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 CO

2

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.