Biogeochemical Constraints On The Development Of High-quality Lacustrine Source Rocks

The study of the formation mechanism of lacustrine source rocks is the theoretical basis to terrestrial oil generation.Although fruitful works have been gained,few consensus has been reached so far.In this study,lacustrine source rocks of the Chang 7Member of Ordos Basin and the Qing 1 Member of Songliao Basin are chosed as research objects.Based on modern molecular isotope organic geochemistrical methodology and techniques,a detailed analysis of the biogeochemical process of source rock development is carried out,and primary understandings are as follows.1.Through the systematic study of sedimentary organic matter in Chang 7 source rocks of Ordos Basin,it is revealed that high productivity caused by the blooming of algae is the decisive factor for the formation of Chang 7source rocks.The high productivity of the lake basin during the Chang 7 period may be related to the input of nitrogen and phosphorus nutrients and trace elements like Fe from the volcanic ash and hydrothermal fluid at the lake bottom.2.The relatively low gammacerane content and relatively high Pr/Ph value in Chang 7 source rocks indicate that the water body of palaeo-lake Ordos was under weak oxidation and weak reduction conditions during the deposition of Chang 7 source rock.The lack of stable water stratification at the bottom of the palaeo-lake may be related to the destruction of the frequent internal gravity flow induced by internal tectonic activity and the lake bottom hydrothermal activity.3.Based on the study of the development and evolution process of organic matter in the first member of Qingshankou formation of the Cretaceous Songliao Basin,it is proposed that the strength of the microbial activity at the water-sediment interface during the formation of lacustrine source rocks can be used as a good function of the surface productivity and organic matter preservation of palaeo-lakes.The high linear correlation between the content of biomarker compounds indicating terrestrial input and the content of biomarker compounds indicating planktonic algae and water salinity,revealing that terrestrial input,which determined by runoff under certain climatic conditions,has controlling effect on lake productivity and water environment.The sufficient runoff can bring more nutrients from land,which promotes productivity like algae and the enrichment of organic matter in the sediments;when the runoff is relatively weak,the salinity of the bottom water of Songliao palaeo-lake increases significantly,which leads to the enhancement of the stratification and reducation of the water body and the more favorable preservation conditions of the organic matter.4.For the first time,3?-methylhopane with carbon number up to C45 was detected in sedimentary organic matter.The high abundance of ¹²C-rich hopanes and 3?-methylhopanes in source rocks of the first mumber of Qingshankou formation indicate the proliferation of microbial communities dominated by methanogens and methanobacteria at the bottom of palaeo-lake Songliao,and strong carbon geochemical cycling at the water-sediment interface.In the section of Qing1 member,the contents of biomarkers such as low-carbon-number n-alkanes and C27 steranes are highly linear correlated with the content of C30 hopanes,which indicates that the input of algae organic matter is the first control factor of the microbial prosperity at the bottom of Songliao palaeo-lake.Meanwhile,the coupling relation of low Pr/Ph ratio,high gammacerane index and high 3?-methylhopane index also reveal that the enhancement of water stratification and reducation had a significant positive effect on the growth of methanotrophes and methanogens.5.Comprehensive research reveals that under the background of warm and humid climate,terrestrial source input which dominated by runoff plays a central role in the development of both semi-deep/deep lake source rocks.Under warm and humid climate condition,the strengthened weathering of parent rock and river input cloud input large amounts of terrestrial nutrients to polaeo-lakes inducing eutrophication.If other geological events,such as volcanoes,also exist,the eutrophication rate of polaeo-lakes would be much faster.In addition,the formation of seasonal stratification of polaeo-lakes is also conducive to the preservation of organic matter.Finally,due to the combined action of these internal and external factors,high-quality lacustrine source rocks can be formed and developed.