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The Time, Environment And Hydrocarbon Potential Of The Lower Cretaceous Black Shales In Southeastern China

Posted on:2013-01-27Degree:DoctorType:Dissertation
Country:ChinaCandidate:G HuFull Text:PDF
GTID:1110330371986137Subject:Energy geology
Abstract/Summary:PDF Full Text Request
A set of clastic sedimentary rocks, particularly black shales, developed widely in the upper Mesozoic, which is dominated by igneous rocks in southeastern China. However, the ages, sedimentary environments, and hydrocarbon potential of these black shales have not been studied in detail. In this work,11typical Lower Cretaceous sections outcropped black shales were investigated. Multidiscipline researches are concentrated on them including radiometric dating, sedimentology, and geochemistry and so on. Five preliminary conclusions were drawn.The formations that contain black shales are the Guantou Formation and the Shipu Group in Zhejiang Province, the Shixi Formation in Jiangxi Province, the Bantou formation in Fujian Province, and the Shuidishan Formation in Guangdong Province. The zircon U-Pb dating on these formations indicated that the Shipu Group is comparable to the upper Jiuliping, Guantou, and the lower part of Chaochuan formations, and the age of the base of Shixi Formation is roughly isochronous to the Guantou Formation in Zhejiang Province. The age of the base of Bantou Formation in the Yong'an basin of Fujian Province is144±2Ma, whereas, the age of the Bantou Formation in the Chong'an Basin is about116±2Ma. Thus we suggested that the Bantou Formation in the Chong'an section should correlate with the beds from No.21to32of the Yong'an section, and is comparable to the Guantou and Chaochuan formations in Zhejiang Province.The results of radiometric dating and stratigraphic research demonstrate that there are in general two sets of black shales, i.e., the lower part and the upper part of Lower Cretaceous. The lower part of Lower Cretaceous black shales are distributed regionally in Guangdong and Fujian provinces, which are represented by the Bantou Formation and the Shuidishan Formation. Their sedimentary ages are134±2Ma and144±2Ma, respectively. The upper part of Lower Cretaceous black shales develop widely in Fujian, Zhejiang, and Jiangxi provinces, i.e., the Bantou, Guantou, and Shixi formations. They were deposited from113±3Ma to100±2Ma.Comprehensive researches of petrography, paleontology and geochemistry were conducted to the upper part of Lower Cretaceous black shales. Results indicated that the black shales were influenced by marine transgression. The northern boundary of transgression extends at least to the28°N, while its western boundary is limited to the southeastern side of the Wuyi Mountains. The regional tectonic subsidence that stimulated by the raped westward subduction of the Paleo-Pacific plate accompanying with the overall high sea level is the main driving force for the transgression.Organic carbon isotope, total organic carbon, and total nitrogen were also analyzed for those black shales from the Shipu and Chong'an sections. At least five δ13Corg negative excursion events (events SI-SV for the Shipu section, events CI-CV for the Chong'an section) can be recognized in both sections. Relevant analyses of TOC, N/C ratios, and δ13Corg suggested that the events SIV, SV, CIV, and CV were caused by regional factors such as an increase in terrestrial plant detritus, whereas events SⅠ-SⅢ and CⅠ-CⅢ might result from global factors. The tentative correlation between the δ13Corg negative excursions SⅠ-SⅢ, CⅠ-CⅢ, and OAE1b in Tethys and Pacific implied that they are similar in stratigraphic carbon isotope. It suggests that the OAElb might be global events, and the black shales SI-SIII and CⅠ-CⅢ in costal southern China should be the equivalents to OAE1b. They are all the products of the disturbance of carbon isotope of CO2reservoir in the global ocean-atmosphere system.By detailed organic geochemical analyses including organic matter abundance, type and maturity of the Lower Cretaceous black shales in Southeastern China, the source rock features of the strata were revealed. The organic matter abundance of the sources reaches the medium to good quality hydrocarbon source rock standards, and the lower part of Lower Cretaceous is better. The organic matter type is mainly III, with types II and I also being found. The type of the upper part of Lower Cretaceous is better. The thermal maturity of most samples is mature to high mature, with some in mature stage by little influence of volcanic. Thus, a relatively comprehensive hydrocarbon resource evaluation indicates that the favorable regions are in the north of Guangdong Province for the early Lower Cretaceous and in the coastal region of Zhejiang Province and the west of Fujian Province for the late Lower Cretaceous.
Keywords/Search Tags:Southeastern China, Lower Cretaceous, black shales, zircon U-Pb dating, transgression, δ13 Corg negative excursion, hydrocarbon potential
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