Longtou Mountain atoll is developed in a open sea of early Middle Devo-nian.The explored reef is a dome-like carbonate body stricking NNW

4.8km long

3km wide and more than 900m high.The reef-building organisms are mainly composed of Stromatopora

Ta-bulata

HelioIites

Chaetetes and a few of Tetracoral.Some reefphilic organi-sms such as Echinodermata

Brachiopoda are also abundant.In reef coreand forereef subfacies

the organism contents in situ are usually 30-40%

locally above 80%.The atoll may be divided into forereef subfacies(including forereef flatbioclastic limestone microfacies):reef core subfacies(including reef base orinterreef sand-size intraclasts-Echinodermata limestone microfacies

irre-gular and coarse arborescent Tabulata-Stromatopora lime bafflestone micro-facies

tabular and dome-like Tabulata-Stromatopora lime framestone micro-facies);reef flat subfacies(including back-reef flat bioclastic limestonemicrofacies

sand-size intraclasts-pellet or lump limestone microfacies);back-reef lagoon subfacies(including Amphipora micrite microfacies

birds-eye micrite microfacies).Nearly all above mentioned subfacies and micro-facies appear to be hemi-circular zones bulging southwest wards.In the reef body

there are rich in nonferrous metals such as cassiter-ire

lead

zinc and a large amount of bitumen.The bitumen

produced between the grains of rock and in the organisms

are controlled by the pri-mary porosity and are mainly distributed in forereef and reef core;and yet

thebitumin

produced in structural fractures

vugs and stylolite

may be relateded to the fissures which are formed by the reef diagentic and epigenetic changes.The presence of large amount of bitumen indicates that there existed an oilpool which has been destroyed now.The writer believes that the openseablack mudstone and argillaceous limestone surrounding and covering the reefwere good source rock and cap-rock.