Organic geochemical study of the Kalirahi formation (Prinos-Kavala basin)
P. Kiomourtzi
Kavala Oil S. A. Hydrocarbon exploration and exploitation, Greece
A. Zelilidis
Department of Geology, University of Patras, Greece
N. Pasadakis
Mineral Resources Engineering Department, Technical University of Crete, Greece
ABSTRACT
Core samples from the first exploration well in
Kalirahi oil formation, located in Prinos-Kavala
basin (North Greece) were studied using
standard petroleum geochemistry methods. The
analytical results revealed that the source
organic matter, the depositional environment
and the reservoired oil in this formation are
similar, to a great extent, with the formations of
Prinos and Epsilon fields located in the same
basin, being at the same time well
distinguishable from the North Prinos
formation. A less significant input of terrestrial
organic matter in oil generation, compared to
the adjacent formations, was observed. The
presented analytical data are essential in order to
understand the oil generation, maturation,
migration and accumulation into formations
such as Kalirahi formation and similar
structures in the Prinos-Kavala basin, which will
aid future oil exploration in the area.
1. INTRODUCTION
In this work standard analytical techniques of
petroleum geochemistry were employed to
characterize twenty six core samples retrieved
from the exploration well K1 in Kalirahi
formation. The main objective of the study was
to reveal similarities and/or differences existing
between the organic content of the cores within
the respective of other adjacent formations
(Prinos, North Prinos and Epsilon) located
within the same basin (Kiomourtzi et. al 2008
a& c, Kiomourtzi et. al 2007, Pasadakis et. al
2005, Georgakopoulos 1998, a,b,c).
2. GEOLOGICAL SETTING
Prinos-Kavala is a tectonic basin located at the
southern margin of the Rhodope Massif, formed
during Palaeogene period, trending on NE-SW
direction, bounded to the south by Thassos
Island and to the north by the mainland in North
Aegean Sea (North Greece), as shown in Figure
1. The basin, with a length of 38 km and width
of 20 km, was rapidly filled with Tertiary
sediments, reaching a thickness of 5.8 km at the
deepest central parts, due to fast subsidence,
starting at Miocene and continuing till today
(Proedrou et, al, 1994 and 2004, Kiomourtzi,
2008, b).
The importance of this small scale basin comes
up of the fact that it is the only hydrocarbon
producing area in Greece. Exploration activity
during 1970s revealed Prinos oil and South
Kavala gas fields. Further exploration activities
continue to nowadays, resulting in the discovery
of new satellite fields within the basin, such as
Kalirahi, formed by an anticline controlled by a
SE-NW direction fault.
Figure 1. Prinos-Kavala basin (improved version
published after Kiomourtzi et al. 2007)
Kalirahi structure has been discovered at SW of
Prinos field and two exploration wells have
been drilled. The stratigraphic column consists
of all three main sequences found in adjacent
fields: the Pre-Evaporitic, the Evaporitic and the
Post-Evaporitic sequences. The upper part of the
Pre-Evaporitic sequence drilled in both wells
consists of inter-bedded sandstones and marine
shales. This Miocene age unit is equivalent to
the turbidite system that forms the oil reservoir
of Prinos field, but in Kalirahi structure it is
found few hundred meters shallower, while the
thickness of this unit is significantly less
compared to Prinos. The Evaporitic sequence of
Messinian age consists of eight evaporitic layers
inter-bedded with clastics, while the PostEvaporitic sequence consists of mainly sands
and some clays, deposited during PliocenePleistocene till today.
Saturates were eluted with n-pentane, aromatics
with toluene and NSO components with a 60:40
v/v mixture of toluene-methanol.
3. SAMPLES AND METHODS
The GC-MS analysis of saturates was carried
out using a HP 7890A-5975C system. A J&W
HP-5MS, 30m x 0.25mm x 0.25μm column was
used with the GC oven temperature
programmed from 60oC to 300oC at a rate of
6oC/min. The injection (1/150 in n-C5, splitless,
1μl) was performed with a split-splitless injector
at 250oC. The MS was operated at 70eV
ionization voltage, with the interface and source
and quantrupole temperatures set at 280oC,
230oC and 150oC respectively, in full-scan (50550 m/z). The hopanes and steranes biomarkers
were determined from the m/z 191, 217 and 218
mass fragmentograms, based on the retention
times of standard components as well as on their
mass spectra.
