Geochemical Characteristics and Generation Process of Mixed Biogenic and Thermogenic Coalbed Methane in Luling Coalfield, China

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This study presents detailed information regarding the organic geochemical and biomarker characteristics of the Permian coal-bearing strata in the Huaibei coalfield. The type of organic matter, sources input, depositional paleoenvironments, and thermal maturity are revealed by the organic molecular signatures study. These are of great significance to understand the coalification process and the generation of the coalbed methane. The extractable organic matter (EOM) in coal and coaly mudstone samples are 1.14% and 0.11% on average, respectively (polar compounds > aromatic hydrocarbons > saturated hydrocarbons). The midchain n-alkane (n-C 20 −n-C 24 ) is the dominant fraction for coal (42.33% on average) and coaly mudstone (44.91% on average). Compared to the continental coal-bearing basins, the Pr/Ph ratios (mostly <1.0) of both the coal and coaly mudstone in the Huaibei coalfield are lower. R o (%), Ts/(Ts + Tm), 22S/(22S + 22R) C 31 homohopane, 20S/(20S + 20R) C 27 diasteranes, and the Rock-Eval pyrolysis results demonstrate the organic matter in coals and coaly mudstones is at the mature to postmature stage. The coal-bearing strata were formed in a transitional zone of an open marine and estuarine environment. The interactive deposition of marine and terrestrial sediments in the weak oxidizing−reducing environment was favored for the accumulation of the organic matter and the appearance of multiple coal seams. The organic matter is primarily from terrestrial higher plants mixed with a certain amount of marine sources input for both coal and coaly mudstone. Biodegradation, waterwashing, and thermal maturation are all important factors which would change the distribution of saturated hydrocarbons significantly. As a result, some of the biomarker parameters may lose the meaning to differentiate sources input, depositional environment, and thermal maturity. Hence, the combination of the geological background and the strict screening of these biomarker parameters are very important and indispensable.

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Biomarker Study of Depositional Paleoenvironments and Organic Matter Inputs for Permian Coalbearing Strata in the Huaibei Coalfield, East China

Chen

et al. 2017

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“…Coalbed methane (CBM), also called coal-seam gas or coalbed natural gas, is a clean and important energy resource . The majority of CBM is absorbed on the inner surface of particles and secondly exists as a free gas in coal pores and cracks. − CBM is formed either biogenically or thermogenically via different processes. , Biogenic (microbial) methane is mainly derived from the low-rank coals during early diagenesis; however, secondary biogenic gas can be generated from any ranks of coals at shallow horizons (burial depth less than 3000 ft) . Thermogenic methane occurs in middle- and high-rank coals, with a R 0 more than 0.6%, due to high temperature and pressure during coal rank advance .…”

Section: Introductionmentioning

confidence: 99%

Potential and Constraints of Biogenic Methane Generation from Coals and Mudstones from Huaibei Coalfield, Eastern China

Bao

Huang

et al. 2018

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Coalbed methane (CBM) resources formed biogenically and thermogenically have been discovered in the Permian coalbeds of the Huaibei coalfield. Four coals, three mudstones, and five coalbed-produced water samples collected from the Linhuan, Luling, and Haizi coal mines of the Huaibei coalfield were characterized geochemically and biologically to gain an understanding of the biogenic methane generation potential and the microbial communities involved in situ and in coalbed-produced water-enriched samples. The 16S rRNA gene high-throughput sequencing results showed that the archaeal communities from in situ and enriched cultures were dominated by Methanolobus and Methanobacterium species. The organic material of coals and mudstones could be biodegraded under an anaerobic incubation. The maximum biogenic methane generation potentials of coal and mudstones were 98.5 and 72.5 μmol/g within 123 days, respectively. Volatile matter and total organic carbon (TOC) content were the most important internal factors affecting biogenic methane generation from coals and mudstones. The Na-SO 4 water type resulted in a low methane generation potential.

“…However, hydrogen and carbon isotopic compositions of thermogenic CBM are controlled by many factors, the most important one being the environmental factors especially for its hydrogen isotopic composition. Previous studies were mainly concerned with the influence of the properties of coal and maturity on the hydrogen and carbon isotopic compositions of thermogenic CBM. − Liu et al performed a comparative thermal simulation study of exinite, vitrinite, and inertinitic materials and found that the δD value of methane pyrolyzed by exinite is the lightest in organic microscopic components. Similar results from pyrolysis experiments of coal with different maceral components were obtained by Duan et al The simulation experiment results of Duan et al indicated that the carbon and hydrogen isotope ratios of the hydrocarbon gases pyrolyzed by the herbaceous swamp peat were lighter than those generated from the forest swamp peat, and the isotopic compositions became heavier upon increasing the maturity.…”

Section: Introductionmentioning

confidence: 99%

Hydrogen and Carbon Isotope Composition of Hydrocarbon Gases Generated during Pyrolysis of Peats from Different Environments

Duan

et al. 2018

For the purpose of understanding whether the formation environment of peat precursor has a meaningful effect on the isotope ratios of thermogenic coalbed gas, pyrolysis experiments were carried out on herbaceous and reed marsh peats derived from high-latitude areas with cold dry climate and a herbaceous peat of low-latitude area with tropical moist climate. Our results show that the hydrocarbon gases generated from the herbaceous swamp peat of the high-latitude area had lighter hydrogen (−56 to −46‰) and heavier carbon (3.3–8.2‰) isotopic compositions than those generated from the low-latitude area peat. As the peat samples were pyrolytically matured to vitrinite reflectance levels of 2.5, 3.5, and 5.5%, respectively, the hydrogen and carbon isotope ratios of the generated hydrocarbon gases increased, and the average hydrogen isotopic differences between them decreased from −32 to −10‰, whereas the carbon isotopic differences increased from 0.9 to 3.6‰, respectively. We suggest that the cause of these differences may originate mainly from the environmental influence on hydrogen and carbon isotopic compositions of the peats. Our results also showed that the influence of the formation environment of the peat precursor on the isotopic compositions of the pyrolysis hydrocarbon gases is less than that of types of peat. A good positive correlation of the δD or δ13C value with maturity as well as the δD value with the δ13C value of the generated hydrocarbon gases exists between the two kinds of samples, respectively, and their mathematical expressions were also established. These results showed that the environmental influence on isotopic compositions of peat should be considered when evaluating the genesis of thermogenic coalbed gas using hydrogen and carbon isotopes.