陆相断陷咸化湖盆细粒沉积地层有机质富集特征及控制因素分析——以东营凹陷沙河街组第四段上亚段纯上次亚段为例

Analysis of Organic Matter Enrichment and Influences in Fine-grained Sedimentary Strata in Saline Lacustrine Basins of Continental Fault Depressions:Case study of the upper sub-section of the upper 4th member of the Shahejie Formation in the Dongying Sag

【目的】湖相细粒沉积泥页岩储层“甜点段”的发育与有机质富集密切相关,但目前针对断陷咸化湖盆泥页岩有机质富集控制因素的探究及富集模式的建立等相关研究较少。【方法】东营凹陷沙河街组第四段上亚段纯上次亚段(Es4scs)时期发育典型的陆相断陷咸化湖盆,为深化对研究该区细粒沉积地层有机质富集规律的认识。从沉积学、地球化学、古生物学及分子地层学等方面入手,利用薄片鉴定、X射线衍射、氩离子抛光—扫描电镜、孢粉鉴定、生物标志物色谱—质谱分析等技术手段,运用各项指标详细探究有机质富集的控制因素,建立适用于研究区各环境演化阶段的有机质富集模式。【结果】(1)有机质具有条带状、充填式、分散式等微观赋存状态,母质来源主要为藻类等低等水生生物,在中上部地层出现少量的陆源高等植物来源;(2)沉积环境经历了较浅水半干旱弱氧环境—深水半湿润还原环境—次深水半干旱贫氧环境三个演化阶段;(3)有机质富集的主要控制因素为古气候、古盐度、古氧化还原性、古生产力和沉积速率,而次要控制因素为古水深、陆源输入、火山作用和热液活动。【结论】本次探究了有机质富集的主控因素,建立了适用于陆相断陷咸化湖盆细粒沉积地层的有机质富集模式,研究成果对我国陆相页岩油气勘探开发具有重要意义,即高生产力、还原环境、适宜的盐度和沉积速率主导了高有机质富集,该环境的沉积产物是页岩油气勘探的有利层段。

[Objective]The development of the“sweet spot”in lacustrine fine-grained sedimentary shale reservoirs is closely related to the enrichment of organic matter(OM).However,few studies on the causes of OM enrichment and the establishment of enrichment models of shale in faulted saline lacustrine basins have been reported.A typical continental rifted saline lake basin was developed for the upper sub-segment of the upper 4th member of the Shahejie Formation(Es4scs)in the Dongying Sag.To further understand the OM enrichment in fine-grained sedimentary shale during this period,[Methods]This study used thin slice identification,scanning electron microscopy,X-ray diffraction,sporopollenin identification,biomarker chromatography-mass spectrometry and other techniques.The controlling factors of organic matter enrichment were explored in detail by using each index,and the organic matter enrichment model suitable for each stage of environmental evolution of the study area was established.[Results](1)OM occurs at the microscopic level as strips,infills and dispersions.The OM source was mainly lower aquatic organisms such as algae,with evidence of small amounts of terrestrial higher plant material in the middle and upper strata.(2)The sedimentary environment evolved in three stages:A shallow-water semi-arid low-oxygen environment,a deepwater semi-humid reducing environment,and a sub-deepwater semi-arid low-oxygen environment.(3)OM enrichment was mainly influenced by climate,salinity,redox condition,productivity and deposition rate.Water depth,terrestrial input,volcanism and hydrothermal activity were secondary influences.[Conclusions]A model of OM enrichment in fine-grained sedimentary strata of the continental rifted saline lake basin was established,which may be highly significant for the exploration and development of continental shale oil and gas in China.To summarize:high OM enrichment in the studied interval was dominated by very productive conditions,a reducing environment for preservation,suitable salinity and suitable deposition rate.