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.
Shale gas is an important field for current oil and gas exploration, as the huge resources of nonconventional natural gas, there exists the highest potential for the study of the black shale in the lower Palaeozoic Erathem of South China. In recent years, a great number of researches have been performed on the marine shale in Longmaxi Formation in the periphery areas of Sichuan Basin and in the southeast of Guizhou Province [
The result shows that the shale in Longmaxi Formation is a critical gas source zone in South China. It contains high organic matter; the thickness of high quality shale with TOC value higher than 2% is generally 10 to 50 m, the type of organic matter is mainly Type I (Its equivalent vitrinite reflectance average Ro is between 2.0% - 3.6%) [
The understanding of geological features of marine shale in Longmaxi Formation belonging to the northwest of Guizhou Province is weaker at present. The successful exploration and development of the same set of shale bed in the periphery area promotes the evaluation of shale stratum in Longmaxi Formation.
The thermal evolution maturity of organic matter is not only an important parameter for the evaluation of conventional source rocks, but also an indispensable geochemical indicator for unconventional shale gas research. In the early Palaeozoic strata, it commonly lacks vitrinite, but more graptolites, bitumen debris were developed, and amorphous bitumens were packed in fractures. The difference between the microscopic shapes is the foundation for detecting the reflectance of different microscopic components.
The samples for detecting the reflectance for microscopic components are taken from black shale zone in Longmaxi Formation of Lower Silurian System of Qilong Village in Xianyuan Town , Xishui County , Guizhou Province. The sampling interval is about 2 m, which is evenly distributed. The bulk samples are screened through slight breaking and grinding, the screened samples of 10 to 20 meshes (its particle size of 2 to 0.9 mm) are poured into a special mold for bonding with epoxy resin, thus a round sample of 2 cm is created, and a bright surface with dense rock sample particle arrangement is developed after grinding and polishing.
A whole rock slice microscopic observation is performed on rock samples from Longmaxi Formaiton in the studying area, from which 2 important and the highest content of microscopic components containing the graptolite and solid bitumen are found out. According to the different shapes of bitumens, they can also be divided into debris bitumen debris and amorphous bitumen.
Under the microscope, it can be observed that the graptolitic chips are originated from their periderm organizations [
The graptolites in Longmaxi Formation are mainly dispersed in chips in the mineral matrix of shales. Compared with solid bitumen, their particle size is generally larger; mostly their shapes are not complete, some of them with good shapes are distributed in the matrix in long tubular and strip shapes (
The non-particulate graptolites and less amount of particulate graptolites are mainly developed in the shale cores obtained from Longmaxi Formation, to ensure the convergence of tested results and more accurate testing results. In the study, non-particulate and homogeneous graptolites are mainly chosen for reflectance determination.
Bitumen generally has the same microscopic optical characteristics with graptolites, therefore it cannot be identified based on its optical characteristics. The morphological difference is a major attribute for bitumen’s microscopic identification. There exists 2 morphological distributions of bitumen in shale from Longmaxi Formation in the studied area. For an easy division, it is defined as Bitumen A and Bitumen B.
The microscopic observation indicates that Bitumen A is in a shape of polygonal chip, and it appears in geometrical shape (
Bitumen B is mainly filled in the micro-fractures of shale or the pores of mineral matrix (
The experimental testing data are taken from the laboratory of China National Geological Experiment Center. The reflectance is measured by using Leica DM4500P polarizing microscope (the objective amplification is 510×, oil immersion condition, the measuring point is no less than 20 for each sample); the result is taken from random reflectance value.
