cambrian sandstones.

The intergranular volumes (IGV) defined accordingto Paxton et al. (2002), i.e. is corrected for secondarypores, point to the importance of mechanical compac-tion in reducing porosity and causing lithification. Com-paction comprised the mechanical rearrangement ofgrains throughout the sandstones as well as the chemi-cal compaction located along shale-sandstone contactsand within the shales. Grain breakage is rare, and nointergranular pressure solution in clay-free clean sand-stones has been observed. In the sandstones, detritalquartz grains mainly have point contacts. The variationin IGV reflects differences in the degree of mechanicalcompaction. This is probably related to both maximumburial depth and variations in the depositional textureand susceptibility of the sand to mechanical compac-tion.

Quartz, in the form of authigenic overgrowths ondetrital quartz grains, is the main cement mineral (Laško-va, 1979; Kilda & Friis, 2002). Quartz cement is regio-nally widespread, but mainly confined to areas wherepresent day temperatures in the Cambrian are over 50to 90 degrees. The amount of quartz cement increasestowards the deeper buried parts of the basin in WestLithuania, but is highly variable on a local scale andeven within individual oilfields. Quartz cement contentsshow a negative correlation with porosity (Fig. 6). Thenumerical values were obtained by point-counting of1000–2000 points on digital SEM-CL micrographs (ave-rage values from several micrographs of a single sam-ple). The quartz cement values range from 9 to 33%(average 22%). The spread in values can be explainedby variation in intergranular volume (corrected for se-condary porosity) (Table 2). Since much more porositydata from special core analyses are available than qu-artz cement percentages from petrographic analysis, po-

0 10 20 30 40 50 60 70 80 90 100

Shale Content (%)

0

0.01

0.02

0.03

0.04

Rel

ativ

e F

requ

ency

N aum iestis w e lls

Fig. 5. Histogram showing shale content in Lower and Mid-dle Cambrian derived from natural gamma radiation logs forNaumiestis wells (West Lithuania)5 pav. Naumiesčio gręžinių apatinio ir vidurinio kambro uo-lienų molingumas, apskaičiuotas pagal gama metodo kreives

0 5 10 15 20

Porosity (%)

5

10

15

20

25

30

35

Qu

artz

cem

ent

(%)

N =83

Fig. 6. Graph showing a statistically significant negative cor-relation between quartz cement and porosity6 pav. Kvarcinio cemento ir poringumo reikšmių neigiamakoreliacija

Table 2. Average sandstone composition based on thin section, SEM-CL and SEM-BS image quantification2 lentelė. Bendra smiltainio sudėtis, pagrįsta šlifų, SEM-CL ir SEM-BS nuotraukų analize

Detrital Quartz (%) Quartz cement (%) Porosity (%) IGV (%)*

Average 70.60 22.62 6.31 26.54Standard deviation 4.35 5.81 4.53 4.10Minimum 60.87 8.59 0.85 17.14Maximum 82.87 33.64 26.67 39.07N 85 85 85 85

* The IGV values are corrected for secondary porosity.* IGV reikšmės koreguotos įvertinant antrinį poringumą.


rosity will be used as an inverse proxy to quartz ce-ment content.

Besides the prevailing quartz cement, some Fe-dolo-mite cement, pyrite and authigenic illite are present.Authigenic illite occurs adjacent to a shale lamina. Fe-dolomite cement occurs as dispersed patches (<1%), withno effect on porosity or permeability. In the eastern,marginal parts of the basin the sandstones are weaklylithified with Fe-dolomite or calcite and minor quartzcement.

Elongate and angular grains with sutured grain con-tacts are common in the shales (Fig. 7) and along sha-le–sandstone contacts (Fig. 8), and point to an extensi-ve pressure solution of detrital grains. Also, at mica-rich laminas, dissolution of quartz grains took place.The micas are mainly muscovite and biotite. These phyl-losilicates are aligned in preferred orientation or ran-domly oriented due to bioturbation. The micas appearto be unaffected by dissolution, their shape being vir-tually intact. Dissolution has produced sutured bounda-ries in the case of clay–quartz contacts (Fig. 9) andstraight boundaries at mica–quartz contacts.