Twenty-six rock samples were retrieved from
available cores of the well K1 drilled in Kalirahi
formation. All samples have been chosen from
the Pre-Evaporitic sequence.
3.1 Rock-Eval Pyrolysis-Total Organic Carbon
(RE-TOC)
The Rock-Eval Pyrolysis of the core samples
was carried out on a Delsi RE2 system with a
TOC unit.
3.2 Core Extraction
Core samples dried at 40oC overnight, were
crushed and sieved using a 60 mesh (250μm)
sieve. Aliquots (about 100g) were extracted for
24h in a Soxhlet apparatus using chloroform.
Copper strips were used to remove elemental
sulphur. De-asphaltening of the bitumen was
performed with excess of n-pentane (40
volumes).
3.3 Group type analysis
The obtained maltenes were separated into
saturates, aromatics and polar (NSO)
component fractions, using open-column
chromatography on silica-alumina mixture (1:3)
according to a modified USGS methodology.
3.4 Gas Chromatography (GC)
The saturated fractions were analyzed by gas
chromatography (GC) using an HP-5890
chromatograph with FID detection. A CP-Sil5
CB MS column (30m x 0.32mm x 0.25μm)
from Supelco was used with helium as carrier
gas. The oven temperature was programmed
from 60oC to 300oC at a rate of 6oC/min. The
samples (1μl) were injected dissolved in nhexane (1/150) in split-less mode. Injector and
detector temperatures were set at 280oC and
300oC respectively. GC data were acquired and
processed using the Millenium-32 software
(Waters Corporation).
3.5
Gas
Chromatography
Spectrometry (GC-MS)
(GC)-Mass
4. RESULTS AND DISCUSSION
The analytical results and the respective
calculated geochemical indices obtained from
the RE-TOC, core extraction and extract
fractionation, GC analysis of the saturates and
the hopanes and steranes biomarker indices,
derived from the GC-MS analysis of the saturate
fraction are presented in Tables 1, 2, 3 and 4
respectively.
4.1 RE-TOC results
The examined cores showed TOC values
ranging from 0.44-2.36 %w., with an average of
1.37 %w., and the exception of K1-16 sample
that showed an abnormal value of 6.30 %w. The
high organic content of this sample, in
conjunction with its high S2 value, may be
interpreted as an indication of a source bed
within the formation. The determined relatively
high S1 values (min 1.13mg/g, max 4.67 mg/g,
average 2.36 mg/g) are considered to be due to
oil contamination. The measured S2 values
(<2mg/g), if attributed solely to the precursor
organic matter, show a poor source rock with
the exception again of the K1-16 sample.
when examined against sample depth, reveals
two pairs of samples located close to each other
(K1-30, K1-21 and K1-16, K1-26), which
according to their RE-TOC values may be
considered characteristic of source organic
matter, being less contaminated from migrated
oil. At the same time, the low Tmax values of
these samples, which are of the same range as of
the whole sample set, do not allow a reliable
estimation of their maturity level. The
calculated values of Hydrogen and Oxygen
indices from the Roc-Eval pyrolysis are shown
on the pseudo-Van Krevelen diagram in Figure
2, indicating a type III kerogen.
