The Mazhuang Gas Reservoir in the Junggar Basin FORMATION CONDITIONS
Gas source rocks of the Mazhuang gas reservoir consist of rich type-III organic matter. Formed in a reducing environment, they lie at the low evolutionary stage. The reservoir is characterized by rapid generation and migration of natural gas and rapid combination of source, reservoir and cap. Natural gas is formed during the subsidence of the depression. But, when the depression rises, diffusion becomes dominant. Its migration continues for a long time and can be divided into several stages. Owing to the diffusion of natural gas, the Mazhuang gas reservoir is a reservoir of multiple layers, and it is always in a dynamic equilibrium of becoming larger and smaller. Only when the amount of gas formed by source rocks and preserved by cap is larger than that of escaped gas can the gas reservoir be formed. Cap is a kind of obstructing layer. The thickness of regional cap is generally more than 50 m. There is a positive correlation between the height of gas reservoir and the thickness of regional cap. Uplift and subsidence are favorable to the formation of microbio-thermogenic gas field. Large delta and submerged fan are good places for the formation of gas reservoir of this kind.