Fig. 7. SEM-CL and BSE micrographs of thin sections, sho-wing pressure solution of quartz grains. Well location Lašai-3, depth 2092 m7 pav. SEM-CL ir BSE šlifų nuotraukose – kvarco grūdeliųtirpdinimas dėl slėgio. Lašų-3 gręžinys, gylis 2092 m

Fig. 8. Micrographs of thin section showing solution of det-rital quartz grain along clay- and mica-rich lamina (top –plane polarised light; bottom – idem with crossed polariza-tion filters). Well location Vabalai-1, depth 2105 m8 pav. Šlifo nuotraukose – detritinio kvarco grūdelių tirpdi-nimas išilgai molio ir žėručiu praturtinto tarpsluoksnio (vaiz-das poliarizacinėje šviesoje ir su poliarizacijos filtrais). Vaba-lų-1 gręžinys, gylis 2105 m

0.46 mm

0.46 mm

0.46 mm

Fig. 9. Micrograph of thin section showing quartz dissolutionat shale–sandstone contact surface and the formation of sutu-red boundaries (plane polarized light and with crossed pola-rization filters). Well location Šilgaliai-1, depth 2067.6 m.9 pav. Šlifų nuotraukose – kvarco tirpdinimas argilito ir smil-tainio kontakto paviršiuje bei dantytų ribų formavimasis (vaiz-das poliarizacinėje šviesoje ir su poliarizacijos filtrais). Šilga-lių-1 gręžinys, gylis 2067,6 m


The intensity and extent of dissolution left only frag-ments of detrital quartz grains (Fig. 10). The solubilityof quartz at mica–quartz and clay–quartz interfaces ad-joining stylolites appears to be more enhanced (Fig. 7)than at quartz–quartz grain contacts. The latter are pointcontacts indicating that intergranular pressure solutionwas absent. It appears that clay or phyllosilicate indu-ced pressure solution of detrital quartz took place be-fore the true penetrative stylolititization or intergranularpressure solution could start. The volume of dissolveddetrital quartz in West Lithuania is estimated in therange of 20–25%, in some places even larger.

Detailed analyses of facies and petrophysical pro-perties show that sandstone intervals containing a thinshale lamina are more intensely quartz-cemented andhave a lower porosity than thick sandstone bodies with-out shale intercalations (Fig. 11). Lithofacies changes

are minor within particular structures. However, evensmall synsedimentary variations have a considerable im-pact on the rate of quartz cementation.

Petrophysical propertiesIn the study area, the top of the Cambrian successionoccurs at a depth of 1700 up to 2230 metres. Its pet-rophysical properties show a rather large range in va-lues (Table 3). The porosity of the sandstones varies

0.46 mm

0.46 mm

Fig. 10. Micrographs of thin section, showing dissolution ofquartz in clay-enriched layers. Quartz grains are largely dis-solved with only small detrital grain fragments left (top –plane polarised light; bottom – idem with crossed polariza-tion filters). Well location Pašaltuonis-94, depth 1646 m10 pav. Pašaltuonio-94 gręžinio šlifo nuotraukos, gylis1646 m. Kvarco grūdelių tirpdinimas moliu praturtintuosesluoksniuose; grūdai stipriai aptirpę, likę nedideli detritiniųgrūdų fragmentai (vaizdas poliarizacinėje šviesoje ir su polia-rizacijos filtrais)

Shale content (% )

Neu

tro

n G

amm

a lo

g (

AP

I)Fig. 11. Neutron gamma log versus shale content (from na-tural gamma logs). Graphs of dry well Girkaliai-411 pav. Neutrono gama metodo kreivės ir molingumo (iš ga-ma metodo kreivės) priklausomybė Girkalių-4 gręžinyje

0 5 10 15 20 25Porosity (% )

2250

2200

2150

2100

2050

2000

1950

1900

1850

1800

1750

1700

Dep

th (

m)

W ells in W est L ithuaniaN =6625

Fig. 12. Porosity versus depth for selected oilfields in WestLithuania12 pav. Poringumo ir gylio priklausomybė pasirinktuose Va-karų Lietuvos naftos telkiniuose


from 0.01 to 24.29% with an average of 7.47%. Theaverage is close enough to the porosity of 9.2% predic-ted using existing models such as by Scherer (1987)(corrected for cement). The horizontal permeability ran-ges from 0.001 to 2152 around an average of 51 mD.The main properties show a wide scatter, and only aweak correlation with burial depth or temperature ex-ists. The maximum porosity and permeability decreasewith increasing depth or temperature (Fig. 12).

