Diagenesis characteristics and diagenetic evolution of organic-rich marine shale

Shale is an unconventional oil and gas reservoir with both generation and storage characteristics. Diagenesis has an important impact on its organic petrological characteristics, reservoir physical properties, pore system structure characteristics, mineral component content and transformation. Diagenesis is of great significance for its porosity and permeability analysis, reservoir comprehensive evaluation and shale gas productivity. At present, the researches on diagenesis and diagenetic evolution mainly focus on conventional sandstone reservoir. Because the application of conventional oil and gas reservoir characterization technology to shale reservoir is limited, and the diagenetic characteristics of shale reservoir are difficult to identify, the researches on diagenetic evolution of shale reservoir are relatively weak, and the comprehensive researches on diagenesis and diagenetic evolution of shale reservoir are relatively scarce.

At present, there are mainly two kinds of research methods on the dual effects of thermal evolution and diagenesis of shale: the first is the direct observation method, which uses high-resolution equipment to analyze shale samples with different maturity and diagenesis to determine the characteristics and development differences of diagenesis. However, this method ignores the heterogeneity and regional differences of samples, and cannot show all the evolution characteristics of shale in the diagenesis process. The second is the physical simulation method, that is, the sample of low maturity is selected, the temperature series is set, and the generation of diagenesis process is induced by heating. This method reduces the heterogeneity of samples and the influence of regional differences on the experimental results to a certain extent. It has strong comparability and can provide the overall characteristics in the process of diagenesis. However, the disadvantage is that it lacks intuitive characterization and cannot clearly and intuitively display the diagenetic evolution characteristics of minerals in the same area.

In view of the above problems, the diagenesis and diagenetic evolution of low-mature organic-rich Marine type II shale in the Middle Proterozoic Xiamalin Formation in Zhangjiakou area of Hebei Province were studied by using the method of high temperature and high pressure physical simulation. The characteristics of diagenesis were observed and characterized in the simulated samples, and the types of diagenesis in the simulated products were identified. A conceptual model of shale diagenetic evolution sequence based on physical simulation is established. In addition, this study also uses direct observation method to characterize the diagenetic characteristics of natural marine shales of Xiamaling Formation in this area. Five diagenetic types including compaction, cementation, dissolution, hydrocarbon generation of organic matter and clay mineral transformation are identified, and diagenetic stages of Xiamaling Formation shales are divided. Furthermore, the marine diagenetic evolution sequence and diagenetic evolution model of the Mesoproterozoic Xiamalin Formation in Zhangjiakou area of Hebei Province are established (Fig.1). This study makes up for the deficiency in the study of shale diagenetic evolution, and has important reference and indicative significance for the development of other high-over-mature Marine shale gas reservoirs in China and the world.

Fig. 1. Diagenetic evolution sequence of the Mesoproterozoic Xiamaling marine shale in Zhangjiakou, Hebei.