High Abundance Diterpenoids and Their Geological Significance in Coal Measure Strata, Liaohe Basin

Key words: 
• Liaohe /
• Basin /
• coal /
• measure

Abstract: Tricyclic diterpanes such as pimarane, dehydrabieatane and simonelli te and tetracyclic diterpanes such as phyllocladene are most common biomarkers i n coal and other terrestrial source rocks and oils, but in most case they are no t high in abundance. In this study we examined some Chinese coals and terrestria l sediments for the highly presence of tricyclic and tetracyclic diterpenoid hyd rocarbons, and have assigned structures to the diterpanes by mass spectra. The s amples were collected from the third Member of Shahejie Formation, Low-Eogene, E astern Depression, Liaohe Basin. The sedimentary environments for the samples ar e fresh water lacustrine bog and flood plain facies. Organic petrologic analyses show that the organic macerales for the coal measure source rocks are mainly co mposed of vitrinite, contented up to 70% or even high to 90 %, while the content s of exinite, inertinite and sapropelic material are very low. The Ro for the sa mples under study varies from 0.475%to 0.539%, so the organic matters from those samples are generally in immature and lower thermal evolution stages. Fo r all of the samples, C₂₀⁺ n-alkanes predominate over C₁₅～ C₁₉ n-alkanes and the nC₂₉ or nC₃₁ n-alkanes are the prominent peaks . All of the OEP values for the samples are large than 7 and the CPI values are larger than 3. Therefore the alkanes also show an obviously immatures to low-mature thermal evolution. In all of the samples pristane is the most abun dant alkane of the saturated hydrocarbon fractions. Pristane/phytane ratios all are larger than 2. This is also the typical isoprenoid hydrocarbon distribution characteristic for coal measure source rocks. High abundance tricyclic diterpane s such as pimarane and dehydrabieatane and tetracyclic diterpanes such as phyllo cladene are detected from the samples. Besides, some special biomarkers such as oleanene, ursene and other pentacyclic triterpenoids are also detected from thos e samples. Although thre kinds of biomarkers are detected in the same strata, t hey come from different plants. Tricyclic diterpanes and tetracyclic diterpanes mainly come from gymnosperm (in particular the conifer), petridophyte, bryophyte and pteridophyte respectively, whereas pentacyclic triterpenoids mainly come fr om angiosperm. show that the input of organic matter in the coal source rocks, the third Member of Shahejie Formation, Low-Eogene, Eastern Depression, Liaohe Ba sin,is of obvions diversification and complexity.