these isomers, most of the so-called naphthalene-phenan-threne maturation parameters were defined. Examination of a great number of crude oils and source rock bitumens, as well as the corresponding geosynthetic reactions, has shown that in addition to a —► P isomerization, alkyla-tion-dealkylation processes were also an important factor in maturation changes[5, 11, 13, 20, 30, 31]. It was therefore suggested that parameters based on this type of reactions may also serve for crude oil maturity estimation [11, 32, 33].
In this sense, searching for more reliable maturation parameters, aimed at improved crude oil classification, naphthalene and phenanthrene isomerization and dealkylation processes, as possible basis for designing new parameters, were studied in detail in this paper. Their applicability was veribied by comparison them with known maturation parameters calculated from distribution and abundance of aromatic hydrocarbons, with the help of factor analysis (programme SPSS 10.0 for Windows; [34-37]).
Crude oil samples recovered from the Banat depression were used as the objects under study. As to the Banat depression it constitutes a major part of the Southeastern Pannonian Basin (Serbia). The Banat depression is divided into two parts. The major part of the Banat depression is located on the territory of the Province of Vojvodina (north of the rivers Sava and Danube). While its smaller, southern part, is located south of Danube, near the City of PoZarevac, and is called the Drmno depression [38]. Most of these crude oils were not exposed to microorganisms in reservoirs [39, 40]. Minimal biodegradation was observed only with several samples originating from the Velebit and Boka oil fields [41, 42]. These crude oils are characterized by mixed origin: Vojvodina crude oils by a larger contribution of marine type biomass and those from the Drmno depression by biomass of terrestrial origin [40, 43]. According to depositional environment, the Drmno depression crude oils were found to differ from each other [44]. Similar differences were also observed, to a certain degree, with some crude oils originating from the Banat depression [40].
So far several bulk parameters of these crude oils were determined. Also, n- and isoprenoid aliphatic alkanes and polycyclic alkanes of sterane and terpane types were analyzed in detail [39, 41, 43]. The corresponding hopane and sterane parameters suggested the Southeastern Pannonian Basin crude oils to have originated mainly from source rocks of tertiary age. They were of medium to high maturity [39, 40, 45, 46], i.e., the Vojvodina oil field crude oils (e.g., Kikinda and Mokrin-south) of higher maturity compared to crude oils from the Drmno depression (e.g., Bra-darac-Maljurevac) [43, 45].
Aiming at improved classification of crude oils, particularly those originating from the Pannonian Basin, the maturity of twenty one crude oil samples from twelve Banat depression oil fields was studied in this paper on the basis of known and new maturation parameters calculated from abundance and distribution of the components in the aromatic fractions.
SAMPLES AND METHODS
Samples and geological setting
The examined crude oil samples originated from the Pannonian Basin. This basin, of an area of approximately 260000 km2, belongs to the category of hyperthermal basins, characterized by high geothermal gradient, in some parts up to 70°C/km. Such a high geothermal gradient provided satisfactory heating even of Pliocene sediments [38]. The Pannonian Basin is of complex geological structure. It is not a uniform sedimentary basin. During Neogene and Anthropogene intense tectonic activity, followed by volcanism of variable intensity, resulted in the formation of a number of tectonic depressions, characterized by specific geological and geotectonic development [47]. The oil window in this basin is considered to begin at a depth of 2-3 km and to end at 3.5 to 5 km (corresponding to a vitrinite reflectance of Ro = 0.60-1.30%). Hence, most of the Middle Miocene and Lower Pannon sediments already passed through the oil window phase [48, 49].
The Serbian part of the Pannonian Basin consists of Tertiary Banat, South Backa, Danube-Morava and Srem sub-basins, each of which having an independent tectonic, sedimentary and geothermal history [50]. The Banat depression, of a area of approximately 13500 km2, is located in the southeastern part of the Pannonian Basin. Tertiary sediments are at a greatest depth north of Kikinda (over 4500 m). Towards the south and west the depression is shallower. Successive occurrences of petroleum and gas in the profile of tertiary sediments of the Banat depression, like in the Pannonian Basin generally, indicate the existence of several migration paths and corresponding hydrocarbon accumulations.
Twenty one samples of crude oils from twelve Banat depression oil fields were investigated in this paper, fourteen samples originating from localities in Vojvodina and seven samples from the Drmno depression localities. These crude oils are produced from reservoir rocks located at depths of 755.0-2304.5 m. All Drmno depression crude oils are found in reservoir rocks of Miocene age, and the crude oils from Vojvodina localities occur in reservoir rocks from Paleozoic, Mesozoic, Miocene and Pliocene age. The Drmno depression stratigraphy has so far been studied in detail and the source rocks of the corresponding crude oils were identified [50-54]. Crude oils from boreholes Bradarac-Maljurevac 2 and 4 originate from Red Formations. By seismic investigation a large fault was discovered between these two boreholes (2 and 4). The boreholes Sirakovo 1, 2 and 18 are located in one zone, i.e., in the deeper, faulty zone, whereas the borehole Sirakovo 20 is situated in another, shallower zone of the Ottnangian-Carpathian sediments. A Badenian oil deposit was found at the depth of 1989-1985 m [46, 52, 53]. Detailed strati-graphic relationships between the Vojvodina Banat depression crude oils have so far not been determined. The locations of their source rocks are still unknown.
