GEOCHEMICAL APPLICATIONS OF POLYCYCLIC AROMATIC HYDROCARBONS IN CRUDE OILS AND SEDIMENTS FROM PAKISTAN

Abstract

Crude oils and sediments extracts from Kohat-Potwar Basin (Upper Indus Basin) were examined for polycyclic aromatic hydrocarbons (PAHs), heterocyclic aromatic hydrocarbons, biomarkers and stable isotope compositions. The first four chapters provide background to the research. Chapter 5 discusses the petroleum geochemistry of Potwar Basin where three groups of oils were recognized on the basis of diagnostic biomarkers, distribution of PAHs and stable bulk carbon and hydrogen isotopes. In chapter 6, PAHs distributions and compound specific stable hydrogen isotope compositions have been used to assess minor biodegradation in Potwar Basin oils. The final chapter of this thesis (chapter 7) describes the formation of heterocyclic aromatic hydrocarbons and fluorenes in sedimentary organic matter through carbon catalysis reactions. Diagnostic biomarker parameters along with stable bulk δ13C and δD isotope abundance reveal three groups of oils in Potwar Basin. Group A contains terrestrial source of OM deposited in highly oxic/fluvio-deltaic clastic depositional environment shown by high Pr/Ph, high diahopane/hopane, high diasterane/sterane, low DBT/P ratios and higher relative abundance of C19 tricyclic and C24 tetracyclic terpanes. Aliphatic biomarkers for rest of the oils indicate marine origin however two ranges of values for parameters differentiate them into two sub-groups (B and C). Group B oils are generated from clastic rich source rocks deposited in marine suboxic depositional environment than group C oils which are generated from source rocks deposited in marine oxic depositional environment. Group C oils show higher marine OM (algal input) indicated by extended tricyclic terpanes (upto C41 or higher) and higher steranes/hopanes ratios. Distribution of PAHs classified Potwar Basin oils into similar three groups based on depositional environments and source OM variations. Abundant biphenyls (BPs) and fluorenes (Fs) are observed in group A oils while group B oils showed higher abundance of dibenzothiophenes (DBTs) and negligible presence of dibenzofurans (DBFs) and Fs and group C oils showed equal abundance of DBTs and Fs. This relative abundance of heterocyclic aromatic hydrocarbons in Potwar Basin oils broadly indicate that the distribution of these compounds is controlled by depositional environment of OM where sulfur compounds (i.e. DBTs) are higher in marine source oils while oxygen compounds (DBFs) and Fs are higher in oxic/deltaic depositional environment oils. Higher abundance of aromatic biomarkers the 1,2,5-trimethylnaphthalene (1,2,5-TMN), 1- methylphenanthrene (1-MP) and 1,7-dimethylphenanthrene (1,7-DMP) indicate major source of OM for group A oil is higher plant supported by abundance of conifer plants biomarker retene. Variations in distribution of triaromatic steroids (TAS) in Potwar Basin oils clearly indicate source dependent of these compounds rather than thermal maturity. Higher abundance of C20 and C21 TAS and substantional difference in distribution of long chain TAS (C26, C27, C28) between the groups indicate different source origin of these compounds. Group A shows only C27 and C28 TAS while group B shows C25 to C28 TAS and absence of these compounds in group C oils revealed that the sterol precursors for these compounds are most probably different. Aliphatic and aromatic hydrocarbon maturation parameters indicate higher (late oil generation) thermal maturity for all oils from the Potwar Basin. The crude oils of group A and C are typically non-biodegraded mature crude oils whereas some of the oils from group B showed minor biodegradation indicated by higher Pr/n-C17, Ph/n-C18 and low API gravity. Distribution of PAHs and stable hydrogen isotopic composition (δD) of n- alkanes and isoprenoids has been used to assess the minor biodegradation in a suite of eight crude oils from Potwar Basin, Pakistan (group B). The low level of biodegradation under natural reservoir conditions was established on the basis of biomarker distributions. Bulk stable hydrogen isotope of saturated fractions of crude oils show an enrichment in D with increase in biodegradation and show a straight relationship with biodegradation indicators i.e. Pr/n-C17, API gravity. For the same oils, δD values for the n-alkanes relative to the isoprenoids are enriched in deuterium (D). The data are consistent with the removal of D-depleted low-molecular-weight (LMW) n-alkanes (C14-C22) from the oils. The δD values of isoprenoids do not change during the minor biodegradation and are similar for all the samples. The average D enrichment for n-alkanes with respect to the isoprenoids is found to be as much as 35‰ for the most biodegraded sample. The relative susceptibility of alkylnaphthalenes and alkylphenanthrenes at low levels of biodegradation was discussed. Alkylnaphthalenes are more susceptible to biodegradation than alkylphenanthrenes while extent of biodegradation decreases with increase in alkyl substitution on both naphthalene and phenanthrenes. A range of biodegradation ratios (BR) are purposed from dimethylnaphthalene (DNBR), trimethylnaphthalenes (TNBR) and tetramethylnaphthalene (TeNBR) that show significant differences in values with increasing biodegradation and are suggested as good indicators for assessment of low level of biodegradation. Laboratory experiments have shown that activated carbon catalyses the reactions of biphenyls (BPs) with surface adsorbed reactants that incorporate S, O, N or methylene forming some common constituents of sedimentary organic matter namely, dibenzothiophene (DBT), dibenzofuran (DBF), carbazole (C) and fluorene (F). A relationship between the % abundance of the hetero element in kerogen and the abundance of the related heterocyclic compound in the associated soluble organic matter supports the hypothesis that these reactions occur in nature. More specific supporting evidence is reported from the good correlation observed between methyl and dimethyl isomers of the reactant BPs and the methyl and dimethyl isomers of the proposed product heterocyclics compounds i.e. DBTs, DBFs, Cs and Fs. It is suggested that these distributions reported for sediments and crude oils from the Kohat Basin are the result of a catalytic reactions of compounds with BP ring systems and surface adsorbed species of the hetero element on the surface of carbonaceous material. Similar distributions of heterocyclic aromatic hydrocarbon from Carnarvon Basin (Australia) were illustrated to show the global phenomenon of this hypothesis. Furthermore, the abundances of these compounds (DBT, DBF and BP) show similar concentration profiles throughout the Kohat Basin sediments suggesting that share a common source. These compounds also correlate well with changes in the paleoredox conditions. These data tends to point towards a common precursor perhaps lignin phenols of land plants. Coupling of phenols leads to BP, which has been demonstrated in laboratory experiments to be the source of C, DBT, DBF, and F.