Abstract
Loalongdong reef,found recently in the Upper Permian Changxing Formation along the southern end of Huayingshan area is 160m thick and tapers out within 2000m.The research of petrography,paleontology and microfacies indicates that this reef belongs to the framewark reef composed mainly of hydrozoan-sponge.The main reef-building organism is calcisponge,Tabulozoa,hydrozoan and bryzoan which make up 20-30% in average,maxium 50%,of the total or-ganism.Other reef-dwelling organism,such as echinoides,crinoids,foraminiforas,gastropods,brachiopods and calcareous algae,can be found quite often.Thereef facies can be distingushed easily from the non-reef ones,the former isgenerally characterized by bright-grey,massive,silica-free,and in situ biolith,i.e.framestone,bafflestone,bindstone,with a updomed morphology,whilethe later is characterized by dark-to-blackgrey and moderatly or thickly bed dedargillaceous rock with biogenic detritus and/or flints.The biogenic and non-biogenic rocks are interfingeringly contacting each other.This reef had went through four diagenetic environments,i.e.the submarinephreatic,the meteoric phreatic and vadose,reginal ground water and deep buried environments.It is obvious that the diagenesis has close relationship to variation in porosity of the oil reservoir.Comparing with the non-biogenicrocks,the biogenic rocks exhibit better space for oil accumulation,but the primary porosities may be diminished to some extent in the diagenetic process,and thus lost their importance.Since the secondary porosities which formed at the same time,including the important dolomitized intercrystal porosity in reefcore facies,the solutionized intergranular and in-granular porosities in reef-flank are favourable for oil accumulation.Sometimes they are infilled with bitumen.