GEOL. C ROAT. 50/1 89· 103 19 Figs. 2 Tabs. ZAGREB 1997 

Pliocene Source Rocks, Miocene Reservoir Rocks and Origin of the Gas 
Accumulation of the Irma Field (Northern Adriatic, Croatia) Based on 

Wireline-Logging 

Ivica VULAMA 

Key words: Pliocene source rocks, Miocene reservoir 
rocks, Messini an unconformity, Wirc line logging, 
Geochemical analysis, Discriminant function, Over­
pressure zone, Biogcnc gas, Middle Adria tic ridge, 
Croatia. 

Abstract 
The Pliocene source rocks of the Middle Adriatic ridge appeared 

to be thermally immature. according to Ihe geophysical and the geo­
chemical anal yses, but they may have produced biogcnc gas . Wirelinc 
logging co nfirmed theil' broad ex tent. Correlation between geophysi­
ca l :H1d geochcmicul data indicated their di stinguishing parameters: 
GR = 55 - 125 AP I, U2.18= 5 - 25 ppm, Rt = 1.5 - 4 Ohmm, 6,= 75-
170 ps/ft (x 0.306 Il s/Ill), Po,(= 1.6 - 1.9 g/cml, ¢IN = 33 - 39%, SP =-44 
(-) -56 mY, Corg. = 0.43 - 3.23%, V",(%) = 0.47·0.66, K{fh = Illite. 
These organic mm ler have been classified as a kerogene type III-II. 
Characteristic geophysical parameters of the Miocene reservoir rocks 
arc: $::: 22%, R ... :::; 0.18, III :::; 1.8, Fr :::; 15.3, Ro::: 2.75 Ohmm, K avg. 
::: 2 1.4 x 10 3 pm!. Favorable lithologic- tectonic conditions (overpres­
sure zones, normal faulting) led to gas accumulation in Miocene cal ­
care nit es ami sandsto nes. The Pliocene source rocks discordantly 
overli e Mioce ne rese rvo ir rocks, separated by the Messi nian uncon ­
formit y. Statistical analyses show that, alt hough the Pliocene shales 
arc thermall y illllllalUre, they fit into the model of source roc k recog­
nition. 

1. INTRODUCTION 

The Irma Oil fie ld is located in the soulh -west part 
of Ihe Middle Adriat ic ri dge alon g the border line 
between Croatia and Italy (r ig. 1). The slructural-con­
tour map fo r the Mess in ian reservoir (se ismi c marker 
"A") shows posi li on where Irma-1 well was dril led, and 
more to the northwest where Irma-2 well was drilled. 
The whole area is represented by an ant ic linal closure 
spread ing sou thward over the inte rnational boundary 
li ne, onto the Barba ra field in Ita ly. To the north , the 
structure is closed by a normal rault wi th 26 m throw 
(Figs. 1 and 2). 

Pliocene sed iment s discordantly ove rlie the 
Miocene deposits beneath the 1v1css inian unconformity. 
In Ihe lower part o f the transgressive seq uence, these 
Pliocene sed im ent s have source rocks characteristics 

INA-Na ftaplin, Subiceva 29, HR- IOOOO Z lgreb. Croatia. 

Kljucne rijeci : pliocenske maticne stijene, mioeenske 
kolek lorske sti jene, mesinijanska diskordaneija, 
gco fi zicka mjerenja, geokemij ske analize, diskrim i­
nan tna funkcija, nadpr iti snute zone, bioge ni plin, 
srednjojadranski prag, I-Irvatska. 

Saietak 
Pliocenske malicnc stijene srednjojadranskog praga pokaza le su 

se prcma geokc mijskim anal izama terrnalno nezreti m, al i imaju 
mogllcnosl pro izvodnje biogenog plina. Primjenom geofizic ki h 
mjcrenja II busot inama lI tvrdena je njihova iiira rasprostranjenost, te 
Sli korelacijom geofiz ickih i geo kcmijsk ih mjerenja utvrdeni s ljedcCi 
paramelri 7-<1 njihovo prepoznava njc: GR = 55 - 125 AP I, U2.ls= 5 - 25 
ppm, Rt = 1.5 - 4 Ohmm, D.,::: 75 · 170 )1s/ft (x 0.306 f-ls/m), POI = 1.6 -
1.9 g/em), ¢IN = 33 - 39%, SP = -44 (-) -56 mY. Corg. = 0.43 - 3.23%, 
V 01(%)::: 0.47 - 0.66, Krrh ::: illite. Klasificirane Sll kao tip III i II 
kerogena. Karak teri sl icni geofi7-ic ki pa ra melri m ioeensk ih rezer­
voarskih slijcna plinskog polja Inna dobiveni iz gco fi zickih mjerenja 
u busol in<lma su: ¢I = 22%, Rw = 0. 18. III = 1.8, Fr = 15.3, Ro = 2.75 
Ohmm, K avg. = 21.4 x 10 3 pm2

• Povoljni litoloski i tcktonski uvjcti 
(nadpri ti snLLtc zone. nonnalno prot li sllljerno rasjedanje) lI zrokovali su 
nakupljanj e plina u mioce nskim ka lk areni tim a i pj e.~cenjac i rna na 
koje nalijczlI Pl ioccnske mati cne stijene cij u grani cll cini lllcsinijan ­
sk<l d iskordaneija . StatistiCka j c a naliza pokaza la da se plioeenski 
sejlovi, iako su tennalno nezrcli, mogu opisanim mctodama prcpoz­
[tati kao maticne sl ijene. 

recogni zed by wire lin e log interpretation, and co n­
firmed by geochemica l ana lyses . The seismic marker 
"An also represents an excellent wireli ne logging mark­
e r in the base of the source rocks recognized on the 
wells Inna· l , 2, 2a, 3, 4, Suz· l, Incs- I, lnga- l , lin- I, J-
7, J·22/l and J 23/1 (Figs. 2 and 3). The source rocks 
probably also extend soulhward across the border with 
ll a ly (Barbara fi e ld) bU I Ihc Iialian well dala arc nOI 
avai lab le fo r study. 

Previous work in these area emphasize the ex istence 
of source rocks in the region of Midd le Adriatic ridge 
(VULAMA, 1991, 1994). T he geological sClling of Ihi s 
a rca was documcntcd by KADIJA c t al. (1991 ), KADI­
JA (1992), T ARI KOVACIC ( 1995) and MILETIC & 
LUGOVIC ( 1996). Geochcmical dala wcrc provided by 
l NA-Nartaplin laboralory. 

The input data base comprised resistivity, acoust ic, 
densit y, apparent neutron poros ity, spontaneous poten­
li a l, rnicrolog and spectralog (K, Th and U) loggi ng 
responses with a secondary geochemi ca l data base: 



90 

Fi g. 1 Locat ion map or the Middle Adriatic ridge source rocks in the 
slUdy arca. 

