HighResolution Sequence Stratigraphy Division of Upper Putaohua Reservoir, Northern Daqing Placanticline

Key words: 
• LaSaXing /
• highresolution sequence stratigraphy /
• base level cycle /
• stratigraphy overriding model

Abstract: Daqing Placanticline was located in the central sag of Songliao Basin. Upper Putaohua oilbearing layer (PI) belongs to a part of the 3rd Member of Qingshankou Formation and 1st Member of Yaojia Formation. According to the data of logging, cores, seismic and highresolution sequence stratigraphy theory, the highprecision sequence stratigraphy framework has been established. The 3rd Member of Qingshankou Formation and the 1st Member of Yaojia Formation are combined to one long base level cycle, which was limited by flooding surfaces. The PI oilbearing layer is judged leaping over the transformation surface, which could be considered as one middle base level cycle and 14 short base level cycles inside. 
Flooding surfaces are considered as marin surfaces of the middle term cycle in the study area, corresponding to the mudstone of front delta facies in top and bottom of PI oilbearing layer. Because of the stable spreading for this front delta mudstone, it is easy to track the surface of middle base level cycle. The transformation surface in the middle base level cycle corresponds to the unconformity between Qingshankou Formation and Yaojia Formation.
The framework of highresolution sequence stratigraphy is established based on the understanding above.
Based on the framework of highresolution sequence stratigraphy, several sections are established. Much bigger thickness of stratigrahpy and more shortterm base level cycles are found in southern of study area. According to the analysis of these highdense sections, three kinds of stratigraphy overriding models are proposed, includes active progradation model in low accommodation condition, forcing progradation model in lowest accommodation condition and congruence model of progradation and erosion. 
The first model formed in  lower accommodation space (A/S<1), which reflected relatively stable lake level in the study area (the increasing rate of lakelevel can not meet the abundant supply of source material), the new accommodation space formed by lake level rising was rapidly filled with sediment, leading the location of shoreline migrate to depocenter. 
The second model formed in much lower accommodation space conditions (A/S<<1), accompaning with relatively decline of lake level in the study area.  PI32 and PI31 layers in Lamadian and Saertu area were belong to the this kind of model. Gentle slope of Shallowwater delta will make this deposition hiatus development widely.
PI43PI33 layers  belong to the third model. With the continuous decline of the midterm base level, PI43, PI42 and PI41 layer showed progradation style, the ratio of sand increase to up. PI43PI33 layers in Saertu and Lamadian area were eroded after the tectonic uplifting in the northern area.
These stratigraphy overriding models are the main facts causing the differences in stratigraphic thickness and number of shortterm cycles.  The establishment of these deposition models is favor of the highprecision strata correlation in LaSaXing area, northern Daqing Placanticline.