Multiphase natural gas migration and accumulation and its relationship to diapir structures in the DF1-1 gas field, South China Sea

Abstract

The Dongfang1-1(DF1-1) gas field is the largest commercial gas field yet discovered in the Yinggehai Basin, South China Sea. The gas reservoirs in the field are normally pressured Pliocene shallow marine facies fine-grained sandstones, and the main source rocks are highly over-pressured Miocene mudstones and shales. The natural gases in the reservoirs consist mainly of hydrocarbon gas, CO 2 and N 2. Geochemical data for the natural gases show that the hydrocarbon-rich gases (except for a small amount of biogas) were derived from these source rocks distributed within over-pressured zones, but gases rich in CO 2 mainly originate from the thermal decomposition of Miocene calcareous shales and pre-Tertiary carbonates at higher temperature. Fluid inclusion data for the reservoirs were used together with the regional geology, distribution of different types of reservoirs and the evolution of diapir structures to determine the episodes of gas migration and accumulation in the DF1-1 gas field. We believe that the field was formed by four episodes of gas migration and accumulation. Fluid paleopressure simulation software, designed and developed by CNOOC China Limited, was used to simulate the paleopressure history of the strata at the top of the present-day high pressure zone (a depth of 2800 m) in the DF1-1 gas field. It predicts that the first episode occurred before about 2 Ma, the second and the third episodes between 2 and 0.5 Ma, and that the CO 2-rich gases migrated into the reservoirs after 0.5 Ma. The formation and evolution of diapir structures in the Yinggehai Basin had an important influence on the migration and accumulation of natural gases in the DF1-1 gas field. Rapid deposition, gas generation in the source rocks and thermal expansion of pore-fluids caused over-pressuring in the Miocene shales in the center of the depression. This over-pressuring together with active faulting, formed diapir structures during 5–0.5 Ma. The faults and fractures related to the diapir structures provided a pathway for gas vertical migration from the deep source rocks into the DF1-1 reservoirs, and controlled the geological distribution of different types of natural gases. On this basis, a model for gas migration and accumulation is suggested for the DF1-1 gas field.