东海陆架盆地西湖凹陷原油“气洗”地球化学特征及成因

Geochemical characteristics and genesis of “gas washing” of the crude oil from the Xihu Sag,East China Sea Shelf Basin

“气洗”是油气藏后期改造的重要营力之一,在东海陆架盆地西湖凹陷曾发生过大规模气洗作用,但现有研究仅限于描述阶段,并未对原油的“气洗”地球化学特征展开系统研究。通过对西湖凹陷平湖斜坡带和天台反转带原油进行地球化学分析,结果表明:原油的甾烷、萜烷指纹特征及碳同位素特征相似,显示出具有相同母源的特征,均为成熟原油,但不同原油的物性特征、轻烃、正构烷烃及金刚烷分布特征存在较大差异,通过计算原油的石蜡度、芳香度和正构烷烃损失量及对金刚烷系列化合物进行定量分析,认为气洗作用应是造成该差异的主要原因,并非生物降解或热裂解。为进一步厘清原油的气洗作用过程及成因,探讨了原油气洗作用的主控因素,并结合油气藏地质和地球化学资料进行了综合分析。结果表明:(1)平湖斜坡带北部原油主要为强烈蒸发分馏后的残余油,晚期可能被东部横向运移而来的高熟天然气混入,使原油中部分轻烃和金刚烷化合物含量升高;(2)平湖斜坡带南部主要受运移分馏作用影响,浅层聚集次生凝析油,分馏后的残余油在深层形成轻质油和蜡质油藏;(3)天台反转带原油主要为“气洗”晚期阶段形成的次生凝析油,其残余油目前尚未发现,暗指其深部储层或许具有一定规模的轻质、蜡质油藏。

Gas washing is one of the important driving forces for the late stage reformation of oil and gas reservoirs.Large-scale gas washing occurred in Xihu Sag of East China Sea Shelf Basin,but the latest research is limited to description,and there is no systematic study on the geochemical characteristics of gas washing.The geochemical analysis of the crude oil from the Pinghu Slope Belt and Tiantai Inversion Belt in the Xihu Sag shows that the sterane and terpane fingerprints and carbon isotope compositional characteristics of crude oil are similar,which can be believed to belong to the same oil family with a similar thermal maturity.However,the physical properties and distribution characteristics of light hydrocarbons,n-alkanes and diamondoids of the oils are different.Based on the calculation of the paraffinicity index,aromaticity index,n-alkane mass depletion and absolute content of diamondoid hydrocarbons,it is shown that gas washing is the main reason for the difference,not biodegradation or thermal cracking.In order to further delineate the gas washing process and genesis of the crude oil,this study discusses the controlling factors of the gas washing,and comprehensively analyzes the geological and geochemical data of oil and gas reservoirs.The results show that:(1)The oils from the northern Pinghu Slope Belt is mainly residual oil after strong evaporation fractionation.Later,it may be mixed with high-mature gas from lateral migration in the east,resulting in the increase of some light hydrocarbons and adamantane compounds in the crude oil.(2)The oils from the southern Pinghu Slope Belt are mainly affected by migration fractionation,resulting in shallow layer accumulation of condensate oil and in deep layer accumulation of light oil and waxy oil.(3)The oils from the Tiantai Inversion Belt are mainly condensates formed in the late stage of gas washing,and the residual oil has not yet been found,which implies that the deep layer may have a certain scale of light and waxy reservoirs.