oa The Discovery and Development of Oil Rim Fields in the Beibu Gulf, China

The Beibu Gulf is a prolific hydrocarbon province on the western coastline of China. The Miocene Jiaowei Formation contains thick sandstone sequences with excellent reservoir quality, a low degree of internal heterogeneity and excellent aquifer support. Structures are low relief causing many discoveries to be classified as thin oil rim fields (<15m oil columns with bottom-water drive). These discoveries have moderate to heavy oils with low gas-oil ratios and moderate to high oil viscosities. The combination of these rock and fluid properties are ideal conditions for rapid water coning, hence early well water breakthrough and low oil recovery factors. However, multiple fields in the Beibu Gulf significantly exceed pre-development production expectations. Closer inspection of core and log data indicates there is often a dolomitic alteration zone at the oil-water contacts with permeabilities typically 2 to 3 order of magnitudes lower than the hydrocarbon-bearing reservoir which act as effective aquitards to slow the onset of water breakthrough. The diagenesis is theorized to be due to microbial decomposition of hydrocarbons at the oil-water interface which accrete dolomitic cements as a by-product. Seismic inversion and amplitude mapping reinforce the view that the alteration zones are pervasive and flat-lying. Case studies are presented covering the discovery, development and production performance of three oil rim fields in the Beibu Gulf.