GEOL. CROAT. 49/2 2 17 · 226 8 Figs. I I Tab. ZAGREB 1996 I 

Expansion of the Old Oil Field of Bunjani on the Basis of 
Geological Reinterpretation 

Niko FILIPOVIC 

Key words: Reservoir developm ent, Geolog ical rei n­
te rpre tation , Sava depression. 

Abstract 
Duc \0 the non-de f ined contouring of the Blllljani field and l h e 

posi tive exploration well Jo-2 at the SQu \hwcs\ of Ihe field outsi de the 
concession, :I need for the geological reinterprewtion of the arca wa" 
discovered in order to solve the petroleum-geological rel ationships in 
the arc;!. and determine field ex pansio n and reservoir contouring in a 
more qu al itati ve way. The fccc nt geo logical reinterpretati on was per­
famed 011 the bas is of a rev iew of all avai lable data from the exist ing 
we lls, lhe res ult s of field dcvc]opmcl11 10 lhe present , the int erprcta ­
tion of ncw scismic profi lcs and thc knowledge of the pctrolcum-gco-
10g ic;1I re l;lli onsh ips in [he pan of the basin previ ously acqui rcd . The 
rcinterprctati on proved to be justified ,md i1 result ed ill the detennin<l­
ti on of ncw wc ll locati ons and their drilling in 1993 and 1994. In thi s 
way, the field ex panded towards the southwest and th c north . With 
the ncw reservoir geometry, beller quality data of the reservoi r char­
acteris tics, anti lyses obtai ned from the new well s, together with the 
reintcrpretation of clect rie log analyses obtained from olel wells, the 
reserves of oi l have been increased by 3 1.4%. 

1. INTRODUCTION 

T he objec ti ve o /" this paper is to explain ho w lhe 
geolog ical re inte rpretation (using the available dala and 
with the minimum of investment) has enabled expan ­
s ion o r the o ld BUlljani oil field and secured the suc­
cessful continuation or petroleum production. 

In o rder to revitali se old oil field s with a low pror­
itability margin , (o r which the Bunjani is one sample), 
at tempts have been made to renew development ac tivi ­
ties . The essen tia l targe t was to make profitable use of 
the available data and to define the possible ex pansion 
01" the oi l field with the minjmulll cost and with the use 
of standard development practices. In view of the above 
c riteria attent ion was directed to the possible expansion 
o r undepicted parts or the rcservoir, or the discovety of 
new hydrodynam ic oil saturated bodies, as well as olher 
development act ivi ti es which would increase o il rccov­
cry from Ihe old part o f the oil fi e ld. The first step was 
the geolog ical reinterpretation which resulted in drilling 
o f the new productive wells , the oil fi eld expansion and 
an increase in the petroleum reserves. 

INA -Natbplin, Subicev;\ 29, IIR 10000 Zag reb. Croatia. 

PROCEEDINGS 

KljLlcne rijcci: razrada lei ista , geoloska re int erpreta­
cija, Savska potolina. 

Saietak 
Zbog nedefiniranog o kon lllrenja po tj a Bunj ani i poziti vllc 

istrazne busoline Jo-2 na jugozapad u polja, izvan konces ije, uk azaJa 
sc pO lrcba Zil geoioskom reinterpretacijol1l s c iljcm kva litctn ijeg 
tjesavanja naftnogeolos ki h odnosa ovog podrucja, prosi rcnja polja i 
okollt urenja leiisla. Najnovi ja gcoioska rein terpret acija izv rscna je na 
temelj u ponO\,11og preglcda svih raspo loliv ih podat aka iz poslojeCiil 
busotina, rezu itata dosadasnjc razrad e lezista, rczu llata intcrprelacije 
novosn imlj enih seizmickih prorila , te dosaclasnji h sawanja 0 naft no­
geol o .~kim ocinosima log dijela potoline. Rci nl erprctacija sc pokazala 
opravdanom i rezultirala lociranjem i izradom llovih b ll.~o tina u 1993. 
i 1994 . godini. Time je poljc prosireno preilla j ugozapadu i sjevcru . 

S novom gcomelrijom leiista i pros irc nj em po lja, Ie kv~ lit ctnijim 

podac ima aIlaliza i svojs t(lva kol e kt ora, s nov ih i starill busotina , 
povccane su rezerve nan e za 3 I .4''10. 

