Quantitative Analysis of Inorganic Mineral Components in Coal Rock from the Longtan Formation, Sichuan Basin by X-ray Diffraction Rietveld Refinement with External Standard Method

Mineralogical and petrological characteristics are key factors controlling reservoir physical properties, fracturing effectiveness, and gas production capacity. As the coalbed methane (CBM) reservoir of the Longtan Formation in the Sichuan Basin is a key target for exploration and development, investigating its mineralogical and petrological characteristics enables accurate reservoir property evaluation, optimization of fracturing designs, and quantification of gas potential, thereby effectively supporting efficient CBM exploration and development. However, traditional methods for coal mineral analysis suffer from complicated sample preparation, long analysis times, or inaccurate quantitative results, making it difficult to meet the demands for detailed evaluation of highly heterogeneous CBM reservoirs. Therefore, it is urgent to establish a new, efficient, and accurate mineralogical characterization method that can eliminate interference from organic amorphous components in coal rock, precisely determine inorganic mineral components and their contents, and provide a data foundation for constructing reliable reservoir evaluation models and optimizing exploration strategies. In this study, XRD-Rietveld analyses were performed on both a corundum standard (100% crystallinity) and the coal rock samples under identical measurement conditions to determine the instrument intensity constant (K value). An XRD-Rietveld refinement with external standard method was thereby established, which does not require tedious and time-consuming pretreatment and can rapidly and accurately determine the contents of organic amorphous components and the absolute contents of inorganic mineral components in coal rock. The experimental results show that the coal rock samples from the Longtan Formation in the Sichuan Basin all contain more than 50% organic amorphous matter. The inorganic mineral components is dominated by kaolinite, with associated calcite, tobelite, anatase, pyrite, diaspore, quartz, halite, and fluorapatite. For all samples, the Rietveld refinement indices with weighted profile residual factor (R_wp) values are below 5%, and the goodness of fit (GOF) values are ≤3.01, all within acceptable ranges, confirming convergence of the Rietveld refinements. The average relative deviation between the amorphous content derived from the Rietveld method and that obtained by low-temperature ashing is 2.00%. For the major elements, the average relative deviations between Al and Si contents converted from the inorganic mineral components and those measured by X-ray fluorescence (XRF) chemical analysis are 6.41% and 1.09%, respectively. These two sets of experimental results validate the accuracy and reliability of the XRD-Rietveld refinement with external standard method for determining inorganic minerals and amorphous components content in coal rock. This method provides a new technical approach for absolute quantitative analysis of inorganic mineral components in the coal rock of the Longtan Formation, Sichuan Basin, and can play an effective role in CBM reservoir evaluation and exploration-development practices.