Geochemical Characteristics and Controlling Factors of Organic Matter Enrichment of Wufeng Formation Shale in ZY3 Well, Northwestern Hunan Province

doi:10.19657/j.geoscience.1000-8527.2020.06.085

Abstract

Abstract:

Based on the analyses on graptolite, lithological characteristics, GR curve and TOC content in the Wufeng Formation (Fm.) shale in ZY3 well in northwestern Hunan, the study area clearly belongs to the sedimentary period of WF2-WF3 shale graptolite belt. 20 samples from the WF2-WF3 in ZY3 well were selected to measure their trace elements and organic carbon isotopes compositions, and their vertical variation trends to reveal the controlling factors of organic matter enrichment in the Wufeng Fm. shale in northwestern Hunan were analyzed. The results showed that the organic carbon content in the shale was lower in WF2 (0.17% to 0.86%) and higher in WF3 (0.57% to 2.28%). The V/Cr, Ni/Co, V/(V+Ni) and U/Th values indicate that WF2 and the late-stage of WF3 were suboxic, whereas the early-stage of WF3 was anoxic. The P/Al value and Ba_xs content in shale indicate that WF2 and WF3 have relatively high paleoproductivity. However, the weak correlation between TOC and P/Al values and Ba_xs content, and the strong positive TOC vs. redox environmental index correlation indicate that the organic matter enrichment is mainly controlled by redox environment. Cerium anomaly and Mo/TOC values indicate the effects of frequent sea-level fluctuations and restricted basins on the paleoredox environment of the Wufeng Formation. Due to the sea-level rise in the early-stage of back bulge basin, WF2 may have formed in a sub-oxic environment. The coupling of sea-level rise and confined retention basin in the early-stage of WF3 resulted in hypoxia in the bottom water. In the late-stage of WF3, relative sea-level dropped due to the elevation peak of the Hunan-Hubei underwater upland, resulting in a sub-oxic environment that was not permissive to organic matter preservation. The results show that the redox environment was controlled by tectonic activity and paleo-sea-level change, which further affected organic matter accumulation in the Wufeng Fm. shale.

Key words: northwestern Hunan, Wufeng Formation, depositional environment, organic matter enrichment, trace element

CLC Number:

TE132.2

LIN Donglin, TANG Shuheng, XI Zhaodong, ZHANG Songhang, ZHOU Shulin. Geochemical Characteristics and Controlling Factors of Organic Matter Enrichment of Wufeng Formation Shale in ZY3 Well, Northwestern Hunan Province[J]. Geoscience, 2020, 34(06): 1144-1152.

Figures/Tables 6

Fig.1 Paleogeographic map of the Late Ordovician Yangtze region [16-17](a) and stratigraphic column of ZY3 well (b) b-1.WF3笔石带中灰黑色页岩; b-2.Anticostia uniformis Mu and Lin, in Mu et al. 1993; b-3.Appendispinograptus venustus, 附刺笔石属; b-4.Rectograptus abbreviatus Elles and Wood, 1907, 直管笔石属; b-5. WF2笔石带中灰黑色页岩

Fig.2 Integrated histogram of paleoenvironmental index distribution of the Wufeng Fm. shale of ZY3 well in northwestern Hunan

Fig.3 Distribution features of trace elements enrichment factors in the Wufeng Fm. shale of ZY3 well in northwestern Hunan

Table 1 Identification results of redox index of the Wufeng Fm. shale of ZY3 well in northwestern Hunan

地层	V/Cr		Ni/Co		V/(V+Ni)		U/Th
地层	范围值	平均值	范围值	平均值	范围值	平均值	范围值	平均值
WF2	2.01~2.27	2.09	5.80~6.40	6.00	0.54~0.58	0.57	0.96~1.13	1.02
WF3	1.18~7.02	4.15	5.00~11.02	8.24	0.52~0.88	0.71	1.08~1.89	1.40

Fig.4 Mo vs. TOC relationship of the Wufeng Fm. shale of ZY3 well in northwestern Hunan

Fig.5 Relationships of TOC versus redox condition and paleoproductivity

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