2. REVIEW OF EXPLORATION 
OPERATIONS TO DATE 

The Bunjani oil fi e ld be longs to th e Pannonian 
region of the Sava depression (Fi g. I ) . The struc tural 
closure of " Krii", where the Bunjani oil fi e ld is located, 
was discovered by the rcgional gravimetric survey per­
formed by the German group "Scismos" from Hannover 
between 1940-1942. 

Since the whole area is covcrcd by dilu vial and allu­
via l deposits, it has been impossi ble to carry out s llrf~lee 
geological mapping. Hcnee during I 94 l -1942, 20 sha l­
low s truct ura l wells were d rilled to confirm the geo­
physical resuits. Onl y onc of these well s ($-12) is locat­
ed wi thin the Bunjani oi l fic ld . Based on the grav imetric 
survey and struc tural weli s, the fi rst deep ex ploration 
well was drilled in 1948, which d iscovered oil at Sume­
cani. For thc purpose of furth er expansion o r the S ume­
calli oil field (Kriz), thc deep expl orati on well Bn- I was 
drilled in Autumn 1951. Thi s well pene trated bcds COIl ­

firmin g the prcsence of an oil reservo ir in Ihis area. T hi s 
is how the Bunjanl oil field was d iscovered , which pro­
duccd oil of a different quality 10 that found in the Kriz 
oil fi c ld. 



21X 

=~"l=~1 -------....bo ,,10 --t---t---t---+-
• ZAGREB I' I' J "eoo" I 

1---1-----1- ... KLOSTAR --

! 
JE1:E~O _ . I 

t 

..... _, .... KR I • 

~'~. \ BUNJANI : --+----'.+--"<11..- ,4 

IVANT ~"~i ZUTICA .... ~ : 

VEZISCE -+----' 
~WU ! 

7.5km 

T 
L 

MR AM OR BRDO ... 

O IL FIELDS _ BUNJANI O IL FIELD 

_ O IL-GA S FIELDS 

f-ig.! Location map of Tile fields in the nonh-west area of the Sava 
depression. 

By 1964,61 wells were drilled through the field, 
without the field contours having been precisely 
defined. The field was put into production in 1952. 
Between 1964-1979, there were no exp loration or 
development operations. In 1979, in the southwestern 
part of the field, (outside the concession) , the Jo-2 

f-i g. 2 Map of all [he Bunjani field reservoirs with marked expansion. 

Geologia Cromica 49/2 

exp loration well was drilled, which was pronounced 
negative, but which later produced good results during 
subsequent testing. It offered new possibilites for field 
extension LO the southwest beyond the boundaries of the 
production field. With this aim development activities 
were resumed in 1992. 

3, GEOLOGICAL REINTERPRETATION 

Undefined field contouring and the positive explo­
ration well Jo-2 (1979) have prompted the resumption 
of development activities on the ficld. On first review­

ing the available field data, it became evident that the 
geological reinterpretation was necessary in order to 
bettcr dcfine the petroleum-geological relationships of 
the area, delineate an area for field extension and to 
fac ilitate precise reservoir contouring. 

The last available integrated geological presentation 
detailed 41 drilled wells (1955) in the archives of the 
Tna-Naftaplin trade and techni cal papers and record s 
library, and in the published paper (GALOYIC, 1951-
1953). In addition to the above mentioned geo log ical 
data, there are also available anonymous maps prepared 
at the reservoir top but which contain insufficient infor­
mation, based on 61 drilled wells. Three scismic lines 
(2 0) of mediocre quality and standard processing were 

LEGEND 

• 
o 

• 

PRODUCING WELLS 

DRY WELLS 

DEVIATED WELL 

EXSPANDEO FIELD AREA ALONG THE 
RESERVOIRS 1+ 11 , III , IV arid V 

OLD PART OF THE FIELD 

Q CONCESSION 

A·A' LINE OF GEOLOGICAL CROSS SECTION 

4 



Filipovic: Expansion of the Old Oil Field of Bunjani on the Basis ofGeologieal R.e interpretation 219 

Fi g . 3 C OnTour map of the top of the "[II " reservoir. 

shot on [he field. Wireline logs eEL, ML) from 61 wells 
were also used, as well as the results of laboratory 
analysis and data on the field production history cover­
ing [he years from 1952101992. 

