The Relationships Between Neogene Shallow Water Deltaic System and Hydrocarbon Accumulations in Bohai Bay, China

Weilin Zhu, Jianping Li, and Xinhuai Zhou
CNOOC, Tianjing, China

Bohai basin is characterized by tectonically-uniform block with steady and slow subsidence, which resulted in low-relief, low gradient, and high-stand lake-level depositional settings during the Neogene. Shallow water delta sandstone is mainly made of distributary channel sandstone, which showed little influence of wave. Excellent reservoir-seal combinations and superb conditions for the formation of hydrocarbon traps in the region. Cyclic depositions of shallow water deltaic facies which influenced by fluvial systems with sand bodies having observable orientations. Delta front sheet sands are well developed, which showed obvious lobe shape on sand isomaps and very good lateral connectivity between lobes. Broadly developed, vertically stacked cyclic thick shale resulted in multiple sets of reservoir-seal combinations. Shallow water deltaic facies are positioned near the prolific hydrocarbon-enriched depressions. The coincidence of neotectonism since Neogene and the peak hydrocarbon generations in the depression has created the excellent basis for the formation of medium-to-large scale fields in the high quality shallow-water deltaic facies offshore. Compared with other Neogene reservoir types, especially fluvial facies, these hydrocarbon accumulations have characteristics of larger scale, simpler structure, better oil, and higher O/G ratio. Under certain conditions, condensate and large gas accumulations could form. Currently, seven accumulations in the Neogene shallow water deltaic reservoirs have been found in northern edge of Huanghekou depression and southern edge of Bozhong depression. The OOIP of these fields surpasses 1 billion m3. Compared with Zhanhua depression of Shengli field, there is still huge potential for the Neogene shallow water deltaic reservoirs in Bohai Bay offshore.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas