Boundary Strata And Paleoclimate Records From Upper Qingshankou To Lower Yaojia Formations In Xu-22 Well Of Daqing Oilfield

Detailed sample collections and descriptions have been made from the upper Qingshankou to lower Yaojia formations in Xu-22 well of Daqing Oilfield. The length of sampled core is 12.20 meters. Careful identification and analysis on Sporopollen and Ostracods fossils have been carried out by present authors, and as well testing and analysis on Carbon stable isotope.At the bottom of this interval is the greyish-black mudstone in horizontal bedding. It riches in Sporopollen and Ostracods fossils. From lower to middle part is the dark grayish-green silt mudstone. The abundance of Sporopollen and Ostracods fossils reduces evidently. From middle to upper part is grayish-green and light grayish-green siltite, but beddings are obscure and fossils rare. The top part is brown-red siltite and silty mudstone within grayish-green siltite. The lower part yields Sporopollen assemblage of Cyathidites- Balmeisporites-Cedripites, and the Ostracod assemblage Cypridea panda-Triangulicypris fusiformis. They occur in the Third Member of Qingshankou Formation in Songliao basin. The upper part, just upon the boundary at 1612.90 meters, the Ostracods absolutely vanished. And a new Cyathidites-Schizaeoisporites Sporopollen assemblage appears. They are the indicators of First Member of Yaojia Formation. However the quantity is lower.Microbiota in the lower part of Qingshankou Formation imply a deepwater and disoxidation environment. There are large amount of Ostracods and other biota. The Sporopollen Cyathidites is dominant. This shows a warmer and moister climate. The color of upper layers turns light, the granularity increases and fossil abundance reduces rapidly. This shows that the lake became shallower, the climate was more dry and warm, and the creatures were hardly hitted. At the boundary of 1612.90m, Ostracods absolutely vanished, and Sporopollen reduce their number. Sediments in upper layers become coarse and in red color. This shows that the lake was much shallower, maybe in a delta silted phase. The climate is more dry and hot, and fluctuates frequently, and fossils are rare. Generally, at the boundary, the climateturned to more dry and hot from warmer and moister conditions, the water tended shallower and more fluctuant upwards, and as a result, biota reduces their abundance rapidly under the climate changes.Organic carbon stable isotope values show three evolvement stages. In the upper part of Qingshankou Formation,δ13C value shows a negative shift, with the lowest of -27.9‰, the highest on -23.5‰and average on -25.4‰. This reflects that climate was comparatively humid. C3 plant is dominant. C3 plant and water phytoplankton are the primary source of organic matter. At the boundary,δ13C values are comparatively stable. Comparatively dry and heat climate is reflected. C4 plant becomes to bloom and predominate. At the lower part of Yaojia Formation, a clear positive excursion ofδ13C value occurs. This reflects more dry and heat climate and C4 plant predominates absolutely. Few fossil has been found. Study results upon, lithologic characters and Sporopollen data are in accordance with the research in other parts of the Songliao besin.At the boundary,δ13C values show a positive shift. The highest value is -22.6‰. This result is in accordance with the research of Wan (2005). However its fluctuant range is much lower than the latter. This is remarkable.Moreover, we also probe into the evolutionary mechanism. The results indicate that high temperature tropic climate occurs during the boundary time. There is a little regression in the First Member of Yaojia Formation. Meanwhile, farther calefacient events make the climate more dry and heat. The change of humidity is evidently larger than the change of temperature.