Evaluation of coal texture distributions in the southern Qinshui basin, North China: Investigation by a multiple geophysical logging method

Abstract

Coal texture is indicative of coal mechanical strength, which is important not only for evaluating coalbed methane (CBM) reservoir permeability but also for evaluating the possibility of coal and gas outbursts in underground coal mining. This paper begins with a foundation of a geophysical logging-based coal texture evaluation method that compares the digital logging data with the results from 108 coring samples. Then, the method was applied to the analyses of coal textures in 39 CBM wells, the results of which are discussed in terms of the vertical distribution of coal textures in the study area. Finally, a ternary diagram based on thickness proportions of coal textures was used to identify reservoir compartments in the Zhengzhuang field of the southern Qinshui basin. The results show that undeformed coals are easily distinguished from cataclastic and granulated coals because of the high DEN and GR of undeformed coals. Meanwhile, cataclastic coal has lower response values of AC and LLD than granulated coal, which can be used in identifying cataclastic from granulated coal. A multiple geophysical logging method combining the log data of DEN, GR, AC, and LLD was provided for predication of the coal textures in the Zhengzhuang field. The predicated results show that the coal seam can be vertically divided into three to eight sub-layers, with a typical interbedded pattern of undeformed and cataclastic coals on the upper and lower parts and granulated coal in the middle at a relatively large thickness. The study area is divided into four major regions and 11 sub-regions based on the classification of coal texture types. The undeformed coal-dominated region is located in the southwestern part of the study area, indicating a low permeability in the coal reservoir. The granulated coal-dominated region is mainly in the northern and eastern parts of the study area and has resulted from deep burial and multi-period structural activities. The cataclastic coal-dominated region is in the middle of the study area; the coal reservoir in this region is possibly the most favorable for CBM production.