Control of accommodation changes over coal composition: case study of Jurassic medium-thick coal seams in the Ordos Basin, China

Abstract

This study was conducted to determine the control of accommodation changes over coal composition. Petrological and geochemical characteristics were investigated for 45 samples collected from three Jurassic medium-thick coal seams (coal thickness > 1.3 m) from the northern and western margins in the Ordos Basin, China. The accommodation trend was identified by relative content of macerals, and then the control of accommodation change on coal composition (organic component, ash, sulfur, and elements) was analyzed. The results showed that (i) according to the vertical variation trend of macerals, four types of accommodation reversal surfaces (ARS) could be recognized. The coal seam formation process can be separated into several types according to these surfaces. (ii) The area near the ARS is not only apt to form vitrinite-rich coal or inertinite-rich coal, but microlithotypes possess special characteristics of paleoenvironmental significance. (iii) Ash yield is mainly under the control of the debris input, and when the supply of debris is stable, there is a strong relationship between ash composition and accommodation. When the total sulfur is less than 0.3%, organic sulfur is dominant; otherwise, pyrite sulfur is dominant, and the larger the accommodation, the higher the content of pyrite sulfur. (iv) Accommodation change has an impact on the salinity and redox of the mire, and then on the occurrence of elements Sr, Ba (sensitive to salinity changes), and U (sensitive to redox).