Abstract:

During the past10years，a number of oil/gas fields of various scale and oil/gasˉbearing structures have been found in the Neogene of the Bohai Bay.The initial oil in place is as high as2billion cubic meters.Most of them are heavy oil fields.A detailed research of these fields gives the following conclusions.（1）The distribution of these heavy oil reservoirs is in apparent negative correlation with their burial depth.Their lower lim it ofoccurrence is at about2000m.（2）The crude samples from these reservoirs are high in density，viscosiˉty（inˉsitu oil viscositywithin70-500mPa•s）and bitumenˉcolloid content（a key factor controlling viscosiˉty），but low in freezing point and gas oil ratio.（3）Oilwasmainly generated by the sources rocks of the Palaeoˉgene Shahejie Formation in the hydrocarbon kitchen.In some areas，source rocks in the third member of the Palaeogene Dongying Formation also contributed some oil to the reservoirs.The generated oilm igrated upwards to and accumulated in the reservoirs above the source rocks.These reservoirs feature in active faulting，late pooˉling，multipleˉstage charging，and m icrobiological degradation.（4）Crude density and viscosity are relatively low at the top of these reservoirs and near them igration paths，a phenomenon closely related to late oil charging and oxidation ofwater bodies at the bottom of the reservoirs.

Key words: crude colloid, faulting activity, m icrobiological degradation, Neogene, heavy oil, Bohai Bay