After having defined the potential prospectivity of 
the field extension, the next question to discuss was the 
profitability of investment in further development oper­
ations, in respect of the field size and the oil reserves 
present within the old part and possible extension there­
of. The objective was to define the field extension and 
to put new wells in production with the minimum finan­
cial investment, using thc available data and standard 
practices usually applied in geological reservoir devel­
opment . Exploration began with the shooting of 10 
short seismic lines (2D) (total of ca. 40 km) across the 
field and marginal parts, covering principally the possi­
ble ext ension areas. The survey and interpretation 
proved to be justified. Based on the interpretation and 
synthesis of all geological data, including the Jo-2 well, 
the locations were proposed and the positive wells En-
02, Bn-63, Bn-64, Bn-6S~ and Bn-67 were drilled, 
thereby extending the rield to the southwest, and Bn-66 
which initiated extension to the north (Fig. 2). 

Geological reinterpretation was based on the 
renewed analysis and inteq)J'etation of all available well 
data (geological monitoring records, DST; physical and 
geochemical rock analyses; biostratigraphic, petro­
graphic and sedimentologic assignments; oil and water 

SCl·53 

Bn·16 
o 

LEGEND 

• OIL WELLS 

• WATER WELLS 

o IMPERMEABLE {wJtllOU! reservoir rocks) 

c:J OIL 

[ZJ WATER 

D IMPERMEABU~{w'thou t reservoir rocks) 

.~~ . .::J DEVIATED WELL 

- - OIL·WATER CONTACT 

- - ISOBATH 

• •• •••• • PROBABLE SANDSTONE PINCH·OUT 

~ NORMAL FAULT 

~ REVERSE FAULT ACC TO SEISMICS 

:: :>< :: SUPPOSED REVERSE ACC TO SEISMICS 

® TECTONIC BLOCK 

O~1....E.~~5'm 

SCALE 

analyses, well completion results, reservoir pressure 
and temperature measurements, as well as field produc­
tion history), and in particular revised the renewed 
wireline log correlation. 

Evaluation of the physical rock properties for the 
Bunjani field was performed using porosity and water 
saturation data obtained [rom wireline log analyses and 
laboratory analyses of rock samples from conventional 
cores. E-Iogs from recent wells (Bn-62, Bn-63 , Bn-64, 
Bn-6S , Bn-6Sa, Bn-6S~ and Bn-66) have been per­
formed within an integrated programme. This included 
different resistivity measurements (ML, D1FL, DLL, 
EL), acoustic logs (ALBf-Ie), natural radioactivity mea­
surements of the rocks (GR), density and apparent 
porosity measurements (CDL/CNL) and formation dip 
measurements. This data combined with the laboratory 
core analyses and geological monitoring data, provided 
a good basis for evaluation of the physical parameters 
of thc reservoir. For this purpose an "EPILOG-eRA" 
computer program set was used. Lithological assign ­
ment and porosity calculation were mainly de fin ed 
from CDL/CNL/GR logs. 

The new reservoir geometry was defined by reinter­
pretation, excluding some of the reservoir rock bodies 
originally believed to be oil saturated, but which in fact 
have been water saturated or which are not reservoirs. 

Until now, the Bunjani field had three classified 
reservoirs ("M", "K" and "Tg"). During the gcological 



220 

Fig. 4 Contour map of the top of the ··V" reservoir. 

reinterpretation and after drill ing the new wells, four 
reservoirs we re identified (I+I1, III, IV and V). The 
reservoir classification was based on data synthes is, 
revised correlation of wireline logs, results of analyses 
of open interval tests in the wells, well pressure mea­
surement data and the production history. 

A synthesis of the utilised data, with interpretation 
of the newly shot seismic lines, together with the 
absence of preeoncived ideas and the aim of extending 
the field resulted in a new picture of the structural and 
tectonic relationships, reservoir division and contours 
involved. By including the belier quality data for each 
new drilled well, a new geological scenario was pre­
pa red for the Bunjani field and the extensions to the 
sout hwest and north were defined (Fig. 2) . 

With the field extension to the sou thwest over the 
reservoir TTl (Fig. 3) and the reservoir V (Fig. 4), the 
presence of separate hydrodynamic units has been iden­
tified. These units have higher reservoir pressu res 
(nearly hydrostatic) , which are independent in respect 
of thc old part of the field (excepting the Bn-2 well 
area), with the trial production of the new comp leted 
wells. The Bn-62 we ll (20 m'/day of water free oil), 
Bn-63 well (IS m'/day of oil) , Bn-6S~ (25 m'/day of 
waler free oi l) and Bn-67 (35 m3/day of water free oil) 
produce yields well above the average of the old weUs. 
The eontouring/bounding of the well to the southwest at 
reservoir HI is not fina l. In this geological scenario, the 

Gcologia Croatica 49n 

• OIL WELL 

• WATER WELL 

0 DRY WELL 

D 0" 

