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Organic Geochemistry Study Of Palaeoenvironment And Palaeophyte Of Middle-Upper Proterozoic In China

Posted on:2010-05-14Degree:MasterType:Thesis
Country:ChinaCandidate:Z W HuFull Text:PDF
GTID:2230360272488052Subject:Mineral prospecting and exploration
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The Middle-Upper Proterozoic strata are widely distributed in China, and Jixian section in north China and Dengying Gorge section in Yichang, Hubei Province of south China are the most well investigated among others. The Jixian section consists of the Great Wall, Jixian and Qingbaikou groups, which contains twelve formations. My field work in south China focuses on Dengying Formation, which belongs to the Sinian system (relatively young in the Precambrian strata) and is well exposed in the studied area.Five samples were collected from Jixian section, among them two are from Hongshuizhuang Formation shale, one from Xiamaling Formation shale, one from Tieling Formation shale and one from Chuanlinggou Formation shale. Twenty-seven samples were collected from Dengying Formation at Mianzhu and Emei area in Sichuan Province, which are composed of dolomite and limestone. Geochemical analysis results show that shales from Hongshuizhuang and Xiamaling Formations are organic rich and can serve as good source rocks, while Tieling and Chuanlinggou Formations are organic poor and have no hydrocarbon generation potential. Since maceral analysis fails to illustrate organic component compositions, type I kerogen among all studied samples was inferred based on molecular compositions and age of organism formed. Only one sample from Hongshuizhuang Formation remains at moderate maturity stage and all others are over matured. The carbonate rocks from Mianzhu and Emei areas in Sichuan Province are lack of organic matter. Fifteen selected samples performed on TOC and Rock-Eval analysis show no hydrocarbon generation potential at all, partially due to high maturity stage. No further analysis was arranged for samples from Dengying Formation.The biomarkers detected from Hongshuizhuang Formation include normal alkanes, isoprenoid alkanes, methylalkanes, alkylated cyclohexanes, steranes, and terpanes. High abundance of normal alkanes, methylalkanes and alkylated cyclohexanes reflect that precursors of organisms are probably related to cyanobacteria, while low abundance of isoprenoid alkanes may indicate the absence of archaea in Hongshuizhuang Formation. The extremely low contents of steranes and terpanes also suggest the absence of eukaryotes and archaea. Coupled with carbon isotopic compositions of alkanes, paleoenvironment and paleontology analysis, the precursors of organic material in Hongshuizhuang Formation are bacteria and blue green algae, which are deposited under highly reducing environment. The biomarkers from Xiamaling Formation are mainly normal alkanes, alkylated cyclohexanes and steranes. The carbon number of n-alkanes is dominated in medium-high molecular weight range, which differs from these in Hongshuizhuang Formation. This might indicate certain contribution of eukaryotic algae. On the base of carbon isotopic compositions of alkanes and published literatures, the precursors of organic material in Xiamaling Formation are red alga, which are deposited under slightly oxidazing environment.
Keywords/Search Tags:Middle-Upper Proterozoic, source rock assessment, biomarker, carbon isotopic compositions of individual alkane
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