莺歌海盆地中央泥底辟带气田水地球化学特征及其油气地质意义

以莺歌海盆地东方区和乐东区气田水样品水化学常规分析为切入点,利用气田水矿化度、水型、钠氯系数(变质系数)、脱硫系数,氯镁系数等参数,深入分析了两区气田水地球化学特征及水化学组合参数特征,并判识对比了两区气藏封闭程度及其保存条件。在此基础上,结合莺歌海盆地天然气成藏地质条件,重点剖析了两区气田水分布规律及成因控制因素,探讨了两区气田水与天然气运聚成藏之间的成因联系。强调指出,两区气田水具有相似的形成演化特征,天然气保存条件好,但对比两区气田水的水化学组合参数,乐东区天然气保存条件稍差于东方区;东方区气田水以NaHCO3水型占绝对优势,其分布规律与断裂系统存在密切联系。该区水文地质开启程度差,气藏盖层封闭性好,有利于天然气富集成藏。乐东区气田水类型多样,成因复杂,天然气保存条件较好但相对东方区稍差。两区气田水具分区分块分布规律,莺歌海盆地泥底辟及其伴生热流体活动对气田水成因及分布规律与天然气运聚成藏等均具有重要控制作用和影响。

GEOCHEMISTRY AND PETROLEUM GEOLOGICAL SIGNIFICANCE OF GAS FIELD WATER IN THE CENTRAL MUD DIAPIRIC BELT OF YINGGEHAI BASIN

Routine chemical analysis was performed for the gasfield water of Dongfang block and Ledong block in the Yinggehai Basin. Together with other analytical data, such as the water type, Na/Cl ratio(metamorphic coefficient), desulfurization coefficient, and Cl/Mg ratio, we studied the geochemical and hydrochemical characteristics of two blocks and their relation with the closure and preservation conditions of the gas reservoir. Based on the research work mentioned above, with the consideration of the geological conditions for accumulation of natural gas in the Yinggehai Basin,we paid more attention to the distribution pattern of gasfield water and its controlling factors in the two blocks, and the genetic relationship between the gasfield water and process of gas migration and accumulation.It is found that (1) the gasfield water of the two blocks is similar in origin and evolution. The preservation of natural gas is basically in good condition. However, the water chemical composition parameters indicate that the preservation conditions in the Ledong block are little worse than those in the Dongfang block.(2) The Dongfang gasfield is dominated by NaHCO3 type of gasfield water. The distribution pattern of the water has close relationship with the fault system. The Dongfang gasfield is low in opening degree and sealing property and the caprock of gas reservoir is good, as the gasfield water indicates. Therefore, it is favorable for the accumulation and preservation of hydrocarbons. The Ledong gasfield water is diverse with complex origin, the preservation condition is little worse than that in the Dongfang block.(3)Both the gasfield water and natural gas reservoirs of the two blocks have strong variations in spatial distribution.(4)The mud diapir and the accompanied thermal fluid in the Yinggehai Basin play an important role in controlling and affecting the origin and distribution pattern of the gasfield water as well as the gas migration and accumulation.