Hydrochemical properties of formation water and its relationship with oil and gas migration and accumulation in Zhenwu area of southern Gaoyou sag

Key words:  Gaoyou sag  globosity chemistry  oilfield water  hydrochemistry of formation water  vertical distribution  horizontal distribution  oil ＆ gas migration and accumulation

Abstract:

Based upon the analysis of vertical and horizontal distribution properties of formation water chemistry in Xuzhuang, Zhenwu and Caozhuang oilfields of Gaoyou sag, the origin of formation water and the relationship between hydrochemistry and oil and gas migration and accumulation were thoroughly studied. The results show that meteoric water infiltrated discontinuously in Xuzhuang oilfield, and the infiltration depth may reach 2 500 3 000 m. The origin of formation water in Zhenwu and Caozhuang oilfields is sedimentary burial water, but their over flow patterns are different. The hydrochemistry field of Zhenwu areas in southern part of Gaoyou sag can be divided into three parts:①freshening zone due to meteoric water leaching downwards; ②concentration zone due to cross formational flow; ③freshening zone due to compaction released water. In the south, meteoric water infiltrated due to the long term activity of Zhenwu fault. The mineralization of formation water is lower as the desalination of infiltrated meteoric water, and the hydrocarbon accumulation and preservation conditions are poor. The northern part, obviously affected by the desalting of compaction released water, has low mineralization of formation water. However, the formation water concentrated and its mineralization increased gradually during the process of centrifuge over flow.The part of the oil and gas could be preserved in appropriate trap in the path of secondary migration along with the formation water. The concentration zone due to cross formational flow in the middle part, with high mineralization degree of formation water, is the common point area of northern part mudstone compaction released water and southern part meteoric water infiltration. It is favorable to oil and gas accumulation, and the degree of enrichment is higher.