Petroleum Geochemistry

Content:

Petroleum Geochemistry

I. Summary

II. Introduction

III. . GC-MS

Results and Discussion
i. TOC
ii. Rock Eval
iii. Bitumen Extraction
iv. Asphaltene Precipitation
v. IATROSCAN (TLC-FID analysis)
vi. GC
viiIV. Conclusion

V. References

VI. Experimental Procedures

APPENDIX

I. Summary

Eight different samples of various locations have been analysed during the practical course. The samples consisted of two source rock samples (Kimmeridge Clay and Blue Lias), two oil sands (Wealden oil sand and Osmington Mills oil sand) and four oil samples including Libya oil, R828, Iraq oil (09_205) and Schwedeneck. The Kimmeridge Clay represents an immature source rock with very good potential. The Blue Lias source rock, kerogen type II, shows the same conditions as the Kimmeridge Clay. The other Kimmeridge Clay sample kimsrf 1 indicates kerogen type I. The Wealden oil sand shows intense biodegradation and plots in the area of a type II kerogen. The Osmington Mills oil sand plots in type I kerogen area, and also shows intense biodegradation. The Libya oil is comprised of a type II/III kerogen, similar to the R828 oil sample which additionally shows oxidizing marine conditions. Iraq oil plots in the area of a type II kerogen and was deposited under marine hypersaline conditions. The last sample, the Schwedeneck oil, was deposited under rather oxidizing conditions and sterane analysis suggests an early oil window stage.

II. Introduction

The aim of the laboratory course „Petroleum Geochemistry“ in the winter term 2013/2014 was to assess the kerogen type, depositional environment, maturity and petroleum potential of source rock and oil sand samples, using standard geochemical screening and characterization methods. The experimental procedures were based on the Norwegian Industry Guide to Organic Geochemical Analyses (NIGOGA). They comprise screening methods on bulk source rocks such as TOC-measurements and Rock-Eval Pyrolysis, as well as semi-quantitative analysis of bitumen composition by means of chromatographic separation. The fractions gained by liquid column chromatography (saturated, aromatic, NSO) were further characterized by GC and GC-MS measurements, allowing the recognition and quantification of biomarker compounds. The schematic work flow according to the NIGOGA guide is depicted in figure 1 (Appendix).

The set of samples (see Table 1) comprises two Source Rocks and two Oil Sands, all collected during the field trip “Reservoir Petrology” in South England in September 2013. The exact locations where the samples were found are shown in figure 2 (Appendix). For the GC and GC-MS analysis some of the samples were substituted by those from the year before because in those the biomarkers were better preserved. In addition, three oils from different locations were examined. Again, for the GC-MS analysis an additional Oil from Schwedeneck was analyzed because it was suitable for exhaustive biomarker analysis.

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Tab.1: Analyzed sample set, comprising two source rocks, two oil sands and four oils.

III. Results and Discussion

i. TOC

The TOC results of the samples vary between L.R. (Lyme Regis/Blue Lias), KC (Kimmeridge Clay) and the oil sand (O.S.) samples. L.R. has around 10 % TOC, which is remarkably high and has been measured twice, to clarify this value. The KC source rock shows also a high TOC result, but has a lower value than the L.R. source rock. Finally, the O.M.OS (Osmington Mills/Corallian Oil Sand) and WOS (Wealden Oil Sand) samples show lower TOC results. Their comparable origin from oil sands builds a separated category, at least for TOC.

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Tab.2: Results of the TOC, TIC and TC measurements of source rocks and oil sands.

(Note: L.R. & L.R. II are two measurements of the same sample, to clarify the high TOC/TIC content)

ii. Rock Eval

For the Kimmeridge Clay (KC) source rock, the Wealden oil sand (WOS), the Corallian oil sand (OM) and the Blue Lias (BL) source rock sample a Rock-Eval Pyrolysis was performed. Measurements include temperatures over time, S1 and S2 in mV (with FID) and the CO2 content in mV (with IR). Through a slightly differing Rock-Eval assembly and a temperature measurement in the stemp, 43 °C have to be subtracted in order to get correct Tmax temperature values. The calculations were performed with two standards (STD).

