Sequence Stratigraphy And Sedimentary Facies Of Es₄ In The Middle Section Of North Region Of Dongying Depression

The sequence stratigraphy and sedimentary facies of Upper Es4 of Paleogene in the middle section of north region of Dongying depreesion have been studied using seismic, well logging, mud logging, testing data and core information.Upper Es4 can be divided into five fourth-order sequences(SQ-1,SQ-2,SQ-3,SQ-4,SQ-5) according to the sequence boundaries recognized by methods of classic sequence stratigraphy using both well logging data and seismic data. Each sequence develops at the half cycle of increasing. On the basis of the fourth-order sequence framework, the short-term base-level cycles are subdivided and correlated by drilling, well logging and 3D seismic data. In the end , the isochronous stratigraphic framework is established.There are three types of sedimentary facies developed in the Upper Es4 including nearshore subaqueous fan, lacustrine facies and slump turbidite fan which are indicated by sedimentary facies marks of both petrology and geophysics as well as the analysis of single-well facies and cross-sectional facies.Four provenances lying in the north of the study area are analyzed by the characteristics of lithology distributions and seismic reflections under the sedimentary background which include well blocks of Tuo174 Tuo175, Tuo75 and Tuo154.The characteristics and distributions of sedimentary facies of upper Es4 are summarized in light of the horizontal distributions of sand-ground ratio, seismic facies and seismic attribute. It is considered that large-scale nearshore subaqueous fans spread widely at the up thrown block of Shengbei fault with the nature of succession, while turbidite fans developed at the downthrow wall swinging from side to side with frequent migration. Both the nearshore subaqueous fans and the internal sequences themselves show an evident retrogradation depositional trend as the fans moved to the north margin during the deposition period of upper Es4. This suggests an intermitted lake expansion.The fan distribution was determined by the paleo-channel and fault. Wherever there was a channel, there was a fan, to be more specific, large fan responded to large channel and small fan responded to small channel. Moreover, the fault controlled the types of fans. At the up thrown block of Shengbei fault, large-scale nearshore subaqueous fans are developed. At the downthrow wall, the instable sediments at the up thrown block are transported along the steep fault plane, which produce slump turbidite fans.