莺歌海盆地富CO_2气藏形成的有利条件及分布规律研究

Favorable Forming Conditions and Distribution Rules of Rich CO_2 Gas Reservoirs in Yinggehai Basin

以莺歌海盆地富CO_2气藏地球化学特征为基础,结合地质条件分析,探讨了CO_2的成因类型,分析了富CO_2气藏形成的有利地质条件和分布规律。结果表明：研究区富CO_2气藏CO_2主要为无机岩石化学成因,有少量幔源气混入;富CO_2气藏主要分布在中央底辟带底辟构造1 000~2 000 m深度范围内,东方区CO_2体积分数分布于0~18%和54%~88%这2个区间,且同一底辟构造内,下部储层CO_2含量高于上部储层,断裂附近储层CO_2含量高于远离断裂储层,乐东区CO_2体积分数介于0~88%,CO_2分布没有明显的规律性,两区底辟发育特征不同是CO_2分布特征差异的根本原因;中央坳陷带底辟区巨厚钙质砂泥岩沉积、高地温场和大地热流背景及泥底辟的周期性活动是富CO_2气藏形成的有利地质条件;中央坳陷带非底辟区和莺东斜坡带无成规模富CO_2气藏分布。

The genetic type of CO2, and the favorable forming conditions and the distribution rules of rich CO2 gas reservoirs are studied based on the analysis of the geochemical characteristics and the geological conditions of rich CO2 gas reservoirs in Yinggehai Basin. It is shown that ： the CO2 in the gas reservoirs is mainly CO2 of inorganic lithochemical origin, and a small amount of CO2 gas of mantle origin is mixed; the rich CO2 gas reservoirs mainly distribute in the depth range of 1 000 - 2000 m of the mud diapiric structures in the central diapir zone. In Dongfang area, the volume fraction of CO2 is in two ranges of 0 -18% and 54% - 88% ,in the same diapiric structure, CO2 content in the lower reservoirs is higher than that in the upper reservoirs, and CO2 content in the reservoirs near to faults is higher than that in the reservoirs far away faults. In Ledong area,the volume fraction of CO2 is in 0 ~ 88% ,there is on obvious rule in the dis- tribution of CO2. The difference of two areas in the diapir development features is the key reason leading to different CO2 distribution characteristic in two areas. In the diaper area of central depression zone,the giant thick calcareous sand mudstone,the high ground temperature field, the high terrestrial heat flow background and the periodic activity of the mud diapirs are favorable geological conditions for the formation of rich CO2 gas reservoirs. The large rich CO2 gas reservoir has not been found in the non - diaper area of central depression zone and the slope zone in eastern Yinggehai Basin.