陆相泥页岩层系岩相特征与页岩油富集条件——以松辽盆地古龙凹陷白垩系青山口组一段富有机质泥页岩为例

Petrological characteristics and shale oil enrichment of lacustrine fine-grained sedimentary system: A case study of organic-rich shale in first member of Cretaceous Qingshankou Formation in Gulong Sag, Songliao Basin, NE China

以松辽盆地古龙凹陷白垩系青山口组一段富有机质页岩为例,对细粒碎屑湖盆页岩的岩石学特征、岩相类型、孔径分布及其与页岩油富集的关系进行研究。结果表明,岩相的空间变化受米兰科维奇旋回控制和沉积物源影响,具有明显的序列性;按照"有机质丰度-岩石沉积构造-矿物组成"分类标准,将青一段划出7类岩相。其中,中等有机质含量纹层状长英质泥岩相具有较好的生烃潜力,储集空间发育,为原地滞留基质型页岩油的优势岩相。在异常高压的背景下,发育于SSC2旋回顶面—SSC3旋回底面的中等有机质含量纹层状长英质泥岩相在三角洲—湖相沉积区横向分布稳定,连续厚度大于30 m,并具有这2期旋回中部发育的高有机质含量块状长英质泥岩相和中等有机质含量块状长英质泥岩相作为顶、底板封盖,具备形成基质型页岩油的有利条件。

Taking organic-rich shale in the first member of Cretaceous Qingshankou Formation in the Gulong Sag,northern Songliao Basin as an example,this study examined the lithofacies classification,petrological characteristics,pore size distribution and their implications on the enrichment of shale oil of lacustrine detrital fine-grained shale.The spatial variation of lithofacies,controlled by Milankovitch cycle and influenced by sediment provenance,has an obvious sequence.The fine-grained sedimentary rocks of studied section could be classified into seven lithofacies according to a three-step classification criterion that consists of total organic carbon(TOC),sedimentary structure and mineral composition.Among them,the laminated siliceous mudstone lithofacies with moderate TOC has high hydrocarbon generation potential and abundant reservoir space,making it the most favorable lithofacies for the enrichment of matrix shale oil.Under the background of abnormally high pressure,the laminated siliceous mudstone lithofacies with moderate TOC deposited between the top of SSC2 and the bottom of SSC3 is stable in lateral distribution in the delta-lacustrine transition zone,with continuous thickness greater than 30 m.The massive siliceous mudstone lithofacies with high and moderate TOC developed in the middle of the these two cycles can act as the roof and floor seal for shale oil,therefore,the study area has good conditions for forming matrix type shale oil reservoirs.