Coal Forming Environments of the Third Member of Yacheng Formation of the Paleogene in Qiongdongnan Basin for an Example

Abstract

Qiongdongnan basin is located in the Southeastern waters area of the Hainan Islands China. In this paper, many methods were used such as sedimentology, coal geology, and geochemistry to study the geochemical characteristics and coal-forming environments of the coal seams in the third member of Yacheng Formation. The results indicate that the coal seams were characterized by thin-beds, multi-layered and unstability forms. Through the analysis of the moisture content, ash content, volatile content, total sulfur, carbons, coal maceral and the maximum reflectivity of the vitrinite, we found that the coals belong to the medium rank bituminous coal, of which the metamorphic grade between the fat coal and coking coal. This result is consistent with that reflected by the volatile content. The coal macerals are mainly desmocollinite, which means the plant lignocellulose tissues have undergone more complete gelation during coalification, so the sedimentary environment should be the light oxidation to reduction. The sediment facies plan of the period of the sedimentation of the third member of Yacheng Formation indicate that the coal seams were mainly formed in the environments both near the sea and the sources such as fan or braided delta plains, or delta front with shallow water where could be easily influenced by the sea water. The general trend of sedimentation showed as transgression, that is, with the intrusion of sea water, the sedimentary environments in the region of interest involved into transitional environments such as fan delta, braided delta and shallow sea, then involved into marine environments such as tidal flat, lagoon and shallow sea.