Application and Research on Macroscopic Identification of Bitumen and Graptolite in Shale and Reflectance Detection

Abstract

Longmaxi Formation of Lower Palaeozoic Silurian System in Northwest of Guizhou Province is an import target layer of shale gas exploration; the study of its organic matter maturity is a necessary indicator for evaluating the resources of shale gas. Because the Longmaxi shale does not contain any vitrinite; its thermal evolution must be tested by the shale asphalt reflectance and the graptolite reflectance. The DM4500 microscope is used to test the shale of Graptolite reflectance and asphalt reflectance. The results show that the shale graptolite reflectance is between 2.91% - 3.20%, average 3.08%; asphalt fragments reflectivity is between 2.93% - 3.19%, average 2.94%; amorphous bitumen reflectance is between 2.89% - 3.18%, average 2.89%. Graptolite reflectance is slightly higher than the reflectance of asphalt. The average of equivalent vitrinite reflectance is 2.06%; the equivalent vitrinite reflectance of asphalt debris is 2.20%, indicating that the shale of Northwest Guizhou Silurian Longmaxi has been in high-over mature stage of thermal evolution. The result of study provides a reliable parameter for the evaluation of shale gas resources in the area, and also shows the applicability of the evaluation of bitumen and graptolite reflectance of the maturity of organic matter of shale gas in the Silurian System.