D WATER 

D IMPERMEABLE (W ithout reservOFr rocks) 

.-'.0 DEVIATED WELL 

OIL- WATER CONTACT 

ISOBATH 

RESERVOIR ROCK PROPERTIES 
DISAPPEARANCE 

Bn-55 = NORMAL FAULT • 
~ REVERSE FAULT ACe_ TO SEISMICS 

::c :>C :: SUPPOSED REVERSE ACC TO SEISM ICS 

TECTONIC BLOCK 

reverse fault is anticipated as a boundary only on the 
basis of 2D seism ic lines which are characterized in this 
area by extremely dispersed seismic rcflections, which 
implics that the seismic interpretation is also unsafe. 

The extension to the north with the new well Bn-66, 
is based upon contouring of all four wells from the old 
field area (Fig. 2), and that of reservoirs I+ll (Fig. 5) in 
particul ar. 

Based on the resu lt s of the reinterpretation, it was 
proposed that the hydraulic fracturing method for reser­
vo ir rocks should be used in order to enhance the pro­
ductivity of 12 wel!s. Three fracturings were performed 
(Bn-40, Bn-S7 and Jo-2) and good results were 
obtained. 

The production history to date indicates tha t the gas­
oil energy and thc waler environment elast ic energy are 
used in reservoir exploitation. An overview of the oil 
production to date and future predictions as the result of 
the geological reinterpretation, are shown in Fig. 8. 

The proven results of the geological reinterpretation 
have opened the possibility for yet another extension of 
the field to the southwest (across rese rvoir TIl), where 
the fault boundary is also uncertain to the north across 
reservoir 1+11. There is a sha llo w dcpth of soi l to the 
north, and it is necessary to es tab lish with more preci­
sion the pinchout boundary of the oi l-bcaring sand­
stonc. It is absolutely necessary [or the fi nal defini tion 
of the reservo ir boundaries to carry out the seismic sur-



Filipovic: Expansion of Ihe Old Oil Field of BUlljani 011 Ihe Basis of Gcologic<ll Reimerprelalioll 221 

° Kr_59 

Bn·63 
o 

Jo·2 
o 

00010.2 0 304 05km 

SCALE 

Fig. 5 Contour map of the top of the ·'1+11 " reservoir. 

Bn·24 

" 

Bn·14 o 

Bn-2B 
o 

vey and 3D seismic interpretation togcther with a rea l­
istic evaluation of furt her deve lopment operations in 
terms of cost-effective feasib ility. 

4. GEOLOGICAL STRUCTURE OF THE FIELD 

The description of the geological structure of the 
field has been prepared on the basis of known data 
obtained during exploration and development opera­
tions to date with special reference to the results of the 
geological reinterpretation. 

4.1. STRATIGRAPHIC RELATIONS 

The stratigraphic age of separate beds of the Bu­
njani field has been established on the basis of palaeon­
tological analysis, supe:·position, lithological character­
istics as well as the wircline log corre lation, and also 
the results of the regional correlation of lithostrati­
graphic unils (SIMON. 1973). 

Based on the available data, the following strati­
graphic units can be distinguished (Figs. 6 and 7) in the 
Bunjani field area: 

Quaternary: Represented by diluvial beds within 
the yellow clay facies, with loess in places in the base. 

Pliocene (Romanian and Dacian): Pliocene beds 
developed within the facies composed of variegated 
clays , coarse grained sands and argillaceous sands. No 
cores have been recovered from these beds, hence there 
is no palaeontological data available. 

0 8 r>- 16 

LEGEND 

• OIL WELLS 

• VATER WELLS 

o IMPERMEABLE (without reserVoir rocks) 

o WELLS IN AREAS WITHOUT RESERVOIRS 

DOlL 

c::5] WATER 

D IMPERMEABLE (without reservoir rOCk"1 

""r .,(J DEVIATED WELL 

- - - - OIL-WATER CONTACT 

-- ISOBATH 

•• ••• • PROBABLE SANDSTONE PINCH·OUT 

~ NORMAL FAULT o TECTONIC BLOCK 

t-50 BS-55 L,---------~----.J 

Bn·47 
o 

Bn-56 

" 
----~8~'.--.'"~. ~5-----­

o 
~~-27 86'-38 

Upper Pontia n: The Upper Pont ian beds are char­
acterized by the alterna tion of gray and brown argilla­
ceous marls, marly sands and sands. T he fauna has not 
been specifically defined, but Limllocardides wi th large 
shells and COllgeriae with C. rho!1lhoidea and C. zagra­
bieHses species have been observed. There was no 
hydrocarbon present. The Upper Pontian beds were 
deposited in a concordant relationship with the Lower 
Pontian beds within the facies of grcy marls and local 
clayey marls. 

