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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. [ABSTRACT FROM AUTHOR]

The Tarim Basin has experienced three tectonic evolutionary phases from the Cambrian to Ordovician: (1) Regional extension from the late Neoproterozoic to Mid-Early Cambrian, (2) Relatively weak regional compression from the Late Cambrian to Mid-Early Ordovician, and (3) Regional compression during the Late Ordovician. Intra-platform tectonic and sedimentary characteristics indicate a clear linkage to the tectonic evolution of the basin margin during early Paleozoic time. During the Cambrian, small intra-platform rift-related depressions formed during an extensional setting. During the Mid-Early Ordovician, a transition from extension to compression caused formation of the Tazhong and Tabei paleo-uplifts and major unconformities T 7 4 (base of the Late Ordovician). The evolving paleo-geomorphology led to differentiation of sedimentary facies, and numerous intra-platform shoals formed during deposition of the Early Ordovician Yingshan Formation. During the Late Ordovician, regional compression began, which changed the platform margin slopes into four slopes that surrounded the three isolated island uplifts of Tabei, Tazhong, and Tangnan in the Late Ordovician. Simultaneously, the basin margin dynamic conditions also changed the relative sea level and filling pattern of the basin. In the Early and Middle Cambrian, the Tarim Basin mainly developed a progradational ramp-type platform due to relative sea level fall. From the Late Cambrian to Early Ordovician the relative sea level began to rise, resulting in an aggradational—retrograding rimmed margins-type platform. In the Late Ordovician, along with a further rise in relative sea level, the basin mainly developed isolated platform. [ABSTRACT FROM AUTHOR]