准噶尔盆地西北缘沸石发育特征及成储机理初探:以玛南地区二叠系乌尔禾组为例

Characteristics and Formation Mechanism of Zeolite in the Northwestern Margin of Junggar Basin:A Case Study of the Permian Urho Formation in Manan Area

沸石类胶结物是准噶尔盆地西北缘玛南地区二叠系乌尔禾组发育的一种特殊胶结物类型。通过岩心观察、薄片鉴定、扫描电镜分析与岩石组分统计,明确了玛南地区乌尔禾组沸石胶结物的矿物特征、空间分布、成因机理与成储机制。研究区乌尔禾组主要发育浊沸石与片沸石,其中片沸石分为两期,早期富铁,晚期贫铁。研究区沸石胶结物的空间分布规律并不明显,仅在垂向上,从乌尔禾组到百口泉组,浊沸石与片沸石含量具有逐渐降低的趋势。沸石类胶结物的形成主要受控于母岩中凝灰岩砾石组分,中酸性火山岩砾石不利于浊沸石的形成,其他火山岩砾石与沸石的形成无明显关系,浊沸石大量发育的次生溶蚀孔隙是研究区油气赋存的主要储集空间类型。沸石类胶结物的沉淀与溶蚀可以简化为两阶段成岩作用过程,早期正常成岩胶结形成各类沸石胶结物,晚期深部成岩流体上涌,对储层进行溶蚀与改造,深部流体改造是成储的关键阶段。

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.