Abstract:

Sedimentary characteristics and depositional model of Dongying Formation in depressed belt of eastern Nanpu Depression were studied based on core, grain size and logging data. The gravity flow sediments, characterized by richness of sandstone lithofacies and lack of conglomerate ones, always interbedded with gray or deepgray mudstone. According to beds number, normal graded bedding sandstone (S3), siltstone (S4) and massive bedding sandstone (S2) were the most developed ones, and the massive bedding pebbled sandstone (S1) took the second place. However, S2 takes the first place in thickness, S1 and S3 took the second one. Considering supporting and depositional mechanism, sediment gravity flows in studied area may be divided into four types, i.e., turbidity flow, sandy debris current, grain current and liquefied current, and specially, sandy debris current was supported by matrix and deposited by frozen massive mode. Every gravity flow developed differently on large scale. According to sandstone beds number, turbidity flow took the first place and sandy debris flow the second one. Considering deposition thickness of single gravity flow, sandy debris flow took the first one with an average of 1.17 m, and the turbidity flow deposit took the last one with an average of 0.25 m which showing turbidity flow usually was very thin. In order to reduce the limit of cores and ignore the effect of different flow types on sandstone size, which decided their importance for hydrocarbon exploration, logging data were used in lithointerpretation. Interpretation results showed that the gravity flows sediments included fine sandstone and siltstone with an average thickness of 2.94 m and a maximum thickness of 9.5 m for a single bed. Among them, there were 22 single sandstone beds with over 3 m thickness, which had a total thickness of 107.5 m. Basically, the sediment gravity flows were sprung by sedimentsslump on the delta front slope in studied area. During the processes, earthquake movements and intermittent volcano eruptions, besides natural sediments slump when the angle of delta front slope overrun the critical slope angel, brought the key dynamical mechanisms. Besides, faulting played an important role not only in providing accumulation space but also in deciding the depositional architecture.

Key words: gravity flow, sandy debris current, turbidity flow, sedimentary mechanism, Nanpu Depression