This paper aims to provide proofs of hydrocarbons migration from petroleum reservoirs towards the surface of discharge playas. This is a case study of the discharge playa of Sidi El Hani, eastern Tunisia. The geochemistry of water of some hydrological drills in the Sahel area and of water from the discharge playa proves relatedness between the deep aquifer and the water of the discharge playa. Thus, the hydrology is now more than likely converging from the subsurface. This convergence may be an agent of transport of hydrocarbons. Concerning the organic matter within the discharge playa, high percentages of different fractions seem abnormal in such a saline context. This maturated organic matter should be viewed in the widest context of a multidisciplinary study taking into account the presence of petroleum potentials in the subsurface, the converging hydrogeology, and the tectonised region. The high percentage of Aromatic Polycyclic Hydrocarbon (APH) may be the result of hydrocarbons migration rather than anthropogenic pollution. As for the reinterpretation of previous works about the organic matter in playas done in sebkha Moknine, the contaminated organic matter, which was interpreted as a human induced activity, may have another origin from a reservoir located in the subsurface of the Sahel area.
Since the study of the petroleum system alone is not enough to achieve a more precise exploration, petroleum migration has recently become an important duty for petroleum exploration and assessment in petroleum basins (e.g., [
The sebkha of Sidi El Hani is a NW-SE lengthened depression in the Sahel area (eastern Tunisia) (Figure
Geographical location of the Sidi El Hani discharge playa and samples locations.
The Sahel domain in eastern Tunisia is separated from the Atlantic domain by the so-called north-south axis (e.g., [
Tectonic summary map of northeastern Tunisia compiled from outcrop and subsurface data: 1, Mio-pliocene; 2, miocene; 3, oligocene; 4, eocene; 5, late cretaceous; 6, thrust faults; 7, major faults; 8, petroleum well; I, zone of Kelbia-Enfidha; II, zone of Kairouan-El Hdadja; III, zone of Sidi El Hani-Chorbane [
On the other hand, gravimetric data prove an important subsidence associated with faulted zones would be guessed from the surface monotony and the deep structure of the domain [
Gravimetric data localizing petroleum potentials in the Sahel area, Tunisia [
It is worth noting also that some petroleum wells are located in the Sahel area, and they have been exploited since the 1980s. For instance, the field of Sidi Kilani is located south of Kairouan, (Figure
As for the genesis of the discharge playa as such, the genesis and evolution of the coastal lagoons and sebkhas of the Tunisian eastern coast were controlled by two successive quaternary tectonic phases. A Postvillafranchian NW-SE compression was followed by a Tyrrhenian NE-SW extension [
Figure
Convergence of Kairouan aquifer towards the surface of Sidi El Hani discharge playa and the possibility of hydrocarbons migration ([
Four drills (Figure
In our case, only hydrocarbons contained in the sediment were studied. One sample was obtained from the mixture of samples from different locations of the surface of the discharge playa (Figure
The sample of sediment was first placed in the steamroom at 40°C to be dehydrated; then it was crushed manually by a mortar. Only the fraction that was smaller than 63
An amount of 10 g of the desulfured fine fraction was placed in a cellulose cartridge of a soxlet. The organic solvent was composed of 100 mL of methanol (MeOH) and 200 mL of dichloromethane (CH2Cl2) evaporate under the heat effect. Hence, it joined the cooling agent while passing by the tube supply main; thus, it condensed to fall within the cartridge of extraction. The sample of sediment macerated with organic solvent. Once the cartridge was full, the liquid accompanied by the extracted organic matter turned over in the balloon; the cycle continued making the balloon progressively enriched by the organic matter. It was necessary to count approximately 7 hours in order to extract by this method all the organic matter of a sample. Thereafter, the balloon was placed in a Marie bath to evaporate solvent and recuperate the organic matter. Then, the concentration contained in a sample was obtained by weighing the dry residual.
The purification of different components of hydrocarbons was done by fractionation. To be able to dose with precision only hydrocarbons, other components should be eliminated from the organic matter; the residual suspended with dichloromethane passed through a burette containing the florisil, which is a gel that can be crossed only by hydrocarbons and others negligible lipidic compounds. The total hydrocarbons were obtained after the passage in the burette. The concentration of these total hydrocarbons is determined also by weighting the dry residual obtained after evaporation. The amount of hydrocarbons in this sample was compared with data of Sidi Kilani field recently published by Boussiga et al. [
The detection of hydrocarbons of the saturated fraction was carried out by gas chromatography (GC) coupled with mass spectroscopy (MS). Thus, these aliphatic hydrocarbons were analyzed by gas chromatography—mass spectrometry HP 6890-HP 5973 MSD combination (Agilent Technologies, Wilmington, DE, USA). The GC was used with a 30 m fused-silica column (0.25 mm i.d.) coated with 5% phenyl methyl siloxane. Helium was used as the carrier gas at a flow rate of 1.4 mL/min. The temperature program was as follows: temperature held at 40°C for 1 min, then increased from 40 to 120°C at 30°C min−1, 120 to 300°C at 5°C min−1 with the final isothermal hold at 300°C for 20 min. The mass spectrometer operated in the electron impact (EI) mode at 70 eV (ionization energy) and scanned from 50 to 650 Dalton. Compounds were identified by comparison with published mass spectra and relative retention times.
