There exists a stable carbon isotope reversal between crude oils and kerogens from the Cambrian-Lower Ordovician source rocks of the Tarim Basin, NW China. To verify the inverse carbon isotopic pattern and probe the possible mechanism, three shale samples with low thermal maturity were used for thermal simulation experiments, two of which were taken from the Neoproterozoic Xiamaling Formation in Xiahuayuan region, North China (one from argil?laceous shale and the other from calcareous shale) and the third one was from the Permian Lucaogou Formation in Santanghu Basin, Northwest China ( argillaceous shale ). A reversed carbon isotope distribution pattern between kerogen residue and its pyrolysates was observed for the Xiamaling calcareous shale, which was not found for the Xiamaling argillaceous shale or Lucaogou argillaceous shale. A stronger carbon isotope fractionation was found for the pyrolysates from Xiamaling calcareous shale kerogen than those from argillaceous shale kerogens, and then accordingly a weaker carbon isotope fractionation was determined for the pyrolyzed residues from Xiamaling calcareous shale kerogen. Combined with organic petrology and thermal simulation experiments, the “Xiahuayuan algal relic” from Xiamaling calcareous shale was supposed to have a lower hydrocarbon generation capacity than the mineral bituminous matrix from Xiamaling argillaceous shale and the laminated algae from Lucaogou argillaceous shale. Combined with biomarker distribution features, it was suggested that the reversed carbon isotope pattern, between kerogen residue and its pyrolysates, may be ascribed to the contribution of some special biomass (primarily constituted by n?alkanes) of early life, or the isoprenoid compounds were less preserved in the process of biomass sedimentation.

Organic compounds-could be adsorbed and even occlutled within the macromolecular Structures of kerogen. Studies concerning thermal evolution of adsorbed/occluded hydrocarbons inside kerogen will be helpful in understanding its structural characteristics and-evolution features and estimating the-effectiyeneSs.olaclsorbed and occluded-fractions. In this work,. adsorbed and occluded hydrocarbons have been released from low-matured kerogen from the upper Permian Lucaogou formation of Santanghu Basin, north-West China; and from its pyrolysis residues by solvent extraction and oxidative treatment, respectively. The results showed that some n-alkanes, terpanes,and stetanes 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 composition's 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 covalefitly 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 bioniarkers 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.

Even carbon numbered n-alk-1-enes, trapped inside organic geomacromolecules such as kerogen, asphaltene and solid bitumen, can survive geologic time because of the effective protection provided by the complex matrix structures of geomacromolecules. These alkenes are presumed to be derived from esters which are commonly present in various organisms. Subjected to either normal maturation increase or the impact of drastic thermal events, these trapped n-alk-1-enes gradually decrease in concentration and eventually disappear.

The geochemical characterizations of adsorbed and occluded components of solid bitumen in the Kuangshanliang area of northwestern Sichuan Basin were studied by comparing the results of conventional extraction and mild oxidative degradation. The results indicate that the solid bitumen originated from Sinian-Cambrian ancient oil reservoirs. The main source of the parent material is lower-order aquatic deposited in a high-sulfur anoxic marine environment. Exposed solid bitumen suffered from severe biodegradation, altering the tricyclic terpane/pentacyclic terpane and (pregnane + homo-pregnane)/regular sterane ratios as compared to the earlier parent material.

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 degrees C. The results showed that the Raman parameters, such as the shift of the spectral peaks G (similar to 1600 cm(-1)) and D (similar to 1350 cm(-1)), DIG 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. (C) 2014 Elsevier B.V. All rights reserved.