关键词: 沸石, 乌尔禾组, 砂砾岩储层, 玛湖凹陷, 成岩作用

Abstract: Zeolite is a type of special cement developed in the Permian Wuerhe Formation in the Manan area of the northwestern Junggar Basin. The mineralogical characteristics, spatial distribution, genetic mechanism and reservoir formation mechanism of zeolite cement in the study area were investigated, by using combined methods including core observation, thin section identification, SEM analysis and rock composition statistics. Results show that laumontite and heulandite are the two main types of zeolites developed in the Wuerhe Formation of the study area. Specifically, the heulandite has two phases, i.e., iron-rich in the early phase and iron-poor in the late phase. They can also be distinguished by the color of the cements under microscope: the early phase in red and the late phase without color. The spatial distribution of the zeolites in the study area does not have an obvious pattern. In contrast, vertically the content of laumontite and heulandite has a gradual decrease trend from the Wuerhe Formation to the Baikouquan Formation. The formation of zeolite is mainly controlled by the components of tuff gravel in the parent rock. The intermediate-acid rock gravel is unfavorable for the formation of laumontite, and there is no obvious relationship between the other volcanic gravels and the formation of all kinds of zeolites in the study area. The widespread secondary dissolved pores of laumontite are the main reservoir space in the study area for hydrocarbon accumulation. The precipitation and dissolution of zeolites are a complex diagenetic effect that can be summarized into a two-stage diagenesis processes. In the early stage, the normal diagenesis formed diverse kinds of zeolite cements under an alkaline environment that rich in metal ions. In the late stage, deep fluid upwellings that contain acidic hydrocarbon-bearing fluids dissolved and altered the reservoirs. The deep fluid alteration is the key stage of reservoir formation in the Manan and its adjacent areas of the northwestern Junggar Bsin, which is important for the formation of commercial oil accumulation.

Key words: zeolite, Wuerhe Formation, conglomerate reservoir, Mahu Sag, diagenesis