Lower Pontian: Palaeontological macrofossil dete­
rm ination has been performed only during geological 
monitoring of wells where specific specimens of the 
genus Paradacna with P . ahichi species were identi­
fied . No hydrocarbon presence was found within these 
beds. 

Upper Pannonian: The lithological composition of 
this strat igraphic member is characterised by greyish 
marls with slightly arenaceous marls with calcareoLl s 
marl intercalations in places. This member exhibit s 
strong uniformity across thc study area. The fauna has 
not been specifically determined. Howcver, Congeria 
spp . fragments and limnocardids dcbris have been 
determined in the field. There is no hydrocarbon reser­
voir within thcse beds. 

Lower Pannonian: This stratigraphic member rep­
resents a facies composed of hard, white to grey cal ­
careous marls and argillaceous limestones. These are 
frequently interlayered with thin sandstone bcds. Usual­
ly, marls are fracturcd. There is a freshwater fauna 



222 

STRATIGRAPHIC CLASSIFICATION 
(STEININGER, ROOL. MULLER 1987) 

W ~ O\ tJ 
if> 

STAGE/ AGE if> 
a: 

~U) \ ~ :r w 
~ 

" 1-0: llJ UJ EUROPE 
0 a: 

Q Lii \ Do- \ 

<Xl <! 

" " 8~t \ ~ 
~ 

GENERAL LOCAL if> w 
~ ~ if> O~ ~ \ LU I CLASSIFICATION CLASSIFICATION 
~-g: , a: \ (TETHYS) {CENTRAL 

if> :J! PARATETHYSI o -- -
HOLOCENE -9 QUATER- HOLOCENE 

PLEISTOCENE 
0 

~ NARY _ PLEISTOCENE 

i T~ PIACENCIAN 

] t
g 

ROMANIAN PI, 

ZANCLEAN >-
~ :::::i 

5 ~ 0.. DACIAN 

MESSINIAN ee 

3 
~ w 

PONTI AN 

"i f-
<! 

i ~ ee TORTONIAN 

1 W 
10 I PANNONIAN 

i Z 
w 

~ 
W " 

11.5 

i z Me (!) SERRAVALLIA SARMATIAN 
w 

-ll w ~ 136 
~ 0 ° 
II ° 

0 " 15 ;; W BADEN IAN M, 

~ 0 LANGHIAN 

~ Z 16.5 
--~~ 

KARPAT1AN M3 
-

i 2 
OTTNANGIAN fA, 

~ 
~ 

BURDIGALIAN ---, a: 19.0 

--:I 
<! , 
w , 

EGGENBURG M, 

~--L 
AQUITANIAN 

W EGERIAN? 
OIM 

CHADIAN 

KISCEtLfAN 

composed or iimneids, planorbids and ostracods. No 
hydrocarbon presence has been identified in these beds. 

Sarmatian?, Badcnian, Karpathian, Oligo-Mioce­
ne?: The lithological composition of this group of stra­
tigraphic members is characterized by an extremely va­
ried sequence of clastic sediments, beginning with coa­
rse deposits directly overlying the basement (breccias, 
granil ic debris, breccio-conglomerates and conglomer­
ates) , followed by conglomeratic sandstones, marly 
sandstones and marls. In the overlying layers there is 
also a calcareous component represented by calcareous 

COLUMN OF 
BUNJAN1·9 
(Bn · 9)wal 

DEPTH (m) 

