GEOL. C ROAT. 49/2 3 I I - 3161 2 Figs. 1 12 Pis. ZAG REB 1996 1 

Sc;ellrific nore 

Porosity, Cementation, Diagenesis and Their Influence on the 
Productive Capability of Sandstone Reservoirs in 

the Sa va Depression (Croatia) 

PROCEEDINGS 

.l asnH TADE.J. Zeljka MARIC-DUREKOVIC anci Renala SLAVKOVIC 

Key words: Secondary porosity, Cementati on, Diage­
nes is, Rese rvoir quality, Petrography, Sed imen to­
logy. 

Abstract 
The sandstone rcsclVoirs in the Sava depress ion lost their prima­

ry porosi ty through compaction and ccmCI1f;:uion in the carly stage of 
diagenesis. Disso lution events enlarged porosity and created reser­
voi rs Ih:\( were composed mainly of secondary porosity. but some 
zones of these reservoirs were destroyed by prec ipitat ion of late Fe­
ri ch c'l lc ilC and dolomite cement. Three groups o f sandstones differ­
ing in reservoir quality arc ide ntified - hi ghly poro us litharcnitcs, 
lilharc nil CS wi th int crmed iat e porosi ty, and wcakly porO ll S, hi ghl y 
ccmcntcd lit harcnitcs. The type of porosiTY depends on the clay con­
tent in sand stones, whi ch is the resu lt of water energy and deposilio­
llill ellv ironment. 

1. I 'TRODUCTION 

Regional influences on reservoir qua lity inc lude 
depos iti onal e nviron ment, initial composition o r Ille 
sand and associated muds, texture, lime, subsidence 
rate, pressure, thermal gradient , pore fluid compositi on 
and diagenetic hi story (LO UCKS et al.. 1985). 

'file main purpose of this study is to investiga te dia­
genetic processes and their influence on reservoir prop­
erti es or o il-gas fi e ld s in the Sava depression (Fi g. 1). 
The similariti es in diagenetic and reservo ir properties 
characterizing these rocks permit the development or a 
diagenetic model that is of general applicability. 

In the cored intervals of all fields, oil -bearing sand­
stones and \ve ll cemented sandstone zones were noticed 
(Fig. 2). These dirrerences were st udied in detail. 

2. GEOLOGICAL SETTING 

The Sava depression is located some 40 km south­
east from Zagreb (Fi g. I) and consists of a few oil-gas 
field s where hydroca rbons accumulated lt1 upper 
Miocene sandstonc-marlstone sequences. 

INA -Nart'lpJi n. Laboratory Research Dept., Lovinciceva I , HR -
10000 Zagrcb, Cro;Hia. 

Kljucne rijeci: sckundarni porozitet, Cemet1lacija, dija­
geneza, kvaliteta rezervoara, pelrogra fija , sedime­
ntologija. 

Sazetak 
VcCi na primarnog porozile ta pjescanih rC.lcrvoara Savske <lepre­

s ijc nCS lal a je zbog kompakc ijc i ccmen l<lcijc II ranom sladiju dija­
ge nc7.e. Naknadni proccsi otapanja doveli SlL do s ivaranja sekun ­
darnog porozilcla, ali nckc zone promatranih rczcrvoara su uni s tcnc 
prccipilacijom kasnodijagenclskog fcro- kalcilnog i clolomitnog 
ce mcn ta. S obz irorn na svojstv a reze rvoara izdvojcnc Sll tri grllpc 
pjcscenjaka - jako porozn i litoareni ti , lit oareni ti sa srcdnjim vrijed­
nosti11la poroziteta i s!abo porozni , cv rsto ccrncnlirani lil 0arcnili. Vri­
jednosli porozit cta ovi sne Sll 0 sadrzaj u gJinc u pjesccnjaei11la, sto jc II 
vC7.i s energijo11l vode i uvjctima talozcnja. 

The dis tribu tion and geomet ry or sandst one bodies 
were strongly in iluenced by depositional palaeoenviron­
ment , delt aic and shelf processes. Depos ition o r Iva­
sandstones from the Ivan ic Producti on Field ocemed on 
a shallow indented shelf, where elonga te, lenticular, 
sheet-like and chanel-fill ed sa nd bodies, as well as 
intcrbar and shelf marl s have been developed (TADEJ 
& KRIZMANIC, 1995 ). The Poljana sandstones of the 
Zu ti ca field are interpreted as a dendritic palaeod rainage 
pattern characterized by the three major channel sand­
stone bodies (SAFflC et a I. , 1995). Thicknesses or the 
separate sand bodies in the Sava depress ion vary from 
few melres lip to several hundred metres. 

3. METHODOLOGY 

Sandstone samples from a few hundred metre cored 
intervals of different productive fonnations were chosen 
[or in tensive study. Standard core descriptions of litho­
logy and structural characteristics were performed. Sev­
eral hundred thin-sections were exam ined wi th optical 
microscopy to determine mineral compos ition and 
porosity. Thin-sect ions were stained with both Alizarin 
rcd-S and potassiulll fcrricyanide 1'01' the purpose or car­
bonate minera l identificati on . Routine point counts were 
done to quantify mineral composition and thin-sec tion 
porosity. Grain s ize and so rting were determined. 



