Occluded Hydrocarbons Research Articles

Biodegradation of occluded hydrocarbons and kerogen macromolecules of the Permian Lucaogou shales, Junggar Basin, NW China

Three kerogen samples (JJZG-1, JJZG-2 and JJZG-3) isolated from the Permian Lucaogou shales of varying biodegradation levels (BLs ≈ 0, 3 and 7, respectively) were subjected to sequential stepwise pyrolysis combined with on-line detection of gas chromatography-mass spectrometry (GC-MS). Occluded fractions (bitumen II) released at low-temperature steps (≤410 °C) show consistent biodegradative signatures with that reported for solvent-extracted fractions (bitumen I) of the original shales, e.g., broad range of abundant n-alkanes, isoprenoids and regular hopanes for the non-biodegraded JJZG-1; trace n-alkanes and abundant hopanes for the moderately biodegraded JJZG-2; and no n-alkanes but still prominent hopanes including the microbially produced 25-nohopanes for the severely biodegraded JJZG-3. This consistency between bitumen II and bitumen I fractions indicates the biodegradability of the kerogen-occluded bitumen II with limited protection from host kerogen. A minor level of protection was suggested by the trace distribution of n-alkanes in the bitumen II of JJZG-2, whereas the bitumen I had no n-alkanes. The kerogen itself was more resistant to biodegradation as reflected by the persistence of high abundances of both n-alkanes and hopanes in the high temperature (≥460 °C) products of all three kerogen samples. However, the relative abundances of these product groups did show some evidence of biodegradation alteration, e.g., ratios of n-C15 alkene/C27 hop-17(21)-ene at 510 °C pyrolysis decreased by order of magnitude from the non-biodegraded (JJZG-1 = 27.4) to highly biodegraded (0.3 for JJZG-3) samples. The reduced biodegradation impact on the kerogen fraction (Cf. bitumen fractions) was also evident by the absence of 25-norhopanes in the high-temperature analysis of the JJZG-3 kerogen.

The hysteresis of asphaltene-trapped saturated hydrocarbons during thermal evolution

Asphaltene-trapped (adsorbed or occluded) biomarkers are considered a valid source of information for crude oil with severe secondary alterations. However, thermal stress might change asphaltene-trapped hydrocarbons to some extent. Therefore, it is indispensable to investigate the thermal evolution behavior of asphaltene-trapped biomarkers. In this study, low-maturity bitumen from the Kuangshanliang area of the Sichuan Basin (China) was selected as a sample. The thermal evolution behavior of free and asphaltene-trapped saturated biomarkers was investigated by thermal simulation experiments. The results revealed, in addition to normal biomarkers, also a series of even carbon number n-alk-(1)-enes in asphaltene-occluded hydrocarbons. All of them were hardly influenced by secondary alterations. Moreover, due to the restriction of macromolecular structure, the thermal evolution of asphaltene-trapped biomarkers lags behind that of free biomarkers. Most biomarker parameters of occluded hydrocarbons changed little with increasing maturity. In particular, the distributions of occluded terpanes or steranes retained the characteristics of early diagenesis of organic matter, even if they suffered from strong thermal alteration.

Nondestructive investigation on hydrocarbons occluded in asphaltene matrix: The evidence from the dispersive solid-phase extraction

Asphaltenes are the heaviest compound group in crude oil. The complex and porous macromolecular network of asphaltenes both adsorbs and occludes small hydrocarbon molecules, which are considered source-related petroleum biomarkers as they are shielded from secondary alterations. The current method to isolate the occluded content involves mild oxidative degradation of asphaltenes with CH3COOH/H2O2, but it is generally not suitable for quantitative studies, as side reactions that chemically alter the bound hydrocarbons are difficult to mitigate. In the current study, we compared the performance of dispersive solid-phase extraction (DSPE) and mild oxidative degradation in isolating asphaltene-occluded hydrocarbons, using Lower Cambrian solid bitumen from northwestern Sichuan and Ordovician crude oil from the Tazhong area of Xinjiang, China. We demonstrated that DSPE was effective in extracting occluded hydrocarbons from asphaltene aggregates without undesirable chemical alterations, thereby allowing subsequent quantitative analysis. Further investigations revealed that the chemical composition of the occluded n-alkanes provided useful information with regard to the origin of the petroleum. Importantly, the enrichment of aromatic hydrocarbons inside the asphaltene aggregates, particularly the relative abundance of different methylphenanthrene isomers, provided crucial experimental evidence in support of our earlier hypothesis that the occluded hydrocarbons were protected from secondary alterations, and therefore had remained relatively stable over a long geologic period.

