DEPOSITIONAL DIFFERENCE OF THE SHAHEJIE FORMATION ONLAND AND OFFSHORE AND ITS BEARING ON OIL AND GAS ACCUMULATION IN THE QIKOU SAG, BOHAI BAY BASIN

Paleogene depositional systems are studied in a unified sequence stratigraphic framework in the Qikou Sag of the Bohai Bay Basin upon the concept of "Great Qikou Sag". Integrated research and comparison are made for both the Shahejie Formation onland and offshore, with emphasis on the difference of the sand bodies and their effects on oil and gas accumulation. The sand rich depositional systems in the Member Sha-3 are dominated by the fan delta followed by the braided river delta, while those in the Member Sha-2 are dominated by the braided delta, followed by the fan delta and subaqueous fan delta. The west part of the CNPC licensed area is mainly affected by large provenances in the north and west where the sand bodies cover a large area, and the east part of CNOOC licensed area is mainly controlled by the small internal provenances in the south and east, where sand bodies only cover rather small areas. The differences in the occurrence of sand bodies has made the h0ydrocarbon accumulation in the offshore and land areas in a ratio of about 1:2, that is one of the key factors controlling the big difference in the exploration results between the sea and the land. However, the offshore area of the Qikou Sag still bears considerable exploration potential, and the Shahejie Formation of the south-western Shaleitian skirt area is a favorable exploration target for CNOOC in the Qikou Sag.