Cargo content , Tmax. from pyrolysis, Hydrogen index, 
vit ri nite re fl ec tance (Ra), and genetic potential S I and 

S2' 
The possibility of the recognition and qualification 

o f immature Pliocene source rocks, which possibly pro­
duce biogen ic gas is discussed toge ther with the origi n 
o r gas accumulation in gas field Imm based on wirel ine 
loggi ng. The applied methods include classical logging 
data computed and interpreted through various res isti vi­
ty, acoust ic, density, neutron e lc . combinations of 
c rossp lols and s tatis tica l anal ys is (discriminant fun c­
tion) corre lated wi th geochemical analyses. 

J-5 
+ 

Geologia Croalica 5011 

2, STRA TIGRAPHY 

The lowermos t Cretaceolls, Palaeocene , Eocene, 
Oligocene, Miocene, Pliocene and Quate rnary sedi­
ments were drilled across the explorati on area. Onl y 
those Miocene, Plioce ne and Quaternary sediments 
which are related to the described source and reservoir 
rocks arc discussed brieO y. 

The Miocene sed im ents are represented by light 
gray limestones and g ray 10 li ght gray fossilife rou s 
marls. The quantity of clay in th e limestones varies 
from 10-80 %, while sandy limes tones (cal ca renites) 
often occur, which represent good collectors, The aver­
age depth of the top of the Miocene sediments is 
approximately 1500 m (KADIJA , 1992), 

Pliocene sediment s arc mostly represented by less 
fossiliferous gray 10 dark gray organic- ri ch shales, with 
inte rbedded sand and silt (MILETI C & LUGOVIC, 
1996), The average depth of the top o r the s ha les is 
approximately 1420 m (KADIJA , 1992). 

Quaternary sediments are represented by marls, 
c lays and poorly to non conso lidated sands and s ilt s, 
and in the shallower part s by coa l int e rbeds. In the 
deeper part (below approximately 1200 m) marls are 
more frequent, where the overpressure zo ne begins. Tn 
the deepest parts above the Pliocene sedim ents , lower 
concentrat ions of organic mailer were observed. 

3, TECTONICS 

T he w ithdrawal of the African plate beneath the 
Euro-Asian plate form ed the Adriatic Depression situa t­
ed between the upl ifted Oute r Dinaridcs anclthe Apen­
nines (PRELOGOVIC & KRANJEC, 1983), Eocene 

CARBONATE PLATFORM 
(MESOZOIC TO MIOCENE) 

.\ffi' 
.\'5~ 

LEGEND .\'560 

E::d SOURCE ROCK DISTRIBUTION 

~ Tor OF" A" RESERVOIR o TECfONIC·EROSION UNCONFORMITY 

~ NORMAL fAULT 

A-A' PROFILE 

Fig. 2 Si ructural contour 
map of Ihe top of the 
"A" reservoir (after 
KADlJA ct aI. , t991) 
of the trma gas field 
with di stribution of 
the source rocks over­
lying the lOp o f Ihe 
reservoir (pro fil e S­
B'). 



Vlll,llll,l: Pliocene Source Rocb. Miocene Reservoir Rocks and Origin or the Gas Accumulation ... 91 

."TrR.Ill[ ~n· 
Ir~MA· l IRtv\A·4 J 23/1 

Q'ALI' 

6000 
1000 

'500 

1100 
;000 

>500 
1200 

;00 
~ 1300 

3~ 
2 
:;:; 1400 

",00 i= 

1500 
3000 

<5"" 1600 

6000 1700 

1800 

Fig. 3 Seismic section profile A-A' with position orthe source rocks and "A" gas reservoir. 

tecton ic movements caused normal faulting (Figs. 2 and 
3) and the uplifting of the "Middle Adriatic ridge" area. 
The Eocene fault zone remained active during the 
Miocene when fault related traps were formed. Erosion 
of the uplifted sediments infilled the Adriatic Depres­
sion. The depositional surface of the Pliocene sedi­
ments was dominated by multi phase tectonic activity, 
as well as Quaternary sediment deposition. These tec­
tonic movements caused [aster subsidence of the 
African plate which exceeded deposition rate, and 
resulted in deposition of transgressive sed iments in a 
North-East direction. This explains the increased sedi­
ment thickness of the South-\Vest and reduced 
sequence 10 the North-East. 

4. GEOPHYSICAL WIRELINE-LOG RESPON­
SES CHARACTERISTIC FOR SOURCE ROCKS 

IN llASE OF THE PLIOCENE SHALES 

All the ava il able logging surveys: natural radioac­
tivity, sonic transit time, bulk density, apparent neutron 
porosity, formation resistivity, spectralog, crossplots 
and statis tic analysis were applicd to determine the 
recognition and classification of the source rocks. 

4.1. NATURAL GAMMA-RAY LOGGING 

The increased natural radioactivity in the clastic and 
carbonate sediments is the first indication of a possible 
source rock, if the sediments arc or marine origin. Fresh 
water source rocks do not show high natural radioactiv­
ity because fresh water docs not contai n uranium ions. 
In the Pliocene basal shale natural radioactivity as high 

as 130- 150 API un its has been observed, while the 
basal and uppermost sediments can be characterized by 
40-I!O API units (Figs. 4. 5 and 16). 

4.2. SONIC TRANSIT TIME LOGGING 

Sonic transit time logging (LJ.), as well as bulk den­
sity logging, indicate differential compact ion between 
sediments poor in organic matter and the source rocks. 
Sonic transit time indicates the organic rich layer within 
the impermeable sediments (Figs. 4 and 5). The sonic 
transit time wireline logging is more useful than bulk 
density logging in the cases where the hole is rugose or 
if heavy minerals (e.g. pyrite) occur. As the sonic tran­
sit time is a function of several factors (water - organic 
matter relation, mineral content, carbonate and clay 
content and pressure), it is always useful to apply com­
binations of the various types of logs. The various 
erossplo t combination of sonic transit time and resistiv­
ity proved very effective. The most often used cross­
plots include the following com binations: sonic transit 
time, formation resistivity, bulk density, natural 
radioactivity, apparent neutron porosity or cores corre­
lation's (calibration), as presented on Figs. 4 and 5. 
Sonic logging can also be applied to calculate the total 
sonic transit time (ms) and the interval velocities (m/s), 
which can distinguish the source rocks (Fig. 6) . The 
interval velocity curve (lTVI ) shows the clear decline 
of the sonic transit time in the source rock zones. 

4.3. BULK DENSITY LOGGING 

Bulk density (Pb) is a function of matrix density 
(Pilla) and tluid density (Pr)' The greater the tluid content 



92 Geologia Croal ica 50/1 

IRMA-I PLIOCENE SOURCE ROCKS 
MIDDLE ADRIATIC RIDGE 

~ f ~ 

I ~ ? 
~ ~ , 
~ 

" 
~ 
~ 

~ 1< 

I ~ ,~ < ~ j) 1=( 
J 

f ? 

