Abstract: Tanan Depression is one of the most important oil and gas exploration areas in Hailaer-Talmud Zagat Basin.Various data including cores,logs and 3D seismic data were integrated to study in detail the sequence stratigraphic framework and sedimentary system characteristics of K₁n¹(the 1st member of Nantun Fm)in Tanan Sag,Haita Basin.Three unconformites and two lake flooding surfaces were identified in K₁n¹,according to which K₁n¹ was divided into three third-order sequences.The types of sedimentary facies and distribution characteristics of sedimentary system were analyzed on the basis of isochronous stratigraphic framework.Three sedimentary systems namely fan delta,sublacustrine fan and lake were identified in the K₁n¹.Fan delta sedimentary system under the control of provenance on the western gentle slope dominated in the study area during the deposition of the middle-lower part of K₁n¹ due to the influences of oscillatory subsidence of "flood basin" during passive rifting.In contrast,lake deposition system was predominant in the study area during the deposition of the upper part of K₁n¹ due to the rapid subsidence in the process of active rifting.At the same time,the secondary depression-controlling faults experienced NW-SE extension and tilted,forming large-scale reverse normal faults.Efficient hydrocarbon expulsion characteristics of interbeded source rock and reservoir combination,high porosity and permeability of distributary channel sands of fan delta front facies and structural high in the upthrown block of fault jointly determined the footwall of antithetic fault in the western subsag to be the main location of oil accumulation.

Key words: sequence stratigraphic framework, sedimentary system, Nantun Formation, Tanan Sag, Hai-Ta Basin