meas-Application of Diamondoids as Maturity Indicators for Condensate from Drava Depression, Croatia

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Introduction
Thermal maturity is an important parameter in assessing petroleum evolution in sedimentary basins. 1 The type of hydrocarbons generated (oil or gas) depends on the type of organic matter (type of kerogen) present in the rock, and to some extent, its maturation history. During sediment deposition, organic matter is exposed to elevated temperatures and pressures. The change of kerogen to petroleum under this condition over a long period is also known as maturation. These changes mainly include temperature reactions of cracking or pyrolysis (thermal cracking) of large molecules, and the formation of smaller molecules with lower energy content and increased stability. Determination of thermal maturity level of the organic matter source rocks and hydrocarbons in the reservoir is very important in source rock-oil correlation studies, and in solving the very complex problem of hydrocarbon migration from the source rocks to the reservoir rocks.
Diamondoids show extreme thermal stability during exposure to high temperatures, therefore these compounds are used in the determination of thermal maturity of high maturity crude oils and condensates. Diamondoid hydrocarbons have general molecular formula C 4n+6 H 4n+12 . The saturated hydrocarbons have three-dimensionally fused cyclohexane rings, which results in highly symmetrical diamond-like structure. The simplest compounds are adamantane (C 10 H 16 ) and diamantane 1,2 (C 14 H 20 ) (Fig. 1).
These compounds are more stable than any other hydrocarbons, and once formed, are also resistant to biodegradation processes. [2][3][4] Most "normal" oil samples have a high concentration of other biomarkers (hopanes and steranes), and a low concentration of diamondoids. Conversely, highly mature samples of oil and condensate generally contain a high concentration of diamondoids, and very low concentration of other biomarkers 5,6 or in some cases are completely absent. During cracking to which hydrocarbons are exposed at high temperatures and pressures, [7][8][9][10][11] diamondoids remain concentrated in condensate samples due to their extreme thermal stability compared to other groups of hydrocarbons. 12,13 Different geochemical methods, such as vitrinite reflectance, pyrolysis, and biomarker maturity ratios from bitumen/oil can be used to indicate the level of thermal maturity and type of organic matter. Vitrinite reflectance (R o ) is the most commonly used thermal indicator, and meas-Application of Diamondoids as Maturity Indicators for Condensate from Drava Depression, Croatia ure of the percentage of incident light reflected from the surface of vitrinite particles by the method of optical microscopy. Vitrinite is a coaly organic maceral derived from the connective tissue of vascular plants. The reflectance of vitrinite increases with heat, with the increase in the degree of maturity of the organic matter. Most maturity parameters derived from bitumen/oil biomarkers are used in mathematical equations to calculate vitrinite reflectance (R c ; calculated vitrinite reflectance) of kerogen at the time of bitumen/oil generation.
The use of diamondoids as an indicator of the thermal maturity of hydrocarbons is based on different thermal stability of the methyl isomer of diamondoids. For example, 1-methyl-adamantane (1-MA) is more stable than 2-methyl-adamantane (2-MA), 4-methyldiamantane (4-MD) is more stable than 1-methyldiamantane (1-MD) and 3-methyl-diamantane (3-MD). Hence, the ratios 1-MA/ (1-MA + 2-MA) and 4-MD/ (1-MD + 3-MD + 4-MD) should increase with increasing thermal maturity. 2 In this study, diamondoids were used along with bulk properties, n-alkanes, isoprenoid parameters, and biomarkers in the aromatic fraction (methylphenantren index) to calculate vitrinite reflectance and estimate thermal maturity of condensates from Drava Depression, Croatia, which is difficult to find otherwise because of extremely low concentration of biomarkers in saturate fractions of these samples.

