TURBIDITY FAN IN THE OIL AND GAS-BEARING BASIN IN BOHAI BAY

Since the end of Mesozoic, Bohai Bay region has been influenced by the diving of Pacific Plate from east to west and acted by the stretching forces so as to form many fault-depressions, in which more turbidity fans are developed. The developing stage of turbidity fans is concordant with that of the strong -sinking of lake basin. In the study region the turbidity fans distribute mainly in the third member of Shahejie Formation. The chief depositional features of the turbidity fans are as follows. 1. The turbidity fans mainly consist of conglomerate, sandy-conglomerate, sandstone, siltstone and interbedded frequently with the dark mudstone. They are predominately unsorted admixtures of sand and pebbles in a muddy matrix, namely greywacke, feldspathic litharenite or litharenite. They are characterized by higher rock fragment contents, ranging from 30% to 50%. 2. The cumulative probability curves of deposits in turbidity fans show a population ( a gentle curve ) without obvious breaker, illustrating that the turbidity is dominantly graded suspension, C-M pattern of the sediments is a rectangle paralle 1 to the basic line(M=C), value C(l%)being 100μ to 800μ, value M(50%)40-800μ. 3.The primary sedimentary structures and their vertical assemblages illustrate that there are two kinds of vertical sequences, that is,classical turbidite sequence (with Bouma sequence ) and unclassical turbidite sequence ( without Bouma sepu-ence). 4. Flute cast, groove cast and load cast are common in the turbidite bottom-side in this area, and sometimes the flame structure can be seen. This paper also deals with the difference between the two kinds of tuibidity fans-flood turbidity fan and slump turbidity fan. Finally this paper discusses the oil-bearing ability in the turbidity fan.