Xiaorong Luo¹, Wan Yang², Jihai Yang³, Weiliang Dong³

(1) Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China
(2) Wichita State University, Wichita, KS
(3) China Offshore Oil Nanhai West Co, Zhanjiang, China

ABSTRACT: Overpressuring Mechanisms in Yinggehai Basin, South China Sea

Yinggehai Basin is an elongate Cenozoic rift basin on the northwestern margin of the South China Sea continental shelf. The thick (~17 km) basin fill is characterized by high thermal gradient and overpressure. Generation and accumulation of natural gas in the basin appear closely related to the overpressure. The processes and mechanisms of overpressuring were studied via numerical models coupling the evolution of basin filling, compaction, thermal and pressure fields. Overpressure associated with non-equilibrium compaction mainly occurs at a depth of ~3000 m at basin center and ~4000 m at basin margin, since Neogene and Quaternary strata are mainly interbedded thin shales and siltstones, and effective seals are rare. This overpressure distribution, however, is disrupted at basin center where high overpressure occurs at a depth as shallow as 1400 m on top of a series of deep-originated diapirs. Model results indicated that increase in natural gas by organic cracking is not effective enough to generate the overpressure because of the limited amount of organic matter. The overpressure in shallow permeable formations could be generated more effectively by a proposed "allogenic" mechanism. First, deep-buried rocks were fractured due to increasing extensional tectonic stress and overpressure. Then, the fractures propagated upward to form some open faults, connecting hydraulically the permeable formations at different depths that separated previously by seals. Such vertical hydraulic connection channeled the deep-seated high pressure into shallow permeable formations to form relative high abnormal pressures at shallow depth, as observed in the diapir-rich basin center.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado