珠江口盆地白云凹陷及其周缘CO_2分布与控制因素分析与启示

DISTRIBUTION OF CARBON DIOXIDE IN BAIYUN SAG AND ITS PERIPHERY,PEARL RIVER MOUTH BASIN

南海北部各盆地普遍存在含CO_2气藏。莺歌海盆地CO_2主要受泥底辟带热流体上侵作用的控制,异常高CO_2仅在泥底辟带发现。然而珠江口盆地白云凹陷的泥底辟带与CO_2的关系暂不明确。通过收集分析珠江口盆地的60组(含/高含)CO_2气藏的气体组分和气体同位素地球化学数据,其R/Ra比值(样品的3 He/4 He与空气的3 He/4 He比值)均>2,CO_2成因主要为火山幔源型,不同于以壳源型为主的莺歌海盆地。位于白云凹陷中央泥底辟带的B1井,其CO_2碳同位素δ13 C值约-10.9‰^-10.5‰,根据自生片钠铝石的出现认为曾有CO_2大量聚集,而且CO_2分压高,推测为有机、无机成因混合气,不同于非泥底辟带的幔源无机成因。通过不同构造位置上的3口典型单井的对比分析及其与莺歌海盆地,以及泥底辟带和非泥底辟带的对比研究,认为凹陷周缘凸起上的断裂带和凹陷内的泥底辟带是高含CO_2气藏的主要分布区,认为白云凹陷泥底辟构造对于CO_2分布具有一定的控制作用。

CO2 gas reservoirs are widespread in the northern marginal basin of South China Sea. The generation of the CO2 in the Yinggehai basin is mainly controlled by the upward invasion of thermal fluids by mud diaper,where the CO2 content is unusually high. 60 data about gas composi- tions and gas isotopes from CO2 gas reservoirs in Pearl River Mouth basin was collected so as to understand the relationship between mud diapir in the Baiyun sag and COe gas. Because the R/Ra ratio is more than 2, the genesis of CO2 is confirmed as mantle sourced volcanic type, different from CO2 in Yinggehai basin. Carbon isotope la C of CO2 in the B1 well which is related to mud di- apir is in the range of 10.9‰- 10.5‰. The appearance of dawsonite indicates that there was a large amount accumulation of CO2 with high partial pressure CO2 and the CO2 is the mixture of organic and inorganic, different from inorganic genesis （mantle source） in the non-mud diapir zone. On the basis of analysis of 3 typical wells in different tectonic positions, compared with Yinggehai basin and combined with comparative study of mud diapir zone and non-mud diapir zone,it is proposed that fault zones on the uplift around the sag and mud diapir zones in the center of Baiyun sag is the main enrichment area of CO2 gas. Mud diapirs play an important roll in con- trolling the distribution of CO2.