Geochemical Characteristics and Organic Matter Enrichment Mechanism in Late Paleozoic Mudstone, Eastern Margin of Ordos Basin

In the past two decades, with the development of CBM technology, the reduction of recoverable resources in shallow coalbed methane and the consumption of unconventional natural gas in China increasing year by year, the development of deep coalbed methane is imperative. The coalbed methane resources in China are about 30.05×10¹²m³, and the coalbed methane resource within the burial depth range from 1000m to 2000m is about 18.87×10¹²m³, accounting for 62.8% of the total resources, which reflects that deep coalbed methane is an important resource foundation for the large-scale development of China’s coalbed methane industry. As the second largest petroliferous basin in China, the Ordos basin has great potential for exploration of deep coalbed methane. In the Eastern Ordos Basin, the coalbed methane resource less than 1500m in depth is about 9×10¹²m³. Although the eastern edge of the Ordos Basin is rich in deep coalbed methane and tight sandstone gas, the enrichment characteristics and controlling factors of organic matter in coal-bearing strata are unclear, which is not conducive to the study on the storage law of resource, evaluation of development potential, and selection of favorable areas.

To reveal the organic carbon content and its influencing factors of mudstones in the coal measure strata and guide the efficient development of coal-measure gas in the eastern margin of the Ordos Basin.

(1) Analytical method: A total of 26 mudstone samples from the Shihezi, Shanxi and Taiyuan Formations were collected in the Shixi area. The content of major and trace elements, organic carbon content and clay mineral characteristics were tested by XRF, ICP-MS and SEM. Carbon isotope of Kerogen was determined by gas chromatography-isotope ratio mass spectrometry (GC-IRMS). The working standard of carbon isotope adopted the international standard PDB. (2) The mode of organic matter enrichment: Based on the experimental results, the Carboniferous—Permian sedimentary environment (including redox conditions, paleoclimate and terrigenous clastic characteristics) was studied with geochemical indicators, then correlation between sedimentary environment and organic matter content was further explored.

(1) Organic and elemental geochemical characteristics of muddy source rocks. Compared with the data of major elements in the upper crust of the North China Plate, the Carboniferous—Permian mudstone samples in the study area show significant enrichment of Al₂O₃ and TiO₂, and Al₂O₃/SiO₂ value ranges from 0.11 to 0.58 (mean value is 0.37), indicating that the sample has a high content of clay minerals. The trace elements are significant enrichment of Li and Cs, slight depletion of Sr, and significant depletion of Zn and Ba. Rare earth elements are highly enriched overall (mean value is 475.4μg/g), and higher than UCC (174.074μg/g) and PAAS (211.78μg/g). The organic carbon content of mudstone samples from the Shanxi and Taiyuan Formations in study area is relatively high (mean value is 2.87%), while the Shihezi Formation is relatively low (mean value is 0.72%).　　(2) Redox-sensitive elements, mainly including Mo, V, U, Ni, Ce and La, are important indicators for characterizing the oxidation environment of sedimentary water bodies. V/(V+Ni) values range from 0.61 to 0.89 (mean value is 0.75), and the differentiation of each layer is not obvious (mean value of Shihezi Formation is 0.79, Shanxi Formation is 0.74, Taiyuan Formation is 0.78), and Ce/La values range from 1.55-2.3 (mean value is 1.93). The above indicators exhibit the characteristics of a poor oxygen environment. The corrected CIA index ranges from 85 to 100, reflecting the strong weathering of parent rock in the source area under a hot-humid environment. Besides, the Sr/Cu and Mg/Ca values of the sample range from 3.16 to 24.89 (mean value is 7.43) and 0.34 to 7.98 (mean value is 2.91), respectively. Fe/Mn values range from 21.35455 to 545.72 (mean value is 202.25), indicating a warm and humid climate during the late Paleozoic. The clay mineral content in the terrestrial debris of the sample is relatively high, which consists mainly of kaolinite (mean value is 39.27%) and illite (mean value is 28.54%).　　 (3) The sedimentary period from the Taiyuan Formation to the Shihezi Formation belongs to a warm and humid climate as a whole, and the bottom of the sedimentary water body is in an anoxic environment. There was no significant correlation among climatic indices of Sr/Cu, Mg/Ca, Fe/Mn and TOC values, however, when the TOC value was more than 1, it was significantly positively correlated with Al and redox sensitive elements (RSEs).

The enrichment of organic matter in the argillaceous rock of the Taiyuan Formation and Shanxi Formation is controlled mainly by water redox conditions and terrigenous debris. The Shihezi Formation inherites the regressive trend of the eastern margin of the Ordos Basin in the late Paleozoic, and its sedimentary environment changes from shallow shelf to marine-continental transitional facies, causing a dynamic ambient and high energy circumstance, in which organic matter is not easily enriched and preserved.