]12 

Osijeka 

Vukovar 

Dubrovnik 

fig. 1 The position or oil-gas liclds in Sava depression. 

Porosity and permeabili ty were measured on corc 
plugs. Selected samples were analyzed with scanning 
electron microscope (SEM) for mineral composition, 
microporosity and di agenetic products, and with X-ray 
dilTractometar [or mineral composition. 

4. PETROGRAPHY 

The sandstone reservoirs in the Sava depression are 
fine to medium grained (0.08-0.4 mm), most ly well 
sorted. No major composit ional variations arc observed 
when two fields are compared . Mineralogically, the 
sandstones arc relatively uniform, aside from various 
amounts of earbonatc cements and clays. Sandstones 
mostly consist of quartz and dolomite rock fragments. 
Less abundant arc micas, altered feldspars, chlorite, and 
chert, quartzite and mica-schist rock fragments. 

Sandstones can be differentiated in their cement 
composition and proportion. Early diagenetic calcite 
cement is rarely preserved. Detrital grains appear to be 
"floating" in calcite (Plate I, Fig. 1). In highly porous 
sandstones (Plate I, Fig. 2) micropores are sporadically 
filled with Fe-dolomite and Fe-calcite, and some clay 
minerals. Late carbonate cements (Plate I, Fig. 4) infill­
ing sccondary i11lergranular pores are commonly Fe­
rich (Pc-calcite, Fe-dolom ite). Unlike early poikilotopic 
Fc-calcite, this late cement inCilled residual pore spaces 
that remained after compaction . 

Point count analyses indicate the typical major com­
ponents of sandstones arc quartz (40-50%), rock frag­
ments ( 15-25%). micas (10-15%), feldspars (5-10%) 
and cement (5-20%). X-ray diffraction gave semiquan­
titative analyses of sandstone comparable with petro­
graphic and point co unt data. SEM analysis displayed 
variat ions of c lay mineral content in the productive 

Geologia Croatica 49/2 

L1lholO{O' porosity P=""',"t~ Ilthorades 

Fig. 2 Schematic column - correlation of sedimentary, diagenetic and 
reservoi r properties or Upper Miocene litharcni[cs in Sava 
depression. Legend: t) lili1arcnilc with less than 5% cement and 
high permeability (30-380 x 10·\t1112

); 2) lilharenite with 5-10% 
cement and intermed iat e permeability (7 -105 x 10 \un'); 3) 
lilharcilite, lightly cemented (up to 20% cement), with very low 
permeability (0.05-10 x 1O-3Mm2

). 

sandstones and well cemented sandstones. In porous oil 
saturated zones illite, in the form of coatings and pore 
fillings (Plale II , Fig. I) , and pore filling kaolinite 
(Plate II, Fig. 2) were determined. In strongly cemented 
sandstones clay minerals are sporadically present. 

S. POROSITY AND DIAGENESIS 

In the burial history of sands much of the porosity is 
lost through compaction and cementation processes. 
Early carbonate cemellt, primarly calcite may be inter­
preted as pre-compaction, inasmuch as the detrita l sand 
grains appear to be "floating" in calcite. These ccments 
are rarely preserved. 

Dissolution of early diagenetic calcite cement and 
unstable grains is the main secondary porosity forma­
tional event. It has been suggested (SCHMIDT et aI., 
1979) that the primary agent responsible for carbonate 
dissolution in the subsurface is carbonic acid formed by 
the generation of carbon dioxide durin g the thermal 
maturation of organic matter in sediments . Shale com­
paction yields large volumes of water that may be avail ­
able for dissolution reactions in sandstones . It is proba­
ble that the more porous horizons were never comple­
tely cemented with calcite, and intergranular porosity 
has been enhanced there by dissolution of some calcite. 

In well cemented zones , late carbonate cements 
infilling secondary pores are commonly iron-rich. Their 
occurrence is probably related to the release of iron and 
magnesium [rom the illitization processes in adjacent 
shales (BOLES & FRANKS, 1979). Carbonate cement 
distribution depends on the clay content in sandstones, 
as weI[ as to the proximity of the shales that presum­
ably sourced the material for cementation, both dcpend­
ing on depositional processes. 



Tadcj, Maric-Durckovic & SJavkovic: Porosity, Cementation, Diagenesis ,md Their Lnr1ucnce ... 313 

Other diagenetic processes such as quartz over­
growths, precipitation of kaolinite from pore fluids , 
alteration or feldspars, late chlorite and pyrite formation 
are volumetrically minor and/or locally significant. 

6. RESERVOIR QUALITY 

Variation in porosity and permeability at depth is a 
function of sedimentary conditions, cementation and 
dissolution. The relationship between sandstone body 
geomet ry and the distribution of diagenetic clements 
are critical aspects of reservoir heterogeneity. 