~ ? 
~ 

,~ ~. ~ < ~ 1 ~ 

~ , <;;; 1< t(, §f- ~ <I ), ~ ~F" 

~ l<- I 

"i---P I !? ,1 ~ 
< !:::: I 

,1M 
r r r-~ !:;- ~ 

!c-I I£:; t ~l' ~ 
po 

f) ~ 1 
I < 

o f the formation, the g reater the rock porosity. The 
equation 1'01' calculating rock porosity ($) is as follows: 

<i>=(p",;,-pb )!(p",,,-p,). 

In marl s (sha les) with almost identical degrees or 
compac tion, all the physical parameters, even the waler 
saturati on (S,...) arc equal. If the source rock density (P,lr) 
is lower than the shale-marl density (P.\h)' then organic 
matte r (Pom) is present , and this implies that these marls 
(shal es) are impermeab le and indicates that the rock 
density decline is not the result of the hydrocarbon (fl u­
id) presence in the rock. 

Within the above mentioned assumptions it is possi­
ble to calcul ate the vol ume percentage of the organic 
matter - V 01(% ): 

VO(%) = (P ~h - Psr)/(Psh - Pom) 

The density of organic matter (Pam) is almost equal 
to the water density (Pwl = 19/cm'. 

The average Vo/%) value for Middle Adria ti c ridge 
source rocks is 0.47 -0.66 %. 

Fi gures 3, 4 and 8 show the decline in bulk density 
in the source rock zone compared to the overlying and 
underlying layers . Bulk density loggi ng in the layer 

1< 

\ 
~ 

~ 
I~ 

~ 
~ ~ 
I} 

~ "-

~ 
p. 

~ S 

Fig. 4 Composit e log responses char­
acteri st ic ror Pliocene sou rce 
rocks. Irma- I. 

above the source roek interva l (Irma- I we ll ; Pliocene 
deposits - silt-shale) sugges ted 2. IS g/cm' as the aver­
age bulk density while in the source interval th is value 
was 1.6 g/cm3

. The average vertical resolution of the 
wire logging tools - FOC (Sch lumberger) and COL 
(Dresser Atl as) for rock dens ity survey is around 60 
cm. 

In recognizing source roeks by bulk density logging, 
specia l aucntion should be paid to the highe r concentra­
tion of heavy mineral s, especially pyrit e indicating 
rcduci ng conditions (as well as the presence or organic 
matter). Radioact ive thorium Th232 is vcry often bonded 
to pyri te, as in the case of the Irma gas fi e ld (Figs. 4 
and 5). Hole rugosity is also important due to the pres­
e nce of fluid in the increased hole diam ete r. This 
applies similarly to strata containing fluid in pore/frac­
ture space. 

4.4. APPARENT NEUTRON POROSITY LOGGING 

Apparent neutron porosity loggi ng (tPN) is primarily 
performed in the evaluation of the porosity and litholo­
gy. The meas urement is pr im ari ly the response of 



Vul:mw: Pliocene Source Rocks Miocene Reservoir Rocks and Origin of the G"IS Accumuhtion 93 

IRMA- 2 PLIOCENE SOURCE ROCKS 
MIDDLE ADRIATIC RIDGE 

I~ 
~ 

;. f2;. 
1 ~ 

l ;:, jj 
\ ~ k'> 

'?<p 
( 

- ') < 
~ po ( ) 

.; 

~ P F- ~ 
t>- I.e I~ ~ ~ 

~ f.'p. 
~ I~~ '-

~ t " 

~ 
~F 
~t 

- :> :> 
~ > ~ 50 

'" ~p 
f---,s ,> 

h f= ~ cr? F:: b, 
1 [;> 

1-:;:; 
~ 

> = \ "" I I':' -= ( D 
.-~ <1 J m I}' { ~p ,;s, 

I~ ~ 
c 

g , 

? 

I 

, 
11f- 1 < 

!" GR ,e , RMlll , , AC CN , . 
,, __ ~P __ 2 ' . , ... R~LM .... " 

, P. t1 LL I - Q 10' 

,1 .0 ••• ~ I.L.'~-." •• !<:' 

,! £ __ R~~~_-~) _! <:.< . 

1 __ ~I:r: __ J~ 

hydrogen atoms present in the rock (they are mostly 
bonded (0 fluids or organic matter - hydrocarbons). 
Whcn the hydrogen atom concentration is high, the 
high -energy neutrons slow down and arc captured in 
the nuclei at a short distance from the logging tool. 
Accordingly, any increase in the concentration of 
hydrogen atoms will cause an apparent increase in neu­
tron porosity, registered by logging tool counting. It can 
be interpreted as the presence of fluids, free hydrocar­
bons and/or organ ic matter in the pore space. The 
shales and silty shales arc impermeable rocks, so it is 
expected that the apparent porosity increase is caused 
by the higher concentration of hydrogen atoms of 
organic matter (Figs. 4, 5 and 8). 

4.5. FORMATfON RESfSTfVITY LOGGING 

Any formation resistivity logging can be used to 
recognize source rocks. For impermcable rocks, both 

:::; r f> ..:; :;, 
IS> \ 

< I> 

5" 
) 

~ ~ i ? 
k 

{ ~ 
~ 

1<; I§. 

~ I ~ ~ F"' 

DEN K 
~ 

, U 
~~ 

Fig. 5 Composile log responses cha-
racteristic for Pliocene source 
rocks,lrma-2. 

shallow and deep surveys should give same or similar 
conductivity values (formation rcsistivity; Figs. 4 and 
5). It is necessary to lise mieroresistivity measurements 
in the cases of thin intercalat ions of organic matter in 
shales, marls etc. These appl iances are characterized by 
considerably better vertical resolution. An example is 
presented on Pigs. 4 and 5 (Irma-l and 2), In the source 
rock zone the deep induction log (RT, RILD) docs not 
show increased resisti vi ty , although, according to the 
other surveys and geochemical analysis the high TOe 
of that interval has been recognized (VULAMA, 1991, 
1994). However, the microlatherolog measurement 
(RMLL) indicated higher resistivity in the same inter­
val. 

Source rocks are often laminated and therefore elec­
trically anisotropic. This anisotropy increases the resis­
tivity of the interbedded organic-rich layers, especially 
when the source rock is mature. Resistivity can indicate 
source rock maturity (VULAMA, 1991, 1993, 1994). 



94 Geologia Croalica 50/1 

IRMA-2 MIDDLE ADRIATIC RIDGE 
PLIOCENE SOURCE ROCKS 

b:: i ~ 
f' ~ 

'" ~ 

I -'~ 
~-

\" • 
I" f-

-
- : 'JJ = 

~ I ~-
-~ 

~ 
-,., =--

~ ~ > : -" =- It"-I-I~ ~ 

~ tI1 1::,::: . § :::c 
'JJ -

== " f-

f-~ ~ -, 
~ "'" I---

< !f 
!i 

.~ 
~ 

~ ~-~f 1-

? ~ 
C/O , ;t ~ C/O 

~g 
-=r § ---t i:=" 

- (l 

:: :r 
~ 1- [JJ 

~~ 

._f'~ T A S ~ Ro~ 
£ iF. 

