GENESIS, ACCUMULATION AND DISTRIBUTION OF CO₂ IN THE YINGGEHAI-QIONGDONGNAN BASINS,

Yinggehai-Qiongdongnan Basins are the major gas producers in the north of South China Sea.However, high content of CO₂ in some areas restricts the economic efficiency of natural gas exploration.Genesis, accumulation process and distribution pattern of CO₂ in natural gas are studied in this paper based on the geochemical characteristics of the natural gas as well as regional geological settings.CO₂ in the natural gases in the Yinggehai-Qiongdongnan Basins changes in a broad range in content and has quite complicated origin.There are three types of natural gas accumulation mechanisms controlling the distribution pattern of CO₂ in different areas.In the Yinggehai Basin, high content of CO₂ in natural gas is controlled by diapir structure and overpressure.In the Qiongdongnan Basin, however, high content of CO₂ is related generally to deep fractures and volcanic activities.Based on the distribution pattern of CO₂, favorable fields are proposed for exploration of high quality natural gas, such as the middle and deep layers in diapir-affected zones with early gas accumulation, abundant micro-fractures and good sealing conditions in the Yinggehai Basin, and the areas with good migration and sealing conditions but lack of volcanic activities and deep fractures in the Qiongdongnan Basin.