In all studied fields sandstones can be divided into 
three groups differing in cementation rate and porosity­
permeability patterns. In first group are sandstones with 
Icss than 5% cement, and with high porosity (23-33%) 
and permeability (30-380 x !O.3~m2) values. Micro­
porosity is high, and pore sizes of 50 ~Lm - 0.1 mm pre­
dominate. The sccond group arc sandstones with 5-10% 
cement and average porosity 19-30% and pcrmeability 
7-105 x 1O-\Lm2

. Pore sizes vary from 10-50 p.m. High­
ly ccmented (up to 20 % cement) and fortu nately lcss 
abundant are sandstones of the third group. Porosity 
and permeability values arc very low (porosi ty 2-3%, 
permeability 0.05-10 x [0-3p.m 2

). Microporosity is poor 
with pore dimensions of 0-1 0 ~lm. 

Although there are no significant variations in detri ­
tal mineralogy of all three sandstone groups, some dif­
ferences can be highlighted. Primarly, it is the clay con­
tent that mostly depends on water energy and deposi­
tional conditions. In general, the fairweather clay rich 
sediments are weakly cemented, and the clay poor 
storm deposits well cemented (KANTOROWICZ et al. . 
1987). 

Carbonate cementcd layers can be recogn ized in 
logs becausc they generate very high resistivity peaks. 
Definition of lateral extension of these cemented layers 
is important because thcy reprcsent barriers for oil dis­
placement. 

7. CONCLUSION 

Using a combinalion of petrographical, petrophysi ­
ca l and sedim cntological techniques, diagenetic 
processes can be identified . The similarit ies in the geo­
logical evol ution of reservoirs resulted in recognizable 
and ubiqu itous diagenetic processes. Definition of 
depositional environments, body geometry, lateral 
extension and diagenetic processes are leading to better 
understanding of reservoir properties, and have to be 
assesscd during the evaluat ion of new exploration 
prospccts . 

8. REFERENCES 

BOLES. J.R. & FRANKS, S.G. (1979): Clay diagenesis 
in Wilcox Sandstones of southwest Texas: Implica­
tions or smectite diagenesis on sandstone ccmenta­
tion. - Jour. Sed. Petrology, 49, 55-70. 

KANTOROWICZ. J.D .. BRYANT, LD. & DAWANS. 
J.M. (1987): Controls on the geometry and distribu­
tion of carbonate cements in Jurassic sandstones: 
Bridport Sands, southern England and Wiking 
Group, Troll Field, Norway.- In: MARSHALL. J.D. 
(cd.): Diagenesis of Sedimen tary Sequences. 103-
118, Blackwell Scientific Publications. Oxford. 

LOUCKS. R.G .. DODGE. M.M. & GALLOWAY. 
\V.E. (1985): Regional controls on diagenesi s and 
reservoir quality in Lower Tertiary sandstones along 
the Texas Gulf Coast.- In: McDONALD. D.A. & 
SURDAM, R.C. (cds.): Clastic Diagenesis. AA PG 
Memoir. 37. 15-46. 

SAFflC. B., VELIC. J .. SIMON. J. & NOVAK. J. 
(1995): High-resolution paleogeographic maps of 
sandstone reservoirs: GSS Poljana (Pontian, 
Miocene), Zutica oil & gas field , Sava Depression.­
First Croatian Geological Congress, Proceedings, 2, 
529-533. Zagreb. 

SC HMIDT. V .. McDONALD. D.A. & PLATT, R.L. 
(1979): The role of secondary porosity in the course 
of sandstone diagcnesis.- In: SC HOLLE. P.A. & 
SCI-ILUGER. P.R. (cds.): Aspects of ciiagenesis. 
Society of Econom ic Paleontologist and Mineralo­
gis t, Special Publications, 26, 185-207. 

TADEJ, J. & KRIZMANIC. K. (1995): Paleoecology 
and depositional environments of the Upper 
Miocene sediments in the NW part of Sava Dcpres­
sion. - First Croatian Geological Congrcss, Proceed­
ings. 2, 605 -608. Zagreb. 



~14 Geologia Cro:lIiC:l 49n 

PLATE I 

Fi g. I Litharenite with early diagenet ic calcitc ccment (reel). Some of the sand grains are nearly surrounded by, or 
appear to bc " floating" in thc calc itc cement. 

rig.2 lli ghly porous, oil -saturated (brown) litharenite. First group of sandstones with high reservoir quality. 

Fi g. ::; Litharen ite with moderate porosity and cementati on. Second group of sandston es wit h medium reservoir 
quality. 

Fi g.4 Hi ghly cemented (Fe-dolom ite, blue) weakly porous litha renites. Third group of sandstones with [ow reser­
voir quali ty. 

PLATE II 

Fi g. I SEM pholOlllie rograph of illi te in the ro rm or a grai n coating. 

Pig. 2 SEM photomicrograph of pore fillin g kaolinite. Note abundant inLergranular porosity. 



T;lUCj, Maric-D urekovic & SI;lVkovic PLATE l 315 

0.5 mm 



3 16 PLATE II Geologi:l CrO:l ticn 49/2