~ 
L 

~~ ~ 
,"'-'i:'_ "-

T 
b-- ~ ~-

~ 
;;l 

'" -II 
- DEN_LLI I. 

1 GR . 

Fig. 6 Composite log responses of the 
Pliocene source rocks on sonic 
(AC), bulk density (DEN) , gam­
ma ray (GR) and sonic in terval 
transit time (lTV I). Note the cbar­
ac teristic sonic and density log 
response (increase of sonic transit 
time, decrease of bulk density) of 
the Overpressure zone beginning 
approximately at 1260 m depth . .... 

Example of an isotropy (laminae) and low maturity 

(low resistivity val ues) can be seen on Fi gs. 4 and 5 
(RMLL). 

The irregular hole diameter (rugosity) can influence 
the formation resistivity. This is morc obvious in case 

of shallow investigation tools. Further more, resistivity 

values can be influenced by rock anisotropy (different 
vertical and horizontal res istivity), tcmpcralUre and 

wire logging tool characteristics (investigation depth, 
invasion diameter, vertical resol ution and the geometri­
cal factor). 

4.6. NATURAL GAMMA RAY SPECTROMETRY 
(SPECTRA LOG) 

Spectralog proved to be the most reliable method in 
recognizing the presence of organic matter. Numerous 

exploralionislS (SCHMOKER, 1981; MEYER, 1984, 
etc.) recogn ized that the hi ghcr radioactivity of the 
source rock zone is related to the uranium (Un x) content 
of the sediments (Figs. 4 and 5). It is assumed tha t 
plankton and various organisms absorb the uranium 
salts (ions) from the sea water togethcr with other rare 
elements and in this way uranium is concentrated in 
source rocks. 

It is also necessary to pay attention to the concentra­
tion of heavy minerals, especially pyritc, as its origin is 
connected to reducing conditions as well as organic 
matter. Radioactive thorium Th 232 is very often bonded 
to pyrite as is the case in the wells of the Irma gas field 
(Figs. 4 and 5). 

Recogni tion of high uranium radioac tivity in marine 
sed iments is almost certai nly indicative of the organic 
richness of those rocks, as it was confirmed by numer-



Vulama: Pliocene SOllrce Hocks, Miocene I{eservoir Rocks <lnd Origin of the Gas AccllJlllllrttion ... 95 

~ 

~ ~ ::: 
20 ;§ I /; 

'8 on 
0. .~ 
-S 16 " " i 0 ·c 
fl 
;l '0 

12 0 
u 

~ " 0 
u t' 
~ '" 

8 ~ '<1 
0 

F: 
~4 

~ • 

0 
0 2 3 4 Fig. 7 Thorium vs. potassium crossplot of 

the Pliocene sou rce rocks. K, Potassium Concentration (%) 

ous studies for the Croatian exploration area (VULA­
MA, 1991, 1993, 1994). There arc published case stud­
ies where uranium is concentrated in fracture systems 
through which connate water flows (uranium is not 
transported by migration processes because it remains 
bonded to the heaviest hydrocarbons in the organic 
maller). However, the existence of such fractures in our 
wells has not been confirmed. 

4.7. SOURCE ROCK IDENTIFICATION BY 
CROSS PLOTS 

Numerous autilors (MEYER & NEDERLOF, 1984; 
FERTL & CHILINGARIAN, 1980, 1990; SCHMOK­
ER & HESTER, 1990, etc.) used various combinations 
of crossplots to identify source rocks. The most com­
mon combinations were: bulk density and true (forma­
tion) resistivity or sonic transit time and true resistiv ity. 

I R M A - 2 (MIDDLE ADRlATlC RIDGE) 

" •• " ~~~ , :, ' " "" , o ¢ ¢¢ cC~o 

~" " "¢,, ~ ~ I , " , , , 
~~ , 

~"" "* • ~:t¢6'S , 
w¢ u ~ci' Dft -¢-.;.<:> ~\ 

" "" Ito 0 9 0 ~l" tlJg ~ "" 
" " .;.,t 0°-¢-'~J: ». , ¢ij~g.. " •. 

'.3 

'ii' 
i\ ", 

~~'>,,~J~¢~ 
, 

1\11 
'G, • 

-¢-.;.-¢-.;. 

o~ , ' , $3 ¢ .;. , , ~ , . • . " , , 
' . tf.;.~ , ' • , ~ , , 

~/.;. • .;.*-\ . '" . . " ';'.;.--:4 -¢-

• 

~ 

ffi '.1 " '" ;:l 
'" 

152S. 1S44m {o) SHALE 
1S44-1SMm (-¢-) SOURCE ROCK 
lS64-1S81m (¢) CALCARENITE 
IS81·1S8&m( O) SANDSTONE 

1.9 

10 15 20 " 30 35 

COMPENSATED NEUTRON POROSITY (%) 

These combinat ions of two-component diagrams 
proved to be useful for qualitative rock evaluation. 

Four types of measurement arc included in the four 
component M/N/(Z) and A/K/(Z) diagrams: bulk dens i­
ty (Pb)' apparent neutron porosity ($N)' sonic transi t 
time (~) and as the fourth component - a measurement 
representing the third dimension - Z (natural radioacti­
vi ty OR - or certain part of the spectrum - K, U or Th , 
or spontaneous potential - SP, as a rock permeability 
indicator). 

M,N and A,K arc practically related to rock porosi­
ty. Organic matter is less dense and compact than the 
rock matrix and gives the reflect ion of apparent porosi­
ty. M, N, A and K arc reached by survey combination 
calculation as follows: 

where ~t = sonic transit time; ~! r = sonic transit lime in 

3 
ZM~ 

o 
ZMin 
0 

" 
Fig. R Hulk density vs. compensated neutron 

porosity vs. Z (u ranium) crossplol of the 
Pliocene so urce rocks. 



96 

2.' 

,-. 

~ 
'" "-
;..r 1.9 
-3, 
d 
0 
....:l 

1.4 
0 

180 

160 

140 
,-. 

~ 
'" :::l 120 
~ 

~ 
100 

80 

Middle Adriatic ridge 
Source rocks 

• {> {> I 000 o~ 0 

(D) Non Source rocks o 

D=-17 .866+8338'log(AC)+ 1.213 ']og(RT) 

2.0 

LOG (RT, Ohmm; 24°C) 

o 
o 

o 00 

o 

o 

a a D=148.44-S.44*MLL+O.119*AC 

1 2 3 

MLL(Ohmm) 
4 5 

'.0 

Zmax 
150 
Zmin 
o 

Geologia Croatica 50/1 

Fi g. 9 Soni c transit time vs. resisti vit y cross-
1'101 ploued all logarithmi c scal e. The 
oblique line is the position of D=O (Dis­
criminant analysis), Points above thi s line 
(D=positivc: star) = source rocks; points 
below this line (D=ncgative: rectangle) = 
non source rock. The analyses arc from an 
area of the Middle Adriatic ridge, Sava 
and Drava depress ion (aftcr VULA MA , 
1994). 

