Depositional Models and Petroleum Geological Significance of Gravity Flows Deposits of the Third Member of the Shahejie Formation in Bozhong 25-1 Oil Field, Bohai Sea Area

Key words: 
• Bohai sea area /
• gravity flows /
• debris flow /
• turbidity current /
• depositional models /
• petroleum geological significance

Abstract: By implication of the latest gravity flow theory, the gravity flow deposits of the second member of Shahejie Formation in BZ 25-1 oil field have been systematically studied for their sedimentary characteristics, types, lithofacies combinations, spatial distribution and depositional models based on corring and seismic data. Three types of gravity flow deposits are recognized as slump, sandy debris flow and turbidity current, which constitute five kinds of lithofacies combinations as the followings:1) lacustrine fine-grained sediments and slump; 2) slump and sandy debris flow; 3) debris flow and turbidity current; 4) turbidity current and debris flow; and 5) lacustrine fine-grained sediments and turbidity current. The process and depositional models of "slump-sandy debris flow-turbidity current" collectively controlled by source, slope, fault activities and fluid characteristics has been established in this paper, and the debris flows formed tongue-like sediments body while the turbidity current result in fan-shaped patterns. Synthetical evaluation of reservoir properties indicates that the debris flow sediments is excellent gravity flow reservoirs normally characterized by coarse grains, low width-to-thickness ratio, low mud content and good reservoir quality. It is of great guiding significance to high quality reservoir prediction of deep water environment by detailed studies on sedimentary characteristics, spatial distribution of sand and reservoir properties.