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Coal Geology & Exploration

Abstract

The vitrinite reflectance is recognized as the only comparable evaluation index of organic matter maturity of source rock in the exploration and development of oil and gas, shale gas and coalbed methane resources. Current research was done to determine the organic matter maturity of sapropel coal in oil layer A in Haishiwan mine field more accurately. Coal samples were collected from boreholes and observation of the optical characteristics of reflected light and fluorescence microscopic components were studied under the microscope. It was found that there are two different characteristic vitrinites in sapropel coal in oil layer A. One was fluorescent vitrinite with low reflection color and low determined reflectivity, the other was non-fluorescent vitrinite with measured higher reflection color and relatively higher reflectivity. The sedimentary environment of oil layer A is obviously stratified. The upper part is sapropelic coal. The maceral composition is mainly alginate, containing a large amount of pyrite, reflecting the strong reducing environment of deep swamp; The lower part is mainly composed of sapropelic groundmass, desmocollinite and cutinite. Compared with the upper part, the contents of alginite and pyrite are significantly reduced, reflecting the change of sedimentary source. The proportion of organism like algae decreases, whereas the proportion of plants increases. This reflects a relatively weak reducing environment. According to the sedimentary environment of the mine field, the comparative analysis was carried out between the microscopic composition of the sapropel in oil layer A and the vitrinite composition maturity of the sapropel in its lower coal seam No.2. Major findings were that the reflectivity of the non-fluorescent vitrinite in oil layer A can reflect its true maturity, and that was in the same metamorphic stage as the lower coal seam No. 2. It provides a new method and new thought for the analysis of organic matter maturity of sapropelic coal. When the content of plants in coal seams is relatively low or there is a significant difference with maturity of adjacent strata, it is recommended to compare the test results of specific layers with the whole coal / rock stratum. Then a comprehensive analysis should be made in combination with its vitrinite fluorescence characteristics and sedimentary environments.

Keywords

Yaojie mining area, sapropelic coal, maturity of organic matter, fluorescent vitrinite, sedimentary environments

DOI

10.3969/j.issn.1001-1986.2021.06.008

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