Towards the east, with decreasing burial depth, theamount of quartz cement decreases drastically, and theporosity is up to 20–25% with a permeability of 100–2300 mD in the shallow eastern part of the basin (Vo-sylius, 1998 and 2000). All stratigraphic levels showthe same pattern (Vosylius, 2000).

Also on a local scale, the main petrophysical pro-perties, such as porosity and permeability, have a com-plicated variability, even within a single oilfield, ma-king the exploration of the reservoir and constructionof its models difficult. Closely spaced wells within asingle oilfield may have different reservoir quality (Fig.13).

Although the succession is relatively simple, com-posed of sheet-like sandstone beds and amalgamatedbodies separated by shales and shaly sandstones, andindividual oilfields can be described as layer-cake re-servoirs (Weber and Geuns, 1990), there is a distinctvertical and lateral variability in petrophysical proper-ties and lithology, namely the sandstone/shale ratio. Inparticular, the properties vary greatly in the vertical sensewithin the reservoir succession in a particular field, butalso within individual sandstone bodies building up thereservoir. However, the properties are clearly related toshale content and thus to depositional facies (Fig. 10,Table 4), and this must be explained in terms of theprocesses involved and the controls on quartz cementa-tion. The shale content is related to porosity as shownby the increasing porosity from neutron gamma logswith increasing natural gamma ray readings. The poro-sity decreases with increasing shale content (derivedfrom natural gamma ray logs) (Fig. 14).

DISCUSSION

Any model for quartz cementation must be able to ex-plain a set of general observations: the onset of quartz

Table 3. Average values, standard deviation and range of main petrophysical properties for all wells with the Cambriansuccession below the depth of 1700 m (West Lithuania)3 lentelė. Kambro uolienų, slūgsančių giliau nei 1700 m, kolektorinių savybių statistikos rodikliai (Vakarų Lietuva)

Porosity (%) Horizontal permeability Vertical permeability Grain density(mD) (mD) (kg/m3)

Average 7.47 51.499 40.833 2412Standard deviation 4.15 144.581 113.434 101Minimum 0.01 0.001 0.001 2121Maximum 24.29 2153 1871 2702Number 6658 5628 4936 6580

0 5 10 15 20

Porosity (% )

0 .00 1

0 .01

0 .1

1

10

100

100 0

100 00

Ho

rizo

nta

l Per

mea

bili

ty (

mD

)

G encia i O ilfie ld

0 5 10 15 20

Porosity (% )

0.01

0.1

1

10

100

1000

Ho

rizo

nta

l Per

mea

bili

ty (

mD

)

Kre tinga O ilfie ld

Fig. 13. Example from Kretinga and Genčiai oilfields sho-wing that porosity values vary even among closely spacedwells within single oilfields or dry fields. Symbols show dif-ferent wells within single oilfield13 pav. Poringumo reikšmių kaita Kretingos ir Genčių naftostelkiniuose. Simboliais nurodyti skirtingi gręžinių numeriai naf-tos telkinyje


cementation at certain advanced temperatures, the inc-rease in the degree of quartz cementation with T (orburial depth), the regional occurrence and the localsmall-scale variability in quartz cement content.

Different temperatures have been noted in variousstudies for the onset and main phase of quartz cemen-tation, mainly using temperatures based on studies offluid inclusions in quartz cement (Walderhaug, 1994).The temperature seems to vary from basin to basin andwithin basins. Temperature, which is merely a functionof burial depth and varies according to a local geother-mal gradient and tectonic setting, therefore in itself may

not be that important. Instead, it implies that other va-riables related to burial depth such as pressure or ef-fective pressure, a more independent variable, are moreimportant than temperature. Effective pressure varies alsowith the texture of the sandstone and the general archi-tecture of the succession and overpressured conditions.This may explain regional variations in burial depthand the onset of quartz cementation, but still cannotexplain the observed local variation in the amount ofquartz cement.

