Sedimentary characteristics of the Lower to Middle Ordovician Calathium reefs in the northwestern Tarim Basin, NW China

Recently, geologists are paying more and more attention to the Ordovician reefs in the Tarim Basin of China for related petroleum potential. This work presents a systematic study on the sedimentary characteristics of two well-exposed Lower to Middle Ordovician reefs of the Tarim Basin, one in the Yingshan Formation and the other in the Yijianfang Formation. The organism types, petrography and its sedimentary characteristics were analyzed based on detailed field and thin-section observations. The reef in the Yingshan Formation is identified as a patch reef that developed in open-platform environment and was distinguished four units, including the base facies, reef-core facies, reef-flank facies and sealing facies. In contrast, the reef in the Yijianfang Formation is a platform margin reef and was distinguished three units, including the base facies, reef-core facies and sealing facies. Both reefs had experienced phases of colonization, vertical aggradation and capping. However, the sea-level trends are different: reefs in the Yingshan Formation appear to reflect the sea-level rise with the subsequent fall, whereas reefs in the Yijianfang Formation appear to have been formed in the reverse order. This study improves the understanding of the sedimentary environments where reefs develop and provides guidance for exploration potential petroleum reservoirs in the Tarim Basin.