Continuous unconventional natural gas accumulations of Carboniferous-Permian coal-bearing strata in the Linxing area, northeastern Ordos basin, China

Abstract

Based on results of laboratory tests and a study of exploration wells, we investigate Carboniferous–Permian unconventional natural gases in lateral extension and vertical sequences in northeastern Ordos basin, China. The primary objectives and techniques are (1) hydrocarbon generation analysis based on Rock-Eval pyrolysis and microscopic observations; (2) reservoir characterization based on depth and thickness extension, nuclear magnetic resonance (NMR) and He porosity/permeability; and (3) gas accumulation analysis based on well cross-sections, field well tests and thermal evolution history.Shale and coal are rich in organic matter and favorable for gas generation, with thermal maturity of approximately 1.4% and shales of type II2 to III kerogen. The gas contents of coal vary from 5 to 20 m3/t with total thicknesses in the Taiyuan and Shanxi Formations of 8–18 m and 4–14 m, respectively, and burial depth increasing from 200 to 2000 m from east to west. Shale and sandstone are also greatly distributed with average total thickness greater than 25 m in both Shanxi and Taiyuan Formations. Porosity is poor for all potential reservoirs, with those of coal, shale, and sandstone being 1.0%–4.9%, 1.0%–1.5%, and lower than 10%, respectively. Three types of unconventional gas accumulations can be distinguished based on differences in source-reservoir combination. The inner source accumulation is composed of coalbed methane (CBM) and tight gas (TG) with direct contact between coal and sandstone, and the strata pressure coefficient (σ) generally varies from 0.8 to 1.0. Close source accumulation is a type of typical continuous TG reservoir with tight sandstone closely related to the source rocks and σ ranging from 0.7 to 1.0. Near source accumulation is relatively distant from source rocks and relies on seal conditions, with σ ranging from 0.4 to 0.7. Due to the gently dipping strata, the eastern area is relatively favorable for CBM development, and the western area is favorable for TG development. Furthermore, shale gas should also be considered in the western area due to good hydrogen generation potential and preservation conditions. Our results are beneficial for the co-production of unconventional natural gas in the entire Ordos basin and other coal-bearing basins.