Microfacies and depositional environments of the Late Ordovician Lianglitage Formation at the Tazhong Uplift in the Tarim Basin of Northwest China

Abstract

The Late Ordovician Lianglitage Formation comprises 13 microfacies (Mf1–Mf13) that were deposited on a carbonate platform at the Tazhong Uplift of the Tarim Basin in Northwest China. Each type of microfacies indicates a specific depositional environment with a certain level of wave energy. Four primary groups of microfacies associations (MA1–MA4) were determined. These associations represent different depositional facies, including reef-shoal facies in the platform margin (MA1), carbonate sand shoal facies (MA2) and oncoid shoal (MA3) on open platforms, and lagoon and tidal flat facies (MA4) in the platform interior. Each microfacies association was generated in a fourth-order sedimentary sequence developing within third-order sequences (SQ1, SQ2, and SQ3, from bottom to top), showing a shallowing-upward trend. High-frequency sequences and facies correlation between wells suggests that the reef-shoal facies more successively developed in the southeastern part of the platform margin, and high-energy microfacies were more strictly confined by the top boundary of fourth-order sequences in the northwestern part of the platform. The highstand systems tract (HST) of the SQ2 is characterized by reef-shoals that developed along the platform margin and tidal flats and lagoons that developed in the platform interior, while the SQ3 is characterized by the oncoid shoal facies that generally developed on the uplift due to a regionally extensive transgression that occurred during the latter part of the Late Ordovician. The results of this study can be used for investigating the development and distribution of potential reservoirs; the reservoirs in southeastern part of the platform margin may be of premium quality because the high-energy microfacies were best preserved there.