GENERATION AND ACCUMULATION OF HYDROCARBONS IN A DEEP “BURIED HILL” STRUCTURE IN THE BAXIAN DEPRESSION,BOHAI BAY BASIN,EASTERN CHINA

Significant volumes of hydrocarbons have been produced from karstified Infracambrian dolomites in a “buried hill” structure at depths of 5860m to 6027m and reservoir temperatures of 190–201°C in well Niudong‐1 in the Baxian depression, Bohai Bay Basin. This is the deepest oil and gas discovery made in eastern China so far and structures at similar depths are targets for exploration elsewhere in the Bohai Bay Basin. However the origin and accumulation of the hydrocarbons at Niudong‐1 is not clear: they may have been generated from highly mature lacustrine source rocks in the Sha‐4 Member of the Eocene Shahejie Formation; or they may have been derived from thermal cracking of previously‐accumulated oil. This paper investigates the organic geochemistry of the Sha‐4 Member source rocks and the crude oils produced from well Niudong‐1. Analyses of molecular parameters show that the hydrocarbons originated from the pyrolysis of organic matter in Sha‐4 Member source rocks, rather than from cracking of previously accumulated oil. Infracambrian dolomites at the Niudong‐1 location may have been charged with low‐maturity oil around 34 Ma ago, when the Sha‐4 Member source rocks were buried to depths of about 3500m and first entered the oil window. During further rapid burial to more than 5500m starting at about 15Ma, these source rocks became highly mature and generated significant volumes of light oil and gas. Overpressures in the source rock interval forced these hydrocarbons to migrate into unconformably‐underlying Infracambrian dolomite reservoir rocks at the Niudong‐1 structure. Significant risks are associated with future exploration of deep “buried hill” structures in the Bohai Bay Basin. Not all the structures were charged with oil, and accumulations were not necessarily preserved during Neogene burial as the reservoirs may have been breached by faulting.