Abstract:

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.