Thermal Maturity And Maturity History Of The Lower Paleozoic Shale In Sichuan Basin

The Paleozoic marine shales are mian source rocks and targets of shale gas exploration in Sichuan Basin.Organic matter maturity is not only the key to evaluating hydrocarbon generation,but also a vital indicator of shale quality evaluation.The maturity of organic matter in Lower Paleozoic has always been a difficult and hot issue in organic petrology due to the lack of vitrinite.This thesis takes the Upper OrdovicianLower Silurian Wufeng-Longmaxi shale and the Cambrian Qiongzhusi shale in Sichuan Basin as the object of the research.The maturity indicators of lower Paleozoic organic matter(graptolite,chitinozoan and solid bitumen)were investigated by means of optical reflectance of organic petrology,co-existing thermal simulation experiment of low maturity Lower Paleozoic shale and coal in a closed system,and laser Raman spectroscopy.Based on a large number of reflecance measurements for the WufengLongmaxi shale,the thermal evolution characteristics of graptolites from a natural evolution sequence samples were characterized by laser Raman spectroscopy,while maturity evaluating indicator of graptolite random reflectance for the Wufeng-Longmaxi shale has been established via a comparison with the previous previous Raman spectroscopic parameters to characterize vitrinite.Based on the maturity indictors of organic matter established in this thesis,combined with burial history and thermal history,the differences of maturity evolution characteristics between the two sets of shale intervals in Lower Paleozoic shale in the key areas of Sichuan Basin were clarified.The main findings are as follows:The random reflectance of zooclasts(graptolite,chitinozoan)and solid bitumen and vitrinite increased with the increase of thermal simulation temperature,and the random reflectance of graptolite is higher than that of solid bitumen,and the random reflectance of graptolite was slightly higher or approximately equal to that of chitinozoan;Morphologically,the wall of chitinozoan observed was able to resist thermal degradation in comparison to graptolite periderm;The reflectance of solid bitumen is lower than the reflectance of vitrinite at low thermal maturities(Ro < 1.0%?1.5%)and more similar at higher maturities(Ro > 1.0%?1.5%).The maximum reflectance of graptolite is approximately twice as much as the random reflectance of graptolite,and both of them increase with the increase of thermal evolution degree.Compared with the maximum reflectance of graptolite,the random reflectance of graptolite is more precise due to smaller standard deviation.As a result,the random reflectance of graptolite is more suitable as a maturity indicator.Owing to the special structure and chemical composition of graptolite,the relationship between graptolite random reflectance and vitrinite reflectance is not a single linear relationship in the whole process of thermal evolution.The thermal evolution of Qiongzhusi shale is higher and the anisotropy of solid bitumren is greater.The bottom of shale may be affected by hydrothermal activity,resulting in high anomaly of reflectance value.By analyzing and comparing the Raman spectra of kerogen and solid bitumen,it is revealed that direct measurement of kerogen by Raman spectroscopy can effectively characterize the maturity of Qiangzhusi shale.The Qiongzhusi shale has entered the early mature stage of hydrocarbon generation from early Ordovician to early Silurian,which is characterized by early hydrocarbon generation and late hydrocarbon generation.The Wufeng-Longmaxi shale is immature in Caledonian or only reaches the threshold of oil-generating window,which is characterized by late hydrocarbon generation.In summary,investigation differences of oil generation history for the two set of shales reveals a favorable time relationship for generation and accumulation of shale gas in Wufeng-Longmaxi shale.