Gas source of BZ19-6 condensate gas field in Bozhong Sag, Bohai Sea area

Abstract

The discovery of BZ19-6 condensate gas field in the Bozhong Sag of Bohai Sea area brought hope and confidence for giant gas fields. Intensive study of the source and genesis of natural gas in BZ19-6 gas field plays an important role for guiding natural gas exploration in the Bohai Sea. Open and sealed system pyrolysis as well as geochemical and numerical simulation show that the high-quality sapropelic lacustrine source rocks with an average TOC content of 3.93% and an IH index of 727 mg/g in the Bohai Sea area mainly generate oil, and only 10% of kerogens in the source rocks crack into gas. For humic-prone source rocks, about 30% of kerogens generate into gas when IH reaches 100 mg/g. However, the humic-prone source rocks still expel gas which is cracked from residual hydrocarbon due to the adsorption of kerogens. The natural gas in BZ19-6 gas field are mainly sourced from Ⅱ1, Ⅱ2 and Ⅲ type source rocks in the third member of Shahejie Formation during mature to high-maturity stages. They are mixtures of kerogen cracked gas and residual hydrocarbon cracking gas sourced from parts of the southwestern Bozhong Sag. The source rocks of the third member of Shahejie Formation in the hydrocarbon supply area mainly discharge oil in the early stage, and changed to gas after 15 Ma, accounting for 82% of the total gas discharge. To find large gas fields in the Bozhong Sag, the targets where source rocks have large gas/oil ratio and mainly expel gas should be focused on.

Keywords

• pyrolysis simulation experiment
• gas-oil ratio of hydrocarbon expulsion
• hydrocarbon supply area
• bz19-6 condensate gas field
• bozhong sag
• bohai sea area