The Study Of High-resolution Sequence Stratigraphy And Sedimentary System In Upper Es4 Of Boxing Sag In Dongying Depression

Boxing sag is located in the southwest of Dongying Depression, Bohai Bay Basin. Extremely rich in oil and gas, Boxing sag is an important exploration area of Shengli oil-field. Based on the theory of high-resolution sequence stratigraphy, this paper will use as much information of well logs, seismic and cores data as possible to do some research on the high-resolution sequence stratigraphy and sedimentary faces of the upper E2s4, which is helpful in predicting potential reservoirs.Integrated study shows the E2s4 formation could be divided into 2 3rd order sequences, the upper E2s4 and the lower E2s4. With the initial flood surface and max flood surface, the upper E2s4 could be decomposed into 3 system tracts including low-standard systems tract (LST), lake-expanded systems tract (EST) and high-standard systems tract (HST). In the light of secondary flood surfaces, 5 4th order sequences could be found in the system tracts. The LST is formed by the upper 3 4th order sequences, while HST and EST by 1 4th order sequence each respectively.Based on abundant well logs analysis and core observations of typical drilled wells, the study identify such sedimentary faces as beach bar, delta, fan delta, and distal turbidity current channel in the upper E2s4 formation of Boxing sag. Since the development of syn-sedimentary faults, provenance, base level changes and paleoclimate in different positions of Boxing sag varied over the process of basin evolution, the types and distribution of sedimentary faces in the system tracts and 4th order sequences of upper E2s4 formation differentiate. During LST period, it developed fan delta and beach bars at the steep slope in the north and west of Boxing sag. In the south, delta sedimentary systems occurred at the gentle slope and transformed into beach bar at the Boxing fault belt under the reconstruction of waves and lakeshore currents. When it came to EST period, it inherited the sedimentary framework of LST more or less, but the water level rise, leading to diminishing of beach bar areas. In the HST period, lake level dropped and resulted in the shrinking of deep lacustrine fields. Coarse fragment systems pushed forward into the lake. Meanwhile, there were large scales of sand beach bars.The development and distribution of sequence-tract systems and sedimentary systems are closely related to syn-sedimentary faults. In the north and west of Boxing sag, fan delta front systems developed near the steep slope, mainly controlled by Gaoqing-Pingnan fault. At the same time, the fan delta systems in the west were affected by the west branch of Boxing fault and did not extend so far lakeward. At the southern gentle slope, it developed delta systems, which was related to the provenance of southern Luxi uplift. At the central of Boxing sag, since the movement of the Boxing fault belt of east-west strike was not so intense, the beach bar, in the higher part of Boxing fault belt developed in the shape of crescent along the coast.Given the sedimentary patterns in the sequence-tract systems and 4th order sequence of upper E2s4 formation of Boxing sag, the paper predicts four potential reservoir faces including the middle fault-step belt, southwest steep slope, south gentle slope and the middle turbidity channel, which provide a significant guidance in the exploration and development of reservoir of upper E2s4 formation, especially that of the lithologic and stratigraphic hydrocarbon reservoir.