Organic matter-hosted pores in the Cambrian Niutitang shales of the Upper Yangtze region: Pore development characteristics and main controlling factors

doi:10.13745/j.esf.sf.2022.5.37

Abstract

Abstract:

Shale gas exploration and development is relatively low level in the Lower Cambrian Niutitang Formation in the Upper Yangtze where organic matter-hosted pores (OMPs) developed unevenly across the region. In this paper, shale samples from three wells with different depositional settings—namely well EYY1 in the western Hubei rift trough of eastern Upper Yangtze, well W001-4 in the shallow-water shelf of northern Yangtze, and well SNY1 in the Hannan ancient land of northern Yangtze—were collected and analyzed, and shales were classified according to mineral compositions. Organic matter types and OMP development characteristics were investigated by scanning electron microscope method, and the main controlling factors for OMP development were discussed. The results show that (1) mineral compositions and types of the Niutitang shales vary between different regions. Shale samples from EYY1 and W001-4 were classified into siliceous, calcareous and mixed shales; while in SNY1 only siliceous shale was found. (2) Organic matters in the Niutitang shales were divided into depositional and migrated organic matters. In the three wells, the carbon-to-oxygen elemental ratio (C∶O) in the depositional organic matter (averaging 6.74) was higher than that in the migrated organic matter (averaging 2.71). (3) OMPs were well developed in EYY1 and SNY1, with relatively high surface porosities of 6%-28%; while in W001-4 they were poorly developed, with surface porosities ranging between 3%-10%. The OMP diameter in the three wells showed a unimodal distribution, centering between 2-50 nm in EYY1 and SNY1 and generally between 5-25 nm in W001-4. Organic matter in siliceous shales had the highest surface porosity, higher than in calcareous shales. (4) Migrated organic matter showed higher level pore development compared to depositional organic matter. OMP development was affected by counteraction between pore production from hydrocarbon generation from organic matter and pore reduction from compaction. Clay mineral is another important factor hindering micro-scale OMP development in shales, and siliceous shales rich in clay minerals may be an important clue for the determination of exploration “sweet spots” for Cambrian shale gas resource.

Key words: Upper Yangtze plate, Niutitang shale, lithology, organic matter-hosted pores, main controlling factors

CLC Number:

P618.13

MA Zijie, TANG Xuan, ZHANG Jinchuan, ZHAI Gangyi, WANG Yufang, LIANG Guodong, LUO Huan. Organic matter-hosted pores in the Cambrian Niutitang shales of the Upper Yangtze region: Pore development characteristics and main controlling factors[J]. Earth Science Frontiers, 2023, 30(3): 124-137.

Figures/Tables 19

Fig.1 Sedimentary facies of the Lower Cambrian Niutitang Formation and locations of research wells in the Upper Yangtze region. Modified after [8⇓⇓⇓⇓⇓⇓⇓⇓-17].

Fig.2 Lithologies, sampling site distributions and TOC depth profiles in the Niutitang Formation of typical wells, Upper Yangtze

Fig.3 Mineral compositions of the Niutitang shales in typical wells, Upper Yangtze

Fig.4 Ternary diagrams of mineral compositions of the Niutitang shale in typical wells, Upper Yangtze

Fig.5 Microscopic characteristics of depositional organic matter in the Niutitang shale in typical wells, Upper Yangtze

Fig.6 Microscopic characteristics of migrated organic matter in the Niutitang shale in typical wells, Upper Yangtze

Table 1 Comparison of C∶O ratios in different types of organic matter in Lower Paleozoic shales from different areas of the Upper Yangtze

页岩样品	原生有机质C/O	运移有机质C/O	资料来源
湖北宜昌牛蹄塘组页岩	6.25	2.78	本次研究
四川威远牛蹄塘组页岩	7.35	2.64	本次研究
湖南永顺龙马溪组页岩	8.32	2.86	文献[29]

Fig.7 Microscopic characteristics of OMPs in the Niutitang shale in typical wells, Upper Yangtze

