Well-logging Inversion On The Palaeoclimate Changes Of The Late Cretaceous In Songliao Basin

The research methods of palaeoclimate changes include astronomy,geography,geology,geochemistry,and geophysics.In recent years,some parameters of geophysics,such as magnetic susceptibility and gamma radiation of rocks,have been widely applied in palaeoclimate researches as some new proxies.Geophysical well logging parameters are comprehensive reflections of the formation rock types,physical properties and geochemical characteristics.They are also important means for developing various fossil energy sources such as solid mineral resources,petroleum,natural gas and coal.The changes of the formation lithologies and sedimentary cycles are directly or indirectly reflected in the forms and amplitudes of the logging curves,so the latter are also important data for studying the paleogeography,paleoenvironment and palaeoclimate of the sedimentary rocks.With the advantages of strong continuity,high vertical resolution and abundant information,the well Logging data can provide valuable data for the scientific drilling research.A systematic logging coring has been conducted in the Well SK-1,which is a scientific exploration well deployed for a special research on the important geologic events and the paleoclimatic changes of the Songliao Basin in the late Cretaceous.The implementation of the well logging engineering provides an important basis to establish relationships between the cores and well logging curves,to carry out core depth correction,to establish the geophysical proxies reflecting the palaeoclimate changes,to establish a complete organic geochemical profiles,and to achieve other scientific research targets.The dominating lithology of mudstone,the high quality of spectral gamma ray(SGR)logging data and the high-density of geological and geochemical data provide exceptional conditions for the study of palaeoclimate changes bu using spectral gamma ray(SGR)logging data.It is of great academic value to give full play to the role of logging data in the palaeoclimate research.Based on the paleoclimatic changes frameworks that the predecessors established by utilizing the data of paleo-plant spore powder and geochemical elements analyses,this paper proves that the Th/K ratio in the gamma ray energy spectra of mudstone is a good proxy reflecting the changes of the moisture of palaeoclimate,through the analysis and statistics of the logging response characteristics of the spectral gamma ray of this well,as well as the analyses and comparisons on the framework of paleoclimatic changes.It is concluded that for the different sedimentary environments underwater and above water,different modes should be adopted to interpret the paleoclimatic moisture.The high value of Th/K in the mudstone formed in the exposed environment above water represents a little humid climatic condition,while for the underwater sedimentary mudstone,the case is quite the opposite.According to the modes,the paleoclimatic moisture changes inversions during the sedimentary period of the Quantou and Nenjiang formations in the Songliao Basin have been conducted by utilizing the spectral gamma ray curves of the Well SK-1.The research shows that during the sedimentary period from Member 3 of the Quantou Formation to the Mingshui Formation,the Songliao Basin had experienced seven cycles of climatic moisture.This result coincides with the previous analysis by using the elements geochemical methods.It also indicates that the spectral gamma ray logging data is a better proxy to analyse the paleoclimatic moisture changes.Comparing with the expensive elements geochemical data,the former has the advantages of economy,rapid and high efficiency.Combined with the previous palaeoclimate frameworks,a quantitative paleoclimatic moisture interpretation standard was established by using the SGR data of the mudstone deposition of under water: when Th/K<4,humid climate;when the Th/K is in 4-5,semi-humid climate;Th/K in 5-6,semi-humid and semi-arid climate;when Th/K>6,is a semi-arid climate.