Patrick Hatcher, PhD, Old Dominion University
The processes involved in the formation of petroleum have been debated for approximately 100 years. Some believe that petroleum forms directly from the source rock kerogen, while others believe that it forms from heavy asphaltic products (NSOs) generated early in the maturation process. Our work brings some new non-invasive NMR and mass spectrometric techniques that bear on testing the possible pathways for petroleum formation. Our overarching hypothesis is that we can resolve the issue regarding the pathway for petroleum formation by comparing the chemical structure of NSOs generated from simulations of petroleum formation and the chemical structure of their precursor kerogen at a molecular-scale specificity that has not been attempted before.
The NMR methods allow us to establish that soluble extracts are representative of the whole kerogen. The chemical and structural comparison between the initial kerogen and solvent extracts of this kerogen has been evaluated using 2D HSQC NMR techniques. This approach allows a direct comparison of insoluble kerogen with soluble extracts. The insoluble kerogen was examined by HRMAS NMR and the extracted materials were examined by traditional liquids NMR. A Type I kerogen from the Mahogany Zone of the Green River Formation has been employed to make that demonstration. The result of this work has been published in Organic Geochemistry (Salmon et al., OG v. 42, p. 301). This approach is also employed to characterize two natural bitumens (gilsonite and wurtzite) from the Green River Formation (Zhong et al., OG v. 42, p. 903 ; Helms et al., OG v. 44, p. 21).
In order to examine the molecular and structural relationships between the kerogen and the products, we conducted low-level maturation studies in the laboratory of F. Behar at IFP (Institut Francais du Petrole) in France by closed-system pyrolysis experiments that simulate long-term maturation of kerogen. We are currently using the analytical strategies described above to establish relationships between the initial kerogen, its solvent-extractable products, and the residual kerogen. Artificial maturation experiments that involve a Type II kerogen have also been conducted at IFP.
The Type I kerogen, after mineral disruption, was extracted successively with n-pentane, and DCM. Several series of polar CHO, CHOS and CHON compounds between C12 and C50+ were found in the initial kerogen analyzed by FT-ICR-MS. In the two types (n-pentane and DCM extractable) of NSOs generated during the artificial maturation and analyzed by FTICR-MS, similar series of polar CHO, CHOS and CHON compounds were found. Compounds containing only CHO elements in their make-up were dominant in each spectrum and were comprised mostly of the series CcH2cO2, CcH2c-2O2, CcH2c-2O4, and CcH2c-10O2 which all correspond to carboxylic acids. One chemical pathway that has been proposed for the generation of hydrocarbons found in petroleum is decarboxylation of acids. For the first time here we have directly analyzed by negative ionization FTICR-MS and have characterized the distribution of carboxylic acids extracted from the kerogen without any alteration or derivatization. We show that the distribution of saturated carboxylic acids obtained by ESI-FTICR-MS is directly correlated to the distribution of saturated hydrocarbons. We also demonstrate using statistical post treatment of the data (principal component analysis) and the carbon preference index (CPI) that the CcH2c-2O4 series are released directly by the kerogen and are the source of the mono-carboxylic acids. Other series that are significant in the NSO extracts are the CcH2c-11O2N1 series of compounds that are likely decomposed into isoprenoid hydrocarbons and CcH2c+zN1 series (z=-11,-15, -17).
By calibrating the FTICR-MS with internal standards, it was possible to assign a response factor for the carboxylic acids and dicarboxylic acids, and consequently obtain a better quantification of the total mono- and di-carboxylic acids. This response factor was previously assumed to be invariant in the carbon number range of C20 to C40. To correct for this misassumption, a calibration using a wide range of carboxylic acids was performed. Also, after calibration, the carboxylic acid yield remains dominant among the other peaks detected by FTICR-MS. The quantitative proportions of these carboxylic acids in the NSOs are then compared to the proportion of hydrocarbons produced. The carboxylic acids are dominated by even carbon numbers while the long chain hydrocarbons are dominated by odd carbon numbers. The carbon preference index (CPI) of carboxylic acids with even carbon number dominance is very close to the CPI of hydrocarbons with odd carbon number dominance. This suggests that kerogen produces carboxylic acids with an odd carbon number preference which undergo decarboxylation during thermal cracking to produce hydrocarbons with odd carbon number dominance. The quantification of diacids showed that diacids are produced early in the maturation and have a minor even carbon preference but this can have a major effect on CPI because diacids can produce odd hydrocarbons after a single decarboxylation reaction or even carboxylic acids after a double-decarboxylation reaction.
We have demonstrated in this work that a molecular-level structural relationship exists between kerogen and the associated NSO compounds naturally formed and produced by artificial maturation under laboratory conditions. Our observations for Type I kerogen shows a genetic link between fatty acids and hydrocarbons: this leads us to consider that a simple decarboxylation reaction may be the most important thermal degradation reaction in petroleum formation for this type of kerogen.
From what we have learned regarding the formation of hydrocarbons based on this PRF-funded study, we are just now turning our attention to other kerogen types to examine if this relationship between fatty acids and n-alkanes holds. Mr. Albert Kamga, a graduate student supported by the PRF will be supported in the future to complete his PhD studies as part of funding being supplied by the French oil company TOTAL. They have indicated a strong desire to expand this work. This project has had a major impact on my research direction as I am realizing that future research along the same lines will become a significant focus of activities and collaborations with TOTAL. Dr. Elodie Salmon, supported as a postdoctoral associate on the grant has returned to France to seek out employment in this area of research.