Table 2 Classification and development characteristics of OMPs in the Niutitang shale in typical wells, Upper Yangtze

井名	原生有机质孔隙	碎屑颗粒间有机质孔隙	黄铁矿晶间有机质孔隙	矿物颗粒内有机质孔隙	黏土矿物伴生有机质孔隙
EYY1	发育较差,非均质性较强,面孔率平均为4.88%,呈拉伸状	发育较好,呈海绵状,面孔率平均为13.49%	发育较好,呈海绵状,面孔率平均为21.88%	发育较好,呈海绵状,面孔率平均为28.49%,数量较少	发育较好,呈海绵状,面孔率平均为18.4%
W001-4	基本不发育,面孔率小于3%	发育小孔,面孔率平均为7.49%	部分发育,呈海绵状,面孔率平均为12.68%	未发现	发育较好,呈海绵状,面孔率平均为10.75%
SNY1	基本不发育,面孔率小于3%	发育较好,呈海绵状,面孔率平均为16.93%	部分发育,呈海绵状,面孔率平均为19.26%	发育较好,呈海绵状,面孔率平均为23.51%,数量较少	发育较好,呈海绵状,面孔率平均为19.33%

Fig.8 Identification and comparison of OMPs in the Niutitang shales in typical wells, Upper Yangtze

Table 3 Development characteristics of OMPs in and relevant pore parameters for the Niutitang shale in typical wells, Upper Yangtze

井名	原生有机质面孔率	运移有机质面孔率/%	有机孔孔径/nm	峰值/nm
EYY1	0%~12%	7~24	2~50	10~25
W001-4	0%~4%	3~10	5~25	5~15
SNY1	0%~7%	6~28	2~60	15~30

Fig.9 OMP pore diameter distribution in the Niutitang shales in typical wells, Upper Yangtze

Fig.10 Histogram of OMP pore diameters for different organic matter (a) and shale (b) types in the Niutitang shales in well EYY1, Upper Yangtze

Fig.11 Plots of surface porosity vs. TOC content in the Niutitang shales in typical wells, Upper Yangtze

Table 4 Comparison of organic carbon isotopes and kerogen types between the Niutitang and Longmaxi shales in the Upper Yangtze region

页岩样品	δ¹³C_org/‰	干酪根类型	资料来源
湖北宜昌牛蹄塘组页岩	-33.49~-29.55(-32.60)	Ⅰ型	本文
四川威远牛蹄塘组页岩	-35.24~-30.20(-33.00)	Ⅰ型	本文
渝东南地区牛蹄塘组页岩	-32.40~-31.10(-31.80)	Ⅰ型	文献[38]
川东南地区牛蹄塘组页岩	-35.80~-29.60(-33.20)	Ⅰ型	文献[39]
遵义松林牛蹄塘组页岩	-34.40~-28.90(-32.60)	Ⅰ型	文献[39]
川南地区龙马溪组页岩	-31.40~-26.90(-29.80)	Ⅰ型、Ⅱ₁型	文献[40]
大巴山南缘龙马溪组页岩	-32.04~-28.78(-30.23)	Ⅰ型、Ⅱ₁型	文献[41]
长宁区块龙马溪组页岩	-31.20~-29.40(-30.10)	Ⅰ型、Ⅱ₁型	文献[42]
大巴山前缘龙马溪组页岩	-31.10~-28.20(-29.90)	Ⅰ型、Ⅱ₁型	文献[43]

Fig.12 Organic matter surface porosity versus felsic minerals content of Niutitang shale in typical wells, Upper Yangtze plate

Fig.13 Correlation of quartz and TOC contents in the Niutitang and Longmaxi shales in typical wells, Upper Yangtze

Fig.14 Effects of clay minerals on OMP development in the Niutitang shale in well EYY1, Upper Yangtze

Fig.15 Comparison of surface porosities of organic matter with different microstructures in well EYY1, Upper Yangtze

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