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Fig. 1: Results of Rock Eval and calculation of necessary parameters showing their classification with color codes.

The Rock Eval analysis for the sample KC2, WOS, OM1 and OM2 did not conduct sufficient results due to Tmax values of >800°C Following equations were used for calculations:

Wealden (oil sand)

For the WOS sample the Tmax value of 832°C is too high and therefore not important for further calculations. The results of the Production Index and Generation Index depict that the sample can be characterized as immature. Furthermore, the results of the Kerogen Type and Hydrogen Index indicate oil. This is supported by the Oxygen Index. The Potential Yield is with a value of 21 ranked as very good and the sample has a Reactive Carbon Index (R.C.I) of 104.73. If Tmax is greater than 475°C or the Generation Index is greater than 0.6, the Reactive Carbon Index becomes less useful. Figure K shows that the S1 peak is greater than S2, indicating the oil window. The S3 peak is significantly smaller than the S1 and S2 peak, so the amount of CO2 formed by thermal breakdown of kerogen is very low. The S2 value >5 mg/g is an indicator for an excellent source potential or high molecular weight soluble organic matter (biodegraded oil) or coal. The first peak is > 1 and therefore an indicator for large amounts of kerogen-derived bitumen or the presence of migrated hydrocarbons.

Kimmeridge Clay (source rock)

The sample of Kimmerdige Clay was analysed two times to have a duplicate. Similar to the Wealden oil sand, the KC1 sample is characterized as an immature oil, but it has a good to excellent source potential. Moreover it has a very good Potential Yield with a value of 45 and a R.C.I value of 66. Here the S2 is greater than the S1 peak and S3 is very small (Figure G and H). The S2 value is > 5 mg/g. S1 is >1.

The duplicate KC2 shows not useful values due to a Tmax of 832 °C (Figure I).

Blue Lias (source rock)

The Blue Lias sample shows a maximum temperature of 411 °C and is therefore immature. This is supported by the Production Index and the Generation Index. The Hydrogen Index and the Kerogen Type indicate oil. The values of the Oxygen Index are high so they indicate a good to excellent source and the Potential Yield is classified as very good. The Reactive Carbon Index shows a value of 65.84. Figure E shows that the S1 and S2 peaks are identically that indicates a homogenous sample and the S3 peak is very small (Figure F). The value of S3 is 3.49 mg/g which indicates relative large amounts of terrigenous matter. The S2 peak has a value > 5 mg/g.

Corallian Osmington Mills (oil sand)

The sample of Osmington Mills was analysed two times to have a duplicate. Both depict a maximum temperature of above 800 °C, which is less useful for interpretation. The Production Index varies (0.353 and 0.335) but both values suggest oil, the same applies to the Kerogen Index. The Hydrogen Index indicates oil as well, but the results of the Generation Index show gas or destruction. The Reactive Carbon Index cannot be analysed because it is useless when the Tmax is above 475 °C. The values of the Oxygen Index are high so they indicate a good to excellent source potential and the Potential Yield is classified as very good. Figures A and C show that the S2 peak is higher than the S1 peak, which is an indicator for an immature sample. The S3 peak is as well as in the Blue Lias sample very low (Figure B and D). The S2 peak has a value above 5 mg/g. The first peak is above 1.

Conducted charts of the Rock-Eval Pyrolysis are attached in the appendix. The data was plotted into a Pseudo-van-Krevelen diagram (Figure N) and a crossplot of S2 versus TOC (Figure M), which shows that nearly all samples plot between kerogen type I and II. WOS is near type II and Om is near to type I. According to Van-Krevelen the sample KC 1 is plotted on type I kerogen, but after Langford and Blanc-Valleron it is in the range of type II. The Blue Lias sample is defined as type II according to Langford and Blanc-Valleron but shows a type I-II after Van-Krevelen.

iii. Bitumen Extraction

The fraction of the sample, which still stayed in solution with DCM after one day, is defined as the possible bitumen. After separating it from the residual by filtrating and evaporating the DCM was in a rotary evaporator, the yield of extraction could be determined by weighing the bitumen and relating it to the bulk sample mass. Unfortunately the Bitumen extraction could not be completed in the laboratory for the Kimmeridge Clay and the Wealden Oil sand due to experimental mishaps. The results for the Blue Lias Source Rock and the Corralian Oil Sand are listed in table 3.

