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Palaeolacustrine Productivity Study of Early Late Cretaceous in the Central Area of Songliao Basin

Author: YinXiuZhen
Tutor: WanXiaoZuo
School: Chinese Geology University (Beijing)
Course: Marine Geology
Keywords: Songliao Basin Late Cretaceous trace element biomarker compound palaeo-lacustrine productivity
CLC: P534.53
Type: PhD thesis
Year: 2008
Downloads: 384
Quote: 5
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Abstract


Palaeoproductivity can be used to not only estimate effective source rocks but also discuss the concentration of CO2. The objects of this study are 101 mudstones collected from the top Member 4 of Quantou Formation(Qt4), Qingshankou Formation(K2Qs) and the bottom of Mem-ber 1 of Yaojia Formation(Yj1) from Well 12 in estern Gulong Sag. The paper first qualitatively resumes and quantificationally calculates paleolacustrine productivity of early Late Cretaceous in the central area of Songliao Basin and establish quantificational relationship between palaeo-productivity and effective source rocks. The study further investigates the association of the concentration of CO2 and palaeoproductivity.The experimentations of 101 mudstones are completed including vitrinite reflectance , or-ganic carbon with its isotope, porosity, density, rock-eval, trace elements, chloroform and bitu-men“A”, chromatogram of saturated hydrocarbons and chromatogram-massspectrom of bio-marker compounds. Based on these data, the paper analyzes palaeolacustrine bottom environ-ment and organic mass source firstly. Pristance/phytant is 1.19-1.83; dibenzothi-phene/phenanthrne is 0.009-0.041; V/(V+Ni) is bigger than 0.82; the content gammacerane is higher and its index is 0.67-2.15 showing the palaeolacustrine water is half-salt to salt. The lacustrine bottom reflects layered and anoxic environment. From top to bottom, the content of C35 hopane and the value of V/Cr gradually increase. The abundane of rearranged hopanes and steranes peaks at Qs2+3, but the lowest is at Qs1. These features mentioned above reflect lightly anoxic on the bottom of Y1 to extremely anoxic in Qs1. The gray mudstones of the top Qt4 imply lightly anoxic.The distribution of n-alkane is characterized by the peak C16 from Qs to the bot-tom of Yj1. C23 is the higer peak in Qs2+3 and Yj1. The distribution of terpanes showsαβC30 ho-pane is the highest peak and the content gammacerane is higher. The distribution of steranes dis-plays that the content of C27 is much higher than that of C28 and C29. The distribution features indicate organic matter originates mainly from half-salt water bacterias and algaes and partly from large hydrophytes and protozoans. The percentage of hydrobiology in Qs1, the under Qs2+3 and the bottom of Yj1 is respectively 81%, 62% and 79%.The value ofδ13C showing positive shift from the bottom up explains the biomass of phytoalgaes whoseδ13C gradually decreases and that of submerged plants whoseδ13C increases gradually.On the premise that the palaeolacustrine bottom is anoxic environment and organic matter originates mainly from aquatic organisms, the method of organic carbon calculates quantifica-tionlly palaeoproductivity. The evolution of productivity shows Qs1> the under Qs2+3> the upper Qs2+3>the bottom of Yj1 > the top Qt4. The productivity of Qs1 and the under Qs2+3 is 337-1082 gC/m2a and their lacustrine nutrition condition is eutrophic to hypertrophic. The other stratums’ productivity is 64-211 gC/m2a indicating a dystrophic lake. Trace U, Mo and Mn change with the trend of palaeoproductivity, so that they can be advantaged parameters for productivity re-search. Based on the source of organic matter, the content of organic carbon, relative abuandance of ostracode andδ13C can be used to study palaeoproductivity.In addition, palaeoproductivity estimated quantificationlly effective source rocks and dis-cusses the changes of concentration of CO2. As the content of total organic carbon(TOC) is greater than or equal to 0.698% and palaeoproductivity is greater than or equal to 337 gC/m2a, the mudstones of the Qs1 and the under Qs2+3are high-quality effective source rocks. TOC of 50% samples is higher than 0.6% and the average of palaeoproductivity is 121gC/m2a. Therefore, the mudstones of the upper Qs2+3 are middle effective source rocks. As the average of TOC is less than 0.4% and that of palaeoproductivity is less than 120gC/m2a, the mudstones of the bot-tom of Yj1 and the top Qt4 are noneffective source rocks. The changes of concentration of CO2 accord generally with those of palaeoproductivity, characterized by the fact of Qs1> the top Qt4 > the under Qs2+3> the upper Qs2+3>the bottom of Yj1. The fluctuations of concentration of CO2 exist in every sedimentary period. Preliminary analysis shows that Qs1 has possibly two heating of atmosphere, Qs2+3 has one heating of atmosphere and the bottom of Yj1 reveals a sudden cooling of atmosphere.

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CLC: > Astronomy,Earth Sciences > Geology > Historical geology, stratigraphy > History of each era to its stratigraphic > Mesozoic ( circles ) > Cretaceous ( the Department )
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