| The Carboniferous strata in Qaidam Basin has great potential for oil and gas generation,but the exploration level is relatively low,and the research on Carboniferous source rocks is not enough.In this paper,core samples are collected from multiple wells in eastern Qaidam Basin,oil and gas samples have been obtained from hydraulic fractured well.This study mainly focusses on molecular geochemistry of source rocks,and it provides sufficient evidence for further research and the further exploration and development in the Qaidam Basin.The following results have been obtained in this study.1.The Carboniferous source rocks in eastern Qaidam basin are composed by silty mudstones,mudstones and carbonaceous mudstones,the average TOC indicate a highly organic matter abundance.The source rocks mainly develop type III kerogen and contain both marine and terrestrial organic matters.The vitrinite reflectance ranges from 0.74% to 1.88%,indicating a high maturity stage.The Carboniferous source rocks in the eastern Qaidam Basin are dominated by low-carbon n-alkanes and a certain even-carbon advantage can be noticed.The source rocks deposited in a weakly oxidative-weakly reducing environment,steranes and terpene compounds are not developed which is caused by the higher thermal maturation stage.Adamantane are widely developed in the samples,indicating that the formation of adamantane is not limited to the cracking of crude oil,and a large amount of adamantane can also be formed in source rocks at high maturity stage.2.Polycyclic aromatic hydrocarbon compounds are widely distributed in the sample,and their distribution is mainly controlled by the degree of thermal evolution.Many aromatic hydrocarbon parameters have good linear relationships with maturity.The presence of phenanthrene in MPI-1 prevented it from indicating changes in the maturity of the samples due to the relative content of phenanthrene changes irregularly due to demethylation.Therefore,MPI-1 cannot be used to predict the maturity of crude oil or source rocks.The rate of generation of methylphenanthrene compounds changes with thermal maturity,resulting in a good correlation between relevant parameters such as MPR and MPI-3.3.There is no vitrinite in crude oil and the maturity cannot be directly measured.The vitrinite reflectance in these samples is usually estimated through an empirical formula.However,this paper proves that it is only partially established and has a certain gap with the measured value,which can only indicate the trend of samples' maturity,and cannot be used as a basis for determining maturity without correction.4.The crude oil has been discovered in Carboniferous Strata of Qaidam basin for the first time,and it is a great significance to the search for strategic oil and gas succession strata in the area.The oil samples in Carboniferous strata mainly develops low-carbon n-alkanes with a mixed organic source.The corresponding vitrinite reflectance is calculated from the parameters like MDR,ranging from 0.77% to 1.0%,which is in the middle and low maturity stage.The natural gas sample is oil-derived gas,and the carbon isotope equivalent vitrinite reflectance is 1.77%.Affected by various factors such as pressure,mineral content,and activation energy,there is a difference in the thermal evolution process between source rocks and crude oil samples,so that low-maturity crude oil can still be preserved in highly mature source rock formations.In this paper,molecular geochemical data are used to describe the samples in the study area clarifying the mechanism of content variation with maturity during alkylation of different compounds,revealing the mechanism of the differential evolution of low-mature crude oil and high-maturity source rocks.Its theoretical significance also plays an important guiding role in oil and gas exploration and development practices,especially in the exploration and development of highly mature formations. |
PDF Full Text Request