The general trends of increasing quartz cement con-tent with increasing the burial depth as in the North

Fig. 14. Graphs of well Genčiai-2 showing porosity decrease with increasing shale volume (shale volume from natural gammalog recalculated for sandstone facies). Intervals containing thin shale laminas in the sandstones are more intensely quartz-cemented than thick sandstone bodies without shale intercalations14 pav. Poringumo ir molingumo priklausomybė Genčių-2 gręžinyje: didėjant molingumui poringumas mažėja (smiltainio facijųmolingumo reikšmės apskaičiuotos iš gama metodo kreivės). Smiltainio intervalai su plonais molio tarpsluoksniais stipriausucementuoti kvarciniu cementu nei masyvus smiltainis be molio tarpsluoksnių

Table 4. Property ranges and variations for the main sandstone lithofacies4 lentelė. Smiltainio litofacijų kolektorinės savybės

Sandstone Porosity (%) Vertical Horizontallithofacies permeability (mD) permeability (mD)

S1 Average 7.37 139.112 53.783Standard Deviation 5.61 254.682 120.987Minumum 0.66 0.015 0.001Maximum 26.69 873.046 672.932N 49 10 35

S2 + S3 Average 4.75 2.004 0.632Standard Deviation 1.98 3.720 2.048Minumum 2.50 0.015 0.018Maximum 10.24 8.630 8.000N 16 5 15


Sea (Ramm, 1992) and Baltic Basin (Cyziene et al.,2001; Šliaupa et al., 2004) suggests that an increase oftemperature and the associated reaction rates or of (ef-fective) pressure, i.e. physical conditions related to bu-rial depth, are involved in the rate of quartz cementa-tion. In the Cambrian sandstones, more distinct is thevariation of porosity values found at any depth range.It is evident that temperature variations cannot causesuch small-scale variability, indicating that factors otherthan general physical and chemical conditions relatedto burial depth, including temperature, effective pressu-re and pore water chemistry, are also involved inquartz cementation.

The regional and local distribution and amount ofquartz cement, and inversely its porosity, point to ge-neral confining physical and/or chemical conditions orthresholds for quartz cementation. Of importance is thefact that although quartz cement is regionally wide-spread, it is only a dominant feature in areas wherepresent-day temperatures in the Cambrian are over 50to 90 degrees. This indicates that conditions defined bytemperature or depth, both showing a positive and sig-nificant correlation in Lithuania, form a threshold toquartz cementation. However, this cannot explain thelocal variation, which is even more typical and evidentthan the general trend. The local quartz cement varia-bility, however, is more likely to be linked to a small-scale variability in sedimentary parameters probably as-sociated with depositional facies. These local factorsare, however, a dominant control of the distributionand quantity of quartz cement.

Contradictory results exist about the various mecha-nisms involved in quartz cementation, comprising silicagenerating processes, transport mechanism, and precipi-tation. Transport mechanism and silica source are inter-dependent processes and need to be fully understoodfor predicting the amount of quartz cement in potentialreservoir sandstones. At least the physical constraintsneed to be known for translating standard available da-ta during exploration of production planning into reser-voir models.

External sources of silica implicitly need a flow ofdiagenetic fluids and transport mechanisms that are dif-ficult to explain. Silica from internal or adjacent sour-ces may be transported to the precipitation site by me-re diffusion, only needing a gradient in the chemicalpotential (Sheldon et al., 2003). Diffusion of silicaavoids the difficulties of large-scale transport, and po-tentially opens a wider diagenetic window during bu-rial history solely related to the existence of chemicalgradients. In fact, only local supply of silica can ex-plain the regional occurrence of quartz cement and thefact that thinner sandstone layers or intervals with alower hydraulic conductivity are most intensely quartz-cemented.

Pressure solution has been recognized as a potentialinternal source of silica. This process usually takes pla-ce in deeply buried sandstones in the form of intergra-

nular solution, stylolitization, and through clay-inducedpressure solution but in less deep burial conditions.

Grain to grain pressure solution will be dependentnot only on effective stress but also on detrital textureand detrital composition, the variability of which maycause local changes related to facies, and this processcould be active within a range of burial depths. Ingeneral, IGV of sandstones do not correlate positivelywith quartz cement as would be expected in case ofintergranular pressure solution generating silica forquartz cement (Paxton et al., 2002), suggesting othermain sources for silica. IGV in fact negatively correla-tes with quartz cement in Cambrian sandstones, sug-gesting that quartz cement started at different times,which is confirmed by burial history modelling (Fig.2), and halted the further mechanical compaction. Thefact that with decreasing IGV less quartz cement isfound in the Deimena sandstone points against interg-ranular pressure solution as a source of silica.

Stylolites have been described in sandstones includingquartzose arenites and quartzarenites, but seem to be re-stricted to a relative deep burial. On the other hand, thepossibility of clay promoting process of pressure solutionhas been studied by a number of researchers. Penetrativestylolites have been suggested by Bj¸ rkum (1996) andWalderhaug (1996) as a silica source. True stylolites arepresent but only in the deepest wells. In fact, most ofthese stylolites concentrate along a thin shale lamina. Dis-solutional features of detrital grains commonly occur alongshale–sandstone contacts and within shales with detritalquartz grains show partial dissolution.

