Depositional characterristics of the first member of Dainan formation in Qingtong sag, Subei basin and their significance to lithological exploration

By using high resolution sequence stratigraphy and sedimentology as tools upon the basis of seismic, coreing and well logging data, we carefully studied the first member of Dainan formation in the Qingtong sag in order to find out the favorable layers for lithology exploration. Two kinds of transitional surface, i.e. the sequence boundary and flooding boundary are recognized. The exploration target layer may be divided into one long-term cycle and five intermediate cycles which may be reduced from five to two from the deep sag to the slop surrounded.The Taizhou uplift was the main source of sediments then and theDainanFormation is a kind of fan deltaic deposits.The distribution of sedimentary facies belts are controlled by both the sediment source and base level changes. In the vertical direction, however, mudstone interlayers were deposited in the stages of MSC1, MSC2, MSC3 as favorable cap rocks.All the data indicates that the 1st member of the Dainan formation is the most favorable exploration interval.Spatially, facies belt distribution is the key controlling factoron hydrocarbon accumulation. Caijiabao-Chenjiashe, Shuaixi, Shuaiduo and Shuaibei blocks on the west slope where fan front sandbodies frequently pinchout are the most favorable region for lithological and/or structural-lithological traps.