Gao No.81 Block in Gaosheng Oilfield of Liaohe basin is a monocline structure plunging towards east

and its confined by faults in three directions of northwest

southwest and southeast. The oil bearing strata is upper sub-member of Es

4

which is mainly composed of fan-delta sediments

and its lower part(Dujiatai oil layer) is fan-delta front subfacies

consisting of underwater distributary watercourse

river mouth sandbar and interfluve microfacies.Whereas its upper part is chiefly made up of profan-delta subfacies which is interbedded by underwater distributary watercourse and river mouth sandbar of fan-delta front subfacies occasionally. Dujiatai oil layer can be divided into three sand groups

located at the upper part of the layer

No.1sand group has great change laterally

whereas No. 2 and No.3 group(below No.1) which are quite stable. The physical property of reservoir of No.3 sand group is believed to be the best

and with the greatest effective thickness of oil bearing on the average and oil bearing area is relatively large too. Therefore

No.3 sand group has made greatest contributions to reserving oil and gas in Gao No.1 Block. No.2 sand group

with small effective thickness of oil bearing on the average

lowest oil-water interface and largest oil-bearing area

has made less contributions to reserving oil and gas on the average than No.3 sand group. No.1. group tends to pinch out towards the higher part of the structure and has made a little contribution to reserving oil and gas due to its highest oil-water interface and smallest oil-bearing area.