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Coal Geology & Exploration

Abstract

Objective Deep coalbed methane (CBM) reservoirs tend to exhibit significant tuning effects of thin interbeds, leading to challenging localization of actual coal seam boundaries, limited identification accuracy of micro-amplitude structures in the coal seam roof, and poor stability of the seismic responses of the reservoirs' gas-bearing properties. To address these challenges, this study investigated the No. 8 coal seam of the Benxi Formation in the Mizhi area, Ordos Basin. Accordingly, a method with the prediction of gas-bearing properties as the primary controlling factor was developed for the seismic prediction and comprehensive evaluation of CBM sweet spots. Methods First, the actual roof boundaries of the No. 8 coal seam were traced using high-resolution reflection coefficient volumes, and low-amplitude microstructures in the coal seam roof were characterized through detrending. Then, the thin coal seam thicknesses and roof lithologies were predicted through facies-controlled geostatistical inversion. Based on core experiments and the crossplot analysis of log parameters, the product of Lamé constant λ and density ρ (λρ) was preferentially selected as a seismic parameter for indicating the gas-bearing properties of the coal seam. The spatial distribution of gas content in the coal seam was derived through pre-stack elastic inversion. Furthermore, in combination with fracture prediction based on wide-azimuth anisotropic inversion, the evaluation based on multi-parameter fusion including coal seam thickness, roof lithology, microstructures, gas-bearing property, and fractures was achieved using principal component analysis and a two-dimensional convolutional neural network (PCA—2D-CNN).Results Interpretations based on reflection coefficient volumes effectively reduced the localization errors induced by the tuning effects of thin interbeds. The coal seam thicknesses of approximately 4 m and above and roof lithologies were effectively predicted, with the λρ-based prediction results highly consistent with the measured gas content in wells. Comprehensive evaluation results indicate that classes I and II CBM play fairways are primarily distributed as NE-trending bands. Given the incomplete nature of existing fracturing data, this study developed a semi-quantitative engineering verification framework involving spatial superposition, fracturing response indicators, and consistent discrimination.Conclusion Using λρ-based prediction of gas-bearing properties as the primary controlling factor, the seismic evaluation workflow that integrates constraints from coal seam thicknesses, roof lithologies, microstructures, and fractures can enhance the stability of sweet spot identification in deep-seated thin coal seams. Furthermore, the evaluation workflow can also provide a basis for the selection of optimal play fairways, arrangement of fracturing intervals in wells, and the verification of subsequent fracturing engineering.

Keywords

Ordos Basin, coalbed methane (CBM), rock physics analysis, geostatistical inversion, seismic prediction of sweet spots

DOI

10.12363/issn.1001-1986.25.07.0509

Reference

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