The list of the investigated samples, including data on depths, temperature, lithology and age of the correspond-
Table 1. The investigated crude oil samples
N° Sample Oil field Bore hole Depth (m) Reservoir temperature (°C) Lithology Age
1 V1 Kikinda 23 1196- -1200 105.0 Sandstone Pliocene
2 V2 Kikinda 49 1730- 1781 / Sand Pliocene
3 V3 Kikinda-Varos 3 1897- 1942 108.0 Shale Paleozoic
4 V4 Velebit 87 753- 759 61.4 Sand Pliocene
5 V5 Velebit 98 752- 758 / Sand Pliocene
6 < g Q V6 Velebit 120 756- 758 / Sand Miocene
7 V7 Mokrin-south 8 2040- 2047 116.0 Conglomerate Miocene
8 О > о V8 Mokrin-south 11 2040- 2045 117.0 Sandstone Miocene
9 V9 Jermenovci 1 896- 899 61.7 Marly sandstone Miocene
10 > V10 Boka 37/2 1196- -1206 76.9 Sandstone, limestone Miocene
11 V11 Karadordevo 10 2557- 2572 139.2 Sandstone Mesozoic
12 V12 Itebej 8 2190- 2198 126.6 Aleurolite Mesozoic
13 V13 Elemir 19 1657- 1668 99.0 Sandstone Miocene
14 V14 Velika Greda-south 20 1006- 1010 60.4 Large-grain sandstone, conglomerate Miocene
15 PO1 Sirakovo 1 1778- 1782 101.9 Sandstone, aleurolite, breccia, conglomerate Miocene (Ottnangian-Carpathian)
16 и < > w < >N PO2 Sirakovo 2 1701- 1704 98.0 Sandstone, aleurolite, breccia, conglomerate Miocene (Ottnangian-Carpathian)
17 PO3 Sirakovo 18 1544- 1548 92.2 Sandstone, marlstone, aleurolite, limestone Miocene (Ottnangian-Carpathian)
18 О см 1 PO4 Sirakovo 20 1440- -1444 87.8 Sandstone, marlstone, aleurolite, limestone Miocene (Ottnangian-Carpathian)
19 О N PO5 Bradarac-Maljurevac 2 2302- 2307 121.9 Crystalline rock Miocene (Red series)
20 & Q PO6 Bradarac-Maljurevac 4 2156- 2170 116.0 Marlstone, sandstone, breccia, conglomerate Miocene (Red series)
21 PO7 Bradarac-Maljurevac 5 1985- 1989 107.0 Sandstone, aleurolite, breccia, conglomerate Miocene (Baden)
ing reservoir rocks, is given in Table 1. The locations of the corresponding oil fields are shown in Fig. 1.
Analytical methods
Saturated hydrocarbons were isolated from the crude oil samples by column chromatography using silica gel as adsorbent and petroleum ether as eluent. Polycyclic alkanes of triterpane and sterane types in the corresponding saturated hydrocarbon fractions were analyzed by gas chromatographic-mass spectrometric technique (GC-MSD). Hewlett Packard 5890, Series II gas chromatograph, with a HP-5MS capillary column, using helium as a carrier gas (1 cm3/min) was coupled with a mass selective detector (Hewlett Packard 5972 MSD, 70 eV). The Single Ion Monitoring (SIM) method was used for the identification of individual components. Ter-panes were identified on the basis of m/z 191, and steranes based on m/z 217 ion fragmentograms.
Naphthalene and phenanthrene isomers were isolated and identified using a specially designed procedure [32, 33, 55], consisting of preliminary elimination of asphaltenes (precipitation by «-heptane), NSO compounds (insolubile in «-hexane; separation by Soxhlet extraction) and «-alkanes (urea adduction), followed by column chromatography (y-Al2O3; 80 : 1) using «-hexane and «-hexane/ben-zene mixture (3 : 1) as eluents for quantitative isolation of di- and tricyclic aromatic hydrocarbons, and benzene and benzene/ethanol mixture (1 : 1) for isolation of polycyclic aromatic hydrocarbons with more than three rings and traces of resins. Eluates were collected in 5-10 cm3 portions whose separation into saturated mono-, di- and tricyclic aromatic hydrocarbon fractions was achieved by the help of UV spectroscopy (Specord, UV-VIS) and thin layer chromatography (Silufol 254; standard mixture of «-hexylben-zene:1,6-dimethylnaphtalene : phenanthrene : chrysene = 20 : 5 : 3 : 1; mobile phase «-hexane: chloroform = 95 : 5; UV lamp, X = 254 nm).
Hungary Subotica
1. Kikinda
2. Kikinda Varos
3. Mokrin-south
4. Elemir
5. Karadordevo
6. Itebej
7. Boka Jermenovci
9. Velika Greda-south
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