| The content and type of bio-precursors are not only important indexes for evaluating the quality of source rocks,but also affect the formation,development and evolution of oganic matter pores.Therefore,the development for different bio-precursors of organic pores are an important part of shale reservoir.The lower Silurian Wufeng and Longmaxi Formation are the main shale gas reservoirs in Changning area.Thin section,pellets,the TOC analysis,low-pressure N2 adsorption,small Angle scattering and Raman spectrum which the shale samples come from the Changning area of Shuanghe section are used to research bio-precursors types,organic pores characteristics,shale porosity containging different bio-precursors of type and content and the influence of solid bitumen content.It is found that the bio-precursors types including benthic algae,phytoplankton,acritarch,spores,radiolaria,bacteria,and graptoliths.Benthic algae,with a small amount of acritarch and almost no phytoplankton,dominate the bottom of the Wufeng and Longmaxi Formation.Benthic algae and acritarch,containing a small amount of phytoplankton,mainly distribute the lower part of the Longmaxi Formation.In the middle and upper part of Longmaxi Formation,there are mainly phytoplankton and acritarch,and a small amount of benthic algae.The development model of bio-precursors is mainly related to sea level change.The pore structure of benthic algae,phytoplankton,solid bitumen and graptolite are studied by focused ion beam scanning electron microscopy and optical microscope.The observation shows that there were net structures in benthic algae,but no obvious structures in planktonic algae,solid asphalt and graptolite.In addition,the pore types of different hydrocarbon generating parent materials are also different.In the benthic algae,there are reticular and circular pores.Not all phytoplankton have pores,only some of the phytoplankton have small pores in the interior or form intergranular pores between aggregate phytoplankton.Solid bitumen contains a large number of circular pores,while graptolite contains almost no pores.Low-pressure N2 adsorption and desorption experiments and small-angle scattering experiments are carried out on four shale samples in shuanghe section.The experimental results showed that the shale dominated by benthic algae had higher porosity,while the shale dominated by phytoplankton had smaller porosity and multimodal pore size distribution.The fractal dimension of shale indicates that the fractal dimension of shale dominated by benthic algae is larger than that of shale dominated by phytoplankton,which may be related to the more complex structure of benthic algae,the complex pore structure and rough pore surface.In order to obtain the relationship between the content of bio-precursors and the content of solid bitumen in wufeng Formation and Longmaxi Formation and the difference of shale gas generation capacity,we made quantitative statistics on the solid bitumen in Shuanghe section..Based on the statistical analysis of solid solid bitumen,the oil retention in shale reservoirs was calculated with a range of 0.98?123.50 kg/m3.According to the parameters of unit TOC retained oil and unit TOC retained efficiency,it is found that the the lower Longmaxi Formation have the maximum retained oil and retained efficiency,followed by the lower middle and middle of Longmaxi Formation This feature may be related to the gradual increase of phytoplankton from bottom to top.Combined with the content of retained oil per unit rock and the amount of retained oil per unit TOC,we believe that the Lower Longmaxi Formation is the best shale gas production horizon. |
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