Sedimentary Geochemistry And Lake-level Changes From The Paleogene Niubao Formation Of Nima Area, Tibet

The lake level fluctuation can be indicated by these lithofacies and geophysical exploration data of lacustrine deposit,but it is enslaved to the condition of field outcrops and coring quality and seismic resolution and so on.Based on these,this paper attempts to reconstruct the lake level by using sedimentary geochemical indexs.The Paleogene Niubao Formation in Nima Basin in certer Tibet contains abundant lacustrine strata.Through the synthetical study in field and indoor,and according to the methods and theories of sedimentology,stable isotope geochemistry,organic geochemistry and palaeolimnology,the sedimentary facies and environment evolvement,the hydrological characteristics,the paleosalinity,the temperature of lake water,the abundance,type and maturity of organic matter were studied.On the basis of these types of analysis,the water level of the ancient lake in Chaangba area when the depositional period of the lacustrine strata in the Niubao Formation was reconstructed by using sedimentary geochemical indexs.Combination with observation in field and experiments indoor,Chaangba profile mainly contains lacustrine facies,and its roof and floor develop fan-delta facies.From bottom to top,there are moderate-deep lacustrine subfacies,shore-shallow lacustrine subfacies,moderate-deep lacustrine subfacies,shore-shallow lacustrine subfacies in the lacustrine strata.Five hydrocarbon source rocks of black calcareous shale record the depth of water are the highest.Stable carbon and oxygen isotopic analyses of lacustrine carbonates in rocks from Chaangba profile have been carried out.The ?¹³C values are between-4.40‰ and 2.20‰,the ?¹⁸O values are between-10.60‰ and-4.10‰,their distribution range broadly in line with the isotopic composition of modern and ancient lacustrine carbonates.The ?¹⁸O values have positive correlation with the ?¹³C values.The Z values chang between 114.50 and 126.53.The lake water temperature change between 8.94? and 39.16?.After comprehensive analysis,the ancient lake is a closed and salinized lake,the distribution range of the temperature is coincide with the water temperature of modern lakes,and it mainly record the characteristics of water temperature in warm seasons.The organic matter is mainly of ?1 and ?2 types,and is little of ? and ? types,it shows the source of organic matter is mainly the inner matter of the lake and is affected by terrigenous organic matter and preservation condition.The TOC values are between 0.10% and 2.97%,the S₀+S₁+S₂ is between 39.34mg/g and 630.22mg/g,they reflect high primary productivity in the lake.Amorphous organic matter content is between 35.00% and 97.00%,and 93.33% of them are more than 50%.Combine with the correlation between HI and TOC,depositional environment is responsible for organic matter content and types,and primary productivity is the mediating factor.The Tmax values is rang from 434? to 494?,the H/C atomic ratio is between 1.09 and 1.77,the color of sapropelite are brown and yellow.These parameters show the maturity of organic matter is from immature to mature which indicate the structure and composition of kerogen are low damaged.Research shows lake water ?¹⁸O values and ?¹³C_DIC values,HI values and ?¹³C_org values and TOC values of organic matter can effectively reflect the water level of the ancient lake.Be affected by the ratio of inflow to evaporation,when the lake level is rise,lake water ?¹⁸O values reduce,and vice versa.When lake primary productivity is high,and become the dominant factors affecting the lake water ?¹³C_DIC values.In this case,when the lake level rises,the lake water ?¹³C_DIC values increase,and vice versa.When lake primary productivity is low,the lake water ?¹³C_DIC values as same as lake water?¹⁸O values,when the lake level rises,the lake water ?¹³C_DIC values reduce,and vice versa.Be affected by the source of organic matter,when the lake level rises,the contribution of terrigenous organic matter is relatively decreasing,HI values and ?¹³C_org values all increase,and vice versa.Be affected by lake primary productivity and depositional environment,when the lake level is rise,TOC values increase,and vice versa.Moreover,by using these indexs,the lake level of the ancient lake in Chaangba profile was reconstructed.It shows symmetric sudden rise and fall in part strata and rising and falling gradually in general trend.All records can reflect the process of ries and fall of the water level,and the records by HI values and ?¹³C_org values are more detail,but the lake level fluctuation is indicated by the ?¹³C_org values from moderate-deep lacustrine subfacies is more effective than it from shore-shallow lacustrine subfacies.Among these indicators,the lake water ?¹⁸O values,?¹³C_DIC values and TOC values of organic matter give a better view of the overall characteristics of the change of the lake level fluctuation.Application conditions of these indicators: closed saline lake,the organic matter is mainly of ?1 or ?2 types,depositional environment is responsible for organic matter content and types,water stratification,the maturity of organic matter are not too high and the lower,the better.In addition,the lake water ?¹⁸O values and ?¹³C_DIC values are mainly reflect the water level change from moderate-deep lacustrine subfacies,but other indicators are not subject to this restriction.It should be noted that the water level indicated by ?¹³C_org values when the stratum belong to shore-shallow lacustrine subfacies need to be cautious.Lastly,research shows the best source rock is located in near 150 m which the highest level reached for the profile.