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Tab.3: Yield from the bitumen extraction on the Blue Lias Source Rock and the Corallian Oil Sand.

iv. Asphaltene Precipitation

Two samples (R828 and 09_205) are analysed with respect to their amount of asphaltenes. The results are listed in the table 4 below. The oil R828 contains 5.286 % asphaltenes, whereas the oil 09_205 contains 32.88 % asphaltenes and the Libya oil has an amount of 11.92 % asphaltenes.

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Tab.4: Table is showing the results of the asphaltene precipitation.

v. IATROSCAN (TLC-FID analysis)

After the Iatroscan measurement, the peaks were categorized into Saturated HC, Aromatic HC, NSO components and Asphaltenes. The comparison of the calculated peak-areas gives the relative composition of the oils. Table 5 shows the results.

The LEK205 oil sample shows a normal composition of a normal crude oil, while the R828 Oil sample and the Libya Oil have a higher content of Aromatic HC. Due to the high aromatic content of the Libya oil kerogen type I can be excluded because no biodegradation is present in the GC. All oil samples show depletion of NSO compounds (resins).

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Tab.5: The IATROSCAN results for R828, LEK205 and Lybia Oil. All samples were measured multiple times. The mean values are based on the most reliable data. The column area shows the measured peak areas. The fraction is the calculated relative composition of the oils.

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Fig.2: IATROSCAN plotted results. The triangles are indicating the mean values for LEK205, R828 and the circle for Lybia oil. The X’s show single measurements.

vi. GC (Gas Chromatography)

With the results of the gas chromatography and some suitable formulas, information can be given about depositional environment, maturity and possible biodegradation. All in all five different samples have been analysed.

The used formulas are as follows:

The Terrigeneous /Aquatic Ratio (TAR) gives information about the depositional environment, whether it is terrestrial or marine.

The Carbon Preference Index (CPI) gives information about the odd/even predominance and thus information about the level of maturity.

The pristane/phytane ratio shows, as well as the TAR, the depositional environment attached to the specific ranges:

>3 terrigeneous

<2 and >1 marine oxidizing conditions

<2 and <1 marine reducing conditions

<0.8 hypersaline conditions

The pristine/nC17 together with phytane/nC18 ratio gives information about depositional environment and level of maturity and biodegradation.

The calculated ratio derived from the results from the gas chromatography can be seen in Table 6. The interpretations which are derived from these ratios can be seen in Table 7.

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Tab.6: The table shows the ratio calculated for the different parameters.

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Tab.7: Interpretation of the different samples based on the calculated values in Tab.6 (above).

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Fig.3: The figure shows where the different samples plot in the pristine/nC17 and phytane/nC18 diagram.

Kimmeridge Clay

TAR shows values for marine conditions as well as the pri/phy ratio. Additionally the pri/phy ratio indicates reducing conditions. The CPI has a high odd predominance with nearly 2.5 (Table 6). Figure 1 confirms these observations with Kimmeridge Clay plotting in the type 2 kerogen field with strongly reducing marine conditions. All these mentioned values do not speak for a high maturity.

Kimsrf1

The gas chromatography interpretation indicates an aquatic source (TAR: 0.387, nC27/nC17: 0.309), the CPI (1) ratio of 1.81 indicates a pre-, to early maturity. Reducing conditions associated to a type II or III kerogen and the early maturity are approved by the prystane-,phytane-, nC17- and nC18 values (pry/ph: 0.939, Plot of Phytane/nC18 vs. Prystane/nC17, fig. A).

Blue Lias

The TAR indicates a marine environment or more precisely as can be seen from the pristane/phytane ratio a hypersaline environment. The CPI shows high odd predominance, similar to the Kimmeridge Clay with nearly 2.5. High values for pristane/nC17 and phytane/nC18 makes a classification with respect to Figure 3 not possible, it plots outside of the classification. But the high phytane value indicates a low level of maturity (immaturity).

