Abstract Deep-seated oil and gas have become the focus of hydrocarbon exploration at home and abroad. To further strengthen the research on the sedimentary characteristics and reservoir distribution are the key to hydrocarbon efficient exploration. Based on core observation, FMI imaging logging data, drilling and logging data, seismic data and trace element analysis, indicating that the paleoenvironment was characterized by strong evaporation and dry-hot environment. The water was relatively shallow, and the nearshore subaqueous fan and sublacustrine fan deposition developed during the deposition period of the lower section of the fourth member of Shahejie Formation (E2S42) in Yanjia area of the steep slope belt in Dongying Sag. Based on the analyses of lithology, grain size and bedding structure, 9 kinds of lithofacies are identified, which are further classified into 4 types of lithofacies combination and 18 kinds of combination characteristics. Research results proved that these lithofacies and combinations type are controlled by base level cycle changes, and developed well in the top-bottom interface of the short-term and super-short-term sedimentary cycle. In the sedimentary semi-cycle descending (A/S decreasing) stage, the lithofacies are mainly composed of medium-coarse glutenite facies, fine conglomerate facies, and pebbled sandstone facies supported by grains, which are characterized by the thick layer, coarse in size, and multistage fan bodies with cutting and superimposed on each other, showing the continuous distribution on the plane. While in the sedimentary semi-cycle (A/S increasing) rising stage, the medium-fine glutenite facies, medium-fine pebbled sandstone facies and argillaceous siltstone lithofacies, which are characterized by the interbedded development with mudstone, and dominated by the isolated or discreted distribution in space. Further analysis shows that the difference in provenance supply caused by paroxysmal floods and the morphological characteristics of ancient gullies(wide/depth ratio, paleoslope) are the main controlling factors to the variation of regional ultra-short-term base level cycle, and have important controlling effects on the vertical thickness and transverse distribution range of the fan sandbodies. Meanwhile, two types of sedimentary models are identified, named high energy flood progradation model and low energy/interval flood retrogradation model. In the process of high energy flood progradation, the fan body was mainly distributed in broom shape and transported at a long distance. And the average thickness of the single fan is about 8.5m, developed in the paleoslope of 30°~43° area. While, in the process of low energy/interval retrogradation, the fan body was distributed in shape of lobes, which is transported at a relatively short distance, and is prone to collapse and from sublacustrine fan at the front. The average thickness of the single fan is about 6.5m, and mostly developed in the region with a paleoslope of 22°~33°. It is of great significance to clarify the development characteristics and distribution of fan body under different sedimentary modes for regional high-quality reservoir distribution and diagenetic evolution difference analysis, and then for efficient oil and gas exploration.
Key words lithofacies, glutenite, base level cycle, Yanjia area, Dongying Sag
About the first author Mao Shu-wei, male, research assistant, is mainly engaged in research on sedimentology, reservoir geology and hydrocarbon reservoir characterization. E-mail: maow100@163.com.