All Ordovician limestones were extracted to get the extractions, and the saturated hydrocarbon of which had been analyzed using a SHIMADZU GC-2010Plus equipped with an FID. The results showed that there are vary abundant n-alkanes, while sterane and terpene/horane compounds are all lower than the lower detection limit (minimal or no detection signal). Why ?
Besides, although the maturity is not too high (Ro ~1.0 %), triaromatic sterane (m/z 231) and triaromatic dinoflagellate sterane (m/z 245) also can not be detected. It's amazing.
Looking forward to your reply. Thank you very much!
Saturated and aromatic hydrocarbons from Ordovician limestone samples have been detected on the Agilent benchtop GC-MS again. Results show that there are abundant n-alkan, pristane and phytane (Pr/Ph = 1.5 ), but very low abundance of sterane and terpene/hopane. And it is a pity that triaromatic sterane (m/z 231) and triaromatic dinoflagellate sterane (m/z 245) in the aromatic hydrocarbons are still not be detected.
What kind of organisms can provide only abundant n-alkan but much few steranes and terpene/hopanes ?
Hi Taohua,
while this is a very interesting observation, having polycyclics in rather low concentration may not be that unusual. For example, even in the AGSO oil standard (e.g. shown in Fig. 1C) of Pawlowska et al. 2012, polycyclics are hardly visible in a total ion/FID chromatogram. As also shown in the same figure (although for the Proterozoic), there are huge difference in polycyclic content between samples from different facies.
One of the most straight-forward explanations would be thermal maturity. This is very nicely shown in Fig. 6 of Petersen (2004) where polycyclics decrease significantly when a sample with very high polycyclic content is heated for 72h at 300C (ie still not terribly mature) with further decrease at more elevated temperatures. I guess above Ro of ca. 1.0 this maturity effect can be very significant. In one study of lean carbonate samples we observed steranes and hopanes to become virtually invisible (completely overprinted by anthropogenic contamination) even in MRM mode at maturities >1.3-1.4 Ro. Similar is nicely shown in pyrolysis experiments of MIssbach et al. 2016 where they estimate vitrinite reflectance of pyrolysates.
Without further details, it is of course difficult to tell if your low polycyclic concentrations are unusual or not (particular for mature samples) - but at least it should be straight forward for you to test if maturity has a significant influence (e.g. if it is slightly higher than your estimated R0 1.0 it likely has). Hopane/cheilanthanes, CPI etc. may be a good starting point.
Best regards,
Benni
Pawlowska, Maria M., Nicholas J. Butterfield, and Jochen J. Brocks. "Lipid taphonomy in the Proterozoic and the effect of microbial mats on biomarker preservation." Geology 41.2 (2013): 103-106.
Henrik I. Petersen, Hans P. Nytoft, Lars H. Nielsen,
Characterisation of oil and potential source rocks in the northeastern Song Hong Basin, Vietnam: indications of a lacustrine-coal sourced petroleum system,
Organic Geochemistry,
Volume 35, Issue 4,
2004,
Mißbach, H
Duda, J-P
Lünsdorf, NK
Schmidt, BC
Thiel, V
2006,
Testing the preservation of biomarkers during experimental maturation of an immature kerogen
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