Characteristics of Turbidity Flow Sedimentation and Effects on Reservoir Property in Early Tertiary of Dongying Sag, Bohaiwan Basin, East China

S. Zhang and Y. Ji
Department of Petroleum Resources Science, University of Petroleum, Shandong, P.R.C. ([email protected])

The turbidity flow sedimentation in early Tertiary of Dongying Sag is mainly distributed within member 3 of Shahejie formation (Es3). The overlying and underlying layers of each turbidity flow layer are annual laminations deposited in normal environment of active period, indicating that the turbidity flows were intermittent and uncontinuous, and should belong to spasmodic and choppy sedimentation.

The flows resulted from flash flood. Mighty torrents rushing down in valleys caused lakeshore landslide and forced the antecedent sediment into mass movement. Erosion accelerated and unconsolidated sediments were made active again. Consequently, slumps formed near the shore, slump structures built up on the lake slope, and turbidity flow sediments settled in the static environment of the lake center. The turbidity flow was triggered by flood current, but the former was not indispensable to the latter, that is to say, not every flood was accompanied by turbidity flow.

The largest of the found slumps is 10 m in length and 1.5 m in thickness and composed of lakeshore sandy conglomerate underlain by mudstone. At the contact with the underlying mudstone, i.e., at the sole of the slump, there is very clear load structure. This is because during mass movement, the slumping sandy conglomerate partly sank into the underlying water-filled, plastic mudstone due to sliding load and at the same time the mudstone was squeezed into the sandy gravel. Between the two, there was no fracture but only deformation, and cloud-like complex pockets occurred. The whole sole was then a flame-like contact plane. The discovery of such a slump has provided convincing evidence for the triggering mechanism of turbidity flows and the occurrence of the sedimentation.

The pore types of the reservoirs are mainly primary pores. The heterogeneity of pore structure is obvious. In different sedimentary microfacies, the pore structure characteristics and heterogeneity degree are also different.

AAPG Search and Discovery Article #90903©2001 AAPG Mid-Continent Meeting, Amarillo, Texas