Characteristics Of Structure Evolution And Pore Development Of Organic Matter In Shale Of Wufeng-Longmaxi Formation,Northern Guizhou

The paper does research on shales of the Ordovician Wufeng Formation and Silurian Longmaxi Formation,which are collected from the northern Guizhou area.The Structural characteristics of organic matter at the post-mature stage can be determined by means of resistivity log,laser Raman spectrum and XRD.The organic matter occurrence and the organic pore development are investigated under FESEM and image processing.In addition,helium pycnometry and low temperature nitrogen adsorption are employed to discuss porosity,pore structure and their controlling factors.The micromechanical properties of organic matter in different occurrence are obtained by AFM.These results will illustrate the role of organic matter in the hydrocarbon generation and pore structure of shale reservoirs in the research area.The resistivity log of the Wufeng-Longmaxi Formation is weakly bell-shaped or flat.The medium-to-high resistivity values are much higher than the corresponding value of the lower limit of hydrocarbon generation.The resistivity has no correlation with both GR and TOC.Two test points have low-amplitude graphite peaks and only one test point has a moderate peak of graphite peak in the laser Raman spectrum.The rest of the test points have no graphite peaks.The shape of the(002)crystal plane diffraction peak of the Longmaxi Formation samples is relatively broad and asymmetric,and the(002)crystal plane layer spacing is not within the range of the graphitization layer spacing.However,the shape of the(002)crystal plane at the bottom of the Wufeng Formation is sharp,the peak intensity is significantly increased,and the symmetry is better.These results show that only a small amount of organic particle in the lower part of the Wufeng Formation has entered the graphitization stage,and the overall risk of graphitization is low in the research area.The organic matter is mainly amorphous bitumen,presenting in the shape of strip,scattered block,interstitial and envelope under the FESEM observation.Pores are more developed within the scattered blocky and enveloped organic matters.The pore diameters are below 6nm,the plane porosity is more than 10%,and the roundness is between 0.6 and 0.7.The micro-mesopores(2-20nm)are main contributors to the average plane porosity.When TOC is lower than 0.5%,porosity has no correlation with TOC;when TOC is between 0.5% and 6.0%,porosity is positively correlated with TOC;when TOC content exceeds 6.0%,the positive trend between these two parameters becomes negative.Compared with inorganic minerals,organic matter dominates the structures and fractal characteristics of shale.Atomic force microscope measurement results show that the Young's modulus of organic matter ranges from 8.3 to 21.9GPa,and the most probable modulus value is basically over 15 GPa.Overall,the Young's modulus of organic matter in the samples is conducive to the preservation of organic pores.