Table 1: RE-TOC analytical results
Sample
K1-1
K1-30
K1-21
K1-19
K1-2
K1-31
K1-3
K1-22
K1-4
K1-23
K1-5
K1-6
K1-7
K1-8
K1-9
K1-24
K1-10
K1-11
K1-12
K1-13
K1-14
K1-15
K1-16
K1-26
K1-17
K1-18
Depth
m
2290.68
2291.56
2291.81
2292.35
2293.13
2293.46
2294.24
2295.36
2296.05
2296.37
2298.28
2354.60
2356.32
2357.95
2359.38
2360.83
2360.90
2362.00
2363.46
2364.84
2366.15
2367.48
2369.17
2370.05
2370.57
2372.37
Tmax
oC
414
388
374
375
351
357
412
358
373
410
353
370
372
360
366
388
359
372
416
366
396
365
432
398
377
377
S1
1.23
2.15
1.27
4.67
2.95
2.55
1.86
1.46
2.56
1.13
1.81
1.98
2.18
1.78
3.65
11.04
3.53
2.63
2.40
2.20
2.81
2.23
4.31
1.89
1.76
1.96
S2
mg/g
0.38
1.51
1.27
1.78
1.07
1.22
0.80
1.13
1.13
0.68
0.70
0.93
0.89
0.63
1.27
3.02
1.27
1.39
1.62
1.08
1.63
1.22
6.04
1.51
1.02
1.01
S3
0.30
0.38
0.32
0.72
0.35
0.46
0.42
0.39
0.49
0.34
0.50
0.59
0.49
0.67
0.59
0.37
0.35
0.38
0.62
0.48
0.54
0.39
0.33
0.29
0.61
0.74
TOC
%
0.82
1.38
0.85
2.17
1.62
1.15
0.84
0.82
1.38
0.44
1.10
1.12
1.17
1.01
1.57
2.36
1.61
1.75
1.60
1.37
1.61
1.45
6.30
2.10
1.51
1.49
The calculated S2/S3 values correspond to a
type III kerogen. This finding is in agreement to
the calculated ratios of the pyrolyzed carbon
(PC) to the TOC, which is found to be less than
30%. The productivity index [PI=S1/(S1+S2)]
Figure 2. Pseudo Van-Krevelen diagram
4.2 Core extraction and fractionation data
The extract yields from rock samples exhibit
high values, ranging between 3.3 and 16.6mg/g
sediment, with an average of 7.1mg/g. The
respective ratio of the extract yield to the
measured TOC show the lower values for the
samples K1-16 and K1-26, indicating that their
organic matter is associated with source
material, while the remaining samples are
contaminated with reservoir oil. The low yields
of maltenes as well as their high aromatic
concentration verify the above statement.
Table 2: Extraction and fractionation analytical results
Sample
K1-1
K1-30
K1-21
K1-19
K1-2
K1-31
K1-3
K1-22
K1-4
K1-23
K1-5
K1-6
K1-7
K1-8
K1-9
K1-24
K1-10
K1-11
K1-12
K1-13
K1-14
K1-15
K1-16
K1-26
K1-17
K1-18
Extract
mg/g_s
5.1
8.3
6.4
10.1
8.6
9.2
4.9
5.7
7.6
3.3
6.6
5.4
6.5
4.6
8.3
16.6
8.4
7.4
6.5
5.6
5.4
5.7
12.0
6.3
5.0
5.7
Maltenes
83
35
33
45
93
41
82
40
90
91
54
89
96
97
93
23
28
95
92
95
93
93
26
60
92
46
Sat. Arom.
% w.
53
35
35
54
35
52
61
27
62
30
51
39
57
31
44
43
68
23
49
34
58
32
66
21
69
20
69
19
67
29
70
21
58
31
63
24
58
30
61
27
57
29
56
31
38
54
46
42
53
34
58
29
NSO
11
11
13
13
9
10
11
13
9
17
10
13
10
12
3
9
11
13
12
12
14
13
8
12
12
14
4.3 GC analytical data
The inspection of the GC profiles indicate a
certain difference between the samples from the
two layers (depth 2290m-2298m and 2354m2372m) where the cores where retrieved.
Indicative distributions of the n-alkanes (C24C34) range are shown in Figure 3. The upper
layer is more enriched in higher (C30) nalkanes, whereas the lower one shows higher
concentrations in the C24 region, which can be
attributed to a more extensive contribution of
terrestrial organic matter in the upper zone of
the formation. The distribution of the n-alkanes
shows an even predominance. Phytane exhibits
the highest
concentration among the
components identified from the GC analysis.
These characteristics of the saturates fraction
have been observed in most oil and core
samples, from Prinos - Kavala basin, according
to previous studies, and have been attributed to
marine organic matter.
