Characteristics and controlling factors of carbonate intra-platform shoals in the Tarim Basin, NW China

Abstract

Abstract Intra-platform shoals are widely developed in the Cambrian-Ordovician of the Tarim Basin, western China, and are prospective for hydrocarbon exploration. Early Paleozoic tectonic evolution of the Tarim Basin was one of the extensions and was followed by compression. The dynamic transformation of the regional plate margins caused differentiation of the intra-platform paleogeomorphology, which imposed changes in the development of sedimentary facies contemporaneous with the development of intra-platform shoals. An integrated model for the recognition of intra-platform shoals was established through outcrop investigation, core observation, logging analysis and seismic data interpretation. Ordovician intra-platform shoals in the Tarim Basin occur as ‘shoal containing reefs’, ‘shoal covering reefs’ as well as ‘shoal wrapping reefs’ and show different characteristics at different periods. Few but large intra-platform shoals are developed in the Penglaiba Formation. Large numbers of small-scale intra-platform shoals are widely developed in the lower member of the Yingshan Formation, whereas larger and thicker intra-platform shoals are widely developed in the upper member of the Yingshan Formation. Intra-platform shoals show successive development characteristics from Penglaiba to Yingshan Formations. This study indicates that the development of intra-platform shoals is controlled by many factors such as platform structure, sea-level change, paleodepth, hydrodynamic condition, and paleogeomorphology, among which sea-level changes and micro-paleogeomorphology play the most significant role. The shoals are found mainly in highstand system tracts of high-frequency depositional cycles. The ideal locations for the development of intra-platform shoals are over micro-uplifts and facing windward paleoslopes.