Upper Eocene coal and coaly shale in the Central Myanmar Basin: Origin of organic matter and the effect of weathering

摘要

Extensive coals and coaly shales were deposited at the western margin of the Central Myanmar Basin (CMB) during the Late Eocene. Carbon, nitrogen and sulfur (CNS) elemental analyses, vitrinite reflectance, Rock-Eval pyrolysis, biomarker analyses and δ~(13)C measurements were made of eight coal and coaly shale samples from the Pondaung and Yaw Formations. Moderate weathering was recognized in three coaly shales, as manifest by microscopic cracks, holes, and rim structures in vitrinite. A sample pair of fresh and weathered coaly shale collected from the same seam showed that HI, Pr/Ph ratio, and concentration of PAHs and long chain n-alkanes (>n-C_(20)) bonded to kerogen in the coaly shales decreased drastically due to weathering, while OI increased. These results suggest that hydrocarbon generation potential of the coaly shale as an oil/gas source rock decreased to about one tenth by the weathering, whereas the free n-alkanes, biomarkers such as steranes and triterpanes, and δ~(13)C ratios of kerogen were not affected. Facies change from the coaly shale layers (Phase-I) to the coal layers (Phase-II) was accompanied by variations in origin of the organic matter. The source of the organic matter was mainly terrestrial herbaceous vegetation and/or aquatic plants, deposited in oxic to oxygen-poor peat swamps associated with an estuarine/fluvial-deltaic setting. Phase-I is relatively rich in gymnosperm biomarkers such as retene and 1,7-dimethylphenanthrene (DMP), whereas Phase-II shows an increase in angiosperm proxies such as oleanane content and oleanane/C_(30) hopane ratio, with δ13C values ranging from -24.6%o to -26.5‰. Based on HI values of unweathered samples, Phase-I is characterized by type II-III kerogen, while Phase-II contains type III kerogen. Higher values of HI, Pr/Ph ratios, n-C_(29)-C_(19) alkane ratios (>1.5), and higher concentrations of conifer-derived 1,7-DMP in Phase-I suggest a significant contribution from material of resinous vascular plant origin. High Fluoranthene/(Fluoranthene+Pyrene) ratio in the uppermost coal suggests occurrence of wildfire, probably related to a dry climate experienced in Phase-II. Unweathered CMB samples show good source rock quality, with potential generation of liquid/gas hydrocarbons.