东营凹陷古近系升藿烷生物标志物参数分布及演变规律

Distribution and Characteristics of the Homohopane Molecular Parameters in Paleogene System of the Dongying Sag

为研究实际地质体或地质剖面中的C31到C3517α(H),21β(H)升藿烷生物标志物构型转化参数22S/(22S+22R)及升藿烷指数C35/∑(C31—C35)等的分布、演化特征,对东营凹陷主要烃源岩层系古近系沙河街组三段(淡水—微咸水层系)和沙河街组四段(咸水层系)的系列样品进行了研究,分析样品来源于1 300～4 000的暗色泥岩,其中2 800～4 000 m的样品对应于东营凹陷古近系烃源岩的生油初期到生油晚期。研究结果表明,C31、C32、C33、C34、C35升藿烷(17α(H),21β(H))构型转化参数的分布除与异构体之间的手性碳立体构型转化作用有关外,不同异构体之间降解或裂解速率和新生成速率的差异也是其主要控制因素,沉积环境(如高盐环境)也在一定程度上影响了其分布;从未熟—低熟状态到成熟状态,相关升藿烷构型转化参数22S/(22S+22R)总体均表现出随埋藏深度增大而加大的特点,并从离散状态聚集到热演化的平衡状态附近;在到达成熟状态后,相关升藿烷的构型转化参数22S/(22S+22R)从缓慢增大过渡到一个持续的热演化平衡状态,并且该构型转化参数的热演化平衡状态对应于生油门限附近;在高盐环境中除C33升藿烷保持不变的热演化平衡状态外,其它升藿烷均呈现不同幅度的逆转特征,表明高含量的盐类矿物对升藿烷成熟度参数亦具有抑制作用或迟缓效应;C31、C32、C33、C34、C3517α升藿烷之间的22S/(22S+22R)分布型式复杂多变,其中对于成熟源岩样品,C35升藿烷22S/(22S+22R)值变化强烈,表现出"翘尾"状的上升型特征和"坠尾"状的下降型特征,而对于低熟—未熟样品,C32升藿烷的22S/(22S+22R)显示相对高值,并且多数样品显示为C31<C32>C33<C34>C35的偶数碳优势的特征;研究显示C31到C33升藿烷的构型转化参数S/(S+R)热演化平衡值均为0.6,而C34S/(S+R)及C35S/(S+R)热演化平衡值相对较高,沙三段、沙四段的C34 S/(S+R)及沙四段的C35 S/(S+R)分别达到0.63、0.62、0.65;升藿烷指数C35/∑(C31—C35)受热演化影响也比较明显,在生油期间随着埋藏深度增加而减小,在高盐环境中表现出相对高值,研究表明该指数的变化与其22R异构体演化有关,并且参数C31/∑(C31—C35)、C32/∑(C31—C35)、C33/∑(C31—C35)、C34/∑(C31—C35)、C35/∑(C31—C35)演化特征截然不同。

The Paleogene system in the Dongying sag is a representative lacustrine basin in the eastern China.To investigate the distribution and characteristics of the homohopane maturation parameters,the main source rocks from Member 3（deposited in fresh and brackish water） and Member 4（deposited in salty water） in Shahejie Formation（burial depth from 1 300 m to 4 000 m） were investigated.The range 2 8004 200 m coincides with the ＂oil window＂ from the early of oil generation to the late of oil generation,which can well represent the evolution of homohopane maturation parameters during the process of oil generation. The results demonstrate that the main controlling factors for the distribution of C31,C32,C33,C34,C35（17α） homohopane parameters are the isomerization and different decomposition rates as well as generation rates between different isomers.Depositional environment（such as high-salt environment） also impacts their distribution to some extent.For immature and low-mature samples,the related parameters 22S/（22S＋22R） increase with burial depth from a disperse state to a equilibrium state,while for the matured samples the parameters increase slowly and then transit to a continuous thermal equilibrium state.In high-salt environment most homohopanes show reversal in different degree except C33 homohopane which remains unchanged in an equilibrium state,indicating that high content of saline minerals also has inhibited or retarded the homohopane maturity parameters.The thermal equilibrium of the parameters coincides with the threshold of oil generation and can be seen as a good indicator for oil generation.The distribution patterns of C31,C32,C33,C34,C35（17α） 22S/（22S＋22R） are complex and diversified.For the matured source rock samples,the C35 homohopane 22S/（22S＋22 R） shows a strong change and is characterized by obvious ＂rise and＂ ＂fall＂.While for the immature samples,C3222S/（22S＋22R） displays some remarkable high value,and most immature samples demonstrate characteristics of C31 〈C32〉 C33 〈C34〉 C35 and exhibits an advantage of even-number carbon.The study shows that the thermal equilibrium values for C31,C32,C33 isomerization parameters are 0.6 consistently,while the equilibrium values for C34 and C35 homohopanes are comparatively high（the equilibrium of C3422S/（22S＋22R） for Member 3 and Member 4 are 0.63,0.62 respectively,and the equilibrium of C3522S/（22S＋22R） for Member 4 reaches 0.65）.The homohopane index C35/Σ（C31-C35） is also impacted by thermal evolution.In the oil window the index firstly decreases as the burial depth increases,and in the 3.6km high-salt environment,the index shows opposite trend and relatively high values.The study indicates that its change is dominated by 22R isomers.The parameters C31/Σ（C31-C35）,C32/Σ（C31-C35）,C33/Σ（C31-C35）,C34/Σ（C31-C35）,C35/Σ（C31-C35） show different characteristics in their distribution and evolution in the profile.