A demineralized Dachengzi oil shale sample, p-kerogen, is obtained through hydrochloric & hydrofluoric (HCl&HF) treatment. Themogravimetric analysis combined with on-line mass spectrometry (TGA-MS) tests on original oil shale and p-kerogen were carried out at two heating rates, 5 °C/min and 15 °C/min, to study the effect of heating rate and mineral matrix on the pyrolysis of kerogen in oil shale. In the pyrolysis products, the amounts of both the organic and inorganic gases generated are significant with the evolution of oil in the temperature range of 370-570 °C. Increasing the heating rate from 5 °C/min to 15 °C/min leads to the decrease of most of the small molecule products of interest in this research, which indicates that in the oil shale pyrolysis the secondary cracking reactions may be inhibited by such increase. With increasing heating rate the thermogravimetric (TG) curves shift to a higher temperature region with an increase of about 10 °C due to the temperature difference between the surface and the center of the sample particles. The amount of alkenes generated is higher than that of alkanes and the alkene/alkane ratio increases with heating rate. At the same heating rate, the amounts of both the inorganic and organic compounds generated in the oil shale pyrolysis are higher than those produced in the pkerogen pyrolysis, suggesting that mineral matrix has an obvious catalytic effect on the pyrolysis of kerogen. Hydrogen release is markedly strengthened in the oil evolution process because of the resultant effect of mineral matter which promotes the cracking reactions in the pyrolysis of kerogen. Compared with oil shale, the TG curves of p-kerogen shift to a lower temperature zone with a decrease of about 10 °C because the pore channels formed in the demineralization treatment intensify the heat and mass transfer in the sample particles.
Quantitative 13 C direct polarization/magic angle spinning (DP/MAS) solid-state nuclear magnetic resonance (SSNMR) was used to characterize type I kerogen isolated from Huadian oil shale. The DP/MAS results showed that this kerogen was highly aliphatic and its aromaticity (f a ) was as low as 20.23%. The average aliphatic carbon chain length (Cn), average aromatic cluster size (C) and substitute degree of aromatic rings (σ) were calculated. The NMR-derived H/C and O/C atomic ratios (R H/C and R O/C ) obtained by DP were in agreement with the corresponding results of ultimate analysis, indicating the accuracy of DP for quantification. Besides, using varying contact times cross polarization (CP) spectra were obtained at the same MAS frequency as the DP spectrum. Regardless of contact time, the aromaticities derived from CP were much lower than that from DP. Consequently, the R H/C value from CP was significantly higher than that of ultimate analysis. The contribution of spinning sidebands could be ignored with the MAS frequency up to 10 kHz. It is concluded that DP with a high MAS frequency is necessary for gaining quantitative structural information about kerogen, especially for its molecular modeling.
This study focuses on nanopores of the productive and non-productive sections within the Lower Silurian Longmaxi shale reservoir in the southern area of Sichuan Basin. 4 rock samples from the productive section and 12 rock samples from the non-productive section were collected from a new exploration well in Changning Area. TOC, XRD and gas adsorption measurements were conducted on these samples. The productive section contains more organic matters, biogenic quartz, but less carbonates and feldspar and is much more porous relative to the non-productive section. Organic matters and clays have important impacts on the development of nanoporosity within the Lower Silurian Longmaxi shale reservoir in study area. Nanoporosity of the productive section is affected mainly by the organic matter contents, and hence implying the predominance of organic-matter pores within this section. Clay minerals have the primary controlling effects on the occurrence of nanopores within the non-productive section, and then indicating the predominance of mineral matrix pores in it. This study should be helpful for us to make a better understanding of in-situ shale gas resources and to optimize targets in this region.
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