COGNITION ON THE SEDIMENTARY ENVIRONMENT OF DA’ANZHAI MEMBER, NORTH SICHUAN, AND ITS SIGNIFICANCE IN OIL EXPLORATION

The lacustrine Da'anzhai member of Northern Sichuan can be subdivided into two different sedimentary facies areas, To the north of Yonghe (Jiulong shan)-Fuyangba, there is the Baitianba facies area; and to the south of it, there is the Ziliujing facies area. The later, from the bottom tothe top, can be divided into three different sedimentary layers. The lower layer is of transgression sediments. It is mainly a suite of interbedding shell limestones and shales with equal thickness. The middle layer was deposited when the lake area reached its climax. It is mainly a suite of black shales intercalated with more or less argillaceous shell limestones. The upper layer is of regression sediments. It is mainly a suite of shell limestones intercalated with argillacous shales. The lacustrine Da'anzhai member of northern Sichuan consists of from north to south a series of beach, shallow water lacustrine, moderately-deep water lacustrine, deep water lacustrine sediments. The first surrounds the second, the second surrounds the third and etc., forming half ring-shaped belts or ring-shaped belts. The inner zone of the shallow water zone, which was adjacent to the moderately-deep water-deep water zones, always emerged above the wave base and received a shell deposit of high energy beach. The shell beach is a good reservoir for oil and gas, and lithological and/or structural-stratigraphic oil and gas pools were formed there. Therefore the objective for oil search in Da'anzhai member of northern Sichuan, should be a good geologic body with proper porosity and permeability in the inner shallow water zone around the moderately-deep water and deep water zones (oil-generating depression). This means the shallow water shell beach deserves to lay emphasis.