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

Key words: 
• transgressive-regressive cycles /
• sedimentary evolution /
• dynamic mechanism /
• Songliao basin /
• Nenjiang Formation

Abstract: 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.