The Organic Petrology Of Graptolite-derived Shale In The Upper Yangtze Region,Southwestern China

The organic-rich shale in the Upper Ordovician Wufeng and Lower Silurian Longmaxi Formations in the Upper Yangtze region is the main target for the exploration of shale gas in China.Graptolites are abundant in this shale system,but the contribution of graptolites to the TOC has not received sufficient attention.In addition,the organic maturity of shales in the Wufeng-Longmaxi Formations is currently controversy,which may affect the evaluation of hydrocarbon generation potential and gas-bearing abilities of source rocks.The Wufeng-Longmaxi Formations in the Upper Yangtze region has experienced very high level of thermal maturity,so it is difficult to study changes of the organic matter's physical and chemical properties with the increase of thermal maturity.The depositional environment,age,lithology and organic components of immature Alum shales in northeastern Europe and over-mature shales in the Wufeng-Longmaxi Formations in the Upper Yangtze region are same.Therefore,a comparative study between them is conducted.Organic petrological and geochemical methods are used to explore the contribution of graptolites to the TOC and optical characteristics of organic microscopic components in shales.Additionally,based on the thermal-pressure simulation experiments and Raman methods,reflectance parameters of shales bearing graptolites in the Lower Paleozoic are proposed and the relationship with vitrinite reflectance is established.Results in this study are as follows:The content of graptolites in the hand specimen has a positive relationship with the TOC of graptolite-derived shales in the Wufeng-Longmaxi Formations,which means that graptolites contribute significantly to the TOC of shales and can reflect the degree of organic matter enrichment.Results of microscopic identification show that organic components of the Wufeng-Longmaxi Formation shales are mainly composed by graptolites and solid bitumen.Non-granular graptolites are abundant in shales,which have typical biostructural features and strong anisotropy in the high or over-mature stage.Solid bitumen in this study refers to the primary bitumen,which is formed by the pyrolysis of oil or kerogen in the source rock.Type I solid bitumen is respectively abundant in shales,which can be distinguished from non-granular graptolites by its rough surface,weak anisotropy,and low reflectance.The immature Alum shale in Sweden and Estonia is mainly composed by mineral-bituminous groundmasses,graptolites,Type III solid bitumen and lamalginites,while the high-mature Alum shale mainly includes non-granular graptolites and Type I solid bitumen.Results of thermal-pressure simulation experiments show that Type I solid bitumen begins to appear in the heated Alum shale.When the thermal maturity reaches high level,the main organic components of the Alum shale become non-granular graptolites and Type I solid bitumen,which is as same as the microstructure of shales in the Wufeng-Longmaxi Formations.Therefore,it is inferred that the microstructure of shales in the Wufeng-Longmaxi Formations at the immature stage is as same as the appearance of immature Alum shale in the present.According to this inference,mineral-bituminous groundmasses,Type III solid bitumen and lamalginites are original materials of Type I solid bitumen in the Wufeng-Longmaxi Formations.Results of laser Raman spectroscopy show that the first-order of Raman spectra of graptolites is obvious.With the increase of maturity,D peak shifts to the low field and the band separation increases,which indicate that the degree of graphitization and aromatization of graptolites increases.Due to obvious changes of chemical structure of graptolites with the increase of thermal maturity,the reflectance of graptolites can be used as an indicator to evaluate the organic maturity of graptolite-bearing shales in the Lower Paleozoic.Results of reflectance measurement show that the average maximum reflectance of graptolite has a positive relationship with the average random reflectance of graptolite.However,due to small dispersion and simple measurement method,the average random reflectance of graptolites is more suitable as an indicator for determining organic maturity of shales.With the increase of time and temperature in thermal-pressure experiments,the average random reflectance of graptolite and vitrinite increases linearly and presents a positive correlation.Based on the average random reflectance of graptolite,an equation EqR_vr=1.055R_gr-0.053 is proposed in this study.Based on the above equation,the organic maturity of graptolites-derived shales in Chengkou,northeastern Chongqing is between 1.09 and 1.14%,indicating that it is still within the oil window.The organic maturity of graptolites-derived shales in Wuxi,northeastern Chongqing is the highest,which is between 3.93 and 4.46%,while shales in southeastern Chongqing,central Chongqing,and western Chongqing have reached the over-mature stage.