Abstract
The dark marine shales which are thick in the Mesoproterozoic Hongshuizhuang formation and Xiamaling Formation in the Yanshan area are the subject of this study. The material compositions and pore characteristics were analyzed, and the reasons for the differences in the adsorption capacity of shale were emphatically discussed through geochemical analysis, X-ray diffraction, scanning electron microscope, low-temperature nitrogen adsorption, mercury injection experiment, isothermal adsorption, and other analysis methods. The results indicate that the samples are rich in high organic carbon content, and vitrinite reflectance suggests that the degree of thermal evolution is in the late-mature to high-mature stage; the mineral compositions mainly consist of quartz and clay minerals. The micropores are relatively developed, which mainly include intergranular pores, intragranular pores, microcracks, and many other pores. Results from the nitrogen adsorption analysis suggest that the pore structures can be divided into three types. Porosity has obvious fractal characteristics. The fractal dimensions of the adsorption pore are larger than those of the seepage hole. The difference of shale adsorption between the Hongshuizhuang Formation and Xiamaling Formation is mainly determined by the volume of micropores and transition pores and the different mineral compositions.
Highlights
Shale gas is characterized by favorable self-generation and self-accumulation, which is becoming more and more important in unconventional areas of oil and gas resources
Abundant organic matter has been observed in the dark shale of the Hongshuizhuang Formation and Xiamaling Formation, and a large number of organic pores were formed
Intergranular pores are commonly found between mineral particles, including intergranular pores formed by the irregular packing of mineral grains (Figure 4(a) and (f)), and the intergranular pores between aggregates of flocculent clay minerals (Figure 4(c)), including intercrystalline pores of mineral particles (Figure 4(e))
Summary
Shale gas is characterized by favorable self-generation and self-accumulation, which is becoming more and more important in unconventional areas of oil and gas resources. Ross and Bustin (2009) found that the adsorption control factors of shale with different maturity are different, the high maturity shale sorption capacity is governed by the characteristics of organic carbon, and clay minerals have a significant impact on shale sorption in shale with low TOC. For Hongshuizhuang Formation, the average TOC content of Kuancheng and Xinlong samples (3.17) is larger than that of other regions (0.05). For Xiamaling Formation, the average TOC content of Xinglong and Huailai is larger than that of other regions (1.27). The whole rock XRD analysis of 7 samples from Hongshuizhuang Formation and 10 samples from Xiamaling Formation shows that the mineral composition of the shale mainly consists of quartz and clay minerals (Table 2).
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