Reef-shoal depositional model and reservoir qualities of middle Ordocician Yijianfang formation in Bachu, Tarim Basin

Key words:  Tarim Basin  Bachu area  Yijianfang formation  reef-shoal  depositional model  reservoir qualities

Abstract:

Fine outcrop description as well as laboratory analysis were carried out to reveal the depositional model and the features of reservoir quality distribution of platform margin reef-shoal depositional system of the middle Ordocician Yijiangfang formation in Bachu Yijiangfang outcrop. The results show that Yijianfang formation is divided into five three-order sequences from bottom to top, each of which is comprised of TST and HST. Formed by three genetic facies namely reef, bioclastic beach and background deposit of platform，the reef-shoal depositional system mainly develops at the carbonate platform margins of sequence 1 to sequence 3. The prograding reef-shoal depositional system is featured by the movement of high energy zone towards basin direction, and developed in the HST of sequence 1 and sequence 2. In the TST of sequence 3，the retrograding reef-shoal depositional system is developed with the constant moving of high energy zone to the platform. In such a sequence，catch-up transgression reef-shoal complex develops in the early period，whereas keep-up transgression reef-shoal complex grows in the later one. In the reef-shoal depositional system, the reef-front interior arenaceous beaches and reef-back interior arenaceous beaches are usually potentially high-quality reservoirs.