A PRELIMINARY STUDY ON THE REEFS OF DEVONIAN QIZIQIAO FORMATION IN MID-HUNAN

Qiziqiao formation is distributed widely in mid-Hunan which lies on the southeast side of Xuefeng old land. Most of the reefs are found in this horizon. There are three kinds of reef bodies within the upper member of Qiziqiao formation (D₂q²). Firstly, the biostrome developed on the carbonate platform within the open platform neritic facies zone during the early stage of D₂q². The representative Qingtanchong biostrome is shaped like a blanket. It is built up in a bound-skeletal or skeletal framework style. The lying wall of the reef was favourable for oil generation and the biostrome itself is a good oil reservoir. The superstratum can serve as a cover. It appears to be a positive source/reservoir/cover combination. Secondly,the marginal reef formed on the carbonate platform within the open platform neritic facies zone during the middle stage of D₂q². It is represented by Jukoupu marginal reef. The reef-building biota of the dome-like reef body are mainly of stromatopora, coral and algae with some crinoidea. It is also built up in a bound-skeletal or skeletal framework style. The formation of the marginal reef controls the differentiation of sedimentary facies on both sides. In front of the reef is deep water facies which was favourable for oil generation and the core part of the reef is adequate for oil reservoir. The back reef fa-cies and the superstratum can serve as covers. It can be identified as a lateral source/reservoir/cover combination. Thirdly, the reef mound formed on the positive element within the platform basinal facies zone during the later stage of D₂q². It is represented by Putou reef mound. The reef body appears as a superimposing mound form. The reef-building biota are mainly of stromatopora and coral. The reef is formed in a skeletal or bound-skeletal style. Around the reef body is deep water facies favourable for oil generation and the reef mound itself is favourable for oil reservoir. It is a closed lenticular source/reservoir/cover combination.