Zhifu Wei

Paleovegetation inferred from the carbon isotope composition of long-chain n -alkanes in lacustrine sediments from the Song-nen Plain, northeast China

Abundant n-alkanes were identified by GC/MS analysis in core sediments from Xianghai Lake and the Huola Basin, on the Song-nen Plain, northeast China. The n-alkanes extracted from Xianghai Lake samples showed unimodal and bimodal distribution. The main peaks of unimodal distribution were at n-C29 or n-C31, and the mid- and long-chain n-alkanes had odd-carbon-number predominance, suggesting they were derived mainly from terrestrial higher plants. Bimodal distributions of n-alkanes had maximum values centered at n-C17 and n-C31 in all samples. The short-chain n-alkanes with a maximum at n-C17 showed no odd–even predominance, however there was a strong odd-carbon-number predominance of long-chain n-alkanes, with a maximum at n-C31. These results suggest that organic matter in Xianghai Lake was derived from mixed sources, including bacteria, algae and terrestrial plants. The n-alkanes extracted from Huola Basin sediments were characterized by a unimodal distribution, with the maximum value at n-C31, and the long-chain n-alkanes had an odd-carbon-number predominance, indicating that they were derived mainly from terrestrial higher plants. In addition, the compound-specific carbon isotope composition was determined for C27, C29 and C31n-alkanes in the core sediments, and the relative contributions of C3 and C4 plants were estimated using a binary model. Calculations indicated that C3 plants were the dominant input during the late glacial and Holocene. The relative abundance of C3 and C4 plants changed significantly through time, likely determined by cool versus warm climate conditions.

Characteristics and origin of desorption gas of the Permian Shanxi Formation shale in the Ordos Basin, China

The Lower Permian Shanxi Formation marine–continental transitional organic-rich shale is one of the most important potential shale gas plays in the Ordos Basin, China. However, the content and origin of desorbed gas from the Shanxi Formation are poorly documented, limiting the understanding of gas generation and potential play elements. Geochemical characteristics of desorbed gas, including content and origin, are analyzed from 17 core samples of the Shanxi Formation from well SL-1. The results show that the Shanxi Formation shales in the study area are characterized by high total organic carbon content of 1.17–2.63%, type III organic matter, and high Tmax between 493 and 513℃. The desorbed gas content of the shale samples varies from 0.22 to 0.50 m3/t, with an average of 0.37 m3/t, and shows a positive correlation with total organic carbon. The gases are dominated by methane (69.57–89.02%), with small amounts of ethane (0.01–0.09%). The carbon isotopic signature δ13C1 ranges from −49.5 to −45.3‰, and the δ13C2 ranges from −23.3 to −14.7‰. In addition, gases released from the Shanxi Formation core samples are thermogenic in origin and possibly coal derived, as the Whiticar chart and the diagram of ethane versus δ13C2 suggest.

Composition of Closed-System Fischer-Tropsch Synthesis Gases and the Constraint Factors

Biogenic gases were reported to exhibit a ‘‘normal’’ carbon isotope order, whereas abiogenic gaseous hydrocarbons in igneous rocks and meteorites exhibit a reversed order. In this study, closed-system Fischer–Tropsch synthesis was conducted at 350°C and 380°C under 30 MPa and 390°C under 30, 100, and 200 MPa, respectively, with magnetite as a catalyst. The results do not show an expected reversed order, even though the features of partially reversed order in carbon isotope values can still be commonly observed. Studies have shown that the essential constraint factor on the carbon isotopic pattern of abiogenic gases is the molar ratio of H2/CO2.

Characterization of n-alkanes and n-alkylbenzenes from different sediments by Py-GC/MS

ABSTRACT Sediments from a selection of source were systematically pyrolysed in two kinds of experiments (single stage and multi-stage) using analytical pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) in order to investigate the n-alkanes and n-alkylbenzenes obtained in pyrolysate volatiles. Our results show that the n-alkanes can be detected by Py-GC/MS, this may be related to the high molecular cleavage of the diverse organic matter constituents under the thermal conditions. And n-alkylbenzenes were also detected, probably produced in connection with n-alkanes, or products of their subsequent evolution. In addition, both sets of experimental results suggested that organic matter adsorbed in modern sediments differs from that in source rocks where organic matter is bound in kerogen, thus the primary stage of products in modern sediments is earlier than that in source rocks.

Characteristics and origin of desorption gas of a transitional shale: A case study from the Lower Permian Taiyuan Formation shale, Ordos Basin, northern China

ABSTRACT In China, marine shale and terrestrial shale have achieved significant breakthroughs. Comparing with marine shale and terrestrial shale, the research and exploration of transitional shale are still in the preliminary stage, and the content and origin of desorbed gas from transitional shales are poorly documented, thus limiting the understanding of gas generation and potential play elements. Geochemical characteristics of desorbed gas, including content and origin, are analyzed from 9 core samples of the Lower Taiyuan Formation shale from well SSL23 in the Ordos Basin, northern China. The results show that the Taiyuan Formation shale in the study area are characterized by high total organic carbon content of 2.70–9.00%, type III organic matter, and high Tmax ranging from 461 to 520°C. The desorbed gas content of the shale samples varies from 0.79 to 2.37 m3/t, with an average of 1.35 m3/t. The gases are dominated by hydrocarbons (83.72–98.62%), with small amounts of non-hydrocarbons (1.38–16.28%) and the methane is the dominant component of the desorbed gases. The carbon isotope δ13C1 ranges from −43.9 to −26.3 ‰, the δ13C2 ranges from −27.2 to −23.1 ‰ and the δ13C3 ranges from −25.3 to −11.6 ‰, respectively. The Whiticar chart and the plot of δ13C1-δ13C2 suggest that gases released from the Taiyuan Formation shale are thermogenic in origin and possibly coal-derived.

Novel maturity parameters for mature to over-mature source rocks and oils based on the distribution of phenanthrene series compounds

Pyrolysis experiments of a low-mature bitumen sample originated from Cambrian was conducted in gold capsules. Abundance and distribution of phenanthrene series compounds in pyrolysis products were measured by GC-MS to investigate their changes with thermal maturity. Several maturity parameters based on the distribution of phenanthrene series compounds have been discussed. The results indicate that the distribution changes of phenanthrene series compounds are complex, and cannot be explained by individual reaction process during thermal evolution. The dealkylation cannot explain the increase of phenanthrene within the EasyRo range of 0.9% ∼ 2.1%. Adding of phenanthrene into maturity parameters based on the methylphenanthrene isomerization is unreasonable, even though MPI 1 and MPI 2 could be used to some extent. Two additional novel and an optimized maturation parameters based on the distribution of phenanthrene series compounds are proposed and their relationships to EasyRo% (x) are established: log(MPs/P) = 0.19x + 0.08 (0.9% < EasyRo% < 2.1%); log(MPs/P) = 0.64x − 0.86 (2.1% < EasyRo% < 3.4%); log(DMPs/TMPs) = 0.71x − 0.55 (0.9% < EasyRo% < 3.4%); log(MTR) = 0.84x − 0.75 (0.9% < EasyRo% < 3.4%). These significant positive correlations are strong argument for using log(MPs/P), log(DMPs/TMPs) and log(MTR) as maturity parameters, especially for mature to over-mature source rocks.