Forming Condition and Enrichment Mechanism of the Nanpu Oilfield in the Bohai Bay Basin,China

Significant progress in oil-gas exploitation has been made in the Nanpu depression of the Bohai Bay Basin,China. Discovery of the Nanpu oil field has made the Nanpu depression one of the most oil-rich depressions in the basin. Voluminous sampling and analysis display that the Shahejie Fm (Es) of Paleogene,especially the third member of Shahejie Formation (Es3x) has developed excellent source rocks with TOC up to 5%,a thickness of effective source beds about 250m and vitrinite reflectance (Ro) of above 0.8%. The amorphous body and phycoplast is the important source of organic material in source beds and of great potential for hydrocarbon generation. The studies suggest that ① the Nanpu depression is oil-rich depression and has good conditions for hydrocarbon accumulation; ② the development of premium oil source rocks in the Shahejie Formation (Es) is the material base for hydrocarbon accumulation; ③ Neotectonic movement led to the development of faultage in main depression,in favor of conduction effectively; ④ shallow fan dalta and fluvial facies depositional system,thickly and wide distributed sand layer and good physical property is beneficial to large-area accumulation; ⑤ one suite of thick and widely distributed basalt,developed over the premium sandstone reservoir of Guantao Formation,constitutes high-quality caprock. All the features mentioned above are important factors for rich accumulation of oil and gas in the Nanpu depression.