Elemental Geochemical Distribution of the Jurassic Yan'an Formation in the Ningdong Coalfield and its Controlling Effect on Coal Reservoir Properties

Geochemical elements are important for judging the coal-forming environments and control on pore structure distribution of coal. Geochemical element and reservoir physical property experiments were carried out on the Jurassic Yan'an Formation coal samples from the Ningdong coalfield. Then, the coal-forming environment of Jurassic Yan'an Formation was distinguished, and the relationship between geochemical elements distribution and pore structure of coals was analyzed. The results indicate that the coal samples have low moisture, low ash, medium volatile matter, and high fixed carbon, with an average R o of 0.58%, belonging to low-rank coal. The contents of major elements CaO, MgO and Na 2 O and trace elements Co, Ni, Cu, Zn, Sr, Mo, and Tl were similar to those of Chinese coal, and the other major elements and trace elements were deficient. The contents and distribution pattern of rare earth elements in each sample were similar, and light rare earth elements were relatively enriched. The average porosity of the coal samples was 13.70%, which was dominated by adsorption pores (< 100 nm), and the pore morphology was dominated by parallel plate pore and ink bottle pore. The Yan’an Formation coal in the Ningdong coalfield was formed in a medium-level peat swamp with relative oxidizing conditions, and the sediments were obviously controlled by terrestrial sources. The element distribution indirectly affected the pore characteristics of coal as indicated by changes of provenance and sedimentary environment. The increase of terrigenous input decreased the porosity of the samples and increased the Brunauer–Emmett–Teller (BET) specific surface area (SSA) and the Barrett–Joyner–Halenda (BJH) total pore volume (TPV); the increase of seawater input increased the porosity of the samples and decreases the BET SSA and the BJH TPV. The sedimentary environment changed the provenance and indirectly affected the pore structure of the coal.