The Seismic Rock Physical Properties Research Of Wufeng And Longmaxi Formation Shales On The Zhaotong Area

Wufen and Longmaxi Formation shales are the most important horizon for shale gas exploration in China.Seismic rock physical properties for the formation are the important basis for "sweet spot" prediction by seismic method.But corresponding researches on the seismic rock physical properties do not consider the influence of diagenetic evolution,causing relative rule knowledge is insufficient in geological implication.In this paper,seismic rock physics properties for the shale gas samples from Wufen and Longmaxi Formation are discussed based on acoustic experiments.Based on X-diffraction analysis,scanning electron microscope(SEM)combined with cathode luminescence(CL)and energy spectrum analysis,depositional and diagenetic evolution for Wufen and Longmaxi Formation shales in different sedimentary environment are determined,and the influence of diagenetic evolution on seismic rock physical properties is discussed.According to the results,we find that bulk density of the shale gas samples is above 1.4g·cm-3,and is close to the upper limit value of 1.6g·cm-3 for organic matter,due to the high thermal evolution.Seismic rock physics properties and its variation is controlled by the sedimentary environment,and different sedimentary environment means different diagenetic evolution procedure,which results in different seismic rock physics properties.Rock texture(elastic properties of load-bearing grain)is the primary factor to determine the seismic rock physics properties for the formation,and porosity and the content of organic matter(TOC)is only the second factor.The upper part of the Wufeng and Longmaxi Formation shales are interpreted as of shallow shelf,mechanical compaction and chemical compaction(silica cementation)are two sequential processes,which results in corresponding shale samples showing much larger change of velocity with porosity,high poisson's ratio and strong anisotropy.The low part of the Wufeng and Longmaxi Formation shales are interpreted as of deep shelf,mechanical compaction accompanied by biogenic quartz cementation,which results in shale samples showing relatively lower change of velocity with porosity,low poisson's ratio and week anisotropy.Our results can provide basis for well-logging interpretation and "sweet spot" discrimination by seismic method for Wufen and Longmaxi Formation shale.The microscopic fracture mechanism of Longmaxi Formation shale is established by static test.It is pointed out that as the effective pressure increases,the brittleness is gradually transformed into plasticity,and the fissure gradually changes into shear fissure.For rock with high mud content,In the presence of significant differences in the stiffness of the particles between;given the criteria for brittle prediction.