At the level of Sidi El Hani discharge playa, samples of sediment were taken from the subsurface (from 150 cm to 300 cm) (Figure
As it is shown in Figure
Correlation between four drills showing the evolution of the shallow subsurface in the vicinity of Sidi El Hani discharge playa.
Concerning Piper diagram (Figure
Piper diagram of samples of water from the aquifer of Kairouan and Sidi El Hani discharge playa.
In such a diagram, similarity between samples generally appears in spite of the different concentrations. Thus, samples having genetic relationship are presented by parallel lines. As for the studied samples, they present for the most an evident similarity (Figure
Schoeller-Berkalof diagram of samples of water from the aquifer of Kairouan and Sidi El Hani discharge playa showing their geochemical relatedness.
The elimination of the elementary sulphur shows that the sample form Sidi El Hani discharge playa is rich in sulphur (5 g/kg). This strong presence may be explained by the activity of anaerobic bacteria in a reduced environment, as it may be explained by sulphur migration from a petroleum reservoir. This high amount of sulphur is noticed in many saline environments such as the sebkha of Moknine (e.g., [
Concentrations of the organic matter (OM) and total hydrocarbons (THC) in the mixed sample of sediment from Sidi El Hani discharge playa.
On the other hand, Figure
Concentrations of the total hydrocarbons (THC), polar fraction (PF), alkane (ALK), and aromatic polycyclic hydrocarbons (APH) in the mixed sample of sediment from Sidi El Hani discharge playa.
The chromatogram of saturated hydrocarbons (Figure
Chromatogram of saturated hydrocarbons in the mixed sample of sediment from Sidi El Hani discharge playa.
From the surface downward, the investigation of heavy metals and other chemical elements (Table
Concentrations chemical elements (ppm) of different sediment samples from Sidi El Hani discharge playa.
Samples | Al | Ca | Fe | Mg | SO4 | V | Zn | Ba | Co | Cr | Cu | K | Mn | Ni | Sr | NaOH |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
S1 | 737 | 2937 | 684 | 432 | 850 | 1.18 | 1.85 | 3.79 | 0.25 | 1.04 | 0.3 | 179 | 9.12 | 0.46 | 29 | 516 |
S2 | 448 | 2206 | 351 | 526 | 4750 | 0.62 | 1.36 | 1.2 | 0.14 | 0.6 | 1.2 | 102 | 3.26 | 0.27 | 66 | 591 |
S3 | 541 | 1715 | 425 | 316 | 228 | 0.85 | 1.44 | 2.67 | 0.18 | 0.84 | 3.3 | 124 | 5.43 | 0.33 | 17 | 409 |
S4 | 514 | 2150 | 560 | 400 | 570 | 1.4 | 1.6 | 3.5 | 0.2 | 1.1 | 0.8 | 138 | 4.2 | 0.5 | 32 | 569 |
S5 | 812 | 2050 | 640 | 520 | 840 | 0.9 | 1.7 | 1.8 | 0.42 | 0.6 | 2.7 | 146 | 4.9 | 0.36 | 49 | 490 |
S6 | 753 | 1960 | 570 | 490 | 790 | 1.1 | 1.6 | 1.6 | 0.7 | 0.4 | 3.2 | 198 | 5.6 | 0.65 | 58 | 567 |
S7 | 492 | 1650 | 580 | 380 | 860 | 1.3 | 1.2 | 2.9 | 0.9 | 1.2 | 2.2 | 173 | 5.3 | 0.25 | 47 | 612 |
S8 | 613 | 1960 | 379 | 420 | 690 | 1.6 | 2.2 | 2.6 | 0.1 | 0.9 | 0.9 | 142 | 5.7 | 0.4 | 45 | 480 |
S9 | 759 | 2140 | 460 | 510 | 760 | 2.1 | 2.4 | 3.1 | 0.13 | 0.7 | 1.5 | 156 | 3.9 | 0.6 | 38 | 561 |
S10 | 548 | 2640 | 510 | 380 | 890 | 1.6 | 1.8 | 2.2 | 0.8 | 0.65 | 2.9 | 215 | 4.7 | 0.45 | 60 | 523 |
S11 | 623 | 2750 | 590 | 470 | 760 | 1.5 | 1.5 | 2.7 | 0.35 | 0.45 | 1.8 | 167 | 3.9 | 0.56 | 69 | 486 |
Evolution of the amounts of chemical elements along an auger drill in Sidi El Hani discharge playa.