Table 3: GC analytical results of the saturates fraction
Sample
K1-1
K1-30
K1-21
K1-19
K1-2
K1-31
K1-3
K1-22
K1-4
K1-23
K1-5
K1-6
K1-7
K1-8
K1-9
K1-24
K1-10
K1-11
K1-12
K1-13
K1-14
K1-15
K1-16
K1-26
K1-17
K1-18
Pr/Ph
0.19
0.40
0.38
0.22
0.28
0.29
0.17
0.27
0.21
0.24
0.13
0.18
0.24
0.22
0.21
0.26
0.20
0.23
0.24
0.20
0.20
0.19
0.35
0.29
0.21
0.18
Pr/C17
0.61
0.46
0.47
0.57
0.37
0.30
0.58
0.37
0.45
0.57
1.37
0.50
0.38
0.42
0.59
0.51
0.66
0.53
0.49
0.99
1.13
0.90
0.38
0.62
0.79
1.09
Ph/C18
2.19
1.33
1.34
1.95
1.10
0.91
2.20
1.29
1.48
1.65
6.30
1.39
1.12
1.37
2.07
1.71
2.27
1.67
1.48
3.26
3.34
2.65
1.65
2.14
2.85
3.35
CPI
0.82
0.95
0.96
0.90
0.80
0.86
0.76
0.71
0.84
0.87
0.75
0.69
0.94
0.80
0.91
0.92
0.91
0.88
0.96
0.83
0.74
0.80
1.13
1.02
0.84
0.74
The even predominance in conjunction with the
low Pr/Ph values indicate anoxic hyper-saline
environment during organic matter deposition.
4
15
x 10
K1-2
10
5
0
C24 C25 C26 C27 C28
5
6
x 10
C29 C30 C31 C32 C33 C34
K1-17
4
2
0
C24 C25 C26 C27 C28
C29 C30 C31 C32 C33 C34
Figure 3. Characteristic distributions of the C24-C34 nalkanes
4.4 Biomarkers analytical data
The characteristic hopanes and steranes
biomarkers ratios, calculated from the GC-MS
analysis of the saturate fractions (Tables 4&5)
are interpreted as follows:
Table 4: Hopanes biomarkers analytical results
Sample OLE GAM C31 C32 C33
K1-1
0.02
1.09
40.0 19.3
9.6
K1-30
0.04
1.16
46.7 21.0
9.9
K1-21
0.07
0.62
39.5 21.1 11.5
K1-19
0.10
0.93
31.6 21.7 12.5
K1-2
0.06
0.43
33.0 19.2 12.2
K1-31
0.05
0.49
31.6 17.7 12.3
K1-3
0.05
1.29
38.3 19.3 11.1
K1-22
0.05
0.34
32.1 20.2 12.8
K1-4
0.03
1.15
34.3 18.2 11.9
K1-23
0.04
1.11
33.0 17.4 11.4
K1-5
0.04
0.96
22.1 24.8 11.9
K1-6
0.32
0.55
29.2 21.5 13.2
K1-7
0.44
0.80
24.1 21.6 13.6
K1-8
0.36
0.51
24.9 21.5 12.9
K1-9
0.13
1.09
27.8 20.9 13.0
K1-24
0.07
0.72
38.9 22.1 11.4
K1-10
0.10
0.92
30.9 21.0 12.6
K1-11
0.09
0.78
32.5 21.5 12.8
K1-12
0.10
0.87
31.9 19.9 12.8
K1-13
0.10
0.87
31.9 19.9 12.8
K1-14
0.33
1.09
30.5 20.2 14.1
K1-15
0.28
1.00
29.8 21.0 12.9
K1-16
0.14
1.89
47.0 19.3 11.2
K1-26
0.12
1.06
38.8 19.6 12.2
K1-17
0.09
0.77
38.9 19.7 12.6
K1-18
0.31
0.66
40.7 22.4 12.8
C34
5.8
7.3
8.4
12.5
6.9
7.5
6.4
8.2
7.5
7.5
5.6
7.4
8.8
10.3
13.2
9.7
11.7
10.3
9.5
9.5
9.3
8.4
8.7
9.3
8.2
7.3
The oleanane index shows, with several
exceptions, low values when compared with
previously published data from other formations
of the same basin. Since this index characterizes
the higher plant input in the source organic
matter, this input should be considered as less
significant in Kalirahi formation. The calculated
gammacerane index values, which are
comparable to the respective of other formations
of the basin (with the exception of North Prinos)
indicate a similar reducing environment. The
moratane index shows a similar behaviour,
being of the same range with the respective of
Prinos and Epsilon formation, thus indicating a
similar thermal maturity level.
