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The semi-closed pyrolysis simulation system under constant pressure was conducted to explore the characteristics and mechanisms of hydrocarbon generation from Xiamaling Formation shale in Xiahuayuan, North China. The experiment results indicate the oil generated by the Xiamaling Formation shale in oil window should be classified as 'aromatic-intermediate' type, whereas the decreasing of dry coefficient can be ascribed to the cracking of residual bitumen in source rock in the stage of high to post maturity. The amount of hydrocarbon gas generated from residual bitumen can be up to 1-2 m per ton rock in high to post mature stage by calculating hydrogen contents in the kerogen, the expelled hydrocarbon, and the residual hydrocarbon. This reveals the importance of residual bitumen as a gas source during high to post mature stage of the kerogen evolution, and also as the broad exploration prospect of these gases. This research highlights the attention should be paid to oil/gas reservoirs sourced from residual bitumen of organic-rich source rock in high mature stage, even the primary oil/gas reservoirs considered as the main exploration targets in middle-upper Proterozoic sediments of North China. [ABSTRACT FROM AUTHOR]

Abstract: Experimental studies of the effects of thermochemical sulfate reduction (TSR) on light hydrocarbons were conducted in sealed gold tubes for 72h at 400°C and 50MPa. A variety of pyrolysis experiments were carried out, including anhydrous, hydrous without MgSO4 (hydrous experiments) and hydrous with MgSO4 (TSR experiments). Common reservoir minerals including montmorillonite, illite, calcite and quartz were added to various experiments. Measurements of the quantities of n-C9+ normal alkanes (high molecular weight, HMW), n-C6–8 normal alkanes (low molecular weight, LMW), C7–8 isoalkanes, C6–7 cycloalkanes and C6–9 monoaromatics and compound specific carbon isotope analyses were made. The results indicate that TSR decreases hydrocarbon thermal stability significantly as indicated by chemically lower concentrations and isotopically heavier LMW saturated hydrocarbons in the TSR experiments compared to the hydrous and anhydrous experiments. In the LMW saturated hydrocarbon fraction, cycloalkanes tend to be more resistant to TSR than n-alkanes and isoalkanes. TSR promotes aromatization reactions and favors the generation of monoaromatics, resulting in higher chemical concentrations and isotopically equivalent compositions of monoaromatics in the anhydrous, hydrous and TSR experiments. This indicates that LMW monoaromatics are thermally stable during the pyrolysis experiments. Acid rather than basic catalyzed ionic reactions probably play a major role in TSR. This is suggested by the promotion effects of acid-clay minerals including illite and particularly montmorillonite. The basic mineral calcite retards the destruction of n-C9+ normal alkanes within the TSR experiments. Furthermore, clay minerals have a minor influence on the generation of LMW monoaromatics and play a negative role in regulating the concentrations of LMW saturated hydrocarbons; calcite does not favor the generation of LMW monoaromatics and plays a positive role in controlling the concentrations of LMW saturates relative to clay minerals. Quartz has a negligible role in the TSR experiments. Due to their differential responses to TSR, LMW hydrocarbon parameters, such as Schaefer [Schaefer, R.G., Littke, R., 1988. Maturity-related compositional changes in the low-molecular-weight hydrocarbon fraction of Toarcian Shale. Organic Geochemistry 13, 887–892], Thompson [Thompson, K.F.M., 1988. Gas-condensate migration and oil fractionation in deltaic systems. Marine and Petroleum Geology 5, 237–246], Halpern [Halpern, H., 1995. Development and application of light-hydrocarbon-based star diagrams. American Association of Petroleum Geologists Bulletin 79, 801–815] and Mango [Mango, F.D., 1997. The light hydrocarbons in petroleum: a critical review. Organic Geochemistry 26, 417–440] parameters and stable carbon isotopic compositions of individual LMW saturated hydrocarbons in TSR affected oils should be used with caution. In addition, water promotes thermal cracking of n-C9+ normal alkanes and favors the generation of LMW cycloalkanes and monoaromatics. The result is lower concentrations of n-C9+ HMW normal alkanes and higher concentrations of LMW cycloalkanes and monoaromatics in hydrous experiments relative to anhydrous experiments with or without minerals. This investigation provides a better understanding of the effects of TSR on LMW hydrocarbons and the influence of reservoir minerals on TSR in natural systems. The paper shows how LMW hydrocarbon indicators in TSR altered oils improve understanding of the processes of hydrocarbon generation, migration and secondary alteration in subsurface petroleum reservoirs. [Copyright &y& Elsevier]

