四川南桐地区二叠系龙潭煤层生物标记物及其地质意义

BIOLOGICAL MARKERS FOR COAL BEDS OF THE PERMIAN LONGTAN GROUP IN NANTONG AREA, SICHUAN PROVINCE, AND ITS GEOLOGICAL SIGNIFICANCE

四川南桐地区二叠系龙潭煤层具有良好的生油潜力。其生物标记物包括正构和异构烷烃,类异戊二烯烷烃,倍半萜烷,二萜烷,三环萜烷,藿烷,一种未知结构的五环三萜烷(C_(30)),甾烷,4-甲基甾烷等。煤层生物标记物特征表明在其形成过程中有藻类和细菌等微生物物质的加入。

The Permian Longtan group in Nantong area, Sichuan Province, is a sequence of coal measure strata composed of cyclically deposited shales, limestones and coal beds. The coal measure strata are 85 meters thick and include two exploitable coal beds No. 4 and 6. The Permian coal beds have a good potential for petroleum generation. The content of liquid hydrocarbon (S1) in coal bed No. 4 from Yanshitai Coal Mine is 19.2 kg/t. and that of the kerogen-cracked hydrocarbon (S2) is 154.4 kg/t. S1 and S2 in coal bed No. 6 from the same mine is 12.6 and 146.6 kg/t respectively. The R°(%) of coal bed No. 4 is 1.08, and the R°(%)of coal bed No. 6 is 1.10. Therefore, the maturity of organic matter in the coal beds is in oil window.In the present study of biological markers, not only the commonly occurred normal al-kanes, isoalkanes, isoprenoid alkanes, sesquiterpanes, diterpanes, hopanes and steranes, but also the tricyclic terpanes, 4-methyl steranes and an unknown structure pentacyclic triterpanes (C30) have been found in the Longtan coal beds. The normal alkanes are mainly in a range of C15-C21, however, the carbon number reaches to 35. Some isoalkane peaks are not a simple iso- or anteiso-alkanes. They are probably composites of multi-monomethyl or dimethyl alkanes. The major acyclic isoprenoid alkanes are pristane and phytane with a long chain series presumedly to carbon number 40. Also, the isoprenoid alkanes include the regular C17. The sesquiterpanes consist of C15 and C16, but they have not 43(H) selinane structure. The diterpanes comprise C19 and C20, but they have not abietane or pimarane structure. The tricyclic terpanes are abundant in the Permian Longtan coal beds with the carbon number from 19 to 29. The hopanes in the coal beds are C27-C33, and an unknown structure C30 compound in coal bed No. 4 has been found, even it is the most abundant component over C30 hopane in the pentacyclic triterpanes, The content of regular and rearranged steranes is higher than that of pentacyclic triterpanes. The C29 sterane is major peak. Interestingly, some C29 and C30 4-methyl steranes have been detected in the coal beds, probably they have 4α, 5α structure.The specific biomarkers, such as the tricyclic terpanes, the extended chain hopanes (carbon number>31) and the 4-methyl steranes, in the Permian Longtan coal beds indicate that the substances of microorganism, especially the algae and bacteria would take part in the coal formation.