Abstract:

The organic reef and bank facies, widely developed in Middle Ordovician Yijianfang Formation in Tahe oilfield, are characterized by point reefs and banks within the caronate platforms, and the reservoir spaces are mainly of microfissures and secondary dissolution pores. Vertically, characterized by their uneven distribution in various horizons, while laterally, as a whole, they appear to be well developed in the west and poor in the east. In addition, the permeable-storage spaces are heterogeneous in the reservoir intervals. The formation of secondary dissolution pores had been controlled by karstification of meteoric water during Caledonian or Early Hercynian, and then had been superimposed by late structural fracturing and deep-lying dissolution, of which late deep-lying dissolution to be the main a-gent. Organic acid and CO₂, generated from thermal degradation of source rocks and thermo-catalytic cracking of hydrocarbons in reservoirs, destabilized and dissolved the carbonates at depth. Thermal convection and circulation system, caused by temperature difference, would be the main dynamic force driving the fluid flow.

Key words: Yijianfang Formation, organic reef and bank facies, deep-lying dissolution, secondary dissolution pore, Tahe oilfield