Sedimentary characteristics and meticulous depiction of Palaeogene lacustrine gravity flow deposits in BZ25 area, Bohai Sea, China

Abstract

Sandstone thickness and reservoir quality in the third member of Paleogene Shahejie formation (E2s3) of different wells in BZ25 oil field are significantly variable. The uncertain cognition of high quality reservoir distribution restricts the development progress. By inspiration and application of latest theory progress of gravity flow, we carried out studies on lithological characteristic, lithofacies combination, paleo-geomorphology, sand period and evolutionary model of E2s3 gravity flow in BZ25 area. We identified three facies types of gravity flows in E2s3 of BZ25 area including slump, sandy debris flow and turbidity current. The above three types of gravity flows formed five typical lithofacies combinations in BZ25 area. Based on research of paleo-geomorphology and lateral facies distribution, we conclude that the process of gravity flow deposition from slope to deep-water environment in BZ25 area can be recovered as “slump - sandy debris flow - turbidity current”, and the sandy debris flow body distributes as special tongue-like shape with two branches, while the turbidity current body is characterized by fan-shaped. The sand-rich sandy debris flow in BZ25 area can be divided into two periods. The east branch of second period of the sandy debris flow is less exploited, and its central body has great development potential thickness.