Mechanism of organic matter enrichment and organic pore development in the Lower Cambrian Niutitang shales in northern Guizhou

doi:10.13745/j.esf.sf.2022.5.35

Abstract

Abstract:

Based on the Lower Cambrian Niutitang Formation profile from a cored well in the northern Guizhou area of the Upper Yangtze region, the organic matter characteristics, sedimentary environment and pore development mechanism of the Lower Cambrian Niutitang organic-rich shales were systematically studied by measuring organic carbon contents, mineral compositions, major and trace elements in and pore characteristics of typical shale samples. Organic-rich shales are well developed in the middle and lower parts of the Niutitang Formation, with the total organic carbon content (TOC abundance) ranging from 0.36% to 6.67%, or 2.53% on average. The organic-rich shale layer with TOC abundance greater than 2.0% has an overall thickness of about 80 m, and are formed in a strong reductive sedimentary environment according to the U/Th, Mo/Al and U/Al ratios. The pore volume and pore specific surface area in clay-rich shales with medium level TOC in the middle Niutitang Formation are higher than those in clay-poor shales with high TOC content in the bottom section. Intraparticle pores within clay aggregates are well-developed in clay-rich shales with medium level TOC in the middle Niutitang Formation, providing reservoir space for crude oil or asphalt migration during hydrocarbon generation. Abundant organic matter pores are formed by secondary oil cracking under higher temperature conditions during thermal evolution. The clay-rich shale layer with medium level TOC in the middle Niutitang Formation is more favorable for shale gas enrichment compared to the bottom high-TOC shale layer.

Key words: northern Guizhou, Niutitang Formation, organic-rich shale, organic matter abundance, paleoredox conditions, pore characteristics

CLC Number:

P618.130.1

WU Chenjun, LIU Xinshe, WEN Zhigang, TUO Jincai. Mechanism of organic matter enrichment and organic pore development in the Lower Cambrian Niutitang shales in northern Guizhou[J]. Earth Science Frontiers, 2023, 30(3): 101-109.

Figures/Tables 6

Fig.1 Distribution of the Lower Cambrian Niutitang organic-rich shales in the Upper Yangtze region. Modified from [8].

Fig.2 Vertical changes of selected geochemical parameters for the Lower Cambrian Niutitang shales in the studied well

Fig.3 Ternary discrimination diagram for the mineralogical constituents of the Lower Cambrian Niutitang shales

Fig.4 Cross plots of V/Cr vs. Ni/Co (a) and U/Th vs. Ni/Co (b) for the Niutitang shales

Fig.5 Cross plots of TOC abundance vs. U/Th (a) and Mo/Al (b) for the Niutitang shales

Fig.6 SEM images showing pore characteristics of the Lower Cambrian Niutitang shales

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