Fig. 10 Sonic transi t time vs. resist ivity c ross­
plol plolted on linear scale. T he oblique 
line is the position o r D=O (Disc ri mi nant 
anal ys is). Point s above thi s line (D=posi­
tive: star) = so urce rocks; point s below 
Ihi s line (D:negat ive: rectan gle) = non 
source rock. The analyses arc from un area 
or the Middle Adriatic ridge. 

water ( for fresh water it is 189 J-Ls/ft , whe reas for the 
saIl water is 185 ).Is/ft · 0.306 ).Is/m); Pb = bulk density 
(g/cm3

); Pr = fluid density (for fresh watcr it is 1.0 
g/cm3, whereas for salt waler it is 1.1 g/cm3

). 

wate r poros ity which is 1, or 100%, so the equation 

should read: 

N = ( I - 'i>N) / (Pb - Prj 

A = (Pb - Pr) / (l - 'i>N ) 

where 4>N:::: apparent neutron porosity (%); 4>Nr = neutron 
K = 0.01 (6,,- 6,) / (I - 'i>N) 

0.6 ,----------,---- ------, 

Non Source rocks (0 ) 

.0 ". DO {> {> {> tf: 
~ 0 ~ 
0lJ -.............. Source rocks (v ) .• 0.25h~Et__ttO_~~=-----=-'-_I¥L'-----------=-==-==>.'-'--1 
.0 

Middle Adriatic ridge 
4----- Source rocks 

I 
0=-5.326+ 19.366*log,iDEN)-1.461 *logLo(Rn 

OL-__ ~~~ ____ ~~ ____________ ~ 
o 2.0 4.0 

Fig. II Bulk density vs. resistivi ty crossp[ot 
plo lt ed on the logarithm ic scal e. T he 
oblique line is the positi on of 0 =0 (Dis­
c riminant analysis). Points below thi s 
line (D=positive: sIal') = so urce rocks; 
point s above Ihis line (D=negalive: rec­
tangle) = non so urce rock. The analyses 
are from a n area or the Middle Adriatic 
ridge. Sava and Drava de press ion (after 
VULAMA,1994). LOG (RT, Ohmm; 24°C) 



Vulama: Pliocene Source Rocks, ,Vlioeene Reservoir Rocks and Origin of the Gas Accumulation .. 

M~ 

S 
ib 
.0 
a. 
~ 

Z 
~ 
Q 

2.4 

2.2 

2.0 

1.8 

1.6 

1.4 
1 2 

D=2.24-6.309*N+3,706*M 

D=1.72+0,098*MLL-IO.15*DEN 

3 

MLL(Ohmm) 
4 5 

Zmax 
150 
Zmin 
o 

o~ ______________ ~ ________________ ~ 
0.5 1.0 1.5 

N 

2.0 

1.8 

1.6 

1.4 

A 1.2 

1.0 

0.8 

0.6 

Middle Adriatic ridge 
Source rocks 

D=-O,27S+ 1.22*K-O.82* A 

Non Source rocks ( ,, ) 

Zmax 
35 
Zmin 
o 

0.4 L-_ ____ ___ _ ---.L ____ ___ ___ --" 

o 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 

K 

97 

Fig. 12 Bulk density vs. resistivity crossplot 
plotted on the linear scale. The oblique 
line is the position of D=O (Discriminant 
analysis). Points below this line (D=posi­
tive: star) = source rocks; points above 
this line (D=negative: rectangle) = non 
source rock. The analysis is from an area 
of the Middle Adriatic ridge. 

Fig. ! 3 M/N(Z) four component cl"ossplOI 
consisting of it combination of sonic tran­
Sil lime , bulk density, compensated neu­
tron porosity and Z=U 238

• The oblique 
line is the position of D=O (Discriminant 
analysis). Points below this line (D=posi­
tive: star) = source rocks; points above 
this line (D=negative: rectangle) = nOll 
source rock. The analyses are from an 
area of the Middle Adriatic ridge (after 
YULAMA. 1994). 

Fig. 14 A/K(Z) four component crosspiot 
comprising combination of sonic transit 
time, bulk density , compensated neutron 
porosity and Z=U~ ·>f\ . The oblique line is 
the position of D=O (Discriminant analy­
sis). Points below this line (D=positive: 
star) = souree rocks: points above this line 
(D=negative: rectangle) = non SOltrCe 
rock. The analysis is from an area of the 
Middle Adriatic ridge. 



98 

.. 
TYPEl R=0.5 

900 . 

INES-I ¢ 

~ IRMA-' 0 

~ IRMA-I 0 
0 IRMA·' -¢-

U IRMA-' 0 

bll SUZANA-l 0 , INGA-l • u 
~ TYPE II 

M 
~ 500 

@ 

~ 
~ 

~ 
R= 1.5 

400 450 500 

Tmax °C 

Fig. 15 Kerogen Iype and maturity of Pliocene source rocks of tile 
Middle Adriatic ridge. 

Figures 7- 14 and 19 show KITh, CN/DEN/Z, 
(Z=U238

), L'>,1Rt(RMLL), Pb/Rt(RMLL), MIN and A/K 
crossplots. 

The diversity of well log parameters produces a 
characteris tic and dist inguishable four point star feature 
on the spiderweb diagram. It is also possible to defi ne 
this as a litho-logging unit (JANCIKOVIC et aI. , 1988; 
VULAMA, 1991 , 1993, 1994). The spec ific example 
related to the source rocks of the Irma gas field is pre­
sented on Fig. 17. The typical geophysical parameters 
for the source rock recogn ition of Irma gas fi e ld are 
clearly visible on the complex diagrams of Figs. 3,4 
and 17. 

The cross plots Krrh showed that the main clay min­
erai is illite together with some mica (illite and mica are 
struc turally very si mi lar) and mixed components of var­
ious clay minerals (Fig. 7). The source rocks of th is 
clay mineral are alum inum and potassium rich rocks 
which can be found in the hinterland of the Middle 
Adriatic ridge, as wel! as volcan ic rocks of Pliocene 
and Plcistoccne age (TARJ-KOVACIC, 1995; MILE­
TIC & LUGOVIC, 1996). 

5. STATISTICAL ANALYSIS 

Statistical analysis was applied in order to empha­
size the simple classification rules to distinguish source 

Geologia Croatica 50/1 

GAMMA RAY CORELATION PROFILE (B-B') 0 
E 

INOA-l mES-! 1RMA_2 IRMA_I 1RMA-4 IRMA-3 SlJZ..l J-22I1 ON·' J-23I1 I P , I I I T 
H 

~ t ~ 
! r " 

1 ~ t r- £-

f I l 
t 

f r ,; t ;; 
\ .7.. t 1 ~ 

0 , 
~ 

0 

t ! I f r , 
f f r ;; 

,f 0 

t ! 
,'c' , 

t l 
} t: 
"{ N 

( { 
0 
0 

r 
< 
f ~ 

( 0 

f 
0 

; 
0 
0 

"A" Reservoire ~ 
0 
0 

) 
~ 
0 
0 

Fig. 16 Gamma ray composile log response of lhc Middle Adriatic 
ridge source rocks. 

rocks from non-sourcc rocks, based on quantitative well 
log parameters. 

