Geochemical characteristics of the Zhaolanzhuang sour gas accumulation and thermochemical sulfate reduction in the Jixian Sag of Bohai Bay Basin

Abstract

Natural gas from the Zhaolanzhuang field of the Jizhong Superdepression, Bohai Bay Basin contains the highest proportions of H 2S (40–92%) among the sour gases encountered in China. The gas payzones include the Eocene–Oligocene Kongdian Formation (Ek) and the Es4 member of the Shahejie Formation. The sedimentary sequence consists of halite, anhydrite, carbonate, sandstone and shale interbeds deposited in the evaporative brackish water lacustrine – salt lake setting. In the deepest part of the Jinxian sag, the total thickness of evaporites is more than 1000 m, of which halite accounts for over 40%. Various organic-rich mudstones intercalated with the evaporites are currently within the conventional hydrocarbon window (with a depth of 2500–3500 m), and likely the source for the oil and sour gas in the Zhaolanzhuang field. The temperatures of the gas reservoirs range from 75 to 100 °C, too low for significant thermochemical sulfate reduction. The co-occurrence of abundant elemental sulfur with the sour gas and the δ 34S values of the various sulfur-containing compounds indicate that the H 2S gases were most likely derived from much deeper source kitchens where significant thermochemical sulfate reduction has occurred.