松辽盆地西部斜坡油砂油地球化学及生物降解特征

松辽盆地中央坳陷原油向盆地边缘运移过程中发生降解,在西部斜坡形成油砂矿藏,油砂油密度和黏度大、不流动。在运移方向上,套保—小太平山—西北沟—图牧吉地区的上白垩统姚家组油砂油随深度变浅饱和烃和芳香烃含量减少,非烃和沥青质含量增加,全油和饱和烃碳同位素变重,以上参数在小太平山单井样品中随深度变浅呈相反变化趋势。气相色谱质谱分析发现,油砂油降解级别为Wenger 2～8级,大多数样品饱和烃中正构烷烃和类异戊二烯烷烃缺失,藿烷部分降解,小太平山地区发现25-降藿烷;套保—图牧吉地区随深度逐渐变浅,生物降解程度逐渐增加,三环萜烷/五环三萜烷、C27重排甾烷/规则甾烷、二苯并噻吩/菲等生物标志化合物参数均呈规律性变化。小太平山地区,25-降藿烷和其对应藿烷的质量分数、二者的比值,均反映出生物降解程度随深度变深而增加,藿烷抗降解能力随碳数的增加而增加,升藿烷22R构型抗降解能力大于22S构型。松辽盆地西部斜坡油砂油降解的规律是:由深部厌氧环境向地表喜氧环境运移的过程中,油砂油降解程度逐渐增大,单井样品中随深度和含水饱和度的增加降解程度逐渐增大。

Characteristics of geochemistry and biodegradation for oil sand extracts in western slope of Songliao Basin

The degradation occurred to the crude oil in the migration process from central depression to the margin of Songliao Basin, thus forming the oil sand reservoir in the western slope, where the extracts of oil sand are non-liquid with high density and viscosity. In Taobao-Xiaotaipingshan-Xibeigou-Tumuji area along the migration direction, for the oil sand extracts in Upper Cretaceous Yaojia Formation, the content of saturated aromatic hydrocarbons decreases with the depth shallowing, the content of unsaturated asphaltene increases, the carbon isotopes of whole oil and saturated hydrocarbon become heavier. The parameters above present an opposite variation trend in the single-well samples of Xiaotaipingshan area. Based on GC-MS analysis, the degradation levels of oil sand extracts are Wenger 2-8, the n-alkanes and isoprenoid alkanes in saturated hydrocarbons of most samples are missing, hopanes are partially degraded, and 25-norhopanes are detected in Xiaotaipingshan area. With the decreasing of depth in Taobao-Tumuji area, the biodegradation levels increase gradually. Biomarkers parameters present regular variations, such as tricyclic terpane/pentacyclic triterpane, C₂₇diasteranes/regular sterane, dibenzothiophene/phenanthrene, and etc. In Xiaotaipingshan area, the mass fractions of 25-norhopanes and corresponding hopanes as well as their ratios present that the biodegradation levels increase with the increasing of depth, the degradation resistance of hopanes increase with the carbon number increasing, and 22R homohopanes have greater biodegradation resistance than 22S homohopanes. at the western slope of Songliao Basin, the degradation laws of oil sand extracts are as below: in the migration process from a deep anaerobic environment to a surface aerobic environment, the degradation level of oil sand extracts gradually increases with the increasing of depth and water saturation in the single-well sample.