Niudong 1 ultra-deep and ultra-high temperature subtle buried hill field in Bohai Bay Basin: Discovery and significance

Niudong 1 Well, which recorded a prolific oil flow of 642.9 m 3/d and gas flow of 56.3×10 4m 3/d, discovered a buried hill field in the Baxian Depression, Jizhong Sub-basin, Bohai Bay Basin. The reservoir of this field is the deepest and its temperature is the highest in the Bohai Bay Basin or even in East China. The well has a total depth of 6027 m at which the reservoir is saturated with oil and oil-water contact is not found. Its bottom hole temperature is up to 201°C. The exploration of this field has experienced 4 stages: Initial failure, suspension of renewed exploration, regained hope and great breakthrough, which last over 30 years. High resolution repeated 3D seismic prospecting and hydro-fracturing with high temperature-resistant acid played vital roles in finding the buried hill trap and obtaining prolific oil and gas flows. The hydrocarbon richness and prolific production rate of this field are attributed to the following three main factors. (1) Thick excellent highly matured source rocks are developed in the deeply buried from Sha 4 Member to Kongdian Formation so that abundant hydrocarbons could be generated. (2) Reservoirs of the buried hill field possess good petrophysical properties, which are generally not affected by the burial depth. (3) The buried hill trap was formed early and characterized by early charging, sustained charging and overpressured charging. The good matching of timing of hydrocarbon charging and trap formation favored the hydrocarbon accumulation and entrapment. The discovery of Niudong 1 Well is of milestone significance in promoting exploration of ultra-deep buried hill fields in the Bohai Bay Basin.