Oil R828

The TAR as well as the pristane/phytane ratio show marine oxidizing conditions. The CPI shows an odd predominance (1.3). In Figure 3 it plots type 2-3 kerogen mixtures area, with the lowest level of maturation is comparison to the other two oils.

Libya oil

This sample shows the same results for TAR and pristane/phytane like the Oil R828 (marine, oxidizing conditions). The CPI has a value of one, which shows that odd and even chain alkanes are in equilibrium. It plots in Figure 3 in the type 2-3 kerogen mixtures area but with a higher level of maturation then the oil R828.

Oil 09_205 (Iraq)

This sample indicates a marine (hypersaline) depositional environment. As the only sample the CPI shows even predominance. The pristane/nC17 to phytane/nC18 ratio (Figure 3) shows that it plots in the type 2 kerogen area with strongly reducing marine conditions. This sample is the most mature sample indicated by low pristane/nC17 and phytane/nC18 values.

vii. GC-MS

Hopanes

In the Blue Lias sample the trisnorhopanes Ts and Tm, as well as hopane and C31-C34 homohopane were identified in the massfragment M/Z = 191. The calculated Ts/(Ts+Tm) ratio of 0.8 suggests a very high maturity. Furthermore the very high value suggests that a peak might have been contaminated with further chemical species. An evaluation of the homohopane index (HHI) was not possible, as C35 was not identified and the S and R hopanes were also not resolved.

For the Corallian oil sand sample the peaks for hopane, norhopane and trisnorhopane, as well as the C31 to C34 homohopanes were found. However, the trisnorhopanes could not be split into Ts and Tm. The C31 to C34 homohopanes could be split into S and R modifications. The calculated S/(S+R) ratios lie between 0.5 and 0.6 with a mean value of 0.5342, which suggests that the sample was close to reaching peak oil window conditions. However, the outcrop conditions at which the sample was taken suggests influence by water washing and biodegradation. Moreover the HHI could not be calculated due to the lack of C35.

For the Schwedeneck sample hopane, as well as the homohopanes C31-C35 were indentified. However, especially C35 must be treated carefully here because its value is close to the detection limit. Therefore the HHI was calculated at 0.05 suggesting a rather oxic depositional environment. Yet, this parameter is also influence heavily by maturation and possibly biodegradation. Ts/(Ts+Tm), as well as S/(S+R) homohopanes were not resolved and could therefore not be calculated.

Kimsrf1 maturity parameters 2S/22S+22R for C31 and C32 17 hopanes (0.697); 17 hopane/ 17 Hopane+17 moretane (0.85); 18 neohopane/ 18 neohopane+17 hopane (0.692) were evaluated. Values correspond to a vitrinite reflectance range of 0.6Vr to 0.9 Vr (Table Y Fig. 14 and Fig.15 (Appendix)). The C35 Homohopane Index is associated with the redox potential in marine sediments. Higher values than 0.1 indicate anoxic conditions and are related to marine carbonates and evaporates. The heights of the homohopane peaks were evaluated manually, because of a better fitting of this scheme. The HHI is 0.083.

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Tab.8: Peak areas of the most important hopane components and maturity parameters based on these biomarkers. At kimsrf 1 from C29 to C35 are the relative abundances given, but not the areas.

Steranes

The data gained from mass spectrometric analysis was processed with the help of the software Excalibur in order to identify and quantify the important biomarkers. Specific m/z ratios were extracted out of the mass spectrum and were plotted against retention time in mass chromatograms. This was done for the m/z ratios 372, 386 and 400, in order to identify cholestane, ergostane and stigmastane, respectively (See Fig. 20 to 22 (Appendix)). The integrated peak areas are listed in table 8. Where possible, different α/β and R/S configurations of the steranes were integrated separately. As maturity parameters serve the ββ-ratio and the 20S-ratio, which as well can be found in table 9, because these configurations have the highest stability. In addition, the relative proportions of C27, C28 and C29 can provide directions to the depositional environment.

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