LITHOLOGY 

:-:_:_:1 CLA Y 

:-=-~ ClAY MAAL 

~-~ . ~':'J SANDY MARL 

:-:-:-:1 MARL 

- -:_ -_] LIMY MARL 

<.:«<.:.:J SANDSTONE 

- 0 

'00 

'00 

·~~:.~:..:; I CONGLOMERATIC SANDSTONE 

~~~:5.~ CONGLOMERATE 

~ 
~ GRANIT[ 

Gcologi~ Croalica 49/2 

Fig. 6 Correlation of 
stratigraphic units of 
the I3unjani field 
with the geological 
column of the 811-9 
well. 

marls and sandstones. It has been impossible to subdi­
vide the stratigraphic members within this sedimentary 
sequence due to the high degree of lithological hetero­
geneity and considerable lateral changes, as well as due 
to their poor fossil content. No fossil remains were 
found in the breccias and conglomerates. The age was 
determined on the basis of a fauna which was found in 
the marl, and less frequently in the sandstone [ayers. 

Within these sediments 4 oil reservoirs have been 
identified (1+11, III, IV and V), whereby the reservoir V 
also includes a part of the "basement". 



" '" ~ 
o 
o 
o 
,g 
[ 
o 

~ 
" :: o· 
o 

'> 

" 2, 

" " 2. 
§. 

'" o 
0: 

sw .. 
,~, ] ® 
·100 ' 

~ ,. , 
+ 0 - II 
- ~ « '> 

~~ 
W~ ~. 

-100 ., 0 W ~~ Of-
:::i « ;)0 

"' J0.6 Q 

300 I 
\ 

'00 ] """ ~, "'''''~ W 
-500 _ z \ 

w 
U LOWER \ 

"' i 0 PONTIAN 

.,,, n 
~ p..J!~~~ 

LO' .... ER 
PANNONIAN 

S"OENI1o.N 
K.l.RPATIi\N 

<0' ] 
STR"TIGRAPHIC 

·300 UNDETERMINED 

-100 0 

1\ lOp , 4j i n> 
HORIZONTAL SCALE 

B~2 

h"14686m 
8n-54 

h"13945m 
8n-19 

h" 14792m 

~~:t'i\I '.:,::I! t;! SAND 
~ ", ~ 

~CLAY ...... "" ... 
[-: .... : - : CLAY MARL 

I ~-~·~-~ SANDY MARL 

[-=-=-= MARL 

'" __ ...::.: LIMY MARL 

:::··· ·: :·1 SANDSTONE ....... 

Bn-J5 
h:130 20m 

· ;.~:·~:.~I CONGLOMERATIC SANDSTONE 

~ CONGLOMERATE 

~-%;:; ROCKFALL BRECCIAS 

~GRANITE . . 

80-46 
h:l1J.69m 

~OIL 

~ ~ ~>;" WATER 

8n-44 NE 
h=I04 Dim 

c=J IMPERMEABLE (MARL) 

J'i i4 "iI IMPERMEABLE (GRANITE) 

C ~ LOWER PANNONIAN DEPOSIT 

c==] UPPER PANNONIAN DEPOSITS 

c= LOWER PONTIAN DEPOSITS 

c= UPPER PONTIAN DEPOSITS 

tIJj NORMAL FAULT 

~ REVERSE FAULT 

~~ SUPPOSED REVERSE FAULT 

__ UNCOFORMITY 

~ 
~ 
~ 

] 
c· 
o 
s,. 

" o 
o 
0: 

2 
~ 
~ 

s, 

" o 
~" 
::" 
g 
~ 
~ 
;;." 
s, 

~ 
g 
[ 
~ 
§ 
.; 
il 
~ 

g" 

~ 
~ 
w 



22·1 

OtL FtELD: BUNJANI 

30000 

""''' .-• 20000 +-~ 

1 
~ 15000 

PAST O IL PRODUCTION 

10000 

sooo 

0 " " 
, H ,~ -1--+-+ ~ , 

~ ~ ~ ~ * ~ ~ g ~ ~ ~ ~ , 
V:"A QS 

T he " Base m ent" (s tratigraphi call y und etermi­
ned): Composed of weathered and frac tured - cataclas­
tici zed, hydrothermally al tered granite and gnei ss . 
Granites occur within the overlying beds of the produc­
tive area, while gneisses occur in the peripheral parts in 
depth. Calc it c may usually be found depos ited in fi s­
sures . In those areas where the frac tured "basemen t" 
lies in a structu rall y favourable posit ion, an oi l reservoi r 
was form ed. 

4.2 LOCAL TECTONIC AND STRUCTURAL 
RELATIONS HIPS 

In the most recent geological reinterpretation of the 
ficld. the solutions involved wit h the structura l and tec­
tonic re lationships we re based on the in terpretation of 
new se ismic lines and correlation of well data. General­
ly, the Bunjani oil field is separated from the southwest 
deep area by a fau lt zone, where tectonic blocks sub­
s ided in a "staircase slope" [or over 1000 m. Structura l­
ly, the rield is represented by a faulted monocl ine trend­
in g NW-SE. The monocl ine plunges to the SW at an 
angle of 10°. Some of the faults have been identified 
through the absence of some of the layers in wire li ne 
record s and from the results of seismic line in terpre ta­
tion , and others have been anticipated. The Bu njani 
fi e ld has been divided in 6 tec tonic units . The fau lt s 
were importan t for the reservoir oil accumulat ions. rep­
resenting barriers in places, as a resu lL of which differ­
ent hyd rodynamic condi ti ons were formed, hence the 
water-oil contac t is a t various levels wi th in sepa rate 
