四川盆地龙潭组高热演化烃源岩有机质生源及沉积环境探讨

四川盆地龙潭组烃源岩R°值主要变化在2.0％～3.0％范围,干酪根H/C原子比值多数集中在0.3～0.5之间,有机质热演化达到过成熟阶段.它们的各类饱和烃生物标志物组成和分布大都发生了异常变化,限制了其地球化学应用.其芳烃组分和干酪根碳同位素及S、N元素组成还包含着标志有机质生源及沉积环境的信息.川东南地区近海湖沼相含...

Biological organic source and depositional environment of over-mature source rocks of Longtan Formation in Sichuan basin

The source rocks of Longtan Formation in Sichuan basin have vitrinite reflectance values ranging mostly from 2.0% to 3.0% and kerogen H/C atomic ratios mainly in 0.3 to 0.5, suggesting a post-mature level with regard to hydrocarbon-generating Distribution of their various biomarkers is found to change abnormally and could be limited to geochemical applications. However, their aromatic hydrocarbons, in conjunction with kerogen carbon isotope and element （sulfur and nitrogen） composition, can be used as indicators for organic source and sedimentary environment. Near-marine swamp-lacustrine facies coal-bearing source rocks in the southeastern Sichuan basin show 2,6-/2,10-DMP （dimethylphenanthrene） and 1,7-/1,9-DMP values higher than 0.65 and 3.0 respectively and 4-/1-MDBT （dibenzothiophene） ratios over 15. The kerogens exhibit isotopically heavy δ13C values mostly in excess of 25‰, along with low S/C and N/C atomic ratios （0.003 to 0.056 and 0.012 to 0.025, respectively）. The combined data indicate a dominantly terrestrially derived source and oxidizing depositional environment with a type-Ⅲ organic matter. Whereas, the bay-lagoon facies intervals in the northeastern region of this basin display 2,6-/2,10-DMP ratios lower than 0.65, 1,7-/1,9-DMP mainly below 3.0 and 4-/1-MDBT mostly under 5. Moreover, carbon isotopes of the kerogens are relatively depleted in 13C with the δ13C values around 27‰, with slightly higher S/C and N/C ratios （0.020 to 0.140 and 0.014 to 0.038, respectively）, implying a type-I kerogen derived from a predominantly aquatic organic source deposited in a reducing or less oxic condition. These geochemical parameters for source rocks from other parts in this basin are believed to vary between the two regions.