Presence of clay has been long recognized as pro-moting pressure solution (Heald, 1956; Thomson, 1959;Weyl, 1959; Saibley & Blatt, 1976). Heald (1956) sug-gested that clay simply acts as a catalyst. Weyl (1959)advanced the idea about solution favoured by a greaterdiffusion through clay between grains, which has beenconfirmed by later studies (Renard et. al., 1997; Re-nard & Ortoleva, 1997). The thickness of water filmsat grain contacts and the pressure play a crucial role inthe process (Renard et al, 2000). The pH may play arole since it determines the surface charge of mineralsand the thickness of water films. At low temperaturesthe reaction kinetics is slow and limits the process,whereas at increased burial depths and increased pres-sure, diffusion is a limiting factor because of thinningof water films, and finally at a burial depth of morethan 3 to 4 km the process is expected to cease (Re-nard et al., 1997; Renard & Ortoleva, 1997; Alcantaret al., 2003).

Pressure solution could generate silica, but its oc-currence, its onset and final effects are not only depen-dent on the effective stress but evidently are also ex-pected to be related to the depositional facies, withclay-induced pressure solution more prominent in sand-stones with interbedded or interlaminated shales. In dis-crete pressure solution, long penetrative stylolites appe-ar active at a later stage during a deeper burial.


The scale at which quartz dissolution has occurred atlithological interfaces and within sandy-silty shales sug-gest that the clay-induced pressure solution with its cha-racteristic sutured boundaries might have been a sourceof the bulk of silica cement found on quartz grains insandstones. Thus, the silty-sandy shale layers were effi-cient silica exporters to the neighbouring well-sorted sand-stones, the clay minerals enhancing quartz dissolutionand inhibiting local quartz precipitation. The water-filmdiffusion (WFD) mechanism is likely the main operativein the clay-enriched portion of sandstone. Clay mineralsare capable of promoting mineral dissolution through wa-ter film diffusion because of the thick water film thatcan be preserved at their surfaces among grain contacts(Renard & Ortoleva, 1997; Renard et al, 2000). This iscoupled to the large surface area and the structural elec-tric charges spread on the surface of clay minerals, whichstabilize the water film and allow the solutes to diffuseeasily to adjacent pores.

Without giving evidence, Füchtbauer (1979) sugges-ted that cementation of sandstones adjacent to shales ismore intense. Some evidence has been presented laterto support this by Sullivan & McBride (1991). How-ever, they mainly showed the distribution of chloritecement to be related to the distance to shales. Besidegrain pressure solution in shales, other processes suchas illitization of smectites, organic matter maturationand associated organic acids from the shales could playa role in sandstone cementation. Export of silica toadjacent sandstones is probably a function of shale thick-ness (Wintsch & Kvale, 1994) and thus of the architec-ture of the siliciclastic succession.

Evidence from the Cambrian shows clay-inducedpressure solution of detrital quartz at shale–sandstonecontacts and within thin shale lamina to be the mainprocess locally yielding silica for quartz cementationbut operating on a regional scale. The supply of silicafor quartz cement in the sandstones is thus dependenton the number of thin clay lamina and clay intercala-tions within the reservoir sandstones. The sandstone/shale ratio and the thickness of sandstone bodies con-trol the location and degree of quartz cementation andthus the reservoir quality.

Pressure solution is determined by temperature butmainly by effective pressure and sediment texture. Va-riation in the latter and the occurrence of overpressuredzones in many basins may explain the observed generalvariation in temperatures of quartz cementation shownby oxygen isotope studies and fluid inclusion studiesof quartz cement (e.g., Walderhaug, 1994; Haszeldineet al., 2000).

CONCLUSIONS

The Cambrian marine sandstone bodies are complexsedimentary successions composed of sandstones andshales with variable lithological architecture. Clay-indu-ced chemical compaction and pressure solution of det-

rital quartz at shale–sandstone contacts and within thinshale lamina was the main process yielding silica forlocal quartz cement. The exact nature of the sandsto-ne–shale alternation controls the potential amount ofquartz cement generated through clay-induced pressuresolution. The general physical conditions, mainly interms of effective pressure, constrain the onset of thepressure solution. The sandstone/shale ratio and the thic-kness of sandstone bodies locally control the degree ofquartz cementation and thus the reservoir quality.

