Calibración del sujeto: modelo lineal para la flexión del tronco

R AYOS X

IV.3 D ESCRIPCIÓN DE LOS DISPOSITIVOS DE MEDIDA UTILIZADOS

IV.3.1 MEDICIÓN DE LA CURVATURA LUMBAR

IV.3.1.4 Calibración del sujeto: modelo lineal para la flexión del tronco

The stable carbon isotopic compositions of n-alkanes (n-C14 to n-C31) were measured using GC-MS-IRMS for 10 source rock samples. The δ¹³C values are plotted in Figure 4.60, which clearly shows that the values from the Sirte Shale and Rachmat source rocks cover a broad range between -38.3 and -26.5‰. The profile of δ¹³C values of n-alkanes from the source rocks samples studied show similar trends for most of the samples analysed, except for well L1-16 where variations are observed in the δ¹³C isotope values in n-C18, while their values after the n-C19 are more similar. This anomalous n-C18 δ¹³C value may be due to the co-elution of phytane with n-C18. However, for other studied wells, the profile show that n-C17 and n-C18 partially coelute with other compounds (probably pristane and phytane, respectively). From the characteristics of the carbon isotope versus carbon number slope profile, most of the source rock samples analysed appear with a nearly flat profile that is generally associated with the characteristics of δ¹³C expelled from sources rich in marine organic matter input with some algae contribution (Murray et al., 1994). This flat profile may be produced by homogeneity in the n-alkanes δ¹³C distribution that is thought to be as a result of the derivation of organic carbon from a closely uniform carbon pool. Under

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6A1-59 B1-NC74F B2-NC74A C2-16 FF14-6 L1-16 L1-17 Z1-11

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such circumstances there is typically few an isotopic difference between long and short chain alkanes (Murray et al., 1994). This similarity in the isotope profiles of most of the Sirte Shale and Rachmat source rock samples analysed may reflect the origin of organic source facies rich in marine organic matter, which has been inferred from the molecular biomarker analyses. The profile of δ¹³C values of n-alkanes from the wells 6A1-59 and C2-16 shows depletion in the δ ¹³C isotope from n-C27 to n-C32, which may due to production of isotopically lighter compounds from the source rock. This data for the Sirte Shale and Rachmat source rock sample extracts are presented in Table 4.4.

For the well C2-16, the Sirte Shale source rock extracts show a trend toward isotopically lighter values with increasing n-alkanes chain length (negative slope) with an isotopic range being typically 3-4‰. This feature may indicate some contribution of the terrestrial organic matter (Bjorøy et al., 1991; Clayton, 1991; Hayes, 1993;

Clayton and Bjorøy, 1994; Dawson et al., 2007). The similarity in the isotope profiles of source rock extract samples analysed from the wells 6A1-59, B1-NC74F, C2-16 and FF14-6 may reflect an origin of organic source facies rich in marine organic matter, which has been inferred from the molecular biomarker analyses, as shown in Figure 4.61. The Sirte Shale source rock samples from L1-16 seem to be completely different isotopically compared with the other source rock samples, and they contain the lightest (most negative δ¹³C) isotope values (as shown in Figure 4.62), which may be consistent with the marine source content, with moderate to high contributions of terrestrial organic matter input. The Sirte Shale source rock from the well B2-NC74A showed higher (isotopically heavier) values of δ¹³C, suggests marine sources with high contribution of algal organic matter input. But this interpretation is opposite of what Aboglila (2011) have reported that the more negative ¹³C values are consistent with marine source, whereas the more positive values are indicative of a terrestrial contribution in the eastern Sirt Basin. From n-C19 to n-C24 a slight trend toward isotopically lighter values was found with increasing n-alkanes carbon chain length, with δ¹³C values varying from 0.80‰ to 2.5‰, and also showing a slight negative slope in this range, suggesting that terrestrial organic matter input is predominant in these extracts organic matter samples. Isotope values with n-C14 to n-C18 being isotopically heavier than n-C19 + n-alkanes are maybe due to the maturity influence of the source rock. The less mature organic extracts tend to have isotopically lighter n-C14 to n-C18, for instance L1-16 well compare to other studied wells or that has been sourced

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differently. It is clear from the slight difference in the isotopic composition of the n-alkanes in the wells studied can be attributed to slight variations in the organic facies and depositional environment, as well as maturity effects. This is also indicated by the previous analyses such as n-alkanes and isoprenoid distributions and saturated and aromatic hydrocarbon distributions.

Table 4-4: Compound specific isotopic analyses δ¹³C values for n-alkanes of the Sirte Shale and Rachmat source rocks.

