Abstract:

The main reservoir type of Dongying Formation in the eastern slope of Chengdao low-uplift belongs to deep-water sediment. Based on core-logging and seismic data, we study the distribution and connection of sandbodies from the developmental background, sedimentation process and response characteristics of the reservoirs. It is revealed that the deep-water reservoir of Dongying Formation is composed of sandy debris flow, argillaceous debris flow, turbidity current, and other gravity flow deposits that were modified by traction underflow. In the various sequence development stages, slope/slump subaqueous fans were developed. During the low systems tract sedimentation, floods carried terrestrial debris from weathering, denudation and collapse on slopes, and formed debris flow with point source supply. The debris flow was accelerated along the slope, and migrated laterally in an inclined, wide, flat, and long-distance underwater trough. Meanwhile, pure sandstone was precipitated by lake-water dilution and sand-mud differentiation, forming braided channels of sandy debris flow. Sandbodies are superimposed in many layers and show a mosaic reservoir structure. During the high systems tract sedimentation, unstable prograding fan delta at sedimentary slope break may have collapsed, forming block flows with line source supply. The block flows seeped into lake water and moved bumpily. Channels dominated by sandy debris flow, turbidity tongues dominated by turbidity and turbidity sheet sand modified by traction underflow were developed by dissociation, gravity variation and density stratification of the block flow in lake water. Sheet sandbodies are isolated from mudstone, forming pie-like reservoir structure. The sandbodies of slope subaqueous fans are distributed regularly, and are large and continuous, whereas those of slump subaqueous fans are distributed randomly, non-fragmented, in large number and with varying thicknesses.

Key words: Chengdao low-uplift, sandy debris flow, turbidity current, slope/slump subaqueous fan, reservoir structure