The source rock well log parameters were logged 
for the Pliocene organic rich deposits of the Irma gas 
field. Non-source rocks parameters were logged a t 
intervals above and below the source rocks. These para­
meters were divided into two classes based on the geo­
chemical analyses of rock sampl es: C lass I = source 
rock; Class 2 = non-source rock. Geochemical analyses 
are based on the results of the pyrolys is, vitrinite reflec­
tion values and the carboni zati on degree of paly­
nomorphs. 

Discriminant analys is (pseudoregression scheme) 
was performed in the statist ical analysis (DAVIS, 1973; 
MEYER & NEDERLOF, 1984; VULAMA, 1991, 
1993, 1994) - Figs. 9, II and 13, Tables 1 and 2. Geo­
physical parameters (formation resistivity, bul k densi ty , 
sonic transit time, neutron apparent porosity and natural 
radioactivity urani um ) are used as coordinates to 
locate the classified rocks. 

To separate C lass I from Class 2 it is necessary to 
establish the Discriminant line. The main characteristic 
of the Discriminant fU Ilction is that the distance 
between the average values of Class I and Class 2 pro­
jections is maximal when the distance between the 
points inside a class is reducing. 

T he pseudoregression method means adding the 
appropriate value "Y" to each class: 



Vubma: Pliocene Source Rocks, Miocene Reservoir Rocks and Origin of the Gas Accumulation .. 

Description 

Sample size 

Source rock 

Non-source rock 

Misclassification 

Separation of class 
means in pooled 
standard deviation 

Class 1 Mean 

Class 1 Stand. Dev. 

Class 2 Mean 

Class 2 Stand. Dev. 

Coefficients ' : 

Beta "0"2 

Beta("M") 

Beta("N") 

Beta sonic ("A") 

Beta density ("0") 

Beta resistivity 

SHALES ONLY 
(Middle Adriatic ridge) 

MIN 

217 

100 

117 

6.6 

1.2 

0.64 

1.44 

0.76 

0.26 

2.24 

3.71 

6.31 

99 

ALL LITHOLOGIES 

(Middle Adriatic ridge, 
Sava & Drava depression) 

AIR D/R 

177 177 

92 92 

84 84 

8.9 10.4 

1.33 1.17 

0.79 0.69 

1.09 0.71 

0.98 0.57 

0.87 1.18 

-1 7.86 -5.33 

1.21 

19.36 

8.33 -1.46 

'Llb!c I Res ult s of Di scriminant analysis. Results are tabulated for different well log combinations (after VULAMA, 1994) : "M " = sonic transit 
time vs. bulk -density: "N" = apparent neutron porosity vs. bulk-density; "A" = sonic transit lime (in logw ).ls/ft); "0" = bulk -den sity (in 10g IO 
g/eIllJ) : "R" = resistivity at 24°C (inlog,o Olnnm). 

Legend: 

1 All Beta coeffi cients of Discrim inam fu nctions are statislic.llly significum; 
2 Beta "0" is the intercept. A typical equation Ixlsed on: 

MIN is: 0 =2.24-6.309'N+3.706-M. (Midd le Adriat ic ridge). 

AIR is: D=- 17.866+8.338-log(AC)+ 1.2 13- 1og( RT) (M idd le Adriat ic ridge, E, NW and SW part of Drava Depress ion, Middl e part of 
SaY:! Depression). 

OIR is: 0 =-5 .326+ 19.366· log(OEN)-1.461 '(RT) (Middle Adriatic ridge, E, NW and SW pari of Drava Depress ion, Middl e part of Sava 
Depress ion). 

Middle Adriatic ridge source rocks 

Description A/K MIN AIR D/R 

Sample size 315 315 315 315 

Source rock 200 200 200 200 

Non source rock 115 115 11 5 115 

Misclassification 3.2 6.6 7.9 8.1 

Mean - X 0.8358 0.7167 2.5195 2.51 

Mean - Y 1.4066 0.6012 127.1564 1.9710 

Slope -2.3156 1.2220 -8.4487 0.09829 
Intercept (Beta ' ) 3.3421 -0.2746 t 48.4436 1.7234 

R2 -0.7615 0.9663 - 0.2491 0.3409 

dX/dY 0.4264 -0.8189 0.11880 -10.153 

X * intercept 1.433 0.2254 17.592 -17.493 

Table 2 Result s of Di scriminant ana lyses (Middl e Adriatic ridge source rocks only). 

Legend : 

1 All l3eta coeffi cients arc statistically sig nificant ; "A"= bul k-density vs. apparent neutron porosity; uK" = sonic tl',:lIl s it tim e vs. apparent 
neutron porosity. 



100 

a) OR 
150 

c~ 

" 
p" 1041N 

'" u~ 

b) GR , 
at 2!lQ 

R , t, 

·N " ~ 

p" 

C~ 
sp 

Fig. 17 Spiderweb (min./max.) (a) and average stee ped (b) logs 
responses and C-organic content of the M iddle Adriatic r idge 
source rocks. 

N2 / (N I + N2) for Class I, ancl 

-N I / (N I - N2) for Class 2, 

where N 1 = source rock and N2 = non-source rock. 
Next, the regression upon vari able "X" is per­

formed, usi ng one of the mUltiplying regressive pro­
grams. The resu lting "pseudoregression" is the Discrim­
inant function, where "z" is replaced by "0". 

The anal ys is resu lt s in the linear value of the Dis­
c riminant "0", w hil e the param ete rs can also be 
expressed in loga rit hms if the range of their values is 
wide. The lyp ica l ca lculation of the crossp lot .1t/R t 
(Fig. 9) is as fo llows: 

D = -17.866 + 8.33S' log,o'AC + 1.21 3' log lO' RT(24°C), 

for the crossp lot Llt/pb (Fig. II ): 

D = -5.326 + 19.366' log,u'DEN - 1.461 ' logIORT(24°C), 

and for the c rossplot MIN (Fig. 13): 

D = 2.24 - 6.309' N+3.706'M 

The well log and geochemical parameters arc equa­
li ons fo r calcu lati ng "D", If "D" is posit ive then the 
rock is most probably a source rock; if "D" is negative, 
then the rock is a non-source rock; and if D ;:: 0, the 
case is considered to be undefined (Figs. l3, 14 and 
19). 

IRMA-2 
o GR 125 
Q.. __ !'.I) __ .?~ 

\ 

( 
~, 

< 
> 

/ 

.... , 
) 

I 
) 

J 
I 

J 
I 
\ 
I 

"' tn 
o 

0> 
o 
o 

Geologi;l Croa,ica 50/1 

MIDDLE ADRIATIC RIDGE 
"A" RESERVOIRE 

o RILM 15 
o RILD 15 --------- --------- ---.----. 

----
-,' 

Rmin 
• 

~ 
Rmax 

Fig. 18 PClmcability estim ation from resistiv ity gradient in Miocene 
rese rvoir rocks of the Irma gas fie ld (a consistent reHill with lab­
oratory analysis), 

Analytical resu lLs show that such a quali fication has 

certain sta ti stical e rrors. The Discriminant analys is 

method used for source rock c lassi fication showed the 

average error between 3.2 - 10.4% (Tables I and 2). 

Errors in classification can be caused by: 

I. Poor correlati on between geochemical and petro­

physical data caused by the presence of shallow 

in terbeds. One shallow intcrbcd of the non- source 

rock, sampled for geochem ical analysis can give 

fa lse data and vice versa (this case occurred during 

the geochemical analyses of gas field Irm a). 

2. Heavy minerals in high concen tration , c.g. pyrite, 

can cause low readings of bulk density and dim inish 

the resistivi ty. 

3. In unconsolidated rocks, the physical contrast 

bctween organic rich and organic poor layers is min­

imal , espec ially in the case of low conccntrations of 

organic matter. 

4. 1n very dense and compact rocks (with high veloci ty 

or the son ic wavc), the measurements of the bulk 

dcns ity can be inaccurate. 



VIJlnmn: Pliocene Source Rocks, Miocene Reservoir Rocks and Origin ofille G;)S Accumulation ... 101 

Zmax 

0.9 IRMA-I 35 

Source rocks Zmin 
0 

0.8 

0.7 
Source rocks (¢--) 

0.6 
M 

-¢- -¢- I 8' 
-¢- ,e; 

0.5 -¢- ;~ 
-¢-

0.4 -¢-

-¢-
0.3 

0.2 

-¢-
-¢--¢-

-¢-
-¢-

D=-O.27+1.22*N-O.82*M 

0.1 

0.4 0.5 0.6 0.7 0.8 0.9 1.1 1.2 1.3 

Fig. 19 M/N(Z) rour component crossplot 
consisting of a combination 0(" sonic lran­
sit time, bulk density, compensated neu­
Iron porosity and Z=U 138

. The oblique 
line is the position or 0=0 (Di scriminant 
analysis). Point s below this line (D=posi­
tive: star) = so urce rocks: points above 
thi s line (D=negative: rectangle) = non 
source rock. The analysis is from IRM A­
I well (Middle Adriatic ridge). N 

6. VERTICAL PERMEABILITY FROM THE 
FORMATION R ESISTIVITY GRADIENT 

Vertical pe1l11eability can be calculated from the dif­
ference in the val ue of the formation resistivity in the 
transition water/gas zone according to the Schlumberg­
er formula: 

K ~ eCa x 2.3/(pw-p,,))', 

where c = Schlumberger constant = 20, a = basic gradi­
ent of fo rmation resistivity Ca ~ t.R/6D x l/Ro6R ~ di[­
ference bet ween max. and min. res istivity in the gradi­
ent , where: t.D ~ different ial depth (in feet), Ro ~ 100% 
water satu rated resistivity; Ro = Fr x Rw, where: Rw = 
formation water resis tivity; Fr = formation factor; Pw = 

water density in formation conditions; Ph = CH density 
in formation cond itions). 

[-or IRM A - 2 (Fig. 18) this translates as: '" ~ 22%, 
R w ~ 1.8, l'r ~ 15.3, Ro ~ 2.75 Ohmm, t.D ~ 1580 m -
1578.5 m ~ 1.5 m x 3.28 ~ 4.9 n, 6R ~ 10 - 4 ~ 6 
Ohmm, a ~ 0.44, Pw - p" ~ 1.02 - 0.0407 ~ 0.98, Kavg. ~ 
21.4 x 10 ' ~m2. 

Evaluated vertical permeability should on ly be con­
sidered for the transit ion zone (T-zone, Fig. 5) and can 
not be app lied to the entire interval. We cou ld calculate 
permeab ility by resistivity gradi ent only when the T­
zone is clearly visible on the res istivity logs. 

7. CORRELATION WITH GEOCHEMICAL 
ANALYSES 

Interpretation of well logging data and crossplot s 
for these source rocks showed good correlat ion with 
geochemical analyses. Logging data show very low val­
ues of formation resistiv ity of 1.5-4 OhmlTI , that, 
accordi ng 10 ex ploration performed e lsewhere (VULA­
MA, 199 1, 1993,1994) indi catcd thc immaturity o[ 
these source rocks. Geochemical analyses confirmed 

this low malurity, but also suggest ed that these source 
rocks produced biogenic gas (on Irma-l well iso tope C 
and I-I depletion showed O" C -73%0 and oD - 188 (-) 
- 192%0, and chromatogram giving >98.5% of methane). 

Based 011 statistical discriminant analysis erossplots, 
6l/R, p"lR, A/KCZ) and M/N(Z) were computcd, and 
the processed geophysical parameters (p", Rt , RMLL, 
Lll' <PN' GR - U23~) were used as coordinates for the clas­
sification. The measured parameters were divided into 
two classes according to the geochemical rock sample 
analysis: Class 1 - source rocks; C lass 2 - non-source 
rocks. 

The followin g me thods were used for correlation: 
pyrolys is, vitrinite re flectance and degree of paly­
nomorph carbonization. If one of these did not meet the 
source rock criteria, this point was automatically classi­
fied as nOll-source rock, or at least listed as unidentified 
if the point was on the function (D~O) itself (Figs. 9, 11 
and 13). 

The correlation of geophysical and geochemical sur­
veys determined the following parameters as being typ­
ical for these source rocks: GR ~ 55-125 API, U2J8 ~ 5-
25 ppm , Rt ~ 1.5-4 Ohml11, 6 , ~ 75-170 I1s/fl (x 0.306 
115/m), Pb ~ 1.6-1.9 g/c m', q,N~ 33-39%, SP ~ -44 (-) 
-56 m V, Corg. ~ 0.43-3 .23 %, KITh ~ Illite. 

They are classified as kerogene type III and II (Figs. 
4,5,8,15-17). 

Characteristic geophysical parameters of Irma 
Miocene gas reservoir rocks are: $ = 22%, Rw = 0.18, m 
~ 1.8, Fr ~ 15.3, Ro ~ 2.75 Ohmm, K avg. ~ 2l.4 x 10-3 

~m2 

8. DISCUSSION AND INTERPRETATION OF 
THE PROBABLE GAS ACCUMULATION 

PROCESS IN THE IRMA FIELD 

Previous work in this area indi cated th e Irm a gas 
accumulation originated by migralion processes from 



102 

deepe r parts of the basin - from older Miocene and 
Mesozoic sed im ents (KADIJA at aI., 1991; KADIJA, 
1992). The origin and source of this gas was not clearly 
defined. Vertical migration processes which led to gas 
accumulation in Miocene reservoirs were also involved. 
These papers summari zed drilling, logging and geo­
chemical analysis data which indicated very short 
downward migrat ion pathways for biogenic gas pro­
duced by low maturity Pliocene source rocks overlying 
the M iocene reservoirs, which were result of overpres­
sure. 