Well Name 6A1-59 B1-NC74F B1-NC74F B1-NC74F B2-NC74A C2-16 FF14-6 FF14-6 FF14-6 L1-16

Depth Feet 7250 7050 7190 7940 9570 9800 11290 11320 11650 7530

Source Rock Rachmale

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Figure 4.60: n-Alkane stable carbon isotope plot for the Sirte Shale and Rachmat samples.

Figure 4.61: n-alkane stable carbon isotope plot for the Sirte Shale and Rachmat samples.

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Stable Carbon Isotope Ratio (‰) d13C12 (‰) d13C13 (‰) d13C14 (‰) d13C15 (‰) d13C16 (‰) d13C17 (‰) d13C18 (‰) d13C19 (‰) d13C20 (‰) d13C21 (‰) d13C22 (‰) d13C23 (‰) d13C24 (‰) d13C25 (‰) d13C26 (‰) d13C27 (‰) d13C28 (‰) d13C29 (‰) d13C30 (‰) d13C31 (‰) d13C32 (‰) d13C33 (‰) d13C34 (‰)

Stable Carbon Isotopes for nC12 to nC34

Well 6A1-59 B1-NC74F B2-NC74A C2-16 FF14-6 L1-16

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Stable Carbon Isotopes for nC12 to nC34

Well 6A1-59 B1-NC74F C2-16 FF14-6

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Figure 4.62: n-alkane stable carbon isotope plot for the Sirte Shale and Rachmat samples.

4.11 Summary

The Sirte Shale and Rachmat formations are considered to be the major petroleum source rocks in the Sirt Basin (Hamyani et al., 1984; El-Alami et al., 1989; Baird et al., 1996; Gumati et al., 1996; Ben Ashour, 2000; Hallett, 2002; Burwood et al., 2003).

Several organic geochemical analyses have been conducted on a set of cuttings rock samples from the central and western part of the Sirt Basin. The palaeoenvironment of deposition has been determined using total organic carbon (TOC) analyses, Rock-Eval pyrolysis, organic petrography, molecular biomarker ratios, and compound specific carbon isotope analyses of n-alkanes from the primary source rocks in the basin.

The dark grey, brown, and black shales of the Kalash Formation are in the early mature stage and have TOC values in the range of 3.04 to 3.81% with an average of 3.42%, which represent good source rock potential. Their Type II kerogen is dominated by yellow to orange fluorescent amorphous marine organic matter. In spite of the high TOC and good kerogen preservation, no significant hydrocarbon generation can be anticipated from this formation at the current low level of thermal maturity in the wells studied.

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Stable Carbon Isotopes for nC12 to nC34

Well L1-16 B2-NC74A

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The Sirte Shale source rock samples of the Upper Cretaceous sequence are found to vary in their organic carbon richness due to changes in the organic facies and/or diluted by a clay mineral matrix. The Sirte Shale source rock samples exhibited fair, good to extremely rich organic matter contents, represented by TOC values in the range of 0.50 to 6.73% with a mean value of 1.41%. The medium dark grey, dark grey to brown and black shale of the Sirte Shale Formation samples are in the early or middle to late mature stages, and have fair to excellent source rock potential. Their Type II to II-III kerogen is dominated by yellow-orange, dull orange to brown fluorescent amorphous organic matter, which is sometimes non-fluorescent at higher maturity levels, and with minor contributions of phytoclast material. It has produced a significant amount of hydrocarbons, particularly oil and with some gas.

The dark grey to black shale of the Rachmat Formation is in the middle mature stage, and has a TOC content ranging between 1.05 to 1.90% with an average of 1.36%, which is rated as having good source rock potential. Their Type II kerogen is dominated by yellow-orange to dull orange fluorescent amorphous organic matter with a minor contribution of terrestrial organic matter. This is in agreement with the results of earlier studies (e.g. El-Alami et al., 1989; El-Alami, 1996; Ben Ashour, 2000), that stated that the Sirte Shale and Rachmat formations are the principal source for most of the oil discovered so far in the Sirt Basin. This has probably produced significant amounts of both oil and gas based on their richness as well as maturity.

The maturity assessment of source rock samples based on the Tmax, production indices, spore colour index, and vitrinite reflectance measurement values for the Upper Cretaceous sedimentary section show that the samples analysed are in the early mature, middle to late mature stage. Vitrinite reflectance measurements show that the Kalash Formation is marginally mature for hydrocarbon generation, while the Sirte Shale and Rachmat formations are of varying thermal maturity throughout the basin depending on their situation. Towards the west part of the basin, they are in the early to middle mature stage, while in the middle to late mature toward in the northeast and central parts of the basin. Furthermore, both formations are in the main oil window, but at higher levels of maturity, they are in the gas to condensate windows.

In document CRITERIOS DE EVALUACIÓN BASADOS EN EL ANÁLISIS DE LA POSTURA (página 117-122)