湖盆缺氧水体与烃源岩非均质性成因机制研究

ource rocks heterogeneity are mainly characterized by heterogeneity of total organic carbon (TOC) contents and components which is related with organic matter input, sediments deposition and redox degree of bottom water, etc. The research indicated that TOC contents of lacustrine source rocks could vary in two orders (within the same layer, TOC value is below 0.5% or above 10%), leading to resource evaluation inaccurately and obvious differences of reservoir capacity and penetration ability. Anoxic degree of bottom water is supposed to be positively related with organic matter accumulation. However, how did lacustrine anoxic conditions of bottom water be formed and evolve? How did anoxic conditions of bottom water control the formation of source rocks heterogeneity? These are the key scientific questions to evaluate source rocks precisely. The targets are taken from the lower part of Shahejie Formation 3rd member (well N38) and 4th member (well Dongfeng 1) source rocks. Functional relationship representing bottom water anoxic conditions is built to describe the anoxic degree of sedimentation water quantitatively. The formation and disappearance mechanism of anoxic bottom water and the relationship between anoxic bottom water evolution and organic matter accumulations would be revealed. Formation model of lacustrine source rocks heterogeneity is constructed to provide quantitative evaluation parameters, new methods and insights for prediction of valued or high quality source rocks.

烃源岩非均质性主要指有机质含量和组成的非均质性，其与有机质输入、沉积物埋藏、沉积水体缺氧程度等相关。研究表明陆相湖盆烃源岩TOC可相差1-2个数量级(如同一层段TOC<0.5%~>10%)，易造成资源评价结果不准确和页岩油气储渗能力差异大，并认为缺氧程度与有机质富集程度呈正相关。那么湖盆缺氧条件是如何形成和演化的？又如何造成烃源岩非均质性的？这是准确评价烃源岩需解决的科学问题。本项目选取东营凹陷N38井沙三下亚段和DF1井沙四上亚段烃源岩为研究对象，定量恢复沉积旋回内的古水体盐度、古水深、水动力波动强度和水体动荡程度等，构建表征水体缺氧条件的函数关系，定量刻画沉积水体缺氧程度，揭示其形成与消失机制，阐明缺氧水体演变与有机质富集和耗散的相关性，建立湖盆烃源岩非均质性成因模式，提供烃源岩非均质性定量评价参数，提高油气资源评价结果准确性，也为预测有效或优质烃源岩提供新的思路和方法。

中国的页岩油资源大量赋存于陆相湖盆中，其页岩、泥岩和灰岩的有机质丰度、岩石矿物组成、沉积构造等非均质性差异对页岩油富集的控制作用显著。综合岩心描述、显微观察、矿物分析和扫描电镜等宏观-微观岩矿分析和有机碳含量、显微组分鉴定和生物标志化合物等宏观-微观有机分析，查明了东营凹陷沙三下-沙四上亚段（Es3x-Es4s）烃源岩存在薄透镜状晶粒灰岩、纹层状泥晶灰岩、厚层块状灰质泥岩、块状泥岩、黑色页岩、粉砂质泥岩或泥质粉砂岩、薄层砂岩、泥质膏岩/或膏质泥岩等五类8种宏观岩性非均质性特征；发育了薄纹层页岩/灰岩（1.86%<TOC<12.22%，208 mg/g TOC<HI<781 mg/g TOC）、厚纹层页岩/灰岩、断续纹层页岩/灰岩（0.52%<TOC<6.40%，162 mg/g TOC<HI<835 mg/g TOC）、块状泥岩（0.19%<TOC<4.49%，92 mg/g TOC<HI<713 mg/g TOC）、富砂质泥岩和富石膏质泥岩（0.51%<TOC<2.95%，79 mg/g TOC<HI<504 mg/g TOC）6种微观非均质性岩相，前四者以I-II型干酪根富集为主，后两者则由II-III型干酪根组成，明确了不同烃源岩的有机质丰度主要取决于有机质纹层的密度。利用能谱测井资料的小波频谱分析建立了研究层段的天文旋回年代标尺，揭示了岁差旋回控制了岩层组的沉积旋回厚度， Es4s存在着80个38ka的斜率旋回，共持续3.04Ma。查明了Es3x-Es4s烃源岩非均质性受天文轨道旋回和日照量控制下的古气候变化、矿物来源与供给量（陆源石英、长石、粘土矿物输入，自生碳酸盐岩矿物）、古湖泊生产力、保存条件等因素综合影响，建立了Es3x-Es4s烃源岩非均质性发育模式。该湖相烃源岩非均质性差异及其成因机制研究可以帮助查明富有机质页岩、泥岩、灰岩对常规油气与页岩油气成藏贡献的差异，为陆相页岩油资源评价标准与后续勘探部署工作提供重要理论支持。