Recovery of tectonic traces and its influence on coalbed methane reservoirs: A case study in the Linxing area, eastern Ordos Basin, China

Abstract

The physical properties of coal reservoirs, i.e., porosity, permeability, fractures, and coal structures, are generally evaluated based on its structural morphology, however, its tectonic evolution history plays a much more important role. The key problems associated include the time when the dominant variations on properties of coal occurs and how to quantitatively characterize the influences of different tectonic movements. Based on field joint observations and well logging (micro-resistivity imaging logging (MIL), interval transit time (DT24), density (DEN), and natural gamma-ray (GR)) interpretations, the paleotectonic stress fields and tectonic traces were recovered and the present coal structures were identified. Subsequently, the quantitative correlation between the tectonic deformation and coal reservoir property were established with fluid intrusion tested porosity and permeability and scanning electron microscope (SEM) results.The results showed that the maximum horizontal compressive stress was oriented at the NW-SE direction in Yanshanian and at the NEE-SWW direction in the Himalayan period as obtained from hundreds of joint measurement and fracture interpretation results. Six combinations (by superposition of anticline, syncline, and the flanks) from the Yanshanian and Himalayan movements were found, which indicate different deformation degrees. The maximum tectonic curvature (r'), which occurred in either the Yanshanian or Himalayan period, shows differences at different structural areas and reflects the real deformation degree of coal seam in geological history. When r'< 12 × 10−6 m−1, primary coals are developed; when 12 × 10−6 m−1 <r' < 35 × 10−6 m−1, cataclastic coals are generally developed; when r'> 35 × 10−6 m−1, granulated coals are dominant. Mylonitized coals are barely developed in the study area. With the increase of deformation degrees of coal seams, the number of microfractures increases and fracture stretches tend to be more complicated. When r' < 25 × 10−6 m−1 (if r' > 25 × 10−6 m−1, the coal cores are much too broken for drilling core samples), with the increase of r', the displacement pressure (Pd) achieved by mercury intrusion method decreases in an linear form, and the permeability tested by helium presents a linear growth. However, the porosity shows a weak correlation with r', mainly caused by the great burial depth and long-term compaction resulting in low porosity ranges (2.8%–8.1%) with little difference shown. This presents an applicable method for evaluating the influence of tectonic movement on the physical properties of coal during different times and would benefit the evaluation of coalbed methane reservoirs in the study area as well as other coal basins in the world.