CALCULATION OF COAL BED GAS DESORPTION TIME AND ANALYSIS OF ITS INFLUENTIAL FACTORS

The widely distributed Permian strata belong to stable carbonate platform facies near the north edge of the Gondwana Land,in which includes several subfacies,i.e.open platform,half restricted platform,grain bank,shelly bank and reef.Hydrocarbon source rocks were developed in open platform and half restricted platform facies with organic matters in high mature phase or post-mature phase.Reserviors,mostly in reef and banks,were not developed very well and fracture was the most possibel space for hydrocarbon storage.The cap rocks were also not developed efficiently.Some middle and small reservoirs related to unconformity may be formed under covering of the Middle and Upper Jurassic and Lower Cretaceous shales.