Sedimentary Characteristics and Depositional Model of Delta and Gravity Flow System of the Middle Member 3 of Shahejie Formation in Niuzhuang Sag

Key words: 
• delta and gravity flow system /
• Niuzhuang sag /
• slide deposits /
• slump deposits /
• debris flow deposits /
• turbidites /
• controlling factors

Abstract: In order to unify the classification scheme of gravity flow in Dongying depression and further improve the depositional model of delta and gravity flow system in rifted lacustrine basin, this paper selected the delta and gravity flow sand bodies in the Mid-E₂s₃ of Niuzhuang sag as research object, then used seismic data, well logs and core observation to analyze the sedimentary characteristics, genetic mechanism, distributions and depositional model of deltaic and gravity flow sand bodies within the framework of sequence stratigraphy. The results show that the Mid-E₂s₃ of Niuzhuang sag can be divided into one T-R sequence, viz one lacustrine transgressive system tract and one regressive system tract. The T-R sequence can be further divided into 6 parasequence sets which correspond to 6 cycles of deltaic sand bodies. The gravity flow sand bodies were widely distributed in regressive system tract, viz parasequence set 5 to parasequence set 1 while the distribution was limited in transgressive system tract. Four types of gravity flow were recognized:slide, slump, debris flow and turbidity current. Slide deposits showed a zonal distribution near delta front sand bodies. Slump and debris flow deposits distributed farther on the slope or near the slope point. Sheet-like turbidites were distributed in the deep part of the sag. Deltaic progradation rate was up to 200 m/10⁴ a. During the E₂s₃ period, the fault activity rate was significantly enhanced. The structural slope break zone developed along the direction of provenance. Sporopollen assemblage was dominated by subtropical plants. The value of δ¹³C_PDB and δ¹⁸O_PDB showed a pronounced negative excursion. All these phenomena reveal that, the high speed deltaic progradation rate, structural slope break zone and fracture movement, warm and humid paleoclimate condition controlled the generation and distribution of delta and gravity flow sand bodies in the study area during the Mid-E₂s₃ period.