Pore Structural Characteristics of Wufeng-Longmaxi Formations Under Biostratigraphic Framework in Northwestern Hunan

doi:10.19657/j.geoscience.1000-8527.2022.055

Abstract

Abstract:

To explore the pore structural characteristics of Wufeng-Longmaxi formations under the biostratigraphic framework, these formations in well SY-3 are divided into LM5-LM7, WF4-LM4 and WF2-WF3, based on the pore structural characteristics and influencing factors. The pore structural characteristics of these three graptolite zones are studied and compared by means of scanning electron microscope, low-temperature nitrogen adsorption, and high-pressure mercury injection experiment. The results show that LM5-LM7 is rich in clay minerals and develops mainly macropores (intergranular pores formed by clay minerals). WF4-LM4 is rich in organic matter and develops mainly micropores (organic pores). WF2-WF3 is rich in quartz and develops mainly micropores (intergranular pores formed by quartz). Most of the intergranular pores are filled with clay minerals and organic matter. TOC and mineral contents are the main influence factors of pore development in the LM5-LM7. TOC content is the main influence factor of pore development in the WF4-LM4, and the WF2-WF3 is affected by mineral constituent, organic matter occurrence and diagenesis. Different graptolite zones have different organic matter abundance and mineral constituents due to the different sedimentary environments, which leads to the pore structural difference.

Key words: biostratigraphy, Wufeng-Longmaxi formations, pore structure, low-temperature nitrogen adsorption, high-pressure mercury injection experiment

CLC Number:

QI Yang, LÜ Chunyan, WANG Yuhui, TANG Shuheng, XI Zhaodong. Pore Structural Characteristics of Wufeng-Longmaxi Formations Under Biostratigraphic Framework in Northwestern Hunan[J]. Geoscience, 2022, 36(05): 1292-1303.

Figures/Tables 17

Fig.1 Lithofacies paleogeographic map and target well location of Wufeng-Longmaxi formations in the upper Yangtze platform (modified from reference [9])

Fig.2 Division of biostratigraphic framework of the Wufeng-Longmaxi formations in well SY-3 (modified from reference [9])

Table 1 Organic carbon and mineral contents of the Wufeng-Longmaxi formations in the different graptolite zones of the study area

笔石带	TOC平均含量/%	石英平均含量/%	黏土矿物平均含量/%
LM5-LM7	0.28	38.58	50.78
WF4-LM4	1.27	47.35	26.20
WF2-WF3	1.08	69.25	16.95
所有笔石带	1.02	50.12	29.43

Fig.3 SEM images of pore characteristics of shale samples from the study area

Fig.4 Types of hysteresis loops (modified from reference [13])

Fig.5 Characteristics of nitrogen adsorption-desorption curves of shale samples in the different graptolitezones

Fig.6 Pore size distribution curves of nitrogen adsorption and desorption branches in the different graptolitezones

Fig.7 Pore size distribution curves of the different graptolite zones calculated by the BJH method

Fig.8 Relationship between cumulative mercury intake and pore size of shale samples in the different graptolitezones

Table 2 Pore structure parameters of the different graptolite zones

笔石带	BET比表面积/ (m²/g)	BJH孔隙体积/ (cm³/g)	平均孔径/ nm
LM5—LM7	5.406 8	0.015 4	11.411 9
WF4—LM4	6.121 3	0.011 1	7.271 8
WF2—WF3	3.655 6	0.006 2	7.142 3
所有笔石带	5.451 8	0.011 0	8.116 1

Fig.9 Percentage distribution of specific surface area and pore volume of shale samples with the different pore sizes in the study area

Fig.10 Correlation of BET specific surface area, BJH cumulative pore volume and average pore size of shale pores in the different graptolite zones

Fig.11 Relationship between TOC content and BET specific surface area and BJH adsorbed cumulative pore volume in the different graptolite zones

Fig.12 Relationship between TOC content and average pore size

Fig.13 Relationship between clay mineral content and BET specific surface area and BJH adsorbed accumulated pore volume in the different graptolite zones

Fig.14 Relationship between quartz content and BET specific surface area and BJH adsorbed accumulated pore volume in the different graptolite zones

Fig.15 Relationship between clay mineral content and average pore size

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