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 helpful in oil (bitumen)–source correlation, identification of mixed-source reservoirs, and characterization of hydrocarbon accumulation and evolution.

Organic compounds could be adsorbed and even occluded within the macromolecular structures of kerogen. Studies concerning thermal evolution of adsorbed/occluded hydrocarbons inside kerogens will be helpful in understanding its structural characteristics and evolution features and estimating the effectiveness of adsorbed and occluded fractions. In this work, adsorbed and occluded hydrocarbons have been released from a low-matured kerogen from the upper Permian Lucaogou formation of Santanghu Basin, northwest China, and from its pyrolysis residues by solvent extraction and oxidative treatment, respectively. The results showed that some n-alkanes, terpanes, and steranes were detected from both adsorbed and occluded fractions. Series of even-carbon-numbered n-alk-(1)-enes were also determined from the occluded components. The early stage of thermal evolution showed similarities in biomarker features for both adsorbed and occluded hydrocarbons; however, variations were noted in biomarker compositions with increasing thermal levels. The obvious changes of C21–22 pregnane/C27–29 sterane ratios from adsorbed to occluded fractions at high thermal evolution suggested that high concentrations of pregnane (containing homopregnane) in soluble components in highly or overly mature stage may be mainly derived from the corresponding components covalently bound within kerogen. Occluded components exhibited stronger thermal stability than adsorbed components. This study showed that evaluation of the thermal evolution characteristics of kerogens/source rocks based on biomarkers compositions from the adsorbed components at highly to overly matured stages was difficult, while we hope this kind of assessment of kerogens/source rocks can be achieved from occluded components.

•Different pyrolysis experiments were performed on kerogens.•Multiple Raman parameters were discussed to evaluate the maturity of kerogens.•Different evolution stages were evidenced from the Raman parameters of kerogens.•Potential Raman thermal evaluation parameters of kerogens were discussed.Potential Raman spectral parameters of kerogens are discussed in this work to evaluate the thermal maturity of organic matter. Four series of simulation experiments were carried out, including the semi-open system, closed golden tube system and two sets of closed glass tube system. Samples were treated for 72 h within the temperatures range 280–560 °C. The results showed that the Raman parameters, such as the shift of the spectral peaks G (∼1600 cm−1) and D (∼1350 cm−1), D/G ratios (based on area, height or width) and the inter-peak interval between peaks G and D, varied regularly with increased thermal maturity. However, some results from the closed glass tube systems were different from the other two pyrolysis systems, which can be attributed to the different cracking characteristics of kerogen macromolecules under different conditions. This observation is quite revealing and we therefore suggest that different simulation models should be employed to study the geological samples from specific geological backgrounds, and multiple parameters should be examined for optimal constraints.

Asphaltenes are the polar macromolecules with complex structures in oils, usually existing as aggregates. Inside the macromolecular structures of asphaltenes, some other fractions can be adsorbed/occluded, and the occluded compounds contain some important geochemical information. However, the adsorption/occlusion phenomena of asphaltenes need verification. Thus, this work was aimed to experimentally study the adsorption/occlusion processes inside the asphaltenes, using the deuterated paraffin n-C20D42 as the target compound under the conditions of high temperature/high pressure, with some chloride salts as additives to probe the mechanism of the adsorption/occlusion inside asphaltenes. The results show that the adsorption/occlusion phenomena take place inside asphaltenes during their thermal evolution. Occlusion inside asphaltenes could occur via two paths, namely, the physical process maybe through the polar interactions among the molecules and the chemical process through condensation or polymerization of the molecules, and then some substantial microporous units could develop to adsorb/occlude other fractions. The temperature and the additives have influences on the occlusion taking place inside asphaltenes. Higher temperatures promoted the pyrolysis of asphaltenes, and the additives play an important role in the properties of adsorption/occlusion inside asphaltenes.

The occluded compounds inside asphaltene aggregates were released from 16 asphaltenes through mild degradation processes oxidized by the H2O2/CH3COOH system. Prior to the oxidation procedure, all of the n-C7 asphaltenes (n-heptane-insoluble asphaltenes) were subjected to Soxhlet extraction by acetone continuously for 240 h to rule out interferences from the adsorbed/co-precipitated compounds, which were likely introduced during the asphaltene preparation processes. Among these occluded compounds, a series of even-carbon-numbered n-alk-(1)-enes were detected from 13 asphaltenes, 22,29,30-trisnorhop-17(21)-ene and 17α-hop-20(21)-ene were simultaneously detected from 3 asphaltenes, and C23 tricyclic terp-12(13)-ene and C29 18α-30-norneohop-13(18)-ene were found co-existent in another 3 asphaltenes. From all 16 asphaltenes, a series of n-alkanoic acid ethyl esters were detected from the occluded compounds, which were generally dominated by the even-carbon-numbered n-alkanoic acids in the esters. The above-reported compounds occluded inside asphaltene aggregates are interpreted as hydrocarbon representatives of materials generated from kerogen at an early stage. These unusual compounds were not detected from the crude oil maltenes or from the acetone extracts of asphaltenes, except for some n-alkanoic acid ethyl esters that were observed in the acetone extracts of a few asphaltenes. The occurrence of a series of even-carbon-numbered n-alk-(1)-enes and some terpenes inside asphaltene aggregates indicated that the macromolecular structures of asphaltenes have protected these compounds from being influenced by the alteration processes in the oil reservoirs and enable them to survive over geological time. In consequence, the geochemical study of these occluded compounds should be very meaningful, especially for heavily altered oils, such as biodegraded oils, from which it is difficult to obtain useful geochemical information directly from the crude oil maltenes because the hydrocarbons have been heavily depleted.