They are in some extend correlated with aluminum evolution (Table
Coefficient of correlations of chemical elements with the first two factors of the principal component analysis.
Factor 1 | Factor 2 | |
---|---|---|
Al | −0.513660 | −0.568188 |
Ca | −0.339586 | −0.395070 |
Fe | −0.552638 | −0.280840 |
Mg | 0.299645 | −0.675830 |
SO4 | 0.810643 | −0.043908 |
V | −0.593311 | −0.245553 |
Zn | −0.526560 | −0.241687 |
Ba | −0.809000 | 0.425156 |
Co | 0.005683 | −0.322803 |
Cr | −0.355922 | 0.800061 |
Cu | 0.227554 | −0.253303 |
K | −0.520373 | −0.550914 |
Mn | −0.636364 | 0.195458 |
Ni | −0.497073 | −0.656034 |
Sr | 0.524855 | −0.697106 |
NaOH | 0.250498 | −0.201187 |
Principal component analysis of chemical elements of different sediment samples from Sidi El Hani discharge playa.
For instance, water recuperated from this field is particularly enriched with zinc (Zn) (0.03 mg/L), lead (Pb) (0.1 mg/L), manganese (Mn) (0.46 mg/L), iron (Fe) (6.28 mg/L), and barium (Ba) (1.3 mg/L). The concentrations of these metals largely exceed the national standards in force and thus constitute a significant polluting content. Nevertheless, once migrated towards the surface of the discharge playa, these components become diluted and represent no environmental threat. But the proportionality between the amounts of different components maintains the traces of the migrating fluid. Table
[Zn]/[Fe] ratio of different sediment samples from Sidi El Hani discharge playa and Sidi Kilani petroleum field.
Location | [Zn]/[Fe] |
---|---|
Sidi Kilani | 0.0047 |
S1 | 0.0027 |
S2 | 0.0038 |
S3 | 0.0033 |
S4 | 0.0028 |
S5 | 0.0026 |
S6 | 0.0028 |
S7 | 0.0020 |
S8 | 0.0058 |
S9 | 0.0052 |
S10 | 0.0035 |
S11 | 0.0025 |
This geochemical exploration of Sidi El Hani discharge playa and its watershed found out favorable conditions for the migration of hydrocarbons. Added to previous geophysical studies of the Tunisian Sahel area, which discuss the possibility of the existence of a potential petroleum reservoir located somewhere in the Sahel subsurface, the geochemistry of water of some hydrological drills in the Sahel area and of water of the discharge proves relatedness between them. Thus, the hydrology is now converging from the subsurface. This convergence may be the agent of transport of hydrocarbons. Concerning the organic matter, high percentages of different fractions seem abnormal in such a saline context. But this maturated organic matter should be viewed in a widest context of a multidisciplinary study. Because the high percentage of aromatic polycyclic hydrocarbon (APH) may be the result of a hydrocarbons migration rather than anthropogenic pollution because in these areas the polluting human induced activity is next to absent. On the other hand, hydrocarbons migration toward such a discharge playa necessitates the coexistence of many conditions at the same time. First, the presence of an actual petroleum reservoir in the vicinity of the discharge playa; second, a fractured zone in order to facilitate the mobility of hydrocarbons, and third, a convergent aquifer toward the discharge playa to unsure the transport of hydrocarbons. These very conditions are verified in the case of Sidi El Hani discharge playa. First, the gravimetric data suggest the possibility of a hydrocarbon reservoir located in evaporitic domes in the vicinity of Sidi El Hani discharge playa [
Chromatogram of saturated hydrocarbons in a sample of sediment from sebkha Moknine ([
But it is worth noting that Amari and Bedir [
Hydrogeological map of Moknine region and the effect of the tectonic structures on the shape of the aquifer ([
This study based on a geochemical exploration of water, organic matter, and heavy metals allowed the investigation of hydrocarbons migration towards Sidi El Hani discharge playa. Such migration is enhanced by a very particular geological framework characterized by the presence of a petroleum potential in the Sahel area providing with migrating hydrocarbons, a subsurface tectonised facilitating the migration through faults and fractures and aquifers converging toward the surface of the discharge playa behaving as agent of transport. In doing so, this study highlights the multidisciplinary approach in petroleum exploration. Moreover, it shows how this approach saves researchers from the misinterpretation of the high content of a contaminateed organic matter.