To understand the geochemical characteristics of late Paleozoic coal in the Changzhi and Jincheng mining areas in the southeastern Qinshui Basin, major and rare earth element analyses were conducted through inductively coupled plasma–mass spectrometry (ICP–MS), X-ray fluorescence spectrometry (XRF), and proximate analysis. The results show that the study coals are bituminous A rank and anthracite C rank ( R o,ran : 1.6–3.24%) with low-ash, low-moisture, low-volatile, and low- to medium-sulfur characteristics. The main forms of sulfur in the study coals are organic sulfur, followed by pyritic sulfur, only some coals with high sulfur contents in the Taiyuan Formation (SGJ, WTP, FHS) are mainly pyritic sulfur, and the contents of sulfate sulfur are extremely low. The major elements of the late Paleozoic coal in the southeastern Qinshui Basin are mainly SiO 2 (4.77%) and Al 2 O 3 (3.64%), followed by Fe 2 O 3 (1.22%), CaO (1.53%), FeO (0.48%), MgO (0.25%), Na 2 O (0.21%), P 2 O 5 (0.18%), TiO 2 (0.15%), and minor K 2 O (0.04%) (on a whole-coal basis). Through correlation analysis and cluster analysis, the occurrence states of major elements in the Shanxi and Taiyuan Formations are different. The average rare earth elements and yttrium (REY) value in the study area is 88.68 μg/g (on a whole-coal basis). The mean light REY (LREY)-to-heavy REY (HREY) ratio is 26.33. The mean values of δEu, δCe, Y, and Gd are 0.60, 0.99, 1.07, and 1.02, respectively. The Shanxi Formation is dominated by the L-type REY enrichment, while the Taiyuan Formation is dominated by the M–H-type REY enrichment. The fractionation degree of REY in the Taiyuan Formation is lower than that in the Shanxi Formation. Rare earth elements in Shanxi coal mainly occur in clay minerals, and some rare earth elements are adsorbed and enriched by vitrinite. Rare earth elements in Taiyuan coal mainly occur in clay minerals and pyrite, and some rare earth elements occur in inertinite. A warm, humid, low-salinity, oxidizing, and acidic environment was favorable for REY enrichment. The coal-forming environment was weakly oxidizing and reducing, and the paleosalinity of the water was relatively high during late Paleozoic coal deposition in the southeastern Qinshui Basin. The paleotemperature of the Shanxi Formation is higher than that of the Taiyuan Formation. The provenance is mainly from an upper crustal felsic source region, the source rocks are mainly post-Archean sedimentary and calcareous mudstones mixed with some granite and alkaline basalt from the Yinshan Upland, and the tectonic setting of the source area mainly includes island arcs and active continental margins.
The southeastern Ordos Basin hosts considerable resource potentials and contains significant hydrocarbon discoveries in the Shanxi Formation and Lower Shihezi Formation, making it a significant petroliferous exploration area. In this study, wireline logs, mud logs, cores, thin sections, scanning electron microscope, heavy minerals, porosity and permeability data, and grain size are employed to carry out research on reservoir differentiation characteristics and associated controlling factors in the Shanxi Formation and Lower Shihezi Formation. The results indicate that reservoir sandstone mainly includes lithic sandstone, quartz sandstone, and lithic quartz sandstone. The pore types are dominated by secondary intergranular pore, intragranular dissolution pore, followed by intercrystal pore and microcracks. The porosity and permeability attain 7.9% and 0.59 10−3 μm2 respectively which shows low porosity and low permeability. The differences of sandstone reservoirs in the Lower Shihezi Formation and Shanxi Formation are controlled by tectonic movements, sedimentary environments, and diagenesis. Tectonic movements not only provide potential provenance but also reconstruct physical property. Sedimentary environments determine sandstone composition and texture and controls reservoir heterogeneity. The diagenesis affects the preservation, evolution, and development of reservoirs and determines the quality of physical property.
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