Abstract:

The high-resolution sequence structure and depositional model of Yingshan and Yijianfang formations were established by the study of outcrops in Shuinichang of Keping and Dabantage of Bachu in northwestern Tarim Basin combined with 3-D seismic data in Tazhong area. Carbonates in the Middle and Lower Ordovician (Yingshan and Yijianfang formations) can be divided into 2 composite (second-order) sequences (CS1 and CS2), in which CS1 is composed of 2 third-order sequences and CS2 contains 3 third-order sequences. In addition, 17 to 19 high-frequency cycles or fourth-order sequences were also identified. Based on macro sedimentary structure and micro characteristics of the outcrops,9 kinds of microfacies and 4 kinds of microfacies combination were concluded to reflect the specific sedimentary environment and reef building characteristics, which contain reef-beach complex, bioclastic beach, intraclastic beach, lagoon and interbank sea. Furthermore, the evolution of microfacies combination was revealed in the sequence structure. Sq1 and Sq2 consist of lagoon and interbank sea with strong evaporation, in which the dolomite and its transition types are common and the bioclast particles are not found. In Sq3 a thick intraclastic beach is developed, and a few bioclasts appear. On the top of Sq4, a middle-thin layer of bioclast beach emerges. In Sq5, the reef-building organism is blooming and consequently the reef-beach complex becomes to be the dominant facies of Yijianfang Formation.

Key words: high-resolution sequence structure, microfacies, carbonate depositional model, the Middle and Lower Ordovician, northwestern Tarim basin