ABSTRACT: Formation and occurrence of hydrocarbons and non-hydrocarbon gases in the Yinggehai and Qiongdongnan Basins/South China Sea

Schoell, Martin¹, Y. Tang¹, R. J. Hwang¹, D. K. Baskin¹, N. B. Schoellkopf², and L. M. Cathles³
(1) Chevron Research and Technology Company, San Ramon, CA
(2) Chevron Overseas Petroleum Inc, San Ramon, CA
(3) Cornell University, Ithaca, NY

The Yinggehai Basin (YB) in the South China Sea has today up to 17 km mostly overpressured Tertiary sediments with temperatures >300oC below 6.5km. Giant mud diapirs in the center of the YB attest to the dynamic processes in the basin. The basin evolved from an Eocene rifting event with the highest b values and highest heatflow in the center of the basin. Today's thermal regime can be best explained by a variable heatflow model (highest in the center of the basin, lower towards the Hainan Island). The Yinggehai petroleum system (YPS) is gas-dominated and can be differentiated from the gas-condensate producing Qiongdongnan Petroleum System (QPS). Gas-condensate-source correlations suggest that the source rocks in the YPS are likely of Miocene age and those of the QPS are of pre-Miocene age. Migration of hydrocarbons and non-hydro-carbon gases in the YPS is primarily vertical. Different gas charges to the reservoirs can be documented by compositional and isotopic variations in the gases in gas fields. Specifically, the Yacheng gas field has been charged from the YPS and QPS petroleum systems. Computer models suggest that CO2 from inorganic high temperature mineral reactions is generated in the deepest parts of the YB predominantly in the last 2my. These deep CO2 kitchens evolve into vertical degassing features that resemble the mud volcanoes in the YB. A high CO2-low methane gas that invades many of the reservoirs as a last migration event is likely related to this last episode in the evolution of the YB.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia