热液环流——侵入岩-外变质带储层发育的重要影响因素

Hydrothermal circular flow——significant influence on reservoirs in intrusion rock-exomorphic zone

渤海湾盆地在第三纪具有频繁的基性岩浆活动,在这些高温岩浆的冷凝成岩及围岩递进变质过程中,所生成的COH热液将明显地改变地层的流体介质性质和运动学特征。渤海湾盆地的热液流动具有环流和单向流两种基本形式,其中,热液环流主要出现在具有连通孔缝体系的(超)浅成侵入环境,或者同生断层附近的基岩裂缝发育带;热液单向流则见于侵入深度大于2500m的第三系沉积地层、致密的前第三系碎屑岩,以及裂缝不发育的潜山基岩等环境。根据对热液径迹的观察和参数估算,这类热液环流的流速大约为中低温地热系统的5～9倍,并且循环作用可导致成岩环境具有较高水岩比。其结果不仅有助于形成较宽的变质晕和发育的变晶颗粒,同时使侵入岩及外变质带在完全还原条件下形成发育的溶蚀孔缝。但在单向流环境下,由于侵入岩浆在水岩比极低的条件下成岩,因此基本不具有储集能力。故而在渤海湾盆地,可以将岩浆侵入时的围岩环境作为侵入岩-外变质带储层预测的主要判据之一。

Basic magma was active in Tertiary in Bohai Gulf basin. During the process of condensation of high temperature magma and enclosing rock metamorphose, thus generated COH hydrothermal flow would substantially change the property and kinematic feature of formation fluid. Hydrothermal flow in Bohai Gulf basin was basically divided into circular flow and uniflow these two forms. Hydrothermal circular flow mainly occurred in (super) hypabyssal intrusive environment with connected pore-fracture system, or basement fracture zone near depositional fault; while hydrothermal uniflow occurred in Tertiary sedimentary formation with invasion depth over 2 500 m, pre-Tertiary tight clastic rock, and buried hill basement environment with undeveloped fractures. Through observation of hydrothermal trajectory and estimation of parameters, the velocity of such hydrothermal circular flow was about 59 times of that of mid-low temperature geothermal system, and circulation might result in high water-rock ratio in the diagenetic environment. Circular flow was helpful for generating wide metamorphic aureole and blastic grains, also made the intrusion rock and exomor-phic zone generate solution pore and fracture under totally oxygen-free condition. However, in the uniflow environment, because intrusive magma became rock at extremely low water-rock ration, there was almost no reservoir capability . Therefore, in Bohai Gulf basin, the enclosing rock environment during magma intrusion may be used as one of the major criteria for prediction of reservoirs in intrusion rock-exomorphic zone.