Mechanisms of Gas Accumulation in the Yinggehai Overpressured Basin, South China Sea

Abstract

Yinggehai Basin is a typical overpressure basin on the northwestern margin of the South China Sea continental shelf. The nature gas reservoirs have been found in the basin today are mainly distributed around the diapir of central depression belt. Those reservoirs are divided into two layers in the depth, the shallow is normal pressure but the deep is overpressure. The overpressure in the basin mainly from the diapir conductions. Basin modelling results indicate that the gas generation of Sanya Formation (N1s) and Meishan Formation (N1m) were beginning in 14 and 8 Ma and the peak of gas generation came in 12 and 4 Ma, respectively. Therefore, the main phases of hydrocarbon generation and expulsion were during the Plio–Q period. The fluid inclusions data indicate that all those gas reservoirs formed in the Pleistocene since the 2 Ma and there were at least two phases: the first phase was the main formation of gas reservoirs in the whole basin and charged priority with the hydrocarbon gas; the second phase was charged with inorganic CO₂ near the diapir belt, and mixed partly with the early charged hydrocarbon gas. The diapir associated fracture in shallow and the hydraulic fractures in deep are two main elements of vertical flowing conduits, large number of dissolution are recognized from the diagenetic observation. Multi-episodes thermal fluid activity controlled by vertical flowing conduits system cause the natural gas migration along fault zone and condensate in the shallow normal pressure reservoirs very rapidly. In the deep overpressure areas, many minor faults that overpressure caused forming the path but the cap rocks were not fracture make gas migration and accumulation.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015