珠江口盆地CO2分布特征与成藏机制浅析

珠江口盆地多个凹陷均有钻井钻遇CO₂气体, 个别井钻遇的CO₂体积分数高达90%以上, 这些CO₂气体直接驱替了早期油藏或影响了油气充注规模, 增加了勘探风险。运用钻井数据、地球化学分析数据、重磁研究结果, 通过分析CO₂分布特征, 总结CO₂分布与断裂、火成岩的关系, 明确CO₂分布的主控因素, 预测珠江口盆地高CO₂风险带的分布。研究表明: 珠江口盆地对油气成藏有影响的CO₂主要为无机幔源成因, 受区域盖层和储层的控制, CO₂主要分布在上渐新统到下中新统地层及以下地层, 向上CO₂逐渐减少, 同时无机幔源CO₂分布明显受控于断裂体系, 与基底断裂和火成岩体具有较好的对应关系。无机CO₂气源主要为幔源岩浆的脱气作用, 垂向运移主要受不同级别断裂控制, 尤其该区深大断裂和基底长期活动断裂起到了很大作用, 因此幔源岩浆活动与断裂体系是CO₂汇聚的主控因素。由于珠江口盆地不同凹陷结构差异的影响, 幔源CO₂运聚成藏机制可概括为两类: 一是在北部坳陷带, 地壳厚度大, CO₂沿着深大断裂逐级向中上地壳运移形成"中转站", 然后在基底断裂的沟通下向浅层运移成藏; 二是在珠二坳陷, 由于地壳减薄作用, 并且发育拆离体系, 幔源岩浆首先到达拆离面, 脱出的CO₂沿着拆离面运移, 当遇到深切至拆离面的断裂后继续向浅层运移成藏。基于以上分析并结合运聚条件预测出珠江口盆地分布着7个高CO₂风险带, 这7个高CO₂风险带均位于深大断裂带附近, 并且周边火成岩较为发育。 

Distribution characteristics and accumulation mechanism of carbon dioxide gas reservoirs in the Pearl River Mouth Basin

Exploration practice has revealed abundant carbon dioxide gas reservoirs in many depressions of the Pearl River Mouth Basin, South China Sea with a CO₂ content as high as 90%. These CO₂ reservoirscan either directly displace the early oil reservoirs or affect the oil and gas charging scale, which further increases exploration risk. Based on the comprehensive analysis of drilling data, geochemical analysis data, gravity and magnetic research results, the migration and accumulation conditions of the CO₂ reservoirs have been investigated. Our results suggest that CO₂ in the Pearl River Mouth Basin is sourced by inorganic mantle, and its distribution locations are controlled by regional caprocks and reservoirs. CO₂ is mainly distributed in upper Oligocene to lower Miocene strata and the overlying strata, with its contents gradually decreasing upwards. Meanwhile, the distribution of inorganic mantle source CO₂ is greatly controlled by the fault system, and its distribution locations have a close relationship with the ranges of basement faults and igneous rocks. The inorganic CO₂ gas is sourced by the degassing of mantle-derived magma. Its vertical migration process is mainly controlled by different scales faults, especially the regional abyssal faults. Therefore, it can be concluded that mantle-derived magma activity and fault systems are the main controlling factors for the accumulation processes of CO₂ reservoirs. The mechanisms of mantle-derived CO₂ migration and accumulation can be summarized into two types by structural differences. In the northern depression zone, CO₂ migrates along deep faults step by step to the upper and middle crust to form a "transfer station". By contrast, the detachment system developed in the Zhu 2 Depression, and CO₂ migrates along the detachment plane to upper strata.