渤海湾盆地东营凹陷烃源岩碳氧同位素组成及地质意义

Composition and geologic significance of carbon and oxygen isotopes in hydrocarbon source rocks,Dongying Sag,Bohai Bay Basin

对渤海湾盆地东营凹陷沙河街组烃源岩层段开展了系统取样和碳酸盐碳氧同位素分析,并结合文献中的数据资料,在层位精细划分基础上,系统总结了碳氧同位素组成特征及纵向的差异,并探索分析了古湖泊环境的演变过程。沙四段下亚段δ^(13)C和δ^(18)O呈现较好的正相关,指示了断陷初始期盆地封闭性水文地质条件,但其碳氧同位素又整体相对较轻,并且具有从下部向上部逐渐变重的特征,表明盐湖水体不稳定并受到不同程度的淡水影响作用。沙四段上亚段δ^(13)C和δ^(18)O整体呈现较好的正相关且明显加重,指示了断陷加速期咸化湖泊水体环境较为稳定和封闭的特征。沙三段下亚段不同岩性碳酸盐碳氧同位素存在一定差异,其中较好反映烃源岩原始碳氧同位素组成特征的非钙片页岩,其δ^(13)C和δ^(18)O值分布非常集中,指示断陷鼎盛期非常稳定、封闭的半咸化湖泊环境。沙三段中亚段与沙三段下亚段非钙片泥岩数据较为接近,从一定程度上继承了沙三下亚段沉积期湖泊水体的碳氧同位素特征。

Lacustrine hydrocarbon source rocks ol the Shahejie Formation in the Dongying Sag were sampled, and the carbon and oxygen isotope composition ol carbonates were analyzed. Com^）ined with previous references and precise stratigraphic division,the isotope composition in different stratigraphic units and the evolution ol ancient lacustrine environment were discussed. The 513C and 5180 of carbonates in the lower section of the fourth member of Shahejie Formation are positively correlated,which indicated that the basin was hydrologically closed during the initial period of rifting. But the carbon and oxygen isotopes are relatively light on the whole,which indicated that the water body of the salt lake was not stable,and was frequently influenced by fresh water. The 513C and 5180 of carbonates in the upper section of the fourth member of Shahejie Formation are relatively heavy and positively correlated,which indicated that the salt lake was closed and stable during the acceleration phase of rifting. The carbon and oxygen isotopes of carbonate rocks with different lithologies in the lower section of the third member of Shahejie Formation varied. The 513C and 5180 of non-calcium shale vary very little and are highly concentrated,which implied that the brackish lacustrine environment during peak rifting period was very stable and hydrologically closed. The middle and lower sections of the third member of Shahejie Formation have similar properties to the non-calcium shale,which suggested that the former may have inherited the isotope features of the latter.