鄂尔多斯盆地二叠系烃源岩地球化学特征与沉积环境的关系

Relationship between the geochemical characteristics and sedimentary environment of Permian hydrocarbon source rocks in the Ordos basin

鄂尔多斯盆地二叠系烃源岩是盆地重要潜在的生烃岩系,以钻井岩心地球化学分析资料为主,对泥岩、泥质碳酸盐岩的微量元素、稀土元素和有机质地球化学特征进行了系统分析,其中微量元素含量与同类岩石的克拉克值相比,差别较大。轻稀土富集程度较高,重稀土相对较低,Ce/Ce*具有较明显的正异常,反映出烃源岩沉积环境总体为还原环境。研究认为,半深湖—深湖相带沉积的灰黑色泥岩类烃源岩还原程度强,有机质丰度高,有机质能被及时埋藏;河流—沼泽相沉积环境沉积的灰色—绿灰色泥岩类烃源岩有机质丰度较低。还原程度强、具一定深度水体的沉积相带是原始有机质沉积的良好场所,有机质能沉积并及时保存下来;而气候条件相对干燥、降水少、陆源碎屑物补充不足或较长时间处于氧化环境条件下沉积的泥质碳酸盐岩类岩石,有机质丰度明显偏差。

Permian source rocks are important and potential source rocks in the Ordos basin. A systematic analysis of the trace element and REE geochemistry and organic geochemistry of argillaceous carbonate rocks and mudstones has been carried out mainly according to the geochemical data of the well core samples. The LREE abundance is high and the HR.EE abundance is low and Ce/Ce^＊ has a relatively pronounced positive anomaly, indicating that the source rocks were deposited in a reducing environment. The contents of trace elements in the source rocks are significantly different from their clarke values in similar rocks in the Ordos basin. Study shows that grayish black mudstone source rocks deposited in semi-deep lake-deep lake facies zones are strongly reduced and rich in organic matter, where organic matter could be buried timely, and grayish black mudstone source rocks deposited in a fluvial-swampy environment have a low organic matter abundance. The strongly reduced depositional facies zone with a fairly deep water body was an ideal locus for deposition of primary organic matter, where the organic matter could be buried and preserved timely; whereas organic matter abundance was obviously low in argillaceous carbonate source rocks deposited where the climate was dry with little precipitation and terrigenous clastic material supply was inadequate or in a long-term oxidizing environment.