Geological characteristics of Jurassic coal rock gas and evaluation of favorable zones in the northern structural belt of the Kuqa Depression, Tarim Basin, China

Abstract

The Kuqa Depression in the Tarim Basin has developed two sets of coal-bearing strata: the Triassic and the Jurassic. The Jurassic coals are characterized by multiple layers, large cumulative thickness, and a wide distribution area, providing a strong material basis for the development of coal rock gas. This study presents a comprehensive evaluation of the Jurassic coal quality and reservoir characteristics in the northern structural belt of the Kuqa Depression. The evaluation is based on intensive core sampling, rock debris data, and the use of analytical techniques, including microscopic identification, industrial analysis, vitrinite reflectance measurement, scanning electron microscopy, conventional physical property analysis, nuclear magnetic resonance (NMR) detection, CT scanning, and nitrogen adsorption analysis. The results show that: (1) The microscopic components are mainly vitrinite, with an average content of 64.06%; coal has the characteristics of medium to high volatile matter content, ultra-low moisture, ultra-low ash content, and overall relatively low maturity. (2) Various matrix pores and fractures are developed, with large microcracks forming the main component of the pore network. These microcracks are interconnected and form stacked networks. The porosity of shallow samples ranges from 4% to 23%, with an average of 9.7%, and is characterized by a high proportion of mesopores and macropores. The porosity below 4000 m deep can reach up to 6.34%. Based on the critical burial depth observed in other basins and the correlation between coal maturity and burial depth, the top critical burial depth for effective coal rock gas accumulation in this area is determined to be at least 2500 m. On this basis, an accumulation model for coal rock gas reservoirs was established, and further, in low-to middling coal rank areas, the method for evaluating favorable coal rock gas zones were put forward. Applied the above methods, the coal rock gas favorable zones in the area were comprehensively evaluated. On the one hand, this study provides various indicator parameters for evaluating coal quality in this area, and the critical burial depth, reservoir formation model, and favorable zones offer significant guidance for the selection of target areas for coal rock gas exploration in the future. More importantly, the proposed comprehensive evaluation method serves as a technical reference for coal rock gas exploration in other basins.