ACKNOWLEDGMENTS

Special thanks for financial support are due to the Nor-dic Energy Research (The Petroleum Technology Coun-cil), Lithuanian State Science and Studies Foundation.

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Jolanta Čyžienė, Nicolaas Molenaar, Saulius Šliaupa

MOLIO ĮTAKA KAMBRO DEIMENOS SERIJOSSMILTAINIŲ KVARCO CEMENTACIJAI

S a n t r a u k aVakarų Lietuvos kambro uolienas sudaro kvarcinis smiltainis,molis ir aleurolitas. Vidurinio kambro Deimenos serijos smiltai-niuose yra surasta 18 naftos telkinių su labai sudėtingu kolek-torinių savybių pasiskirstymu, apsunkinančiu angliavandenilių pa-iešką ir gavybą. Regeneracinis kvarcinis cementas, pasižymintis


atvirkščia poringumo ir skvarbumo koreliacija, turi didžiausią įta-ką smiltainių kolektorinėms savybėms. Autigeninio kvarco kie-kis Vakarų Lietuvos Deimenos serijos smiltainiuose kinta nuo 18iki 33%. Regioniniu mastu apkvarcėjimo procesus nulemia slė-gio ir temperatūros pokyčiai, tačiau šie parametrai negali paaiš-kinti labai kaitaus cementacijos laipsnio atskiruose gręžiniuose irlokaliuose plotuose, kur autigeninio kvarco kiekis kinta nuo 28–33% iki 8–12% (poringumas atitinkamai kinta nuo 3–6% iki 12–14%). Detalūs petrografiniai tyrimai rodo, kad sukvarcėjimo in-tensyvumas tiesiogiai priklauso nuo kolektoriaus sinsedimentaci-nių ypatumų. Smiltainio intervaluose, kuriuose yra daugiau mo-lingų sluoksnelių ir būdinga smulkiai horizontaliai sluoksniuotatekstūra, sukvarcėjimo intensyvumas yra didesnis nei masyviamesmiltinyje. Detritinių kvarco grūdų tirpimas (cheminis sutankė-jimas) smiltainio ir molio kontaktuose yra pagrindinis silicio šal-tinis. Molingų sluoksnelių ir smiltainio kontaktuose formuojasistilolitai, kurie yra silicio šaltiniai ir turi įtakos cementacijos pro-cesams. Petrografinių tyrimų duomenimis, būtent čia vyksta in-tensyvus kvarco grūdelių tirpimas. Šis sąryšis patvirtina vietinįsilicio šaltinį kambro smiltainiuose, o difuzija čia yra pagrindi-nis procesas perskirstant ištirpusį silicį gretimuose smiltainiuose.Taigi formuojant kambro kolektorių modelius, pagrindinis dėme-sys turi būti skiriamas sinsedimentacinių uolienų ypatumų ana-lizei (smiltainio ir molio santykiui, smiltainio storiui, litofaci-joms).

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ÐÀÑÒÂÎÐÅÍÈÅ, ÑÂßÇÀÍÍÎÅ Ñ ÃËÈÍÈÑÒÛÌÂÅÙÅÑÒÂÎÌ, ÊÀÊ ÈÑÒÎ×ÍÈÊÊÐÅÌÍÅÇ¨ÌÀ ÄËß ÊÂÀÐÖÅÂÎÉÖÅÌÅÍÒÀÖÈÈ ÊÅÌÁÐÈÉÑÊÈÕÏÅÑ×ÀÍÈÊÎÂ ÄÅÉÌßÍÑÊÎÉ ÑÅÐÈÈ

Ð å ç þ ì åÊåìáðèéñêèå îòëîæåíèÿ Çàïàäíîé Ëèòâû ñëîæåíûêâàðöåâûìè ïåñ÷àíèêàìè, àðãèëëèòàìè èàëåâðîëèòàìè. Â ïåñ÷àíèêàõ ñðåäíåãî êåìáðèÿÄåéìÿíñêîé ñåðèè íàéäåíî 18 íåôòÿíûõìåñòîðîæäåíèé, êîòîðûå îòëè÷àþòñÿ

