Sequence stratigraphy and sedimentary characteristics of Paleogene hydrocarbon reservoirs in eastern Chengdao offshore oilfield of Bohai Gulf

Paleogene ore accumulations in eastern Chengdao Oilfield are controlled by lithology and concealed at depth, and hence sequence stratigraphy seems to be the best means in search for beneficial exploration areas of concealed oil accumulations. Therefore, it is crucial to carry out the research on sequence stratigraphy in this area. In view of the low exploration extent and unclear sedimentary facies belt and sandstone characteristics of this area and with the purpose of accelerating the exploration work in this area, the authors carried out a study of sequence stratigraphy and sedimentary characteristics on the basis of previous research work both at home and abroad. First, tectonic base-level cycles and long-term base-level cycles were identified according to grade division of base- level cycles based on high precision sequence stratigraphy. Then the Paleogene Dongying Formation was further divided into four mid-term base-level cycles. High resolution sequence stratigraphic framework of Dongying formation was established. It can be seen that different sequences or different parts of the same sequence of Dongying Formation have different distribution characteristics, with the general distribution pattern following the principle of volume distribution of sediments. The sedimentary system of the Dongying Formation circles had different characteristics in different periods: deep lakes and turbidite fans were mainly developed during the rising of the base level, the fan-delta system was developed at the early base level descending stage, the braided river delta system was mainly developed at the middle stage, and the low -tortuous river and alluvial plain system covered the whole area at the late stage. The results obtained by the authors are of active significance in analyzing the oil-gas distribution and controlling factors and guiding oil-gas exploration arrangement in this area.