Evolution of pore structure in organic shale with type III kerogen and high kaolinite content in Ningwu Basin

doi:10.1007/s11707-021-0927-4

Front. Earth Sci.

2021, Vol. 15

Issue (4) : 831-848 https://doi.org/10.1007/s11707-021-0927-4

RESEARCH ARTICLE

Evolution of pore structure in organic shale with type III kerogen and high kaolinite content in Ningwu Basin

Qiang XU¹, Hangbing LIN²(

), Yue ZHAO¹, Bo WANG³, Bin MA⁴, Rong DING⁵, Jianxin WANG⁶, Tao HOU⁷

¹. General Prospecting Institute, China National Administration of Coal Geology, Beijing 100039, China
². Purdue University Northwest, Hammond IN 46323, USA
³. Information Institute of the Ministry of Emergency Management of China, Beijing 100029, China
⁴. Shanxi Lanhua CBM Co. Ltd, Jincheng 048026, China
⁵. PetroChina Coalbed Methane Co. Ltd, Beijing 100028, China
⁶. Research Institute of China National Offshore Oil Corporation, Beijing 100028, China
⁷. PetroChina Huabei Oilfield Company, Renqiu 062550, China

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Abstract

Special deposition environment makes organic-rich shales in Ningwu Basin have type III kerogen and high kaolinite content, which are also famous as the kaolinite ore. The specific composition of shale in Ningwu Basin can change the pore structure and thus, influence the shale gas storage and transport. This study focuses on the pore structure and its evolution in shales with type III kerogen and high kaolinite content. In this study, 14 Upper Paleozoic shale samples, whose total organic matter contents (TOC) range from 0.39% to 5.91% and maturities (represented by vitrinite reflectance) range from 1.22% to 2.06%, were collected. Scanning electron microscopy (SEM), high-pressure mercury injection, and low-temperature N₂ adsorption experiments were used to analyze pore structure of these shale samples. Results show that when the TOC content is smaller than 1.4%, the kaolinite content decreases linearly and quartz content increases linearly with increasing the TOC content. In contrast, when TOC content is>1.4%, the clay content tends to increase with increasing TOC. Based on the SEM images, organic pores and clay pores were identified in shale samples with type III kerogen and high kaolinite content. During the maturation process, the kaolinite content decreases and illite content increases with increasing the vitrinite reflectance. At the same time, the pore volume and pore surface area both increase with increasing the vitrinite reflectance, and it may be because more organic pores and clay pores in the illite were generated during the maturation process. This study can provide further understandings of shale gas accumulation in shale with type III kerogen and high kaolinite content.

Keywords pore structure type III kerogen kaolinite low-temperature N₂ adsorption high-pressure mercury porosimetry influencing factors

Corresponding Author(s): Hangbing LIN

Online First Date: 29 October 2021 Issue Date: 20 January 2022

Cite this article:

Qiang XU,Hangbing LIN,Yue ZHAO, et al. Evolution of pore structure in organic shale with type III kerogen and high kaolinite content in Ningwu Basin[J]. Front. Earth Sci., 2021, 15(4): 831-848.

URL:

https://academic.hep.com.cn/fesci/EN/10.1007/s11707-021-0927-4
https://academic.hep.com.cn/fesci/EN/Y2021/V15/I4/831

Fig.1 Schematic map showing sampling wells location (Xu et al., 2018).

Fig.2 The influencing factors on pore structure (Li et al., 2017, 2020).

Tab.1 Mineral compositions and characteristic parameters of shale samples

Fig.3 Organic pores in SEM images. (a) and (b) different shapes of OM pores, well S301, 1214 m; (c) slit OM pores, well S301, 1214 m; (d) circular and elliptical OM pores, well H201, 1117 m; (e) densely distributed circular organic pores, S301,1214 m; (f) organic matter without pore, well N901, 829 m.

Fig.4 The inorganic pores and micro-fractures in the shale samples. (a) Kaolinite pores, well H201, 938 m; (b) kaolinite pores, N901, 829 m; (c) intra-particle pores, well N901, 829 m; (d) micro-fractures and inter-particle pores between organic matter and clay minerals, well H201, 914 m; (e) inter-particle pores in pyrite, well S301, 1214 m; (f) pores formed by pyrite, well S301, 1332m; (g) pores in clay minerals, well X702, 1078 m; (h) inter-particle pores, well H201, 938 m; (i) dissolution pores, well H201, 1049 m.

Fig.5 The mercury saturation under different mercury intrusion pressure.

Fig.6 Pore size distribution with the mercury intrusion method.

Tab.2 Pore structure parameters of shale obtained by MIP

Fig.7 Representative N₂ adsorption/desorption isotherms and the corresponding pore shapes of different samples.

Tab.3 Pore structure parameters of shale obtained by N₂

Fig.8 pore volume and surface area distribution of the shale samples.

Fig.9 Pore size distribution of different shale samples: (a) incremental SSA vs. pore width, (b) incremental PV vs. pore width.

Tab.4 Fractal analysis of the samples from N₂ adsorption

Fig.10 Plots of lnV vs ln(ln(p₀/p)) from N₂ adsorption data.

Fig.11 Correlation between fractal dimensions and pore structure parameters.

Fig.12 The relationship between clay content, quartz content and organic matter content.

Fig.13 the relationship between kaolinite content, illite content and vitrinite reflectance.

Fig.14 The relationship between HI and T_max of Carboniferous-Permian shale in the study area.

Fig.15 The impact on pore structure of the shale from organic geochemical characteristics: (a) TOC vs porosity; (b) TOC vs TPV; (c) TOC vs TSSA; MIP: data obtained based on mercury injection experiment (MIP); N₂ adsorption: data obtained from the low temperature N₂ adsorption experiment.

Fig.16 The relationship between TOC content and fractal parameter D₁.

Fig.17 The effect of vitrinite reflectance on surface area obtained from the low temperature N₂ adsorption experiment.

Fig.18 The correlation between pore volume and vitrinite reflectance in shale samples.

Fig.19 The correlation between kaolinite content and surface area.

Fig.20 Comprehensive analysis of pore structure in shale.

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