Evaluation of coal macrolithotypes distribution by geophysical logging data in the Hancheng Block, Eastern Margin, Ordos Basin, China

Abstract

Macrolithotypes control the porosity and permeability heterogeneity in coal reservoirs, influencing coalbed methane (CBM) production. The traditional method of macrolithotype determination relies on coal cores, but these are expensive and often have limited availability. Geophysical logging data is far more common but few studies have evaluated coal macrolithotypes using these data. Here 57 macrolithotype samples from 16wells (coal cores) informed the relationship between coal macrolithotypes and select logging parameters. The ash yield and density of coal increased while the vitrinite content, pores and fractures reduced from bright coal to dull coal, correspondingly the density (DEN) and natural gamma (GR) logging value gradually increased, while the acoustic time difference (AC) and deep lateral resistivity (LLD) logging value gradually reduced. Most of the macrolithotype identification could be achieved with regional or block specific density cut-off values however, to distinguish between semi-bright and semi-dull a combination of GR and AC values were utilized. The logging evaluation method for coal macrolithotype identification was used to determine their distribution from 67 drilled wells in the Hancheng Block. From that data it was possible to distinguish the macrolithotype vertical distribution and block macrolithotype thickness contour maps for three seams. There was considerable variability in coal thickness and macrolithotype distribution. The greater contribution was typically from the combination of dull, semi-dull, and semi-bright macrolithotypes. The dull coal macrolithotypes often had contributions adjacent to the top and bottom of the seam mudstones. For all of the seams the bright coal contribution was low, and when present occurred in elongated lenses. From the regional contour maps, the semi-bright, semi-dull, and dull coals were well distributed. The macrolithotypes variation reflects the inputs and depositional environment transformations. Here the contribution is mainly from dry peat marsh and the living water peat marsh facies. Incorporation of macrolithotype heterogeneity will aid in reservoir characterization and CBM development.