Geological settings
The Pannonian Basin is the largest extensional basin in a back-arc tectonic setting that formed during the Miocene in Central Europe. It is bordered by the Alps to the west and Carpathian Mountains to the east and north [14][15][16][17] (Fig. 2).
The Croatian part of the Pannonian Basin covers an area of approximately 26,000 km 2 , and is divided into the Dra-va, Sava, Mura, and Slavonija-Srijem depressions (Fig. 2).
Each of these basins also has its own local characteristics, similarities, and differences in evolution, sedimentary environment, transport mechanisms and tectonics, and the boundaries between the depressions are mountains. 18,19 The gas and gas condensate fields of Molve, Kalinovac, and Stari Gradac are located in the northwestern part of the Drava Depression (Fig. 2).
The development of the basin includes the periods of the Neogene and Quaternary, and the deposition, thickness, and deformations of rock structures are indicators of tectonic activity. All rocks are classified according lithostratigraphic nomenclature (Croatian lithostratigraphic nomenclature) for the western part of the Drava Depression 19 (Fig. 3). Described rocks are divided in two groups; the first group comprises younger Neogene-Quaternary (Tertiary) sediments, while the second group consists of Paleozoic-Mesozoic basement. Tertiary sediments are clastic rocks, while basement rocks are represented by carbonates, magmatic and metamorphic rocks.
There are several lithostratigraphic formations defined within the Neogene and Quaternary sediments. The oldest one, Moslavačka gora Formation (Fig. 3) is divided into two lithostratigraphic members (Mosti member is approximately of Lower/Middle Miocene age, and younger Križevci member is approximately of Lower Pannonian age). The sediments of the Ivanić-Grad Formation Ivanić-Grad are often labelled as "Banatica" deposits, due to the characteristic fossil shell Congeria banatica. The environment is predominantly fresh water lakes. Sediments of Lower Pontian, Kloštar-Ivanić Formation, (Fig. 3), are labelled as "Abichi" deposits according to the characteristic fossil shell Paradacna abichi. Sediments of Upper Pontian -Bilogora Formation, (Fig. 3), are also called "Rhomboidea" deposits according to fossil shell Congeria rhomboidea. [18][19][20][21] Depositional environments are divided in several fresh-water lacustric and fluvial areas. The sediments are represented by clayey marlstones, marlitic clay, and clay (depending on compaction), and lesser parts of sandstone or sand. Lonja Formation is mostly represented by clay, sandy clay, and sand. In the deepest part deposited are silt, marly clay or sandy marlstone, with lignite intercalations,. [15][16][17] The producing horizons lie at depths exceeding 3000 m and are characterised by specific pressure and temperature anomalies. The reservoir rocks are naturally fractured and the system of fractures controls the production rate and hydrocarbon migration trend. The reservoir is characterised by a very high initial pressure, up to 500 bar and reservoir temperature range between 180 and 198 °C. Measurement of temperatures in the same well exceed 230 °C at depth of 4000-4300 m. 14-17 3 Experimental

Samples
Condensate samples were collected in glass bottles from the producing fields northwestern part of the Drava Depression, and stored in the refrigerator at 4 °C. Before analysis, the samples were stabilised at room temperature. The locations of producing fields are shown in Fig. 2.

Bulk properties
API gravity was determined using ASTM D5002 method on an Anton Paar DMA 4500 digital density analyser. This test method covers the determination of density, relative density, and API gravity of samples that may be handled in a normal fashion as liquids at test temperatures between 15 and 35 °C.
Pour point was determined using ASTM D97 method on manual tester INKO LAB. After preliminary heating the sample was cooled at a specified rate, and examined at intervals of 3 °C for flow characteristics. The lowest temperature at which movement of the specimen is observed is recorded as the pour point.
Paraffin wax content was measured using UOP 46 test method. Sample was dissolved in petroleum ether and clarified using fuller's earth. Petroleum ether was evaporated and the clarified oil redissolved in an acetone-petroleum ether mixture. This solution was then chilled to −18 °C and filtered through a cold filter funnel, the wax being collected on mat in the funnel. The wax was then washed from the mat into a weighed flask, using hot pe-troleum ether. Petroleum ether was evaporated, and the wax weighed.
Sulphur content was measured on instrument Leco SC-144DR. Sample was weighed into a combustion boat and placed in pure oxygen environment at 1350 °C. After complete combustion process, concentration of SO 2 from sulphur was measured using infrared detection cell. The instrument converts this value to percentage value using sample weight and appropriate calibration curve.

Capillary gas chromatography (GC)
GC analysis of condensate was performed on an Agilent 7890A gas chromatograph fitted with 50 m × 0.25 mm i.d. DB-petro column with film thickness of 0.5 µm, and using helium as carrier gas. A constant flow mode and flame ionisation detector were employed. The gas chromatography oven temperature was held initially at 35 °C for 10 min, then ramped to 40 °C at 1 °C min −1 , increased to 320 °C at 8 °C min −1 , and maintained at this temperature for 60 min.