The bitumen particles are very small in shale, therefore the method of grey- scale value for organic matter detection is used for reflectance calculation [
sample ql-2), and the average value of each microscopic component is calculated (
The results of tested reflectance of microscopic components in the shale samples from Longmaxi Formation are summarized and appropriate equivalent vitrinite reflectance model is used for solution (
Obviously, there exists difference in the test results of 3 microscopic components that experienced the evolution of the same geological history. It is indicated that because of the difference of sensitivity to the temperature, the evolutional velocity is variable in different microscopic components in the same temperature condition. As a whole, the reflectance of Bitumen B ranges from 2.89% to 3.18%; the reflectance of Bitumen A ranges from 2.93% to 3.19%, while the reflectance of graptolite is mainly concentrated from 2.91% to 3.20%. The tested reflectance of Bitumen A and graptolite are on the average higher than that of Bitumen B. The graptolite, Bitumen A and Bitumen B are from the original organic matters of different chemical components. They have different response characteristics of the thermal effect, because there exists the difference in chemical components. The microscopic test of graptolite, that is, the periderm organization of graptolite, has higher aromaticity compared with the reflectance [
On the other hand, the reflectance of Bitumen A and graptolite is more closed, and higher than that of Bitumen B. It is possible because Bitumen A and graptolite experience the same geologic evolutional history, while for the low reflectance of Bitumen B, it is possible because the geologic evolutional history that Bitumen B experiences is relatively short. In consideration of the situation that Bitumen B is the most flowing and amorphously distributed in the fractures of mineral matrix under microscope, it is speculated that Bitumen B is located in shale fractures through migration and filling at the late stage.
The measured value of graptolitic reflectance has strong convergence, the convergence of Bitumens A and B is the next (
In summary, to truly and reliably characterize the thermal evolutional process of black shale in Longmaxi Formation, in the study, the reflectance of Bitumen A and graptolite are equivalently calculated, and the difference between the 2 equivalent vitrinite reflectance is contrasted.
The condensation level of the aromatic structure of organic matter determines its reflectance [
At present, the vitrinites of graptolites are deeply studied by scholars both at home and abroad. Large numbers of studies are made by Cao Changqun [
lg V R eq = 0. 57 2 lg G R + 0.0 21
where, VReq is the vitrinite reflectance, GR is the graptolitic reflectance.
The correlativity between the graptolitic reflectance and the pyrolysis temperature of rocks in the source rocks of Cambrian-Silurian System was studied by Petersen et al. A model was established for the equivalent vitrinite reflectance of graptolite through the relation between vitrinite reflectance and rock pyrolysis temperature. But in the study, the vitrinite-like maceral from alga was mistaken with graptolite, therefore the measured graptolitic reflectance was unreliable, and the model could not be used to truly characterize the maturity of rocks. So in this study, the model established by Cao Changqun is chosen for calculating the equivalent reflectance of graptolite (
For the wide existence of bitumen in source rocks, there is less study on the equivalent reflectance of bitumen [
It can be known from
During the experimental process, for the purpose of reducing their testing errors, although the graptolite and bitumen can be distinguished based on the difference between their shapes, the anisotropy of optical property in graptolite will certainly affect their test results [
1) The Silurian Longmaxi shale does not contain any vitrinite, except for the enrichment of graptolites and asphalt; it provides the feasibility conditions for determining the degree of thermal evolution of hydrocarbon source rocks. 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.
2) The equivalent vitrinite of graptolite reflectance and equivalent vitrinite reflectance of asphalt debris is higher than 2.20, indicating that the shale of Northwest Guizhou Silurian Longmaxi has been in high-over mature stage of thermal evolution. The equivalent vitrinite reflectance of asphalt is lower than the equivalent vitrinite of graptolite reflectance. The reason may be the Anisotropy of asphalt ort the samples. The equivalent vitrinite of graptolite reflectance and equivalent vitrinite reflectance of asphalt debris show the equal result, which can describe the maturity of shale more realistically. The further study of the reflectivity of Graptolite reflectance and the reflectance of asphalt will more effectively indicate the thermal evolution of organic matter characteristics.
Mao, J., He, W.X., Che, Q., Sun, W.L. and Xu, X.M. (2018) Application and Research on Macroscopic Identification of Bitumen and Graptolite in Shale and Reflectance Detection. Open Journal of Yangtze Gas and Oil, 3, 11-20. https://doi.org/10.4236/ojogas.2018.31002