Sedimentary characteristics and formation mechanisms of the Ordovician carbonate shoals in the Tarim Basin, NW China

Abstract

The Ordovician carbonate shoals are extensively developed in the Tarim Basin, NW China, and are taken as important petroleum reservoirs. The sedimentary characteristics and controlling factors of these shoals are poorly studied. Multiple means, including field geological investigations, core observations, thin section identifications by scanning electron microscope and cathodoluminescence microscope, granulometric analyses, and geochemical analyses, are conducted. The classification of limestones and shoals in the Ordovician of the Tarim Basin is established. Four types of shoals are developed, namely, intraclastic shoals, bioclastic shoals, oolitic shoals, and algal shoals. It is demonstrated that these four shoal types are deposited in progradation sequences, among which, the low energy bioclastic shoals are in the sequence of retrogradation. Different sedimentary environments develop various sedimentary combination patterns: the platform margin facies mainly develop reef–shoal combinations followed by shoal–shoal combinations; the open platform facies mainly develop shoal–shoal combinations and then reef–shoal combinations; the restricted platform facies develops restricted shoal combinations. Intraclastic shoals are the most developed shoals, which are directly controlled by hydrodynamic force that caused by sea level changes and paleogeography that dominated by tectonic movement. In the platform margin facies, shoals are well-developed with large thickness and limited horizontal extent, and the main shoal type is bioclastic shoal, followed by algae shoal. In the open platform facies, Shoals usually present thin-layered with large horizontal extent. The main shoal type is intraclastic shoal that shows a gently moundy or sill-like structure, the followings are bioclastic shoals and oolitic shoals that developed in the highland near the platform margin facies. In the restricted platform facies, a small number of shoals are developed, which distributed in striped shape paralleling to the shoreline. This study provides great guidance for predicting shoals in different sedimentary facies and build good foundation for future shoal-related reservoirs assessment and prediction.