松辽盆地嫩江组泥页岩有机质富集模式探讨——以嫩江组一、二段油页岩为例

通过理论模型推演与地质研究实例相结合的方式,以松辽盆地白垩系嫩江组一、二段发育的富有机质页岩(油页岩)为例,重点利用松科1井(南孔)嫩江组取芯井已有测试分析资料,综合有机碳(TOC)含量、黄铁矿硫同位素、全岩元素,以及相关测井、地震资料,综合探讨分析了嫩江组油页岩的成因机制以及油气勘探意义。研究认为:(1)将松辽盆地嫩江组油页岩的形成机制完全归因于海水侵入湖盆很可能是不全面的。虽然海侵能够导致湖盆水体咸化以及藻类勃发,但由于海侵时富氧表层海水对湖盆底部氧化还原环境的改变,"海侵"发生时可能在一定程度上抑制有机质在沉积物中的富集保存,这一点可以从松科1井连续地化剖面上得到验证;(2)海侵对富有机质页岩沉积的促进因素很可能主要体现在海侵结束后湖盆的持续咸化带来的藻类勃发;(3)嫩江组泥岩内存在全盆地广泛分布的火山凝灰岩沉积,且通常与油页岩呈共生关系。古松辽湖盆生产力的提升与火山灰输入所带来的营养物质,如磷、铁等直接相关;(4)嫩江组一、二段油页岩的形成可能是海侵结束后湖盆水体咸化与火山灰瞬间大量输入所诱发的湖盆水体富营养化事件多因素共同作用的结果。油页岩含油率分布规律显示在湖盆生产力普遍较高的情况下,沉积区水体深度越深、越远离河流碎屑输入,则有机质的富集保存条件越好,油页岩品质越高。

A discussion on the organic matter enrichment model of the Nenjiang Formation, Songliao Basin: A case study of oil shale in the 1st and 2nd members of the Nenjiang Formation

The formation mechanism of the oil shale developed in the 1st and 2nd members of Nenjiang Formation in Songliao Basin was studied by combining theoretical model with geological data such as total carbon content, sulfur isotope of pyrite, and whole rock element from the Songke-1 Well (south hole). Some conclusions have been reached:(1) It may not be fully correct to attribute the formation of oil shale to seawater intrusion into the lake basin. Although seawater invasion can lead to salinization of the lake and subsequent algae blooming, the change of bottom water redox condition caused by the invasion of oxygen-rich seawater into the bottom lake can inhibit the accumulation and preservation of organic matter in sediments; (2) The impact of seawater transgression on oil shale formation was mainly related to algae blooming caused by the continuous salinization of the lake water after transgression; (3) Volcanic tuff deposits are widely distributed in the Songliao basin and usually associated with oil shale deposits. The increase of productivity in ancient Songliao lake basin was directly related to nutrients such as phosphorus and iron brought by volcanic ash input; (4) The formation of oil shale in the Nenjiang Formation was controlled by a combination of mechanisms such as water eutrophication events induced by instantaneous mass input of volcanic ash and seawater invasion. It is concluded that organic matter enrichment in the oil shale is controlled by both water depth and the influence of sediment input from rivers.