Organic matter provenance and accumulation of transitional facies coal and mudstone in Yangquan, China: Insights from petrology and geochemistry

Abstract

The coal measure strata of the late Paleozoic Taiyuan Formation in northern China, which are mainly composed of coal, mudstone, limestone, and sandstone, with obvious cyclothems, are important with respect to the development of unconventional natural gas. However, the organic matter provenance and enrichment mechanism in transitional facies are still controversial. In this study, the organic geochemistry, elemental geochemistry, and petrology of coal and mudstone in Yangquan were investigated. The Pr/nC17 values of n-alkanes as well as the proximate analysis indicate a high level of organic matter maturity. Organic petrology and biomarker compounds show that terrigenous higher plants represent the primary source of the organic matter in coal, with mudstone having larger aquatic organism contribution in lagoon facies. The redox index represents the larger Ceanom, U/Th, and δU values in lagoon facies mudstone, indicating weakly reducibility. Large diameter regular hexagon-shaped pyrite particles were identified in coal and coaly mudstone, indicating oxygen-containing bottom water condition. The rapid deposition rate promotes the accumulation of organic matter in coal and coaly mudstone, and the preservation of organic matter in coal is related to the redox shock. When the climate became warm and dry gradually, the input of terrestrial debris increased, and coal deposits were interrupted. The sudden transgression promotes the stratification of water column, forming oxygen-deficient environment and the growth of algae. The shallow water sediments were covered with gray-black mudstone, and the enrichment of organic matter was controlled by redox conditions.