陆相拗陷湖盆细粒沉积有机质富集机制研究:以松辽盆地长岭凹陷青山口组为例

Mechanism study of organic matter enrichment in fine-grained sediments in continental depression lacustrine basin:an example from the Qingshankou Formation in Changling sag,Songliao Basin

松辽盆地作为白垩纪形成的大型陆相拗陷含油气盆地,沉积了巨厚的湖相细粒沉积,近年来在非常规油气方面展现了巨大的勘探潜力,其中青山口组是松辽盆地页岩油的重点攻关层系,细粒沉积有机质富集机制对于非常规油气勘探至关重要。在松辽盆地南部长岭凹陷分别选取位于不同沉积区的3口井,针对其青一段水进体系域的细粒沉积岩,借助岩心观察、有机地球化学和生物标志化合物等技术手段,讨论在不同沉积环境和水进过程中,有机质来源和有机质保存条件的变化,分析有机质富集的关键控制因素,并建立相关的有机质富集模式。结果表明:研究区青一段细粒沉积岩有机质丰度较高,有机质类型以Ⅱ型干酪根为主,整体呈现有机质丰度南低北高、下低上高的特征;饱和烃中正构烷烃主要表现为单峰、前峰型,具有微弱奇数碳优势;萜类化合物中以三环萜烷和五环三萜烷为主;甾类化合物中C_(27-29)规则甾烷以C_(27)规则甾烷丰度最高;芳香烃中三芴系列化合物以二苯并噻吩为主。研究区青一段细粒沉积岩有机质来源以湖泊内源菌藻类为主,含有一定的陆源高等植物输入,古水体整体为弱还原的半咸水环境。其中,有机质来源和古盐度是控制有机质富集的关键因素。在平面上,半深湖—深湖沉积区相较三角洲外前缘沉积区,受河流影响较弱,陆源输入较弱,水体盐度较高,有机质类型较好,有机质更加富集。在垂向上,随着水体加深,河流影响减弱,水体盐度呈升高的趋势,有机质类型变好,有机质富集。

As a large continental depression oil-and gas-bearing basin formed during the Cretaceous period,Songliao Basin contains a huge thickness of lacustrine fine-grained sediments,characterized by great exploration potential for unconventional oil and gas in recent years.The Qingshankou Formation is the key breakthrough layer of shale oil in Songliao Basin,and the mechanism of organic matter enrichment in fine-grained sediments is crucial for unconventional oil and gas exploration.Three wells located in different sedimentary areas in Changling sag of southern Songliao Basin were selected for detailed studies in this study.Based on core observation,organic geochemistry and biomarkers analysis of the fine-grained sedimentary rocks of the Member 1 of Qingshankou Formation(K_2qn~1),the transgressive systems tract,and the source and preservation conditions of organic matter in different sedimentary environments and processes were analyzed.Then the key control factors of organic matter enrichment were discussed,and the relevant enrichment models were established.Our results show that the organic matter abundance of finegrained sedimentary rocks of K_2qn~1 was relatively high,and the kerogen is dominated by type Ⅱ.The overall organic matter abundance was low values in the south but high values in the north,low values in the lower part and high values in the upper part.Normal alkanes in saturated hydrocarbons predominantly exhibit single-peak and pre-peak patterns,with a slight odd-carbon preference.The terpenoids are primarily composed of tricyclic terpenoids and pentacyclic triterpenes.C_(27-29) regular steroids are dominated by C_(27) regular steroids.The major compound of the trifluorene series in aromatic hydrocarbons is dibenzothiophene.The organic matter in fine-grained sedimentary rocks of K_2qn~1 is mostly derived from the endogenous bacteria and algae in lakes,containing a certain amount of terrestrial higher plants.The water body is a weakly reduced,brackish environment.The source of organic matter and salinity are key factors controlling the enrichment of organic matter.Horizontally,the semi-deep lake to deep lake is less affected by riverine influences compared to the delta's outer front,exhibiting weaker terrestrial input,higher water salinity,superior organic matter types,and greater organic matter richness.Vertically,as the water depth increase、,riverine influence diminishes,the water salinity trends upward,the quality of organic matter improves,and organic matter becomes more enriched.