blocks. 

4.3. GEO LOG ICA L DESCRIPTION OF 
THE RESERVOIRS 

In te rms of their stra tigraphic assignment and li tho­
log ica l characterist ics , the reservoir rocks of the Bu­
njani fi e ld arc represented by two d ifferent reservoir 
types : a) " the basement" consisting of eataclasticized 

FUTURE OIL PRODUCTION 

Geologia Croallca 4lJ/2 

Fig. 8 Oit production of the 
Bunjani fi eld past, present 
and future (based on predic­
tions from thi s study). 

and hydrothermally al tered gran ite and gneiss, b) con­
glomerates, breccio-conglomerates, breccias of granitic 
debris, and Miocene sandstones (probabl y also O ligo­
Miocene) . The cap roc ks are marly sedim ents . The 
pet ro leum -geolog ical re lationships, in respec t of the ir 
occurrence, distribution and lithologica l characteris tics, 
<.Ire moderate ly comp lex withi n th e ind ividua l strat i­
graphic members. Th e rese rvoir data summary of the 
Bunjani oi l field is shown in Table I. 

Reservoir l+1I: Com posed of fin e 10 eoarse~ 

grained, poorl y cemented, quartz-micaceolls sandstones 
and conglomeratic sandstones . The reservoir is of st rati ­
g raph ic type, deli neated by a lit hologica l bou ndary -
marl ificatioll and pinchollt , and by a tectonic boundary 
(Fig. 5). 

Reservoir III: In genera l, reservo ir rocks of reser­
vo ir III are fi ne to coarse grai ned, poo rly to medium 
cemented quartz-micaccous sandstones and conglomer­
ate sands tones . Reservo ir beds are as a ru le thin and 
di sco ntinuous and intercalated wit h marl s . The reser­
voir is strat igraph ic in type, de lineated by a li thological 
and tectonic boundary. ]t is characterized by zonal mar­
lifieation and pinchouts in combination with tectonic 
boundaries - faults, with separate water satura ted zones 
(Fig. 3). 

Reservoir IV: Also represented by zonal evolu tion. 
The reservoir rocks are composed of somewhat larger 
coarse-gra ined clasts, consisti ng of coa rse-g rained 
sandstones, conglomeratic sandstones, and sandstones 
with fragments of large granit e pebbl es . They were 
depos ited in thin disco ntinuous laye rs and laminates . 
The reservoir is strat ig raphic in type, delin eat ed by a 
litholog ical and tectonic boundary. The reservoir 
boun daries have sim il ar characte risti cs to those of 
rese rvoir nT, since the resc rvoir rocks were deposited 
with in the same sedimentary cycle. 

Reservoir V: This is formed of two rcservo ir types : 
a) conglomerates, breccia-conglomerates. brecc ias of 



Filipovic: EXp;ulsion o f the Old Oil Field of Bunjani 011 the I3nsis of Geological Re inlcrprctation 225 

Discovery Date: 

Discovery Well: 

Productive Formation: 

Depth of Formation: 

1951 
Bn- I 

Preeec 

700 m (average) 

Reservoir Type : 

Reservoir Age: 

[+ J1 , 1[1 and IV - stratigraphic with the lithologic and tectonic boundaries 

V - stratigraphic-massive type bounded by the impermeable zones 

Karpathian , Badcnian, (Oligo-Miocene?) 

Depositional Environment: marine and fresh-water 

Drive Mechanism: depletion drive and water driv e 

Source Rock: 

Cap Rock: 

Miocene and Pliocene marly sediments in th e Sava depression 

marly sediments 

Pre-Reservoir: gran ite and gneiss 

Orig inal Res. Pressure: 

Present Res. Pressure: 

67.2 bar (9 .7 bar/lOOm) (reduced to the weight level of -579.0 m) 

different along the reservoirs 

Orig. Oil / Water Contact: different in lhe reservoirs and tectonic blocks or withollt contact 

Rock Properties: 

Ave rage Porosi ty : 

(pans of unit) 

lnil ia l Water Saturation: 

(pans of unit) 

1+ 11 

0.139 

1+11 

0.405 

III 

0.130 

III 

0.409 

IV 

0.122 

IV 

0.409 

V 

0.111 

V 

0.424 

Average Permeabi l it)': 

Reservoir Temperat ure: 

insufficient data 

SS.2°C (4.55 °C/IOOm) 

A vernge Oil Propert ies: 
Volume Mass of Oil (20 DC): 

Volume Ratio of Oil (Bo): 
Viscosily (20°C): 

Para fin COlltenl: 

Consolidation Point: 

R75 kgim 1 