All the geophysical parameters proved that the basal 
part of the Pliocene shales is rich in organic matter. It is 
a source rock confirmed by geochemi cal analysis. At 
first, the survey of the induction resistivity loggi ng tool 
(DIFL) was misleading because these curves did not 
show resist ivity increasing in the source rock zone, 
although resis tivity is an important indicator of a source 
rock. However, using the micro-resistivity curvc (MLL) 
the problem can be resolved (Figs. 3 and 4). The organ­
ic matter within the shales occurs as laminae, and could 
not have been detected by the low reso lution (D1FL) 
wire logging tool. The MLL curve clearly shows the 
th in lam ination of the source rock and e lectric 
anisotropy (Figs. 3 and 4). Very low formation resistiv­
ity val ues (1.5 - 4 Ohmm) indicate the immaturity of 
this source rock (VULAMA, 1991 , 1993, 1994). 

In the sou rce rock zone, overlying the Messinian 
calcarenites, during the drilling of Inna-I, a gas show 
was recognized at depth of 1520 m (7%; 1.7% C,) and 
at 1530 m (40%; 18% C,), and also in the Irma-2 well 
at 1588 m. The shales above source rock zone also rep­
resent a very good seal. Geochemical analysis proved 
that this was an immature source rock with the possibil­
ity of biogenic gas production (VULAMA, 1991). 

In the Irm a- I well the Overpressure zone (:1: log) 
was confirmed in the overlaying Pliocene and Quater­
nary shal es . The basal section of these shales is the 
source rock ( 1445 - 1543 m). This zone is also clearly 
vis ible on the 10lal transit time curve (,6.[) and interval 
velocity curve (lTV I), according to the deviation from 
the nOimal compaction trend of Ihe hydrostatic pressure 
(Fig. 6). The ,6.[ increase is the result of water trapped 
within the shales. The Overpressure zone is also clearly 
indicated on the Pb curve where the dev iation from the 
normal compaction trend occurs. Curve ,6.[ and Pb shows 
that the Overpressure zone begins at about 1260 m. 

The hydrocarbons migrate (and fluids in general ) in 
the Ovcrprcssure zone toward the lower pressure zone 
(MAGARA, 1978). Under hydros tatic conditions migr­
ation trends are upward and sideward s. In the Irma 
field, where overlayi ng shales were overpressured, the 
migration trend is downward to the zone of lower pres­
sure - in the calcarenites and sandstones of the Messin­
ian unconformity, where gas accumulated. Consider 
also that the capi llary pressure in the reservo ir rocks 
(especially in sandstone) is lower than in the imperme­
able - non-reservoir rocks. 

Geologi<l Croat ica 50/1 

Geochemical analyses proved that the gas of the 
Irma field is methane (up to 99.4%) of biogenic origin, 
and that it was generated fro m type III-II kerogene (Fig. 
12). MATTAVELLI & NOVELLI (1988) point out that 
80% of all gas fields in Italy contain gas of biogenic 
and/or diagcnetic origin (most of these fields are in the 
Adriatic off-shore), while 20% contain gas gcnerated 
by thermal degradation of organic matter. The existence 
of thin laminated source rocks of Plio-Pleis tocene age 
is also documented and it is assumed thal the primary 
migration s of the biogenic gas were short and that 
migration was connected to the more porous parts of 
the source rocks (MATTAVELLI & NOVELLI, 1988). 
It is also suggested by the Sp curve and gas shows on 
Figs. 4 and 5. 

In the Irma field there is direct contact between the 
source rock producing biogenic gas and the calcarenites 
and sandstones of Messinian unconformity as the favor­
able reservoir (Figs. 3-5) and there is an Ovcrpressure 
zone. The gas from those source rocks probably migrat­
ed downward - into the Miocene calcarenites and sand­
stones. It can also be assumed that parlial fracturing of 
source rocks occurred in the contact zones and gas 
migrated through these fractures. 

In the North-East part of the area gas shows have 
been observed in shallower sediments (Quaterna ry) 
above the carbonate platform (Wells: Inga-l, J-8, 123/1 
and 11n-l ; Fig. 2). Gas migrations in these locations are 
vertical (through fault's zones - KADIJA, 1992), but 
there is no vertical migration in the South-West area 
(Irma gas field). During drilling and logging overpres­
sured zones were not indicated in North-East area and 
the Pliocene (source rock) is usually missing (Figs. 2 
and 3). 

9. CONCLUSION 

The resu lts of geophysica l, geochem ical and geo­
logical investigations indicate the presence of source 
rocks of Pliocene age in the area of the Middle Adriatic 
ridge. These were immature for oil and gas generat ion 
by thermal degradation, but they were capable of pro­
ducing biogenic gas. 

The geological conditions (organic ri ch overpres­
sured shales/silty shales bearing the organic matter) and 
structural-tectonic setting (sealing normal fau lt ) of the 
source rocks, indicate a short downward migration gas 
pathway between the porous - fractured Pliocene source 
rocks and underlying Miocene calcarenites and sand­
stoncs rescrvoir. 

The correlation of gas recove red during drilling 
from source rocks and from the gas of reservoir Irma 
("AU) and the laboratory results also indicate the origin 
of the gas from the overlying Pliocene source rocks. 



Vularna : Pliocene Source Rocks. Miocene Reservoir Rocks and Origin or the Gas Accumulation ... lOJ 

Acknowledgments 

I would likc to thank dr. Gyorgy POGACSAS for 
his helpful adv ises and comments on manuscript. Spe­
cial thanks gocs to dr. losipa VELIC for hers always 
very precious comments and critic ism. Also thank to 
Vlasta TARI, M.Sc., for her comments on manuscript. 
For preparing the English man uscript thanks also to 
Borka VUCINIC and to Katiea M. KRPINA and Z. 
BUDANOVIC [or technical support. 

This investigation was approved and supported 
thanks to INA-Naftaplin oil company. 

10. REFERENCES 

DA VIS , J.C. (1973): Statistics and data analysis III 

geology.- John Wiley and Sons, Ncw York, 550 p. 

FERTL, W.H. & CHILINGARIAN, G.V. (1988): Total 
organic carbon content determined from wcll logs.­
SPE 1562, 61st Annu.Tech.ConL, 407-419, Hous­
ton. 

FERTL, W.H. & CHlLINGARIAN, G.V. (1990): 
Hydrocarbon resorce evaluation in the Woodford 
shale using welllogs.- J. Pet. Sci. Eng., 4,347-357. 

HERNITZ, Z., VELIC, l. & BARIC, G. (1995): Origin 
of the hydrocarbons in the eastern part of the Drava 
Depression (Eastern Croatia).- Gcol. Croat., 48/1, 
87-95. 

JANCIKOVIC, B., VULAMA, I. & JUNGWIRTH, M. 
(1988): Nove spoznajc 0 dubokim naslagama 
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Manuscript received November 4, 1996. 
Revised manuscript accepted April 28,1997. 



104 Geologia Croatica 50/1