THE OIL-GAS SOURCES OF THE LATE PERMIAN ORGANIC REEFAL OIL-GAS POOLS IN THE PANLONGDONG, XUANHAN, SICHUAN

The present paper propose a newly discovered sponge-reef fossil oil-gas pool in the ChangxingFormationin the Panlongdong,Xuanhan,Sichuan.The organic reefs are developedin the middle and upper parts of the Upper Permian Changxing Formation,with a thickness of about 80 m.The rock typeswithin the Panlongdong organic reefs consist significantly of bafflestone and skeletal li mestone.Theorganic reefs are made fromsponge,hydra and polyzoa et al.The occurrence of abundant asphalt in the organic reefs and the strata above and belowthemhas disclosed the migration of oil and gas.The asphalt often fills into pore spaces,fissures and cracks,as well as into brachiopods and sponges.Macroscopically,three phases may be identified for the filling processes of the asphalt :the one is the filling into organicframeworks,intergranular pores,sponges and other organic coelomata such as brachiopods,asindicatedinthe field sections.In this phase,the asphalt occurs chiefly in the organic reefs and reef caps;the secondphase is the fillinginto paulopost fissures(the diameter of the fissures>2mm).In this phase,the asphaltis widespreadin the organic reefs and their cap rocks(including spararenite and dolostone) and secondarypores.The other is the fillinginto paulopost fissures(the diameter of the fissures<2 mm).Onthe basis ofcomparative analysis of δ¹³C,saturated hydrocarbon chromatogram and biomarkers,the authors contendthat the oil-gas sources vary probably fromthe source rocks of the Changxing Formation.