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.