松辽盆地滨北地区生物气源岩酸性含氧化合物的分布及其地球化学意义

DISTRIBUTION OF ACID BEARING-OXYGEN COMPOUNDS AND ITS GEOCHEMICAL SIGNIFICANCE IN SOURCE ROCKS OF BIOGAS, BINBEI AREA, SONGLIAO BASIN

松辽盆地滨北地区白垩系源岩有机质演化程度低(Ro值为0.33%～0.70%),可溶有机质酸性非烃主要由正构一元酸、正构二元酸、姥鲛烷酸、植烷酸、藿烷酸和甾烷酸等化合物组成。其中正构一元酸含量占绝对优势并呈两种分布形态(一为单峰型,以C16为主峰,C18、C14丰度较高,生源母质主要为藻类等水生生物;二为双峰型,前峰群以C16为主峰,后峰群为次主峰,以C24为主,前峰群表征水生生物输入,后峰群表征有陆源高等植物输入),各形态均呈现偶奇优势(与生物脂肪酸合成酶合成双碳乙酰单元有关);正构二元酸含量较低;藿烷酸含量高于甾烷酸。随着埋藏深度的增加,多数样品的姥鲛烷酸/植烷酸的比值及甾烷酸/藿烷酸的比值均减小,表明沉积环境逐渐向还原环境转化。认为由于酸性含氧化合物尤其是正构烷基脂肪酸的脱羧反应,故在形成正构烷烃的同时形成CO2,不仅为产甲烷菌提供了大量营养能源,而且也提供了呼吸代谢的基质;在青山口组和嫩江组时期,湖盆面积扩大,水体加深,区域内出现滨浅湖—半深湖相沉积,因此滨北地区局部时空范围内可形成商业性生物气田。

The immature or lower-mature source rock was collected from the Cretaceous of Binbei area in Songliao Basin, with low vitrinite reflectance of 0.33%~0.70%. Acid bearing-oxygen compounds in non-hydrocarbon consisted of normal-monocarboxylic acid, normal-binarycarboxylic acid, pristane acid, phytane acid, hopanoid acid and steroid acid. Normal monoacid had a higher content and occurred two modeles of peak distribtuion, the one was single peak group with C_(16) dominant position and C_(18), C_(14) higher aboundance, which indicated the input of algae, the other had two peak groups, the fromer with C_(16) main peak characterized by aquatic organisms source matirials, the latter group was secondary, with C_(24) higher peak, this suggests the input of terrestrial higher plants transformed by bacteria. Those acid non-hydrocarbon was dominated by even carbon number, due to the fatty acid synzyme of organisms synthetized acetyl unit with two carbon atom. Normal binaryacid had a lower content and the amount of hopanoid acid was more than steroid acid. In the most of samples, those ratios of Pr/Ph acid and ster/hopan acid decreased with increasing of depth, this suggests that depositional envirenment changes to reducing conditon. As de-carboxyl reaction taked place, acid bearing-oxygen compounds, in particular nonmal alkyl fatty acid was formed into hydrocarbon and CO_2, not only supplied for the large amount of nutritive energy-carbon origin, also supplied for the largest substrate of respiratory metabolism to methanogenic bacteria During Qingshankou and Nenjiang periods of Cretaceous, water system changed to deep with expanding of lake area, shallow and mid-deep lacustrine facies deposit occurred at parts of Binbei area, leading to biogas field was formed.