OIL TYPE AND GENESIS ANALYSIS OF CRUDE OIL IN THE DAWANGBEI SUBSAG OF THE CHEZHEN SAG, THE BOHAI BAY BASIN

Crude oil from the Dawangbei Subsag of the Chezhen Sag of the Bohai Bay Basin can be divided into 2 types,i.e.the medium-low density,low viscosity,low sulfur content oil,and the medium-high density,medium-high viscosity,high sulfur content oil,through study of physical properties,sulfur content and oil group-type.Carbon isotopes and biomarkers are used to study the genesis and accumulation of oil,and the result shows that type Ⅰ origins from fresh-weak salt water,medium-deep water genesis mature source rock of the 3rd segment of Shahejie Formation,which is well preserved and has not experienced distinct secondary change.Type Ⅱ is mixed origin oil of at least 2 origins and is preserved under different conditions.Low maturity and high sulfur content oil from the 4th segment of Shahejie Formation of salt lacustrine-salt water lacustrine environment provides the 1st origin.After accumulation,oil has experienced biodegradation since it is buried low.Then oil from mature source rock of the 3rd segment of Shahejie Formation mixes in.Low-mature oil from the 1st segment of Shahejie Formation also contributes to type Ⅱ.Hence composition and distribution of oil type Ⅱ are complicated.