Jiang, Ye¹, Qiang Xu¹ 
(1) Peking University & CNOOC Research Center, Beijing, China

ABSTRACT: Charge Factors in Gas Migration Passway in the Yinggehai Basin, South China Sea

Yinggehai basin is an elongate Cenozoic strike-slip extension basin on the northwestern margin of the South China Sea continental shelf. On the background of high geothermal gradient, high overpressure, quick and thick basin infilling, the natural gas migration passway is supposed to not only be controlled by carrier rock property, caprock, fault, microfracture, unconformity surface, but also by hot fluid episodic activity. 
Since the Yinggehai basin fill is mostly composed of muddy and fine sediments, convergence and spreading of the gas migration passway is controlled by the property variety in carrier rock, due to different diagenetic evolution, sedimentary facies and hot fluid activity. The advantageous route is also controlled by sealing-surface. The viewpoint about the sealing surface is proposed by many geologists and is testified by seismic time structure map in the Yinggehai basin. 
Several unconformity surfaces including S40, the largest scale one by regression, are puzzling because only a few gas accumulations were discovered in the lowstand system tract sands. The lateral gas migration passway is somewhat due to the relationship between the lowstand tract sandstone area and mudstone one on the unconformity surfaces. 
Vertical gas migration usually happened in the faults of diapir. However, because of the activities of the hot fluid, between diapiric structures and strike-slip boundary fault, the vertical gas migration still occurred via the microfractures which can not be clearly detected and traced on the seismic profiles but can be found in core and thin section analysis.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.