歧口凹陷沙河街组海陆沉积差异及其成藏意义

立足"大歧口"凹陷,基于海、陆资料,在统一地层格架的基础上对沙河街组的沉积特征进行了整体研究,并对比了海、陆砂体发育的差异性,探讨了砂体发育差异性对海、陆油气成藏的影响。研究认为,歧口凹陷沙三段富砂沉积体系类型以扇三角洲为主,其次为辫状三角洲;沙二段以辫状三角洲为主,其次为扇三角洲和远岸水下扇。西侧中石油矿区主要受北、西两大盆外物源的影响,沙河街组砂体延伸远、规模大;东侧中海油矿区主要受南、东2个较小盆内物源影响,砂体延伸较近,规模较小。沙河街组砂体发育的差异性使得歧口主凹向海、陆两侧的汇烃比例约为1:2,这也是海、陆目前油气发现悬殊的主要原因之一,但海域仍然有较大的勘探潜力,沙西南裙边带沙河街组是中海油下一步在歧口凹陷勘探的有利方向。

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.