Characteristics and genetic mechanisms of widely distributed zeolites in the Wutonggou Formation reservoir in the southern Junggar Basin
SHI Yanqing, JI Hancheng, ZHANG Guoy , WANG Jian , SONG Xingguo, LIU Jinxian
College of Geosciences, China University of Petroleum-Beijing, Beijing 102249, China 2 State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum-Beijing, Beijing 102249, China 3 Exploration & Development Research Institute, PetroChina Jilin Oilfield Company, Sonyuan 138000, China 4 Research Institute of Experiment and Detection, PetroChina Xinjiang Oilfield Company, Karamay 834000, China 5 Oil Production Plant of East China Branch Company, SINOPEC, Taizhou 225300, China
Widely distributed zeolites were identified in the upper Permian Wutonggou Formation in the east slope of the Fukang sag of the southern Junggar Basin, NW China. Zeolites are usually found in clastic rocks, especially in pyroclastic rocks. Zeolite has an important influence on the reservoir properties of oil and gas reservoirs, however, the genetic mechanisms are still controversial. Four types of zeolites were identified by using scanning electron microscopy (SEM), electron microprobe analysis and optical microscopy. The content of laumontite and analcite are greater in the study area, accounting for 60% and 30% of total zeolite, while the content of heulandite is relatively lower, and clinoptilolite the lowest. The distribution characteristics of zeolites in the Wutonggou
Formation are mainly affected by sedimentary microfacies, provenance and stratum water properties. There is more zeolite in the microfacies with better physical reservoir properties, such as subaqueous distributary channels and river mouth bars in a deltaic sedimentary environment. And in the area close to the southern Bogda mountain source area, this is beneficial to the formation of zeolites. Meanwhile, in the CaCl2 stratum water environment, the content of zeolites (mainly laumontite) is obviously higher than that in other stratum water environments. When the zeolite content is greater than 5%, the content of zeolite is negatively correlated with porosity and permeability, indicating that the dissolution of zeolite cement could significantly improve the physical properties
and storage performance of the reservoir. It is generally accepted that Permian zeolites in the Junggar Basin are derived from volcanic material alteration. In a recent study, zeolites in the study area are subdivided using the two kinds of genetic mechanisms, high and low temperature. The high temperature genetic mechanism is related to the alteration of tuff material, while the low temperature genetic mechanism is considered as crystallization caused by pore water. In the early stage, zeolite was dissolved by acid fluid; and in the later stage, pore water was enriched and crystallized to form the second stage zeolite. SEM images and energy-dispersive X-ray analysis show that it is different from the traditional sense of volcanic material hydration and zeolites transformed into other zeolite minerals. Zeolites in the Wutonggou Formation are more likely related to the transformation of different types of feldspars. Laumonite and heulandite mainly come from the albitization of plagioclase in an alkaline diagenetic environment, whereas analcime may come from the conversion of potassium feldspar in alkaline conditions.
SHI Yanqing, JI Hancheng, ZHANG Guoyi, WANG Jian, SONG Xingguo, LIU Jinxian. Characteristics and genetic mechanisms of widely distributed zeolites in the Wutonggou Formation reservoir in the southern Junggar Basin. Petroleum Science Bulletin, 2021, 01: 1-15.