The experimental data yielded by Rock-Eval pyrolysis, kerogen atomic H/C, Py-GC and gold-tube sealing thermal simulation on the marine Cambrian-Ordovician source rock from the Tarim Basin revealed that the upper limit of maturity for natural gas generation or the "deadline of gas generation" for marine types I and II kerogens is equal to 3.0% of vitrinite reflectance (Ro); while the "deadline of gas generation" for type III kerogens typically like coals is as high as 10% Ro. Thus, different organic matter has obviously different utmost maturities for gas generation. The mass-balance calculation by kerogen elements showed that when Ro>1.5%, the utmost amount of gas generation for the marine type II kerogen is less than 185 m³/t TOC, accounting for less than 30% of its total hydrocarbon generative potential; when Ro>2.0%, it becomes 110 m³/t TOC, less than 20% of the total hydrocarbon generative potential. The amount of the gas generative potential obtained by Rock-Eval is only around one tenth of the calculated value by the mass balance of kerogen elements at the same thermal evolutionary stage, while those by Py-GC and gold-tube sealing simulation are intervenient between the above two. The utmost of gas generative potential at the over 1.3% Ro stage is around 60-90 m³/t TOC, therefore, the amount of gas generation obtained by Rock-Eval is the minimum of gas generative potential, while that by the mass-balance calculation of kerogen elements is the maximum that the actual amount of gas generation should not exceed. [ABSTRACT FROM AUTHOR]

Using molecular geochemical data from infrared spectrometer and pyrolysis gas chromatography-mass spectrometry, this paper investigates the petroleum generation characteristics of Jurassic coal measures from Kuche depression, Tarim Basin, NW China. The results showed that the Jurassic coaly rocks with medium maturity (Ro%: 0.8– 1.1) were enriched in gas-prone functionalities (-CH3) and low molecular weight pyrolysates (

Previous studies have showed that the hydrocarbon biomarkers bound in asphaltenes can preserve original geochemical information during subsequent thermal alteration and biodegradation relative to their free counterparts in crude oil. In the present study, asphaltenes separated from a set of heavy oils which possess an identical source origin and similar thermal maturity, but have experienced different degrees of biodegradation, were analysed to study the influence of biodegradation on the bound biomarkers of asphaltenes. On-line flash pyrolysis directly coupled with gas chromatography–mass spectrometry (Py–GC–MS) was used on these asphaltenes to release the hydrocarbon biomarkers bound within the asphaltene structure. Additionally, the free biomarker profiles in these oils were investigated by GC–MS for a comparison with the bound phase in the corresponding asphaltenes. The results reveal that most of the free biomarkers in the reservoired oils were significantly altered at heavy to severe biodegradation stages, and thus lost much original geochemical information. However, the biomarkers bound in the asphaltenes show very little alteration due to biodegradation, and they also appear to be less mature than their free counterparts in the corresponding oils. This indicates that the bound biomarker profiles in asphaltene flash pyrolysates can represent “original oils” at an early and low thermal maturity stage. Therefore, the biomarker parameters on the basis of asphaltene flash pyrolysates can still be used as indices for correlation studies even though the oils were heavily to severely biodegraded. [ABSTRACT FROM AUTHOR]

• Torbanite pyrolysis produces inter- and intramolecular 13C anomalies. • Continuing decrease of δ13C 3 after %EasyRo > 3.5 indicates multiple sources of C 3. • Negative ‰SP values before %EasyRo = 1.1 result from aliphatic moieties cracking. • δ13C-depleted aromatic compounds were generated from decomposition of n -alkanes. Abnormal stable carbon isotopic (δ13C) compositions, deviating from the conventional order of δ13C 1 < δ13C 2 < δ13C 3 , are frequently observed in natural gas reservoirs. For thermogenic gas, these anomalies, such as δ13C 1 < δ13C 3 < δ13C 2 or δ13C 1 > δ13C 2 > δ13C 3 , have multiple formation mechanisms including gas mixing in conventional systems and desorption processes of gaseous hydrocarbons in unconventional shale gas systems due to their inconsistency with Rayleigh fractionation processes. Considering distinct reaction pathways (e.g., C 1 polymerized to C 2), these aberrant δ13C signatures are often construed as intrinsic hallmarks of extensively evolved natural gas. However, on the basis of gas generation simulation, not all findings exhibit abnormal δ13C values, hinting at multifaceted and intricate mechanisms governing the isotopic fractionation of alkane gas components. This study conducted gold tube pyrolysis of an Australian torbanite, revealing four distinct types of δ13C anomalies in hydrocarbon classes. Polycyclic aromatic hydrocarbons (PAHs) exhibited δ13C values more negative than co-occurring n -alkanes. δ13C 3 displayed a negative trend shift from EasyRo = 3.5 %, resulting in a partially δ13C-reversed gas (δ13C 1 < δ13C 3 < δ 13 C 2) formed at EasyRo ≈ 4.1 %. Moreover, intramolecular δ13C 3 (both terminal and central carbons, termed δ13Ca and δ13Cb, respectively) reversed alongside the overall δ13C 3 trend. Additionally, the evolution of site preference in δ13C 3 (termed ‰SP = δ13Ca – δ13Cb) transitioned from progressively negative to positive. The results of this study demonstrate that the potential conversion between saturated hydrocarbons, aromatic hydrocarbons, and alkane gases is at least partially responsible for δ13C anomalies, but also that gaseous hydrocarbons formed from other fractions at high maturity cannot be ruled out, such as kerogen and pyrobitumen. [ABSTRACT FROM AUTHOR]