Characterization of heavy oils from the Campos Basin, Brazil: Free biomarkers in oils, adsorbed and occluded in asphaltene structures

The Campos Basin is considered an important Brazilian petroleum province, with the most of its oils consisting in a mixture of biodegraded and non-biodegraded oils, which is related to more than one generation and migration pulse coupled with biodegradation events during successive stages of reservoir filling. Its turbidites reservoirs are also structural controlled through faults generated by halokinesis comprising the main route of oil migration from underlying source rocks. Throughout the biomarkers analyses of free hydrocarbons in oil, adsorbed, and occluded in the asphaltene structures, samples from different oilfields, and distances from the main fault responsible for its reservoirs filling, were comprehensive studied. The gas chromatography coupled mass spectrometer (GC-MS) methodology was applied. The results of biodegradation, source, and thermal maturation parameters reveals that the free and adsorbed saturated hydrocarbons are similarly altered, but differ in biodegradation degrees and pathways between oils, while occluded hydrocarbons and all fractions of aromatic compounds are well preserved. The predominance of biodegradation process over the recharge is suggested for distant samples from the main fault in each area, contributing to the local models of oil migration. The occluded hydrocarbons showed source similarities regardless of oilfield. According to thermal maturity based on unaltered aromatics compounds, the occluded hydrocarbons showed lower mature ratios than those free. This study introduces the application and limitations concerning biodegradation, source, and thermal maturation inferences from occluded hydrocarbons in asphaltenes covering Brazilian oils, in view to improve a concise geochemical tool in petroleum exploration.

EVALUATION OF THE CHEMICAL COMPOSITION AND STRUCTURE OF ASPHALTENES FROM THREE OFFSHORE BRAZILIAN BIODEGRADED HEAVY OILS

Asphaltenes fractions were extracted and purified from three heavy Brazilian oils. Their mass compositions of C, H, N, Ni and V were obtained from elemental analysis and S and O atomic percentages from EDS. The H/C ratios showed high degree of unsaturation, while the O atomic percentages indicated more pronounced biodegradation effects on two samples. Quantitative data on N, Ni, and V and semi-quantitative data on S were related to oils origins. The structural data of asphaltenes were explored by combining Fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance (1 H NMR). The oil with the lower degree of biodegradation contained asphaltenes with a lower level of condensed aromatic rings and longer aliphatic chain substituents. The asphaltenes obtained from the two most biodegraded oils showed similarities of polar groups and the presence of carboxylic functions, as well as lower contents of aliphatic substituents. The quality and quantity of occluded hydrocarbons were assessed after the mild oxidation of the separated asphaltenes fractions. It was suggested that the severe biodegradation which altered these structures may also be responsible to affect their occluded hydrocarbons.

Geochemical Evolution of Occluded Hydrocarbons inside Geomacromolecules: A Review

Within geomacromolecules, such as kerogen, asphaltene, and solid bitumen, other compounds can be adsorbed and even occluded as free molecules. The occluded components have been well preserved by the macromolecular structure, and retain some of the primary geochemical information. In this work we try to probe the geochemical evolution of occluded hydrocarbons inside geomacromolecules associated with the geomacromolecule evolution from kerogen → asphaltene → solid bitumen. The results show that occluded hydrocarbons can be transferred steadily from kerogen → asphaltene → solid bitumen. Later-evolved geomacromolecules not only inherit the occluded hydrocarbons from the former ones, but can also occlude some new free molecules. Occluded hydrocarbons are subject to a relatively independent thermal evolution, whereas the evolution of adsorbed molecules is constrained by other factors besides thermal stress. Elucidation of the geochemical evolution of occluded hydrocarbons inside geomacromolecules will be helpfu...

Multiple charges to Sinian reservoirs in the middle Sichuan basin, SW China: Insight from the adsorbed/occluded hydrocarbons in solid bitumens

Mild oxidative degradation was used to release the aliphatic hydrocarbons occluded inside the macromolecular structure of solid bitumens collected from Sinian carbonate reservoir, Sichuan basin, SW China. In contrast to the adsorbed aliphatic hydrocarbons extracted from the solid bitumens, the mild oxidative products showed unaltered n-alkanes distributions, which had been trapped within the macromolecular structure of the bitumens and protected from biodegradation processes. These observations are quite informative and suggest that at least, two phases of hydrocarbon charging to the Sinian reservoirs might have occurred. The occluded hydrocarbons seem to represent the first phase of hydrocarbon charge while the adsorbed and biodegraded hydrocarbons correspond to the second phase of hydrocarbon charge. However, terpane and sterane distributions in both fractions were preserved and similar, and the carbon isotope analysis results obtained in this work along with the previous geological information, corroborates the idea that Sinian reservoirs hydrocarbons have been sourced from the black shale of Lower Cambrian Qiongzhusi Formation. This study reveals the geochemical significance of adsorbed/occluded aliphatic hydrocarbons in understanding the charging history of oil into reservoirs.

