Sedimentary facies and palaeogeography of the third member of the Shahejie Formation in the Lengjia Oil Field,Liaohe Basin,Liaoning

During the early stage of the deposition of the third member of the Shahejie Formation in the Lengjia Oil Field,Liaohe Basin,Liaoning,the deeper lake water facilitated the formation of fan deltas and basin-floor fans in which the braided distributary channel and beach-bar reservoir microfacies are accentuated.Till the late stage of the deposition of the third member of the Shahejie Formation,the shallower lake water shoud be responsible for the development of lacustrine sandstones.According to the distribution of the sedimentary facies in the study area,the early,middle and late stages of the deposition of the third member of the Shahejie Formation may be involved into two phases of lake spreading and collapse.The Lengjia Oil Field in the western depression of the basin occurs as a half graben depression being steeper in the east and gentle in the west.The palaeogeographic framework is interpretated as the key factor which has an important effect on the sedimentary facies distribution in the third member of the Shahejie Formation.The sizes and scales of fan deposits depend on the energy intensity in the rivers and lakes.Influenced by the lake-level changes,the longitudinal deltas were formed in the spreading phases of the lakes,and characterized by narrower facies belts,sharp gradation and coarser clastics.The fan deposits consist dominantly of frontal distributary channel deposits.The basin-floor fans are often developed on the beaches and bars and profan deltas in the frontal parts or flanks of the estuary.