Gas chromatography-mass spectrometry (GC-MS)
Gas chromatography-mass spectrometry (GC-MS) of the saturated and aromatics fractions from condensate was carried out using quadrupole mass spectrometer Agilent MS 5975C interfaced with an Agilent 7890A gas chromatograph. GC 7890 A was fitted with 30 m × 0.25 mm i.d. HP-5MS column with a film thickness of 0.25 µm, and using helium as carrier gas. The GC oven temperature was ramped from 60 °C to 145 °C at 15 °C min −1 , increased to 315 °C at 2 °C min −1 , and maintained at this temperature for 15 min. The mass spectrometer was operated in electron impact (EI) mode at 70 eV. Saturated and aromatics fractions were dissolved in isooctane and analysed in full-scan mode (50-560 scan range). Diamondoids were examined from saturate fraction using m/z 187 (methydiamantane). Identification of phenantrene and methylphenantrene from aromatic fraction was based on multiple ion fragmentogram m/z 178 and 192, respectively.
3 Results and discussion

Bulk properties
The condensate well depths and group parameters of investigated condensate are shown in Table 1. Crude oils are classified into heavy, medium, and light, based on API gravity. Heavy crude oils have API gravity ≤ 20°; medium 20-40°, and light crude oils 40-45°, while condensate have API gravity >45°. 2 The samples analysed in this study have API value between 48-57° ( Table 1) and are classified as condensates. API gravity is also a bulk physical property of oils that can be used as an indicator of thermal maturity. 9,10 The pour point (PP) is generally associated with paraffinicity or the content of solid paraffin. The term highly paraffinic oils and condensates mainly refers to the high concentration of C 20+ n-alkane in the sample. Also, crude oil derivates from terrigenous organic matter (OM) general-ly have high paraffinic content and high PP. The data listed in Table 1 show that samples Severovci-1 (Sev-1) and Stari Gradac-6 (Stg-6) have high value of PP of up to +15 °C and +6 °C, respectively. These sample also have high concentration of paraffin wax and are waxy in nature, while the samples with low concentration of paraffin wax show very low PP and are nonwaxy in nature: Molve-31R (Mol-31R), Molve-25 (Mol-25), and Kalinovac-15 (Kal-15) ( Table 1).
Sulphur compounds are undesirable in crude oil because of the costs of sulphur removal and environmental problems associated with sulphur compounds. The crude oils are classified as sour or sweet, based on sulphur content > 1 % and < 1 %. 2 The sulphur content of analysed samples was low (< 0.25 %), thus, these samples are sweet and commercially valuable.

n-Alkanes and isoprenoids distributions
The distribution of n-alkane provides useful information on the source of organic matter, thermal maturity, and biodegradation. 22,23 For all investigated condensate samples, the distribution patterns of n-alkane was very similar; these samples mainly have bimodal n-alkane distribution.
Distribution of n-alkanes obtained by gas chromatography as well as Total Ion Chromatogram (TIC), obtained by GC-MS chromatography show the bimodal distribution of n-alkanes, with a maximum at n-C 16 and n-C 30 . Fragmentograms of n-alkanes m/z 71 are shown in Fig. 5. The maximum in the distribution pattern of n-alkanes in high molecular weight reflects that the source generating these samples had high contribution of terrestrial organic matter.
Carbon preference index (CPI) is a maturation parameter, and it shows the ratio of odd and even long-chain n-alkanes. Immature organic matter is characterised by the dominance of odd n-alkanes, and as the degree of maturity increases, the dominance of odd n-alkanes decreases, and the CPI is close to 1. The analysed samples showed no odd predominance of n-alkane, CPI is close to 1. This feature is attributed to high thermal maturity of these samples. The absence of unresolved complex mixture (UCM) in distribution pattern of n-alkane, indicates that samples had not undergone biodegradation.
Pristane to phytane (Pr/Ph) ratio in crude oil and source rocks reflect redox potential in the depositional environment and nature of organic matter. The Pr/Ph ratio (from 1.45-1.91; Table 1) indicates the oxic conditions in the depositional environment, as well as significant terrigenous influence. The ratio of isoprenoids to the corresponding n-alkanes (Pr/nC 17 and Ph/nC 18 ratio; Table 1) is primarily a maturity parameter and is expected to be low in all condensate samples (0.20; Table 1). On the other hand, biodegradation processes cause an increase in this ratio due to the faster removal of n-alkanes compared to isoprenoids; however, these samples did not undergo biodegradation processes. The Pr/nC 17 versus Ph/nC 18 plot proposed by Peters et al., also provide useful information of kerogen type and depositional conditions of organic matter. The source of organic matter is mainly mixed type II-III kerogen deposited under suboxic/oxic conditions (Fig. 4). Such organic facies in most cases generates condensates at high temperatures and pressures.
The gasoline fraction of the condensates has a high concentration of light aromatics (benzene, toluene, and xylene). High content of light aromatics is particularly characteristic of condensates from Severovci-1 and Molve fields (Fig. 6).

