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ORIGINAL ARTICLE

Sedimentary characteristics of the Lower to Middle Ordovician Calathium reefs in the northwestern Tarim Basin, NW China Miaomiao Meng1 · Tailiang Fan2 · Ian Duncan3 Accepted: 1 November 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract 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. Keyword  Calathium reef · Carbonate · Sedimentary characteristic · Ordovician · Tarim basin Abbreviations O1-2y Yingshan Formation in the middle-lower Ordovician series O2yj Yijianfang Formation in the middle Ordovician series NYG profile Nanyigou profile YJF profile Yijianfang profile

* Miaomiao Meng [email protected] 1



Guangzhou Marine Geological Survey, China Geological Survey, No188 Guanghai Road, Huangpu District, Guangzhou 510760, China

2



School of Energy Resources, China University of Geosciences, Beijing 100083, China

3

Bureau of Economic Geology, The University of Texas at Austin, Austin, TX 78758, USA



Introduction Understanding the sedimentary characteristics of reefs can provide important insights into the understanding of their paleoenvironments and paleoecology. The reefs in the Tarim Basin are also considered economically significant as they host large oil and gas reserves. A giant reef-related oil field has been discovered in the Tazhong uplift of this basin. This discovery, which has been considered significant globally (AAPG 2005), has lead to a renewed interest in exploration for reef-related reservoirs in the Tarim Basin. The Ordovician was a unique period in the development of Paleozoic reef ecosystems (Wilson 1975; Scrutton 1997; Webby 2002). During the Early Ordovician, the reef builders were mainly microbial-dominated lithistid sp