Diagenesis and Its Impact on the Reservoir Quality of Continental Shales: A Case Study of the Lower Jurassic Da’anzhai Member of the Ziliujing Formation in the Sichuan Basin, China

Abstract

The Jurassic continental shale oil (gas) is a favorable unconventional resource in the Sichuan Basin of China. In this paper, analysis methods such as core and outcrops observation, thin section identification, scanning electron microscopy (SEM), and X-ray diffraction (XRD) are used to describe the characteristics of lacustrine fine-grained rock reservoir in Da’anzhai Member of the Ziliujing Formation in Sichuan Basin, and it discussed the influence and control of lacustrine fine-grained rock diagenesis on the quality of the reservoir, in order to determine the shale reservoir control factors. The results show that there are three types of rocks in the Da’anzhai Member, which are mudstone, siltstone, and limestone, and the rock combination of shell shale intercalated with shell limestone is developed in the Da 2 submember. Fine-grained sedimentary rocks in this section have undergone compaction, cementation, dissolution, metasomatism, transformation of clay minerals, and hydrocarbon generation of organic matter, which are currently in the middle diagenetic stage A or B substage. Compaction and cementation are the main factors that control the physical use of shale and limestone, and strong cementation is the main reason for the tightness of limestone reservoirs in the Da’anzhai Member in the northern and eastern parts of Sichuan Basin. The difference between dissolution and cementation is the main control factor for the formation of limestone or argillaceous limestone reservoirs in the same section in central Sichuan Basin. The organic-rich shale and shell shale of the Yuanba and Fuling area are the most favorable reservoirs of the Ziliujing Formation in this region. The shell limestone that experienced favorable dissolution in the Da’anzhai Member in the central of the Sichuan Basin has become a limestone reservoir. Discussing the impact of continental fine-grained rock diagenesis on the reservoir can better explore and develop similar intervals, enrich unconventional shale oil and gas accumulation and storage theories, and provide basic theoretical support for finding favorable unconventional shale oil and gas reservoirs.

Clay mineral transformation mechanism modelling of shale reservoir in Da’anzhai Member, Sichuan Basin, Southern China

“…The stratigraphic thickness of the Da'anzhai Member ranges from 70 to 138 m and can be divided from bottom to top into the third sub-Member (5-20 m), the second sub-Member (24-65 m) and the first sub-Member (40-63 m) (Zheng et al, 2013;Zhu et al, 2022). Due to the frequent changes in the lake level, the Da'anzhai Member has caused the interbedded deposition of shale and thinbedded limestone and sandstone.…”

“…There are many types and stages of diagenesis in Da'anzhai Member, including compaction, cementation, dissolution, metasomatism, recrystallization, the transformation of clay minerals and diagenetic evolution of organic matter (Sun et al, 2020;Zhu et al, 2022). Among them, compaction, metasomatism and recrystallization are the main destructive effects, and dissolution and fracture are the main constructive effects.…”

“…The reservoir lithofacies types of Da'anzhai Member in the Sichuan Basin are complex, and the heterogeneity is strong, clay shale and mixed shale are the most developed (Sun et al, 2020;Liu et al, 2021;Yang et al, 2022;Jiang et al, 2023a;Xiong et al, 2023). The mineral content varies little in layers but significantly in regions and is dominated by calcite-based carbonate, clay minerals and quartz, the shale reservoir in the Da'anzhai member underwent an intense transformation of clay minerals, which is crucial to forming and evolving pores (Zhu et al, 2022). However, mineral transformation is controlled by several factors, such as temperature, fluid, and chemical action, the intricate relationships limit the understanding of mineral transformation in the Da'anzhai member, which hinders the study of the reservoir diagenesis and formation mechanism (Bjlykke, 1998;Zhang et al, 2010).…”

Comprehensive Comparison of Lacustrine Fine-Grained Sedimentary Rock Reservoirs, Organic Matter, and Palaeoenvironment: A Case Study of the Jurassic Ziliujing Formation and Xintiangou Formation in the Sichuan Basin

“…As one of the largest oil and gas bearing basins in China, the Sichuan Basin has proved unconventional oil and gas reserves, in which the Jurassic stratigraphy is widely developed with terrestrial lacustrine fine-grained sedimentary rocks [5,6]. This area serves as an important research object for the exploration and evaluation of unconventional oil and gas resources [7][8][9].…”

A New Method for Comprehensive and Quantitative Characterization of Shale Microfractures: A Case Study of the Lacustrine Shale in the Yuanba Area, Northern Sichuan Basin

“…Several wells drilled in the Yuanba area have yielded industrial gas flow in the lacustrine shale of the J 1 z 4 , but the enrichment of shale gas varies considerably [27,28]. The microfractures in the J 1 z 4 shale are well developed [29], but a lack of comprehensive quantitative characterization of the microfractures in this area limits our understanding of the percentage of the microfracture porosity to the total porosity and the distribution of the microfractures, which limits our understanding of the relationships between the shale gas accumulation and enrichment and the shale microfractures. In this paper, we propose a new method that combines RTS and SEM observations to comprehensively characterize the microfractures in the J 1 z 4 shale in the Yuanba area.…”