Characterization of Coal Structure of High-Thickness Coal Reservoir Using Geophysical Logging: A Case Study in Southern Junggar Basin, Xinjiang, Northwest China

Coal structure is an important parameter affecting coalbed methane (CBM) production and reservoir stimulation. It shows significant heterogeneity in the vertical direction of the coal reservoir. However, the current research focuses mainly on coal seams with thickness of 1.3-8.0 m, and there is little research on the vertical distribution of coal structure in high-thickness coal reservoir (HTCR) (> 8 m), and the coal structure distribution law is not clear. Therefore, taking the Fukang West Block with a single-layer coal thickness of > 15 m in the southern margin of Junggar Basin as example and based on core observation and comparative analysis of logging response interpretation, this study analyzed five logging parameters (i.e., deep lateral resistivity logging (LLD), density logging (DEN), compensated neutron logging (CNL), natural gamma-ray logging (GR), and sonic-interval transit time logging (AC)) using principal components analysis to obtain the cutoff value of coal structure division of logging response and to predict the vertical distribution characteristics of coal structure in HTCRs. The results showed that different coal structures have different logging response characteristics; that is, with increase in fracture degree, the response values of LLD, CNL, and AC increase and those of DEN and GR decrease. A single logging response cannot identify effectively the type of coal structure, but after combining the data of the five logging responses by principal component analysis, the coal structure can be inverted accurately with accuracy of 82.4%. The results showed that the HTCR in the Fukang West Block has the most cataclastic coal and the most minor undeformed coal. Still, undeformed coal gradually increased with burial depth. The HTCR is vertically composed of 5-10 sub-layers with different coal structures. The undeformed coal is relatively evenly distributed in the upper, middle, and lower parts in the vertical direction, cataclastic structural coal is distributed mainly in the upper part, and the granulated coal is distributed in the middle and lower parts of the reservoir. The fault leads to no significant difference in the distribution of coal structure, but the folds control the development of cataclastic coal. Compared with other study areas, the number of vertical sub-layers of coal structures of HTCRs is much larger than that in thick coal reservoirs, and the distribution is more discontinuous. The heterogeneity of the vertical coal structure of HTCR will affect the fracturing effect of the coal reservoir, so it needs to be studied.

Automated summary

Coal structure is an important parameter affecting coalbed methane production and reservoir stimulation. However, there is little research on the vertical distribution of coal structure in high-thickness coal reservoirs (> 8 m) and the distribution law is not clear. This study analyzed logging parameters and used principal component analysis to obtain the cutoff value of coal structure division and predict the vertical distribution characteristics of coal structure. The results showed that different coal structures have different logging response characteristics, and the coal structure can be accurately inverted by combining multiple logging responses. The vertical coal structure of high-thickness coal reservoirs affects the fracturing effect and needs to be studied.