Being the heaviest fraction of crude oils, asphaltenes are liable to aggregate, and other molecules in the oils can be steadily adsorbed onto, and even occluded inside the macromolecular structures of the asphaltenes. These occluded compounds inside the asphaltenes can survive over geological time in oil reservoirs owing to effective protection by the macromolecular structures of the asphaltenes. The asphaltenes of a crude oil (ZG31) from the central Tarim Basin, NW China, were hierarchically degraded by increasing the amount of H2O2/CH3COOH to release the occluded compounds. Besides the common components, series of even numbered n-alk-1-enes and 3-ethylalkanes were detected among the occluded compounds. Comparison of the biomarker distributions and the compound-specific C isotopic results between the compounds from the maltenes and those from the occluded fraction, the ZG31 reservoir was suggested to have been charged multiple times, with different charges being derived from different strata of source rocks.Research highlights► Some other molecules are occluded inside asphaltenes ► These occluded molecules can be released from asphaltenes by H2O2/CH3COOH oxidation ► These occluded molecules are representatives of the original oils from kerogens

Located in the center of the Tarim Basin, NW China, Tazhong area was found to be enriched in oil and gas resources, and the resources are mainly originated from two sets of marine source rocks, namely, the Cambrian-Lower Ordovician (ϵ-O1) source rocks and the Middle-Upper Ordovician (O2+3) source rocks. Due to complex post-depositional alterations such as multiple episodes of hydrocarbon generation, migration and accumulation, most oil reservoirs in this area occur as mixed oils.Results of geochemical studies of crude oils, combined with bulk stable carbon isotope data of 108 oil samples, indicate that most marine oils from Tazhong area are mixtures sourced from both the ϵ-O1 and O2+3 strata. Based on bulk stable carbon isotope data of the oils, quantification studies were made to evaluate the relative contributions of different source rocks to the mixed oils, and the results show that the Middle-Upper Ordovician source rocks are the main contributor for the oils in the Tazhong area, despite fewer exceptions that some oil reservoirs were mainly originated from the Cambrian-Lower Ordovician source rocks. However, for the oil reservoirs occurring in different subunits in the Tazhong area, the relative contributions from different source rocks do not show regular distribution patterns, implying that Tazhong area was subject to multiple episodes of hydrocarbon generation, migration and accumulation in geological history.Highlights► Occluded compounds inside asphaltenes verified the mixed features of Tazhong oils. ► Stable carbon isotope ratios were used to quantitatively evaluate the mixed oils. ► Most Tazhong marine oils were contributed from both the ϵ-O1 and O2+3 source rocks. ► The O2+3 source rocks were the main contributor for the Tazhong marine oils.

•The stable carbon isotope excursion event of SPICE is found in Tarim Basin.•Isotopic results confirm the heterogeneous occurrence of Cambrian source rocks.•The 13C-enriched oils such as TD2∈ may be from some localized Cambrian source rocks.This study investigated the geochemical features of the lower Paleozoic strata of Yaerdang Mountain outcrop along with the core samples from well TD2∈ in the eastern Tarim Basin, NW China. The total organic carbon abundance, hydrocarbon-generating precursor biospecies, and stable isotope ratios of organics and carbonate (δ13Cker, δ13Ccarb and δ18Ocarb) were comprehensively studied for their possible correlative constraints during sedimentary evolution. The results revealed that the δ13Cker (VPDB) of Cambrian kerogens along the outcrop section varied from −34.6‰ to −28.4‰, indicating an increasing tendency from the lower Cambrian to the upper Cambrian. This was on the whole accompanied by the variation in the δ13Ccarb and δ18Ocarb along the profile, which might be associated with the changes in the sea level and also in the compositional variation of benthic and planktonic biomass. The large variation in the stable carbon isotope ratios up to 6‰ along the outcrop section reflected the heterogeneity of the Cambrian source rocks from the eastern Tarim Basin. Hence, the 13C-enriched crude oils from well TD2∈ might have been derived from a localized stratum of Cambrian source rocks. The results from this study showed the possibility of multiple source kitchens in the Cambrian–lower Ordovician portion of Tarim Basin.Download high-res image (332KB)Download full-size image

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.Highlights► Adsorbed/occluded fractions inside asphaltenes were obtained. ► Occluded compounds inside asphaltenes suggested an earlier charging of oils. ► Different fractions from crude oils verified the mixing features of oil reservoirs. ► Both ϵ-O1 and O2+3 source rocks contributed to the mixed oils in the Tarim Basin.