J .074 m3/m 3 

2 1.6 mPas 

8.00% 
Il oe 

Table I Reservoir data su mmary of the Bunjani oil field. 

granitic debris, conglomerate sandstones and b) weath ­
ered, fractured - catac lasti cized and hydrothermally 
altered granite and gneiss ("basement"). Most of the 
well s penetrated the "basement " whereas in several of 
the wells the drilling te rminated in the basal Miocene 
sed im ents and therefore the reservoir V was not com­
pletely uncovered. The basa l sediments as the integral 
reservoir pans, are transgressively overlying the "base­
mcnt". They havc variable thickness, and in some 
plaees they are comp letely absent, depending on the 
palacorclief. These sediment s are composed of pebbles 
and granite fragments, and to a lesser extent gneiss. The 
pebbles vary in size. Roundness and sorting arc poor. 
Large pebbles arc frequentl y incorporated into coarse­
grained sandstones. The matrix is argillaceous. 

In lhe southwest, newly extended part of the Bunj ­
ani ficld, these reservoir rocks consist of granite debris 
breccias, unevenly cataclasticized with a dense mineral 
compou nd matrix consisting of sericite, chlorite, 
feldspar and quartz. Rare sedimentary bodies composed 
of si lty-grained fossiliferous marl , with traces of coal, 
can be found within these breeias . 

The reservoir V (Fig. 4) is of mixed type (massive 
and strat igraphi c), bounded by zones with no reservoir 

properties and by fault s wh ich are probably barriers 
considering the different oil-water contacts within the 
separate tectonic blocks. 

5. CONCLUSION 

Based on the geological reinterpretation carried out 
on the old Bunjani oi l field, lhe following conclusions 
may be drawn: 

I. The geological reinterpretation was justified and per­
formed with the minim um financ ial investment (the 
cost of approximately 40 km of seismic lincs and the 
work of the geological project manager). 

2. Standard reservoir develop ment practices wcre used 
with the available data during the reinterpretation. 

3. Inst ead of the three reservoirs discovered to date , 
four reservoirs were identified. 

4. The evaluation of the reservoir paramet ers for the 
old field area was morc realistic , since it was based 
on tile reservoir rock analyses from new well data 
from which the oil reserves were recalculated. 

5. The dete rmination of the structural and tectonic rela­
tionships through interpretation of the new seismic 



226 

lines result ed in the be tter evalu ation of rese rvoir 
cont ouring/bounciaries. 

6. The field was cx tcndcd to the southwest and north , 
wh ich increased thc rccoverable oi l reserves by 
31.4%, and fac ilitates the success ful continuation o f 
oil producti on between 1995-2014. 

7. The fie ld extension to southwest identified reservoirs 
with hi gher formation pressures which are indepen­
dent o f the old pan of the field and the eruptive well 
productio n. The extension to the north is based o n 
the reservoir contouring from the old ficld. 

8. The reint erpretation also proved the suitabi lit y o r 
hydraulic fracturing of reservoir rocks as a means of 
inc reasi ng produ ctivit y of the well s Bn-40 , Bn -57 
and Jo-2. 

9. The possi bility has been opened for further ex tension 
o f the fie ld by application of 3D seismics assum ing a 
detailed study proves the cost effec ti ve feasibility of 
slich a projec t. 

Geolog ia CrO:lIica 49n 

6. REFERENCES 

GALOYIC, S. (1951- 1953): Prilog geoloskolll pozna­
vanju strukturc Kri i.- Geol. vjesnik, 5-7, 255-272. 

ST EININGER , F. , ROGL, F. & M ULLER. 1-1. (1988) : 
Correlation of Central Paratc th ys, Eas tern 
Paratcthys and Mediterranean Neogene stages .- Tn: 
ROYDEN, L.H. & I-lORY ATI-I , r. (cds.): The Pan­
nonian Basin. AAPG Memoir, 45, 79-8 1. 

SIMON, .T. (1973): 0 nekim rcz ultatim<l reg ionalne 
korclacije litostratigrafskih j eclini ca Ll jugozapad­
nom poclrucju Panon skog bazena. - Na ft a, 24/12, 
623-630, Zagreb.