Natural gas in duzhai area was formed the earliest . inclusion enclave temperature testing not only determined the first stage for natural gas formed to be 28 - 20ma but also the second stage to be from quaternary ( 2ma ) 包裹体测温法不仅确定了第一阶段天然气成藏期为28 - 20ma ,也确定了第二阶段天然气成藏期,即第四纪以来( 2ma以来) 。