松辽盆地嫩江组凝缩层形成机制及其地质意义

本文应用层序地层学、沉积地质学及盆地动力学分析方法,阐述了松辽盆地嫩江组凝缩层形成机制及其意义。松辽盆地嫩江组底部发育10~15 m厚的凝缩层,其形成机制是在嫩江组强制性水退旋回作用下导致湖盆萎缩,在进积型层序远端湖相区形成浓缩水体的高盐度还原环境,由此沉积了以油页岩为标志、以钟形高电阻为特征的凝缩层。该凝缩层底界面以SB₀₇最大湖泛面为等时边界,向上包含了进积型层序较长的时间跨度,不具等时性,同时来自东部物源向西的强烈进积堵塞了海侵通道,也使松辽盆地嫩江组发生过"海侵事件"的推论无法成立。因此,凝缩层形成机制的认识在松辽盆地层序地层学和沉积地质学领域具有重要的意义。成藏分析认为嫩江组凝缩层既是烃源岩也是区域性盖层,在其上下蕴藏着丰富的油气资源,同时由于强制性水退型层序在钻井及地震上具有易于识别的结构特点,是含油气盆地评价及层系优选的结构性标志。因此,凝缩层的认识不论在层序地层、沉积环境及成藏分析方面都具有重要的理论和实际意义。

The Mechanism and Significance of the Cretaceous Condensed Section in the Nenjiang Formation,Songliao Basin

This article reveals the mechanism and significance of the condensed section developed in the Nenjiang Formation of Songliao Basin by applying the methods of sequence stratigraphy, sedimentary geology and basin dynamics analysis.10~15 m thick condensed section developed at the bottom of the Nenjiang Formation in Songliao Basin was formed under the action of shrinking process of the lake basin caused by the forced regression. When the water body in the distal end of the progradational sequence became into the highly-salinity and strong-reduced environment, the condensed section deposited as oil shale and was marked by the high bell shaped resistance in the well log. And the bottom of the condensed section took the SB07 maximum lake flooding surface as the isochronal boundary. The upward progradational sequence last a long time span and had no isochronism, meanwhile the strong progradation from the east to west blocked the transgression channel. Therefore, the inference that "transgression event" ever happened in the Nenjiang Formation of the Songliao Basin is false. Consequently, the study of the mechanism of the condensed section in Songliao Basin makes sense for sequence stratigraphy and sedimentary geology. By the analysis of hydrocarbon accumulation, we come to the conclusion that the condensed section in the Nenjiang Formation is both the source rock and the regional cap. There are abundant oil and gas resource covering and underlying this layer, which can be easily identified from the seismic data and well log as forced regression sequence. The condensed section is also a good structural mark to evaluate petroleum basins and to optimize the sequence. Thus, the awareness of the condensed section has theoretic and practical meaning for sequence stratigraphy, sedimentary environment and hydrocarbon accumulation analysis.