洞庭盆地沅江凹陷油气成藏条件分析

AN ANALYSIS OF HYDROCARBON POOLING CONDITIONS IN YUANJIANG SAG,THE DONGTING BASIN

洞庭盆地沅江凹陷古近系总体表现为一个北东向展布的东南断裂、西北超覆的断陷—断坳型准箕状凹陷。中、新生界主要为陆相洪积扇、洪泛平原、滨湖—浅湖—半深湖沉积的碎屑岩及少量蒸发岩。凹陷内有利烃源层为层序Ⅰ的湖侵体系域古近系沅江组暗色泥岩;白垩系—古近系陆相碎屑岩和沅江组碳酸盐岩是主要的储集岩;沅江组—新河口组大套泥质岩系是良好的区域盖层;在有利的油气聚集成藏区内发现了一批近油源的、构造较落实的圈闭。凹陷内具备一定的输导条件,发生过一定规模的油气生成、运移和聚集成藏过程,其中沅江组自生自储型裂缝型油气藏和上白垩统分水岭组—始新统汉寿组侧储型油气藏具有较好的油气勘探前景。

The Paleogene in Yuanjiang sag of the Dongting basin occurs as a NE-trending quasi-dustpan-shaped sag being faulted in the southeast and onlapping in the northwest.The Meso-Cenozoic is mainly composed of clastic rocks of continental alluvial fan,flood plain,shore-shallow lake-semi-deep lake facies with small amount of evaporate.The Paleogene Yuanjiang Formation dark mudstones of the lake transgressive system tract in Sequence Ⅰ are favorable source rocks,the Cretaceous-Paleogene continental clastic rocks and the Yuanjiang Formation carbonates are major reservoir rocks,and the thick Yuanjiang-Xinhekou Formation argillaceous rocks are good regional seals.Several traps being near the hydrocarbon kitchen and relatively ascertained in structures are recognized in the favorable hydrocarbon accumulation zones.The sag has favorable conditions of migration and has experienced relatively large scale hydrocarbon generation,migration and accumulation.The Yuanjiang Formation and the Upper Cretaceous Fenshuiling Formation-Eocene Hanshou Formation are prospective in petroleum exploration.