Maturity assessment from methylphenantrene index
Biomarkers in the aromatic fraction are preferably used as parameters for calculating the maturity assessment of hydrocarbons, and the source rocks that generated these hydrocarbons. To calculate the maturation parameters, phenanthrene and methylphenanthrene were used as the most abundant compounds in the triaromatic hydrocarbon fraction. 24 Maturity parameters derived from phenanthrene and methylphenanthrene biomarkers are used for calculat- In order to interpret the obtained results, it is important to list the processes that take place during thermal maturity of hydrocarbons. Parameters based on isomerisation (α → β) and methylation-demethylation reactions of methylated phenanthrenes are used to assess the degree of maturation of bitumen in source rocks and oil. In the early and main phase of the oil generation zone, alkylation (methylation) reactions of the aromatic ring predominantly take place while demethylation processes begin at higher stages of maturity (R o ≥ 1.3 %). The oldest defined phenanthrene maturation parameter is the methylphenanthrene index 1, MPI 1 = 1.5 x (2-+3-MP)/(1-+9-MP+P) ( Table 2). It is based on the isomerisation of α to β methylphenanthrene and on the possible formation of 2-and 3-MP by direct methylation of phenanthrene. However, the phenanthrene has high thermal stability, and when the α isomers begin to degrade at higher stages of maturity, only the phenanthrenes will remain. Consequently, the MPI-1 will decrease again at a late stage of the oil window. The MPI-1 can be converted into vitrinite reflectivity and have a positive linear relationship between 0.65 % to 1.35 % R o and a negative linear relationship from 1.35 to 2.00 % R o .
However, the 3-methylphenanthrene and 2-methylphenanthrene are thermally more stable, and in all samples, the peaks were expected to be much higher than 1-methylphenanthrene and 9-methylphenanthrene.
All condensates were expected to show a high degree of maturity based on methylphenanthrene index (MPI-1, MPI-2 and MPI-3). MPI-1 was also used to calculate the corresponding vitrinite reflection, assuming high maturity R c > 1.3 (2.3-0.6 × MPI-1) ( Table 2). All analysed condensates showed high maturity (R c > 1.5) except condensate Stg-6 (R c = 0.7). The obtained value is unrealistic, and could be result of the methylation and demethylation processes of phenanthrene that occurred in parallel at such a high degree of maturity.

Diamondoid hydrocarbons as indicators of thermal maturity
The use of diamondoids as an indicator of the thermal maturity of hydrocarbons is based on different thermal stability of the methyl isomer of diamondoids (4-methyldiamantane shows higher thermal stability compared to 1 and 3 methyl isomers of diamondoids).
Diamondoids were identified in the Severovci-1 (Fig. 7) and the Molve condensate samples (Mol-31R and Mol-25), which is additional evidence of the connection of the Severovci-1 condensate with the Molve field condensates. These condensates were probably generated at the same stage of thermal maturity from the same type of kerogen. Based on the calculated methyldiamantane index (MDI ;  Table 3), and calculation of vitrinite reflection, these samples are highly mature condensates (Table 3; R c = 1.3-1.6). In the Kal-15 and Stg-6 samples, diamondoids were not found, or were present in too low concentrations to be identified by this analytical method.

Conclusions
Based on geochemical correlation of Severovci-1 and the nearest condensate fields (Molve-31R, Molve-25, Kalinovac-15, and Stari Gradac), it can be concluded that these condensates were generated at the same or similar maturity level of organic facies corresponding to mixed type kerogen II/III with a pronounced dominance of terrestrial facies in suboxic/oxic conditions.
Diamandoids are still relatively unexplored biomarkers for determination of thermal maturity of Pannonian Basin condensates. Therefore, more oil and condensate samples, and further geochemical petroleum investigations are needed to gain a better understanding of the application of diamondoids for determination of the thermal maturity of organic matter, which would lead to more confident conclusions.