êîëëåêòîðñêèìè ñâîéñòâàìè. Ðàçíîîáðàçèå ýòèõñâîéñòâ îñëîæíÿeò ïîèñê è äîáû÷ó íåôòè.Ðåãåíåðàöèîííûé êâàðöåâûé öåìåíò, êîòîðûé èìååòîáðàòíóþ êîððåëÿöèþ ñ ïîðèñòîñòüþ èïðîíèöàåìîñòüþ, îêàçûâàåò áîëüøîå âëèÿíèå íàêîëëåêòîðñêèå ñâîéñòâà ïåñ÷àíèêà. Êîëè÷åñòâîêâàðöåâîãî öåìåíòà â ïåñ÷àíèêàõ Äåéìÿíñêîéñåðèè Çàïàäíîé Ëèòâû ìåíÿåòñÿ ñ 18 äî 33%. Âðåãèîíàëüíîì ïëàíå ïðîöåññ êâàðöåâàíèÿ çàâèñèòîò äàâëåíèÿ è òåìïåðàòóðû. Îäíàêî ýòè ïàðàìåòðûíå ìîãóò îáúÿñíèòü ðàçíóþ ñòåïåíü öåìåíòàöèè âîòäåëüíûõ ñêâàæèíàõ è íà ëîêàëüíûõ ïëîùàäÿõ, âêîòîðûõ ñîäåðæàíèå êâàðöåâîãî öåìåíòà ìåíÿåòñÿ ñ28–33 äî 8–12% (ïîðèñòîñòü ñîîòâåòñòâåííîìåíÿåòñÿ ñ 3–6 äî 12–14%). Äåòàëüíûåïåòðîãðàôè÷åñêèå èññëåäîâàíèÿ ïîêàçàëè, ÷òîèíòåíñèâíîñòü êâàðöåâàíèÿ ïðÿìî çàâèñèò îòñèíñåäèìåíòàöèîííûõ îñîáåííîñòåé êîëëåêòîðîâ. Âèíòåðâàëàõ ïåñ÷àíèêîâ ñ ïîâûøåííûì ñîäåðæàíèåìãëèíèñòûõ ïðîñëîåâ, äëÿ êîòîðûõ õàðàêòåðíàìåëêàÿ ãîðèçîíòàëüíàÿ ñëîèñòîñòü, èíòåíñèâíîñòüîêâàðöåâàíèÿ çíà÷èòåëüíî áîëüøå, ÷åì â ìàññèâíûõïåñ÷àíèêàõ. Ðàñòâîðåíèå äåòðèòîâûõ çåðåí êâàðöà(õèìè÷åñêîå óïëîòíåíèå), ïðîèñõîäÿùåå ïðèêîíòàêòå ïåñ÷àíèê–ãëèíà, ÿâëÿåòñÿ îñíîâíûìèñòî÷íèêîì êðåìíåçåìà. Ïðè êîíòàêòàõ ãëèíèñòûõïðîñëîåâ è ïåñ÷àíèêîâ îáðàçóþòñÿ ñòèëîëèòû,êîòîðûå òàêæå ÿâëÿþòñÿ èñòî÷íèêàìè êðåìíåçåìà èâëèÿþò íà ïðîöåññ öåìåíòàöèè. Ïî äàííûìïåòðîãðàôè÷åñêèõ èññëåäîâàíèé èìåííî çäåñüïðîèñõîäèò èíòåíñèâíîå ðàñòâîðåíèå êâàðöåâûõçåðåí. Ñâÿçü ýòèõ ïðîöåññîâ ïîäòâåðæäàåò ìåñòíûéèñòî÷íèê êðåìíåçåìà â êåìáðèéñêèõ ïåñ÷àíèêàõ, àäèôôóçèÿ çäåñü ÿâëÿåòñÿ îñíîâíûì ïðîöåññîìïåðåíîñà ðàñòâîðåííîãî êðåìíåçåìà â áëèçëåæàùèåïåñ÷àíèêè. Àâòîðû ïðèøëè ê âûâîäó, ÷òî ïðèôîðìèðîâàíèè ìîäåëè êåìáðèéñêèõ êîëëåêòîðîâîñíîâíîå âíèìàíèå äîëæíî áûòü óäåëåíîñèíñåäèìåíòàöèîííîìó àíàëèçó îñîáåííîñòåé ïîðîä(ñîîòíîøåíèå ïåñ÷àíèêà–ãëèíû, ìîùíîñòüïåñ÷àíèêîâ, ëèòîôàöèè).

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