The pore structure and fractal characteristics of shales with low thermal maturity from the Yuqia Coalfield, northern Qaidam Basin, northwestern China

doi:10.1007/s11707-016-0617-y

Front. Earth Sci.

2018, Vol. 12

Issue (1) : 148-159 https://doi.org/10.1007/s11707-016-0617-y

RESEARCH ARTICLE

The pore structure and fractal characteristics of shales with low thermal maturity from the Yuqia Coalfield, northern Qaidam Basin, northwestern China

Haihai HOU¹, Longyi SHAO¹(

), Yonghong LI², Zhen LI¹, Wenlong ZHANG², Huaijun WEN²

¹. School of Geoscience and Surveying Engineering, China University of Mining and Technology, Beijing 100083, China
². No.105 Exploration Team, Qinghai Bureau of Coal Geological Exploration, Xining 810007, China

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Abstract

The continental shales from the Middle Jurassic Shimengou Formation of the northern Qaidam Basin, northwestern China, have been investigated in recent years because of their shale gas potential. In this study, a total of twenty-two shale samples were collected from the YQ-1 borehole in the Yuqia Coalfield, northern Qaidam Basin. The total organic carbon (TOC) contents, pore structure parameters, and fractal characteristics of the samples were investigated using TOC analysis, low-temperature nitrogen adsorption experiments, and fractal analysis. The results show that the average pore size of the Shimengou shales varied from 8.149 nm to 20.635 nm with a mean value of 10.74 nm, which is considered mesopore-sized. The pores of the shales are mainly inkbottle- and slit-shaped. The sedimentary environment plays an essential role in controlling the TOC contents of the low maturity shales, with the TOC values of shales from deep to semi-deep lake facies (mean: 5.23%) being notably higher than those of the shore-shallow lake facies (mean: 0.65%). The fractal dimensions range from 2.4639 to 2.6857 with a mean of 2.6122, higher than those of marine shales, which indicates that the pore surface was rougher and the pore structure more complex in these continental shales. The fractal dimensions increase with increasing total pore volume and total specific surface area, and with decreasing average pore size. With increasing TOC contents in shales, the fractal dimensions increase first and then decrease, with the highest value occurring at 2% of TOC content, which is in accordance with the trends between the TOC and both total specific surface area and total pore volume. The pore structure complexity and pore surface roughness of these low-maturity shales would be controlled by the combined effects of both sedimentary environments and the TOC contents.

Keywords shale gas pore structure fractal dimension Yuqia Coalfield Jurassic northern Qaidam Basin

Corresponding Author(s): Longyi SHAO

Just Accepted Date: 29 November 2016 Online First Date: 26 December 2016 Issue Date: 23 January 2018

Cite this article:

Haihai HOU,Longyi SHAO,Yonghong LI, et al. The pore structure and fractal characteristics of shales with low thermal maturity from the Yuqia Coalfield, northern Qaidam Basin, northwestern China[J]. Front. Earth Sci., 2018, 12(1): 148-159.

URL:

https://academic.hep.com.cn/fesci/EN/10.1007/s11707-016-0617-y
https://academic.hep.com.cn/fesci/EN/Y2018/V12/I1/148

Fig.1 The location of the studied site and tectonic position of the Yuqia Coalfield, northern Qaidam Basin.

Fig.2 The lithology and sedimentary environments of the Shimengou Formation, as well as the sampled positions of the YQ-1 borehole.

Tab.1 TOC contents, vitrinite reflectance, and pore parameters of the shale samples determined with the nitrogen adsorption experiment.

Fig.3 The nitrogen adsorption-desorption curves of typical shale samples from the Shimengou Formation.

Fig.4 Double logarithmic curves between adsorption volume and P_o/P for typical shale samples.

Tab.2 Fractal dimensions of the shale samples of the Yuqia Coalfield calculated with the fractal FHH equation

Fig.5 Relationship between the TOC contents of shales and burial depth based on the sedimentary environments in the Yuqia Coalfield of the northern Qaidam Basin.

Fig.6 Relationships between the specific surface areas, pore volumes, and average pore sizes of the shale samples from the Yuqia Coalfield, northern Qaidam Basin.

Fig.7 Relationships between the specific surface areas, pore volumes, the average pore sizes, and the fractal dimensions of the shale samples from the Yuqia Coalfield, northern Qaidam Basin.

Fig.8 Relationship between the fractal dimensions and TOC contents of the shales from the Shimengou Formation in the Yuqia Coalfield, northern Qaidam Basin.

Fig.9 Relationships of the TOC contents with the total specific surface areas and the total pore volumes of the shales from the Shimengou Formation in the Yuqia Coalfield, northern Qaidam Basin.

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