鄂尔多斯盆地西南地区延长组长7段细粒沉积物特征研究

Characteristics of Fine-Grained Sediments from the 7th Member of the Yanchang Formation in the Southwestern Ordos Basin

鄂尔多斯盆地西南地区广泛发育细粒沉积物。在大量钻井、测井资料的基础上,通过岩芯分析、地球化学测试等方法,从湖泊演化过程中水体盐度、氧化还原条件、生产力变化及地化特征等方面研究细粒沉积物特征与分布规律,进而分析细粒沉积物主控因素。结果表明:鄂尔多斯盆地西南地区延长组长7沉积期,气候温暖潮湿,半深湖—深湖大面积分布,富有机质细粒沉积物发育。该期形成的富有机质页岩主要受湖盆底形、砂体沉积模式的影响。尤其在长73沉积期,快速湖侵,湖水深度和范围急剧增加,古生产力高,湖泊表层水体与底层水体存在盐度分层,形成大面积缺氧环境,有利于富有机质页岩大规模发育与保存,在深湖区平缓地带,陆源碎屑供给不足,有机碳含量高,有机质类型好,以Ⅰ、Ⅱ型为主,斜坡带半深湖区,陆源碎屑影响明显,粉砂质泥岩发育,干酪根以Ⅱ型为主,局部地区发育Ⅲ型干酪根。

Fine-grained sediments extensively occur in the southwestern Ordos Basin. Based on a volume of drilling and logging data, redox condition and geochemical features of core, this study discusses the distribution rule and major controlling factors of the fine-grained system. The results show that the 7th Member of the Yangchang Formation in the southwestern Odors basin deposited in a warm and humid condition and is extensively distributed in semi-deep or deep lacustrines, with organic fine-grained shale well developed. Organic matter-rich shale formed on the bottom of lake basin and was affected by sedimentary modes of sand body. During the deposition of the Chang 73 Member, lake water invaded rapidly, increasing depth and size of the lake. The high productivity resulted in the layering of water body （salinity） and formed extensive anoxic environment, which are conductive to development and preservation of organic matter-rich shale. In silent deep-flat lake, insufficient clastic supply and high organic carbon gave rise to type Ⅰ and type Ⅱ kerogens, while in slope semi-deep lake close to the delta front, sediments are affected by terrestrial clastics and dominated by silty mudstone, with type Ⅱ kerogen and development of type Ⅲ kerogen.