Multiple-sourced features of marine oils in the Tarim Basin, NW China – Geochemical evidence from occluded hydrocarbons inside asphaltenes

The Tarim Basin, NW China, is a large composite basin with multiple sets of petroleum source rocks. The basin has undergone numerous episodes of hydrocarbon generation, migration and accumulation, making it difficult to assess the source rocks and oil–source correlation for the widespread marine oils in this area.Protected by the molecular structure of asphaltenes, occluded hydrocarbons can provide information about the early source rocks. In this work, the occluded hydrocarbons released from the asphaltenes by a mild chemical degradation method were compared with the crude oil maltenes and the adsorbed compounds from asphaltenes. Analysis of biomarker distribution and the carbon isotope composition of individual n-alkanes suggests that the widespread marine oils in the Tazhong Uplift, Tabei Lunnan Uplift and Halahatang Depression were contributed by Cambrian–Lower Ordovician source rocks at an early stage, and later mixed with hydrocarbons derived from Middle-Upper Ordovician source rocks. The marine oils in the Tarim Basin demonstrate extensive characteristics of having been derived from multiple source rocks.

A new low-interference characterization method for hydrocarbons occluded inside asphaltene structures

Some hydrocarbons occluded inside asphaltene structures can be considered to be “original oil”, and are very important especially for severely post-altered crude oil in related geochemical studies such as oil/oil, oil/source correlation. The use of oxidising reagents could properly release these occluded hydrocarbons, and make possible direct study of these compounds without interference from the segments chemically bonded to the asphaltene molecule. Interference from adsorbed and/or co-precipitated compounds can be avoided by applying an asphaltene purification procedure.

Saturated hydrocarbons occluded inside asphaltene structures and their geochemical significance, as exemplified by two Venezuelan oils

Inside asphaltene aggregates in crude oils, the adsorbed hydrocarbons exist in the packed periphery while the occluded hydrocarbons exist inside the core of the asphaltene structures. Occluded hydrocarbons may be well protected from secondary alteration events in the reservoir due to the colloidal nature of asphaltenes, and can be representative of older oil than the reservoir oil, and so may be very useful for reservoir geochemical studies. In this work, two Venezuelan oils derived from the same source rock have been studied. The results indicate that the oxidative reagent H 2O 2/CH 3COOH can release the occluded hydrocarbons from the asphaltene aggregates, making it possible to selectively study these compounds without interference from the moieties that are covalent-bonded to the asphaltene structures. In order to obtain the precise information from the occluded hydrocarbons, prior to the oxidation processes, acetone extraction of the asphaltenes has been applied to rule out interferences from the adsorbed compounds. Furthermore, pyrolysis of asphaltenes has been performed for comparison with the occluded saturated hydrocarbons. From the oxidation products of asphaltenes from these two crude oils, the saturated hydrocarbons (from the occluded original oil) have very similar distributions, including terpanes and steranes, and almost the same biomarker parameters. These results are in accord with the fact that oil from the same source rock was occluded by both Ven1 and Ven2 asphaltenes. The geochemical information obtained from the occluded saturated hydrocarbons suggests that the source rock depositional setting for these two crude oil samples was an anoxic, hypersaline/evaporitic environment.

Adsorption/Occlusion Characteristics of Asphaltenes: Some Implication for Asphaltene Structural Features

In the crude oil, the other fractions such as hydrocarbons could be adsorbed and/or occluded inside asphaltene structures. In this work, the deuterated compound n-C20D42 has been used to simulate the adsorption/occlusion characteristics of asphaltene in toluene solution. Based on the highly polydispersed structures of asphaltenes, the experimental results indicate that the adsorbed hydrocarbons could be exchanged with the outside bulk phase, whereas the occluded ones inside the core of asphaltenes could not be exchanged. These results suggest that substantial microporous units exist inside asphaltene structures, and these structural units are suitable to adsorb and occlude the other fractions in the crude oil. The carbon number of the occluded hydrocarbons could be up to C40, and the weight could be more than 0.5%, based on the initial asphaltene weight. The adsorption should occur at the periphery of asphaltene aggregates (the loosely packed ones), and then they are liable to be exchanged with the outsid...

Characterization of nC7-soluble fractions of the products from mild oxidation of asphaltenes

A significant quantity of hydrocarbons (including alkanes) is occluded in the skeleton of the asphaltene molecule. The hydrocarbons are probably remnants of the “original oil” which had been retained within the asphaltene matrix and protected from the secondary alteration processes that occurred subsequently in the oil reservoirs. In this work we report that oxidation of asphaltenes by stirring with 30%H2O2–HAc or NaIO4–NaH2PO4 can release nC7-soluble oxidized products, including the occluded hydrocarbons. Characterization of the nC7-soluble fractions of oxidized products can be applied to highlight some geochemical problems, such as in studies of oil–oil correlation, oil–source correlation and secondary alterations of oil reservoirs. It will be especially useful to recover the original geochemical information of some oil reservoirs heavily degraded by post-depositional processes.