Oil And Gas Origin And Accumulation Mode. North Qaidam Nanbaxian - Mohair

In the paper ,we introduce the mechanism of natural gas fraction entrapment and apply the whole hydrocarbon geochemistry method to studying the source, the mode of reservoir forming and exploration prospect of oil-gas in the Jurassic-Tertiary reservoir ,Mahai area , north marginal area of Qaidam basin.We discuss the genetic type of natural gas, and study the migration and accumulation of hydrocarbon in the Mahai area , by means of evaluation of petroleum source rocks, the study of comparing biological markers of reservoir bitumen. The result idcate that there are three reservoir formation stages in the Mahai area, Which are:①When stratum of lower ganchaigouzu formation deposite, migration and accumulation of lacustrine oil occur in middle Jurassic stratum of Saishiteng depression. Reservoir forming temperature is 35-55℃. Because early oil is destroyed without effective trap, there is a little irreducible oil in Mahai area. ②Reservoir paleo-tempeature of the second stage in Nanbaxian structure is 65-85℃ . The stage oil from coal come from mainly Yibei depression , and mixed part early lacustrine oil. Without effective trap, Oil-gas of the stage couldn' be captured in Mahai gas field. ③Reservoir forming temperature of the third stage in Nanbaxian structure is higher than Mahai's. Average paleo-tempeature is about 100℃. The high mature natural gas in Nanbaxian structure come from mainly Yibei depression.There are two beneficial exploration area in Mahai area. The north of Nanbaxian - Mahai structure is the most important exploration region . Favorable exploration prospect of oil and gas from coal is showed in Nanbaxian - Mahai structure.