Prediction ideas and methods for the deep coal resources: a case study of the Car-boniferous-Permian system in the Ordos basin

Background Deep coal resources in China have significant potential and offer promising prospects for development and utilization. Assessing the deep coal resources is a prerequisite and fundamental work for formulating our national coal resources strategy and scientifically developing and utilizing deep coal resources. Research Thinking Based on the situation of coal resources in China, the occurrence characteristics of deep coal resources was analyzed and the types of deep coal resources were categorized, the prediction ideas and workflow for deep coal resources was proposed, and the prediction of deep coal resources in the Carboniferous-Permian of the Ordos Basin was carried out. Results and Conclusions The results indicate that deep coal resources are the ultimate products of coal measures after undergoing multi-phase tectono-thermal events. The term "deep" refers to the current preserved state of coal measures, not the origi-nal coal-forming basin. From the perspective of coal occurrence regularity, there is no fundamental difference between deep and shallow coal resources. The deep and shallow coal resources have basic characteristics such as comparability, continuity and gradual change. However, the unique geological conditions of "deep" environments, such as temperature, pressure, and fluid dynamics, determine significant differences in development methods, mining geological conditions, and detection techniques compared to "shallow" resources. Based on this, deep coal resources are classified into: cur-rently exploitable deep coal resources and potentially exploitable deep coal resources two major categories and five sub-categories. The currently exploitable deep coal resources refer to the coal resources that can be mined by underground methods at present or in the near future. According to the "Medium and Long-Term Plan for Deep Coal Resources Min-ing (2025-2035)", the currently exploitable deep coal resources are further classified into two subcategories: 1 000~1 200 m and 1 200~1 500 m deep coal resources. The potentially exploitable deep coal resources are strategic resources for the coal industry and coal resources that rely on technological breakthroughs and are mined using in-situ fluidization tech-nology. According to the burial depth and development prospects, the potentially exploitable deep coal resources are fur-ther classified into 1 500~2 000 m, 2 000~3 000 m, and 3 000~5 000 m three subcategories. The main constraints in the deep coal resources prediction include the limited understanding of the resource occurrence state, the scarcity of known information, the difficulty in detection, and inadequate sharing mechanisms for multi-source heterogeneous data. There-fore, the prediction ideas should be guided by modern mineral resource prediction and coalfield geological theories, with the research on coal formation and tectonic control of coal seams as the core, and based on the study of regional and shallow coal resource occurrence patterns. It is necessary to extensively collect deep geological and geophysical data, fully utilize deep drilling and deep geophysical exploration data, and pay attention to the integration of multi-source het-erogeneous data. Follow the principle of progressing from large to small, from shallow to deep, and from coarse to fine, the prediction work is carried out step by step. The workflow is summarized as " multi-source data collection → research on regional geology and shallow coal resources → research on the occurrence laws of deep coal resources → deep coal resource prediction," with key tasks including the parameter acquisition and resource classification determination. Based on the above ideas and methods, the deep coal resources for the Taiyuan Formation and Shanxi Formation in the Ordos Basin were predicted. The predicted resources data for the buried depth range of 1 000 to 5 000 meters was updated, and the quantity of predicted coal resources within the buried depth range of 2 000 to 5 000 meters was obtained for the first time. The total predicted resources for the Taiyuan Formation and Shanxi Formation amount to 18,835 billion tons within the buried depth range of 1 000 m to 5 000 m in the Ordos Basin. The vertical resource abundance is the largest in 2 000~3 000 m, followed by 3 000~5 000 m, and the smallest in 1 000~2 000 m. The results of this resource prediction have decreased compared to the national coal resource potential evaluation (2013), which is related to the thickness of the coal seam and the predicted area.