The Formation And Distribution Prediction Of The Beach-bar Inter-bedded Sand Body In The Key Blocks Of Boxing Sag

In this paper, one part of key blocks of Boxing Sag in Dongying Depression is the main research area and the upper Es4 Formation of Paleocene is taken as the target interval. Based on the stratigraphic features and tectonic setting of the study area, with the guide of theories of sequence stratigraphy, sedimentology, logging geology and basin modeling and the use of drilling core, well logging, seismic and test data, the single well and cross section sequences are divided and the regional sequence framework is established. Then, the sedimentary characteristics of beach-bar are analyzed within the sequence stratigraphic framework which mainly focus on the vertical evolution and plane distribution characteristics of different types of sandbodies in the main beach-bar layers. Integrated the above analysis, the main parameters controlling the development of sand-bodies are quantitatively recovered to predict the sand-bodies’ distribution. Main research results include the following aspects.The first part is sequence stratigraphy study. According to sequence stratigraphy principles and related methods, the upper Es4 Formation in research area is divided into one third-order sequence consisting of three systems tracts, from bottom to top are successively lowstand systems tract, transgressive systems tract and high system tract. LST is divided into three parasequence sets and nine parasequences, the stacking pattern of which is progradational. TST is divided into two parasequence sets, the stacking pattern of which is retrogradational. HST is divided into two parasequence sets, the stacking pattern of which is progradational or aggradational. Sequence stratigraphic framework is established on the basis of division scheme.The second part is sedimentary characteristic study. The main sedimentary environment is lacustrine, and the characteristics of beach bar are obvious in research area. Gravelly beach bar is developed along the coastal zones. And inside the basin, sandy beach bar is mainly developed under obvious influence of waves. The sand zonation is evident from lake shore to interior. The well core, logging and seismic characteristics of different types of beach bar sandbodies are summarized according to comprehensive comparison of core, well logging and seismic data.During the LST deposition period, base level falls and provenance supply is abundant, there are lots of beach-bar sandbodies developed inside the basin. During the TST deposition period, base level rises and provenance supply is poor, there are little beach-bar sandbodies developed inside the basin. During the HST deposition period, base level changes from slow rise to decline and provenance supply is relatively abundant, there are some beach-bar sandbodies distributed inside the basin.The third part is quantitative analysis of main controlling parameters. The formation of large-scale beach bar needs adequate provenance supply, suitable palaeo-bathymetry and hydrodynamic conditions. At the same time, palaeo-geomorphology influences the sand’s distribution in the lacustrine and the wind power also controls its formation and distribution. On the basis of sequence division and sedimentary characteristic analysis, this paper uses different quantitative methods to recover every controlling parameter. During depositional period, the Luxi Uplift was the major provenance. The palaeo-bathymetry is calculated to be 2~9 meters combining ripple and bar thickness methods, so the research area’s water environment is relatively shallow reflecting a strong hydrodynamic. In terms of palaeo-geomorphology, strata incline from south to north and overall slope is gentle. North is the secondary sag which is the main depositional area. According to the calculations of wind, the power during LST depositional period was 5~7 level, and this moderate wind force is an ideal condition of forming beach bar inter-bedded sandbodies.The fourth part is prediction of beach bar’s distribution. Integrated all major controlling parameters, the predictions of sand body’s distribution extent and maximum thickness in research area are achieved.