Application of seismic sedimentology in predicating sedimentary microfacies and coalbed methane gas content

Abstract

The identification of sedimentary facies and their lithological distribution characteristics is very important for exploration and development of coalbed methane (CBM). However, the thin interlayer that is common in coal-bearing formation, makes it difficult to predict the favorable facies zone of high frequency sequences based on the conventional methods. In this paper, a seismic sedimentology method was founded to reveal the sedimentary microfacies of high frequency sequences in the coal-bearing Shanxi Formation of the Zhongyu area in the central-western of the Qinshui Basin. The first step of this method is the selection of equal-scale stratal slices and extraction of seismic attributes, then several useable seismic attributes were optimized from all seismic attributes based on the relationship between the seismic attributes and net-to-gross ratios of several single-wells, and finally a model was established to depict the spatial distribution characteristics of sedimentary bodies. Results show that the sedimentary environment of SQ8 and SQ9 in Shanxi Formation is lower delta plain and the upper delta plain respectively. Moreover, the distribution characteristics of net-to-gross ratio across the study area were predicted based on the provided seismic sedimentology. Subsequently, the relationship between the net-to-gross ratio and CBM content of several sing-wells was used to predict the CBM gas content in the study area. The results show that the CBM gas content of No. 2 coal seam ranges from 5 to 20 m3/t, and it decreases with the increase of the net-to-gross ratio in SQ8 and SQ9. The relative error for the predicated gas content results ranges from −14.48% to 3.94%, indicating that the method is effective. Finally, based on the distribution of CBM gas content, the study area is divided into four types, in which the muddy sediment control area (Type IV, located in the central and eastern parts of the study area) is the most favorable for CBM preservation. This study provides a methodology to predict the sedimentary microfacies and CBM gas content in the coal-bearing strata, which extends the application of seismic sedimentology in coal and CBM geology.