The observed higher C29 steranes content
compared to the C27 steranes determined in the
samples from the upper layer, may be
considered as indication of a more pronounced
contribution of terrestrial organic compared to
the lower one. This finding is in agreement with
the n-alkanes distribution as determined from
the GC analysis.
Table 5: Steranes biomarkers analytical results
Sample
R1
R2
R3
C27
C28
K1-1
0.59 0.14 1.17 34.75 30.33
K1-30
0.56 0.11 1.18 38.98 30.11
K1-21
0.57 0.11 0.87 39.40 27.89
K1-19
0.57 0.25 0.31 32.74 26.87
K1-2
0.50 0.10 0.45 29.35 31.78
K1-31
0.57 0.10 0.55 28.27 33.03
K1-3
0.46 0.12 0.57 31.72 31.87
K1-22
0.59 0.12 0.72 52.31 21.98
K1-4
0.58 0.08 0.35 26.22 31.21
K1-23
0.45 0.11 0.31 25.34 29.92
K1-5
0.57 0.10 0.28 30.08 31.12
K1-6
0.58 0.16 0.20 27.56 34.02
K1-7
0.56 0.36 0.17 30.49 29.77
K1-8
0.56 0.40 0.17 30.18 28.26
K1-9
0.58 0.26 0.26 32.49 28.32
K1-24
0.51 0.23 0.47 31.75 27.19
K1-10
0.53 0.30 0.38 33.62 27.86
K1-11
0.53 0.19 0.27 34.03 25.86
K1-12
0.57 0.20 0.26 33.73 26.67
K1-13
0.59 0.14 0.25 38.40 25.21
K1-14
0.58 0.12 0.26 40.08 24.20
K1-15
0.57 0.17 0.25 26.34 29.63
K1-16
0.54 0.22 1.26 36.01 25.91
K1-26
0.47 0.21 0.80 32.96 29.41
K1-17
0.56 0.12 0.47 34.49 26.14
K1-18
0.57 0.13 0.29 35.58 26.38
C29
34.92
30.90
32.71
40.39
38.87
38.70
36.41
25.70
42.57
44.74
38.81
38.43
39.74
41.57
39.18
41.06
38.52
40.11
39.59
36.39
35.71
44.03
38.08
37.63
39.37
38.04
3. CONCLUSIONS
Earlier geochemical studies on Prinos field and
exploration wells into the basin characterized
the source rock of Prinos Oil as waxy sapropelic
oil prone kerogen, formed from aquatic higher
plants in a strong reducing environment. The oil
has a high aromatic and polar components
concentration. The clay layers within the
reservoir formation of Prinos field were
recognized as potential source rocks.
The results of the current study show that
Kalirahi
formation
exhibits
similarities
regarding the source organic matter, the thermal
maturity and the depositional environment with
previously studied formations of Prinos and
Epsilon fields. A marine organic matter origin is
indicated, with less terrestrial organic input
compared to adjacent fields, while the
depositional environment was reducing, hypersaline. Kerogen of type III is implied, while the
potential source rocks identified into the
formation are characterized as poor and of low
thermal maturity. In addition, oil contamination
due to the presence of migrant oil is recognized.
Consequently, it is evaluated that hydrocarbons
from Kalirahi formation must have been
generated in deeper stratigraphic horizons that
obtained higher thermal maturity, and migrated
into the formation. However, thin zones into the
formation could also generate hydrocarbons
when they reach adequate thermal maturity.
On-going work in the fields of organic
geochemistry, sedimentology, petrography,
structural geology and basin analysis will reveal
changes on organic matter input, thermal
maturation level, source rocks and depositional
environments within the basin, and will improve
the evolutionary model of Prinos-Kavala basin.
ACKNOWLEDGMENTS
The authors wish to acknowledge Kavala Oil S.
A. Company, for making the cores available and
permitting the publication of this study.
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